cinrs-core 0.2.0

//! Code generation: the typed [`ir`] becomes Rust tokens.
//!
//! Every token this module emits is stamped with the span of the C construct
//! it came from, resolved through the [`SourceMap`]. That is what makes an
//! error `rustc` raises about the *generated* code — a call with the wrong
//! argument type, say — point at the C the user actually wrote.
//!
//! # Shape of the output
//!
//! One expansion produces, in this order: the alignment wrappers an
//! over-aligned object needs, the `struct` and `union` items for every tag and
//! the flexible-array companions that go with them, the `enum` aliases and
//! their constants, the file-scope `typedef` aliases, an empty `extern` block
//! per library the unit links, one `extern` block for everything the unit only
//! declares, the `static mut` items, and finally the functions. A C function
//! becomes
//!
//! ```text
//! pub unsafe extern "C" fn name(mut p: ::core::ffi::c_int) -> ::core::ffi::c_int {
//!     unsafe { … }
//! }
//! ```
//!
//! `pub` is dropped for a C `static` function, which is private to the module
//! [`crate::expand`] wraps all of this in — that module is what makes C's
//! internal linkage mean something, and what lets two `c99!` blocks in one
//! Rust module both `#include` the same header. `#[inline]` is added for an
//! `inline` function, and the body is wrapped in a single `unsafe` block
//! because edition 2024 no longer treats the body of an `unsafe fn` as an
//! unsafe block. Nothing here carries a lint exemption of its own: everything
//! this module generates goes inside that one module, whose head carries a
//! single `#![allow(…)]` — an inner attribute, so every item under it
//! inherits it — for everything a naive translation provokes: unused
//! bindings, redundant parentheses, non-Rust naming, code a human can see is
//! unreachable, and so on. The list, and the reasons for each entry, are with
//! the code in `lib.rs` that wraps a unit in its module.
//!
//! A function the unit marked [safe](crate::sema::check_safe) —
//! `[[cinrs::safe]]`, `__attribute__((cinrs_safe))` or `#pragma cinrs safe` —
//! is the one exception: it is generated as `pub extern "C" fn` and its body is
//! *not* wrapped, so `rustc` checks every operation the translation of the C
//! needs and refuses the ones that are unsafe, with the caret on the C.
//!
//! A unit that asked for `#pragma cinrs export` gives everything with external
//! linkage `#[unsafe(no_mangle)]` on top of that, so that its functions and
//! objects are real C symbols another unit can link against — with C's own
//! risk of two definitions of one name, which only the linker will see.
//!
//! # Pointers, arrays and records
//!
//! Pointers are raw pointers — `T *` is `*mut T`, `const T *` is `*const T`,
//! `void *` is `*mut c_void` — and pointer arithmetic is `offset`, so nothing
//! in the output holds a reference and none of Rust's aliasing rules are
//! involved. Arrays are `[T; N]` and decay to a pointer through
//! `(&raw mut a).cast::<T>()`, which is a raw pointer from the start; a `&mut`
//! would both change the meaning and trip `static_mut_refs` on a global. A
//! function pointer is `Option<unsafe extern "C" fn(…) -> R>`, so that a null
//! one is representable, and a call through it unwraps first.
//!
//! # Places
//!
//! Assignment, compound assignment, `++`/`--` and `&` all go through this
//! module's `place` lowering, which turns an [`ir::Place`] into a *setup*
//! (statements that must run first, where the pointer arithmetic lands) plus an
//! *access* (a Rust place expression that may be evaluated more than once).
//! `p[i()] += 1` therefore evaluates `i()` exactly once, and `s.f`, `p->f` and
//! `a[i][j]` are all the same three lines of code.
//!
//! # Bit-fields
//!
//! A bit-field has no address, so it is not a field of the generated item: a
//! run of them shares one `[u8; K]`, and sema has already worked out which
//! bytes and bits each member owns (see [`crate::sema`]'s layout). This module
//! turns that into a pair of inherent methods per named member — plain inline
//! integer code, no helper type — and a place whose *access* is the record
//! rather than the member, read with `.f()` and written with `.set_f(v)`. A
//! constant initialiser is folded into the storage bytes here, which is what
//! lets a `static` hold one; a non-constant one becomes a zeroed literal
//! followed by setter calls. A place rooted in a `static mut` goes through
//! `&raw mut` first, because the accessors borrow.
//!
//! # Loops
//!
//! Every loop gets a unique Rust label so that `break` and `continue` never
//! depend on where they sit relative to a `switch`:
//!
//! ```text
//! while (c) B      'lN: while c { B }                continue → continue 'lN
//! do B while (c);  'lN: loop { 'lN_body: { B }       continue → break 'lN_body
//!                             if !(c) { break 'lN } }
//! for (i;c;s) B    { i; 'lN: loop { if !(c) { break 'lN }
//!                                   'lN_body: { B } s; } }
//! ```
//!
//! The body label exists exactly where `continue` has work to do afterwards —
//! re-testing the condition of a `do`/`while`, or running the step of a `for`.
//!
//! # Switch
//!
//! Fallthrough is what makes `switch` interesting: control enters at one label
//! and then runs *through* every group after it. Rust has no such construct,
//! but labelled blocks compose into one. For groups `g0 … gN` the output is
//!
//! ```text
//! 'swK: {
//!     'swK_cN: { … 'swK_c1: { 'swK_c0: { match v { … } } g0 } g1 … }
//!     gN
//! }
//! ```
//!
//! with the dispatch `match` innermost: `break 'swK_ci` lands immediately
//! before group *i*, and control then falls out of each enclosing block in
//! turn, running the groups after it in order. `break` inside the switch is
//! `break 'swK`, and the `_` arm goes to the `default:` group, or out of the
//! whole statement when there is none.
//!
//! # Functions that jump
//!
//! Most `goto`s stay here too. [`regions`](crate::regions) has wrapped the
//! statements a label divides in an [`ir::Region`], which is one of
//!
//! ```text
//! 'done: { … break 'done; … }      'retry: loop { … continue 'retry; … break 'retry; }
//! ```
//!
//! named after the C label — with the number of the label appended when Rust
//! cannot spell the name as a label (`loop:` is a keyword) or when this module
//! already gives that name to a loop of its own. A [`Stmt::Goto`] left inside
//! one is the `break` or the `continue`; which it is comes from the region it
//! stands in.
//!
//! A body sema lowered into a [control-flow graph](crate::cfg) instead —
//! because it holds a jump Rust cannot make at all — is emitted as a state
//! machine over its blocks, with the function's locals defined once at the
//! top. Everything below the statement level is shared: the expressions,
//! places and conversions are generated by exactly the same code in both
//! modes.
//!
//! # Calls without a prototype
//!
//! `int f();` says nothing about the parameters (C99 6.7.5.3p14), so the item
//! generated for it is `unsafe extern "C" fn() -> R` — that is all the
//! declaration said. What the *call* passes is decided at the call site
//! (6.5.2.2p6): sema has already applied the default argument promotions to
//! every argument, and this module transmutes the callee to the signature they
//! make before calling it,
//!
//! ```text
//! ::core::mem::transmute::<unsafe extern "C" fn() -> R,
//!                          unsafe extern "C" fn(T1, …, Tn) -> R>(f)(a1, …, an)
//! ```
//!
//! with the `Option` unwrapped first for a function pointer, and no cast at all
//! when there are no arguments. Reinterpreting a function pointer like this is
//! exactly the contract C's own ABI rests on: on every ABI this crate targets
//! the address is the same one, and the call is defined precisely when the
//! callee really was defined with parameters of those promoted types —
//! undefined otherwise, which is the risk the program took by leaving the
//! prototype out. The same route is taken whenever the argument count and the
//! parameter count disagree at all, which keeps a call checked
//! against an empty list from being emitted against a prototype a later
//! declaration supplied.
//!
//! # Variadic definitions
//!
//! `R f(T a, ...)` becomes `unsafe extern "C" fn f(mut a: T, __cinrs_va: ...)`.
//! The extra parameter is the argument list as the caller left it and is never
//! advanced; `va_start` and every `va_list` local copy it, `va_arg` is
//! `next_arg`, `va_copy` and passing a list on are `clone`, and `va_end` is
//! nothing at all, because the list ends when its value is dropped. See
//! [`crate::sema`]'s `va` module for the model.

use std::cell::{Cell, RefCell};
use std::collections::{BTreeSet, HashMap, HashSet};
use std::str::FromStr;

use proc_macro2::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
use quote::quote_spanned;

use crate::Options;
use crate::capture::{SourceMap, SourceRange};
use crate::cfg::{BasicBlock, BlockId, Cfg, Terminator};
use crate::ir::{
    self, AtomicClass, BinOp, Body, BreakTarget, Callee, CmpOp, ConstValue, Expr, ExprKind,
    Function, LogicalOp, LoopId, NEVER_RAW, Place, PlaceKind, Program, RecordKind, Stmt, Storage,
    Switch, Ty,
};
use crate::reloop;

/// Generates the Rust items for a fully checked program.
pub fn generate(program: &Program, map: &SourceMap, options: &Options) -> TokenStream {
    let mut cg = Codegen::new(program, map, options);
    let mut out = cg.type_items();
    out.extend(cg.extern_block());
    for var in &program.statics {
        out.extend(cg.static_item(var));
    }
    let mut initialisers = TokenStream::new();
    for func in &program.functions {
        if func.body.is_some() {
            out.extend(cg.function_item(func));
            if let Some(kind) = func.init_kind {
                initialisers.extend(cg.init_array_item(func, kind));
            }
        }
    }
    if !initialisers.is_empty() {
        out.extend(cg.init_array_guard());
        out.extend(initialisers);
    }
    if out.is_empty() {
        // A unit that declares nothing expands to nothing at all — not even a
        // module — and there is no code for the data model to be wrong about.
        return out;
    }
    let mut items = cg.data_model_check();
    if cg.uses_cleanup.get() {
        items.extend(cg.cleanup_guard_item(Span::call_site()));
    }
    if cg.uses_arena.get() {
        let span = cg.arena_span.get().unwrap_or_else(Span::call_site);
        items.extend(cg.arena_items(span));
    }
    // The shims the unit needs for the intrinsics whose address it took; see
    // [`Codegen::address_taken`]. Collected while the bodies were generated,
    // so this has to come after them.
    items.extend(cg.intrinsic_shim_items());
    items.extend(out);
    items
}

/// Generates signature-only items for a program that did not type check.
///
/// The bodies are `::core::unreachable!()`: the expansion also carries
/// `compile_error!`s, so nothing here can ever run. Their purpose is to keep a
/// Rust call site that mentions one of these functions from producing a second,
/// unrelated "cannot find function" error on top of the real one.
pub fn generate_stubs(program: &Program, map: &SourceMap, options: &Options) -> TokenStream {
    let mut cg = Codegen::new(program, map, options);
    let mut out = cg.type_items();
    out.extend(cg.extern_block());
    for var in &program.statics {
        out.extend(cg.static_item(var));
    }
    for func in &program.functions {
        if !func.is_extern() {
            out.extend(cg.stub_item(func));
        }
    }
    out
}

// ---------------------------------------------------------------------------
// precedence
// ---------------------------------------------------------------------------

/// Rust's expression precedence levels, tightest last.
///
/// Emitted expressions carry the level they parse at so that parentheses are
/// added exactly where they are needed and nowhere else — the generated code is
/// meant to be read.
mod prec {
    /// A block-like expression (`if`, `{ … }`): usable on its own, but needing
    /// parentheses anywhere an operator or a statement boundary follows.
    pub const BLOCK: u8 = 0;
    /// The weakest level an operand can be handed to without parentheses.
    pub const LOWEST: u8 = 0;
    pub const OR: u8 = 2;
    pub const AND: u8 = 3;
    pub const CMP: u8 = 4;
    pub const BIT_OR: u8 = 5;
    pub const BIT_XOR: u8 = 6;
    pub const BIT_AND: u8 = 7;
    pub const SUM: u8 = 9;
    pub const PRODUCT: u8 = 10;
    pub const CAST: u8 = 11;
    pub const UNARY: u8 = 12;
    pub const CALL: u8 = 13;
    pub const ATOM: u8 = 14;
}

/// An emitted expression together with the precedence it parses at.
struct Value {
    tokens: TokenStream,
    prec: u8,
    /// Set when the tokens are a bare non-negative integer literal, whose Rust
    /// type is therefore still open to inference.
    bare_integer: bool,
    /// Set when the tokens end with the type of an `as`.
    ///
    /// `x as i32 < y` does not parse: Rust reads the `<` as the start of the
    /// generic arguments of `i32`. (`>`, `<=`, `>=` and `==` are unambiguous
    /// and need no help.)
    ends_with_type: bool,
}

impl Value {
    fn new(tokens: TokenStream, prec: u8) -> Self {
        Self {
            tokens,
            prec,
            bare_integer: false,
            ends_with_type: false,
        }
    }

    fn atom(tokens: TokenStream) -> Self {
        Self::new(tokens, prec::ATOM)
    }

    /// Marks the tokens as ending with the type of an `as`.
    fn type_end(mut self, flag: bool) -> Self {
        self.ends_with_type = flag;
        self
    }

    /// The tokens, parenthesised if they would not survive being used as an
    /// operand at `min`.
    fn at(self, min: u8, span: Span) -> TokenStream {
        if self.prec >= min {
            return self.tokens;
        }
        parenthesize(self.tokens, span)
    }

    /// The tokens, parenthesised only if they are block-like.
    ///
    /// Used where Rust starts parsing an expression that is followed by a block
    /// — the condition of an `if` or a `while` — and would otherwise mistake
    /// our expression's own braces for that block.
    fn at_condition(self, span: Span) -> TokenStream {
        if self.prec > prec::BLOCK {
            return self.tokens;
        }
        parenthesize(self.tokens, span)
    }
}

fn parenthesize(tokens: TokenStream, span: Span) -> TokenStream {
    let mut group = Group::new(Delimiter::Parenthesis, tokens);
    group.set_span(span);
    TokenStream::from(TokenTree::Group(group))
}

/// Whether an emitted expression opens with a unary minus.
///
/// `as` is the one operator in front of which that matters: `rustc` reads
/// `-1 as u32` as the negation of a `u32` rather than as a cast of `-1`, and
/// refuses it with `E0600`.
fn starts_with_minus(tokens: &TokenStream) -> bool {
    matches!(
        tokens.clone().into_iter().next(),
        Some(TokenTree::Punct(punct)) if punct.as_char() == '-'
    )
}

fn braced(tokens: TokenStream, span: Span) -> TokenStream {
    let mut group = Group::new(Delimiter::Brace, tokens);
    group.set_span(span);
    TokenStream::from(TokenTree::Group(group))
}

fn bracketed(tokens: TokenStream, span: Span) -> TokenStream {
    let mut group = Group::new(Delimiter::Bracket, tokens);
    group.set_span(span);
    TokenStream::from(TokenTree::Group(group))
}

// ---------------------------------------------------------------------------
// the Microsoft library's inline printf family, and the names it exports under
// a different spelling
// ---------------------------------------------------------------------------

/// The library that has out-of-line definitions of the `printf` and `scanf`
/// families for the Microsoft C runtime.
///
/// It is part of the MSVC toolset —
/// `VC/Tools/MSVC/<version>/lib/<arch>/legacy_stdio_definitions.lib`, one copy
/// per architecture — so it is there on every `*-windows-msvc` target and on no
/// other, which is why [`TargetModel::is_msvc`](crate::TargetModel::is_msvc) and
/// not merely Windows is what asks for it.
const LEGACY_STDIO_DEFINITIONS: &str = "legacy_stdio_definitions";

/// The names that need [`LEGACY_STDIO_DEFINITIONS`] on an MSVC target, each with
/// the library that defines it.
///
/// In the Universal CRT — the Microsoft C library from Visual Studio 2015 on —
/// the `printf` and `scanf` families are **inline functions in `<stdio.h>` and
/// `<wchar.h>`**, written over `__stdio_common_vfprintf` and its relatives, so
/// the import library exports no `printf` at all: an `extern "C" { fn printf(…);
/// }` is `LNK2019: unresolved external symbol printf` at link time. Microsoft
/// ships `legacy_stdio_definitions.lib` with out-of-line definitions for exactly
/// that case, and it is what Rust's own `libc` crate links for the same
/// declarations (`#[cfg_attr(all(windows, target_env = "msvc"), link(name =
/// "legacy_stdio_definitions"))]`). See "The printf and scanf family of
/// functions are now defined inline" in Microsoft's change history:
/// <https://learn.microsoft.com/en-us/cpp/porting/visual-cpp-change-history-2003-2015>
///
/// The rule this table serves is [`Codegen::legacy_stdio_libraries`], and it is
/// keyed on the **symbol a generated declaration links by** rather than on a
/// header: a unit may declare `int printf(const char *, ...);` itself, or reach
/// the function through `__builtin_printf`, and never include `<stdio.h>` at
/// all.
///
/// One row per name, so that a name can be given a different answer on its own
/// should a toolchain ever disagree: a row may name a different library, or go,
/// and nothing else changes. What it must never become is an alias to
/// `_snprintf`, whose truncation semantics are not C's.
///
/// Every name here was read out of a real copy of the library —
/// `dumpbin`/`nm` over `legacy_stdio_definitions.lib` from MSVC 14.44.35207,
/// x64 and x86 both — so the list is the library's own contents and not a
/// guess. `snprintf` and `vsnprintf` are in it, which is worth saying because
/// they are C99 additions the Microsoft library had no out-of-line form of
/// *before* the UCRT and so, unlike the rest, never "became" inline; `libc`
/// declares `snprintf` in the very block it links this library for. The library
/// also holds the `_l`, `_p`, `_s` and `_snprintf` variants, which are
/// deliberately not here: they are Microsoft's functions rather than C's, and a
/// unit that declares one has named the platform already and can say
/// `#pragma cinrs link "legacy_stdio_definitions"` itself.
const LEGACY_STDIO: &[(&str, &str)] = &[
    ("printf", LEGACY_STDIO_DEFINITIONS),
    ("fprintf", LEGACY_STDIO_DEFINITIONS),
    ("sprintf", LEGACY_STDIO_DEFINITIONS),
    ("snprintf", LEGACY_STDIO_DEFINITIONS),
    ("vprintf", LEGACY_STDIO_DEFINITIONS),
    ("vfprintf", LEGACY_STDIO_DEFINITIONS),
    ("vsprintf", LEGACY_STDIO_DEFINITIONS),
    ("vsnprintf", LEGACY_STDIO_DEFINITIONS),
    ("scanf", LEGACY_STDIO_DEFINITIONS),
    ("fscanf", LEGACY_STDIO_DEFINITIONS),
    ("sscanf", LEGACY_STDIO_DEFINITIONS),
    ("vscanf", LEGACY_STDIO_DEFINITIONS),
    ("vfscanf", LEGACY_STDIO_DEFINITIONS),
    ("vsscanf", LEGACY_STDIO_DEFINITIONS),
    ("wprintf", LEGACY_STDIO_DEFINITIONS),
    ("fwprintf", LEGACY_STDIO_DEFINITIONS),
    ("swprintf", LEGACY_STDIO_DEFINITIONS),
    ("vwprintf", LEGACY_STDIO_DEFINITIONS),
    ("vfwprintf", LEGACY_STDIO_DEFINITIONS),
    ("vswprintf", LEGACY_STDIO_DEFINITIONS),
    ("wscanf", LEGACY_STDIO_DEFINITIONS),
    ("fwscanf", LEGACY_STDIO_DEFINITIONS),
    ("swscanf", LEGACY_STDIO_DEFINITIONS),
    ("vwscanf", LEGACY_STDIO_DEFINITIONS),
    ("vfwscanf", LEGACY_STDIO_DEFINITIONS),
    ("vswscanf", LEGACY_STDIO_DEFINITIONS),
];

/// The C functions the Microsoft C runtime exports under **another name**, each
/// with the symbol a declaration of it has to link by on an MSVC target.
///
/// `LEGACY_STDIO` above is about a family the UCRT defines inline and ships an
/// out-of-line copy of in a library of its own. This is the other shape of the
/// same problem: the function *is* in `ucrt.lib`, under a name that is not the
/// one C gives it, and Microsoft's own headers paper over the difference with a
/// macro (`#define time _time64`) that cinrs's bundled headers, which are the
/// same on every platform, do not write.
///
/// Every row was read out of the real import libraries with `nm` — the Windows
/// SDK's `ucrt.lib` and the MSVC toolset's `msvcrt.lib`, 10.0.26100.0 and
/// 14.44.35207 — and not inferred. `ucrt.lib` exports each symbol on the right
/// and none of the C names on the left.
///
/// The `<time.h>` rows are **not** merely a link error, which is what makes
/// them worth a table rather than a paragraph in the documentation. The MSVC
/// toolset's `msvcrt.lib` holds an "alias map" object per name that defines
/// `time` as a *weak* external for `_time32`, and the rest likewise — so a
/// declaration of `time` does link, silently, to the **32-bit** `time_t`
/// function. cinrs's `<time.h>` makes `time_t` 64 bits on Windows, as the UCRT
/// does, and the two disagree in ways a program sees: `_mktime32`'s `(time_t)-1`
/// comes back as `0x0000_0000_FFFF_FFFF`, and `_gmtime32` answers `NULL` for
/// every date after 2038 rather than a `struct tm`. Naming `_time64` and its
/// siblings is therefore a correctness fix and not only a convenience.
///
/// `hypotf` is the one `<math.h>` name with an exported equivalent: `_hypotf`
/// is a real export with C's signature (Microsoft's `<math.h>` makes `hypotf`
/// inline over it). `fabsf`, `frexpf` and `ldexpf` have no symbol at all in any
/// library on an MSVC link line — they are inline over the `double` forms — and
/// so are not here; `doc/cross-compilation.md` records them, with the
/// workaround. The same goes for `<wchar.h>`'s `wmemcpy`, `wmemmove`, `wmemset`,
/// `wmemcmp`, `wmemchr`, `mbsinit` and `fwide`.
///
/// The rule this table serves is [`Codegen::msvc_symbol`], which applies to a
/// **declaration** and after an `__asm__("…")` label: a program that has named
/// the symbol it wants by hand has said the last word, which is also how a unit
/// asks for `_time32` on purpose.
const MSVC_RENAMED: &[(&str, &str)] = &[
    ("time", "_time64"),
    ("difftime", "_difftime64"),
    ("mktime", "_mktime64"),
    ("localtime", "_localtime64"),
    ("gmtime", "_gmtime64"),
    ("ctime", "_ctime64"),
    ("timespec_get", "_timespec64_get"),
    ("hypotf", "_hypotf"),
];

// ---------------------------------------------------------------------------
// identifiers
// ---------------------------------------------------------------------------

/// Every Rust keyword, strict and reserved, in every edition up to 2024.
const RUST_KEYWORDS: &[&str] = &[
    "abstract", "as", "async", "await", "become", "box", "break", "const", "continue", "crate",
    "do", "dyn", "else", "enum", "extern", "false", "final", "fn", "for", "gen", "if", "impl",
    "in", "let", "loop", "macro", "match", "mod", "move", "mut", "override", "priv", "pub", "ref",
    "return", "self", "static", "struct", "super", "trait", "true", "try", "type", "typeof",
    "unsafe", "unsized", "use", "virtual", "where", "while", "yield", "Self",
];

/// Names that a `let` binding or a parameter must not carry, whatever the C
/// program calls them.
///
/// Rust resolves a binding pattern against the value namespace first: a name
/// that already means a unit variant there is a *pattern* that matches, not a
/// new binding, and Rust refuses the ambiguity outright with `E0530`. The four
/// below are in scope in every Rust file through the prelude; the rest of the
/// set is computed per translation unit in [`Codegen::new`], because a C
/// program may name a local exactly like one of its own globals or
/// enumerators:
///
/// ```c
/// int counter;
/// int f(int counter) { return counter; }   /* two different objects */
/// ```
const PRELUDE_PATTERNS: &[&str] = &["Some", "None", "Ok", "Err"];

/// Whether `segment` is an ordinary Rust identifier — one that can stand as a
/// path segment spelled verbatim rather than as `r#…`.
///
/// Its one caller is [`is_crate_path`], whose value is pasted into the
/// expansion as tokens; the three keywords a segment may nevertheless be are
/// allowed there rather than here.
fn is_path_segment(segment: &str) -> bool {
    let mut chars = segment.chars();
    chars
        .next()
        .is_some_and(|c| c.is_ascii_alphabetic() || c == '_')
        && chars.all(|c| c.is_ascii_alphanumeric() || c == '_')
        && !RUST_KEYWORDS.contains(&segment)
        && !NEVER_RAW.contains(&segment)
}

/// Whether a string is usable as the Rust path of a crate:
/// `#pragma cinrs crate "…"`.
///
/// A sequence of segments joined by `::`, optionally starting with one, where
/// each segment is an identifier — with `crate`, `self` and `super` allowed as
/// segments because a re-export is often reached through one. Deliberately
/// strict: the value is pasted into the expansion as tokens, and anything else
/// there would be a syntax error in generated code rather than a message about
/// the pragma.
pub fn is_crate_path(path: &str) -> bool {
    /// The three keywords a path segment may be even though an ordinary
    /// identifier may not.
    const PATH_KEYWORDS: &[&str] = &["crate", "self", "super"];

    let body = path.strip_prefix("::").unwrap_or(path);
    if body.is_empty() {
        return false;
    }
    body.split("::")
        .all(|segment| PATH_KEYWORDS.contains(&segment) || is_path_segment(segment))
}

/// Turns a C identifier into the Rust identifier that stands for it, before
/// the rest of the translation unit is taken into account.
///
/// C names are kept as they are, because that is what makes the expansion
/// readable and what lets Rust call the functions by the names their author
/// gave them. A name that collides with a Rust keyword becomes a raw
/// identifier (`match` → `r#match`); the five names that cannot even be raw
/// get an underscore appended instead. A `$` — which C takes as an identifier
/// character and Rust has no spelling for — is written [`DOLLAR`].
///
/// The last two rules are the only ones that can hand two different C names
/// the same Rust one, which is what [`Names`] exists to prevent: everything
/// the generator emits goes through [`Names::ident`] rather than through this.
fn c_ident(name: &str, span: Span) -> Ident {
    let spelled = rust_spelling(name);
    let name = spelled.as_ref();
    if NEVER_RAW.contains(&name) {
        return Ident::new(&format!("{name}_"), span);
    }
    if RUST_KEYWORDS.contains(&name) {
        return Ident::new_raw(name, span);
    }
    Ident::new(name, span)
}

/// What a `$` in a C identifier is written as in the generated Rust.
///
/// `$` is an identifier character here — GCC takes it unconditionally and
/// Clang by default, which is what WG14 DR027 allows and what
/// [`crate::Options::dollar_in_identifiers`] switches on — and Rust has no
/// spelling for it at all, not even a raw identifier. The C name is still what
/// the symbol links by: an object or function that is not defined here carries
/// it in `#[link_name]`, and one that is carries it in `#[unsafe(export_name)]`.
pub const DOLLAR: &str = "_dollar_";

/// A C identifier as Rust can spell it, which is the same string unless a `$`
/// is in it.
fn rust_spelling(name: &str) -> std::borrow::Cow<'_, str> {
    if name.contains('$') {
        std::borrow::Cow::Owned(name.replace('$', DOLLAR))
    } else {
        std::borrow::Cow::Borrowed(name)
    }
}

/// Whether Rust spells `name` as something other than itself, so that another
/// C name could be spelled the same way.
///
/// A keyword is not one of these: `r#match` is a token of its own and no name
/// but `match` is written that way.
fn respelled(name: &str) -> bool {
    name.contains('$') || NEVER_RAW.contains(&name)
}

/// The text [`c_ident`] gives a name, without the `r#` a raw identifier is
/// written with.
fn plain_spelling(name: &str) -> String {
    let spelled = rust_spelling(name);
    if NEVER_RAW.contains(&spelled.as_ref()) {
        return format!("{spelled}_");
    }
    spelled.into_owned()
}

/// The Rust spelling of every name the translation unit gives to something,
/// made unique across the whole unit.
///
/// Two of the rules in [`c_ident`] change the spelling of a name, and a
/// program is free to use the changed spelling itself:
///
/// ```c
/// int f(void) { int self = 1; int self_ = 2; return self * 10 + self_; }
/// ```
///
/// Both locals would be `self_`, the second binding would shadow the first,
/// and `f` would quietly return 22 instead of 12. The same collision between
/// two members is `E0124`, between two file-scope items `E0428`, and `a$b`
/// next to `a_dollar_b` is the same story again.
///
/// So the spelling is settled once, here, for the unit as a whole. Every name
/// the unit spells is collected — objects, functions and their parameters,
/// tags, members, bit-field accessors, enumerators and `typedef` names, in
/// every name space at once, because one C name must read as one Rust name
/// wherever it appears. A name Rust spells as it is written keeps it; a name
/// whose spelling changes takes the usual one and grows another `_` for as
/// long as something else already has it. Nothing moves for a program that
/// does not have the collision, which is very nearly every program.
///
/// Labels are left out: `'self_` is in a name space of its own, and
/// [`Codegen::name_regions`] already keeps the labels of a function apart.
struct Names {
    /// The Rust spelling of each name whose spelling is not the name itself.
    ///
    /// Empty for a unit that writes no such name, which is the usual case.
    renamed: HashMap<String, String>,
}

impl Names {
    /// Works out the spellings of one translation unit.
    fn new(program: &Program) -> Self {
        let mut spelled: Vec<&str> = Vec::new();
        for object in &program.objects {
            spelled.push(&object.name);
            match &object.storage {
                Storage::Static { item_name, .. }
                | Storage::ThreadLocal { item_name, .. }
                | Storage::Extern { item_name } => spelled.push(item_name),
                Storage::Automatic => {}
            }
        }
        for func in &program.functions {
            spelled.push(&func.name);
            spelled.push(func.item_name());
            spelled.extend(func.param_names.iter().flatten().map(String::as_str));
            // The names the state-machine lowering hoists the locals under,
            // which are what a CFG body's bindings are generated from.
            if let Some(Body::Cfg(cfg)) = &func.body {
                spelled.extend(cfg.locals.iter().map(|local| local.rust_name.as_str()));
            }
        }
        for record in program.types.records() {
            spelled.extend(record.tag.as_deref());
            spelled.push(&record.rust_name);
            for field in &record.fields {
                spelled.push(&field.name);
                if let Some(bits) = &field.bits {
                    spelled.push(&bits.getter);
                    spelled.push(&bits.setter);
                }
            }
            for field in &record.rust_fields {
                match field {
                    ir::RustField::Bits { name, .. }
                    | ir::RustField::Pad { name, .. }
                    | ir::RustField::Align { name, .. } => spelled.push(name),
                    ir::RustField::Member(_) => {}
                }
            }
        }
        for def in program.types.enums() {
            spelled.extend(def.tag.as_deref());
            spelled.push(&def.rust_name);
        }
        for constant in &program.enum_constants {
            spelled.push(&constant.name);
            spelled.push(&constant.rust_name);
        }
        for typedef in &program.typedefs {
            spelled.push(&typedef.rust_name);
        }

        Self {
            renamed: unique_spellings(spelled),
        }
    }

    /// The Rust identifier a C name is generated as, everywhere it appears.
    fn ident(&self, name: &str, span: Span) -> Ident {
        match self.renamed.get(name) {
            Some(unique) => Ident::new(unique, span),
            None => c_ident(name, span),
        }
    }

    /// The same as text, without the `r#` a raw identifier is written with.
    fn spelling<'n>(&'n self, name: &'n str) -> std::borrow::Cow<'n, str> {
        match self.renamed.get(name) {
            Some(unique) => std::borrow::Cow::Borrowed(unique.as_str()),
            None => std::borrow::Cow::Owned(plain_spelling(name)),
        }
    }
}

/// The rule [`Names`] is built on: what each name whose Rust spelling is not
/// itself is spelled as, given everything the unit spells.
///
/// A name Rust writes as it stands claims that spelling first — two such
/// names are distinct exactly when the C names are — and what is left grows
/// another `_` for as long as something already has its spelling. The names
/// that need one are settled in sorted order, so that the answer never
/// depends on the order the arenas happen to be in.
fn unique_spellings<'n>(spelled: impl IntoIterator<Item = &'n str>) -> HashMap<String, String> {
    let mut taken: HashSet<&str> = HashSet::new();
    let mut changing: BTreeSet<&str> = BTreeSet::new();
    for name in spelled {
        if respelled(name) {
            changing.insert(name);
        } else {
            taken.insert(name);
        }
    }
    let mut renamed: HashMap<String, String> = HashMap::new();
    let mut assigned: HashSet<String> = HashSet::new();
    for name in changing {
        let mut candidate = plain_spelling(name);
        while taken.contains(candidate.as_str()) || assigned.contains(&candidate) {
            candidate.push('_');
        }
        assigned.insert(candidate.clone());
        renamed.insert(name.to_owned(), candidate);
    }
    renamed
}

/// Whether `name` can stand as the label of a Rust block or loop.
///
/// A label is not a raw identifier, so a Rust keyword — `loop:` is a perfectly
/// ordinary C label — cannot be one, nor can the five names that cannot even
/// be raw. The labels this module builds itself are kept clear too, so that a
/// `break` in a region never names a loop instead. A C label that is one of
/// them keeps its own name with the label's number appended.
fn is_label_name(name: &str) -> bool {
    let mut chars = name.chars();
    let starts = chars
        .next()
        .is_some_and(|c| c.is_ascii_alphabetic() || c == '_');
    starts
        && chars.all(|c| c.is_ascii_alphanumeric() || c == '_')
        && !RUST_KEYWORDS.contains(&name)
        && !NEVER_RAW.contains(&name)
        && !is_generated_label(name)
}

/// The labels this module gives its own loops, `switch`es, state machine and
/// [recovered shapes](crate::reloop): `'cfg`, `'lN`, `'lN_body`, `'swN`,
/// `'swN_cM`, `'bN` and `'rN`.
fn is_generated_label(name: &str) -> bool {
    fn digits(text: &str) -> bool {
        !text.is_empty() && text.bytes().all(|b| b.is_ascii_digit())
    }

    if name == "cfg" {
        return true;
    }
    if let Some(rest) = name.strip_prefix('l')
        && digits(rest.strip_suffix("_body").unwrap_or(rest))
    {
        return true;
    }
    for prefix in ['b', 'r'] {
        if let Some(rest) = name.strip_prefix(prefix)
            && digits(rest)
        {
            return true;
        }
    }
    if let Some(rest) = name.strip_prefix("sw") {
        if digits(rest) {
            return true;
        }
        if let Some((switch, case)) = rest.split_once("_c") {
            return digits(switch) && digits(case);
        }
    }
    false
}

/// Every [region](ir::Region) of a body, outermost first, with the C label it
/// is named after.
fn collect_regions(stmts: &[Stmt], out: &mut Vec<(ir::LabelId, String)>) {
    for stmt in stmts {
        match stmt {
            Stmt::Region(region) => {
                out.push((region.label, region.name.clone()));
                collect_regions(&region.body, out);
            }
            Stmt::Block(items) => collect_regions(items, out),
            Stmt::Label { body, .. } | Stmt::Case { body, .. } => {
                collect_regions(std::slice::from_ref(body), out);
            }
            Stmt::If {
                then_branch,
                else_branch,
                ..
            } => {
                collect_regions(std::slice::from_ref(then_branch), out);
                if let Some(branch) = else_branch {
                    collect_regions(std::slice::from_ref(branch), out);
                }
            }
            Stmt::While { body, .. } | Stmt::DoWhile { body, .. } => {
                collect_regions(std::slice::from_ref(body), out);
            }
            Stmt::For { init, body, .. } => {
                collect_regions(init, out);
                collect_regions(std::slice::from_ref(body), out);
            }
            Stmt::Switch(switch) => {
                collect_regions(&switch.prelude, out);
                for group in &switch.groups {
                    collect_regions(&group.body, out);
                }
            }
            Stmt::SwitchTree(switch) => collect_regions(std::slice::from_ref(&switch.body), out),
            _ => {}
        }
    }
}

/// The lint exemptions every generated item carries.
///
/// A transliteration of C is unidiomatic Rust by construction: names are not
/// snake case, locals are `mut` whether or not they are assigned again, a
/// `switch` leaves labels that nothing jumps to, and a function that ends in an
/// infinite loop leaves code behind that cannot run. `unknown_lints` comes
/// first so that the list may name a lint an older compiler has never heard of.
///
/// Two of them are about the `extern` block. A C program declares the library
/// its own way — `int strlen();` with no prototype is what a C89 program
/// writes, and an implicit declaration is exactly that — so the declaration
/// `cinrs` generates may disagree with the one Rust's own standard library
/// uses for the same symbol (`clashing_extern_declarations`, and Rust 1.99's
/// deny-by-default `invalid_runtime_symbol_definitions`). Nothing is
/// *defined*: the symbol is the C library's either way, and a call through a
/// type with no prototype is transmuted to the signature its arguments make
/// before it is made, which is the contract C's own ABI runs on.
///
/// Two more are the deny-by-default `arithmetic_overflow` and
/// `unconditional_panic`, which fire when `rustc` can see that an operation
/// would trap: a division whose divisor it has const-propagated to zero, a
/// shift past the width of the type, an overflowing constant. Each of those is
/// *undefined behaviour* in C, so the C program is valid whatever it does and
/// the operation is very often in a branch that cannot be taken —
/// `execute/pr97888-1` is `if (h > -173) e = d / i;` with `i` a zero the
/// program never reaches. Refusing to compile valid C is not an option;
/// panicking at run time if it is ever reached is a perfectly good answer to
/// undefined behaviour, and is what the same code already does when the
/// divisor is only zero at run time.
/// The label a [label address](ir::ExprKind::LabelAddr) names, through any
/// conversions around it.
fn label_state(expr: &Expr) -> Option<ir::LabelId> {
    match &expr.kind {
        ExprKind::LabelAddr(id) => Some(*id),
        ExprKind::Cast(inner) => label_state(inner),
        _ => None,
    }
}

/// The name of the wrapper type an object aligned to `align` is generated
/// inside; see [`Codegen::align_wrapper_items`].
fn align_wrapper_ident(align: u64, span: Span) -> Ident {
    Ident::new(&format!("__cinrs_align_{align}"), span)
}

/// The length of an array type, or `None` for anything else.
fn array_len(types: &ir::Types, ty: Ty) -> Option<u64> {
    match ty {
        Ty::Array(id) => Some(types.array_type(id).len),
        _ => None,
    }
}

/// The name of a unit's `cleanup` drop guard type, in this crate's own
/// hygiene: nothing a C program can write reaches it.
fn cleanup_guard_ty() -> Ident {
    Ident::new("__cinrs_cleanup", Span::mixed_site())
}

/// The name of a unit's bump arena type, in the same hygiene; see
/// [`Codegen::arena_items`].
fn arena_ty() -> Ident {
    Ident::new("__cinrs_vla_arena", Span::mixed_site())
}

/// The name of a [CFG-mode](crate::cfg) function's array of arena marks, one
/// slot per variable length array it declares; see [`Codegen::vla_def`].
fn vla_marks_ident() -> Ident {
    Ident::new("__cinrs_vla_marks", Span::mixed_site())
}

// ---------------------------------------------------------------------------
// the generator
// ---------------------------------------------------------------------------

/// How `continue` leaves a particular loop.
#[derive(Clone, Copy, PartialEq, Eq)]
enum ContinueStyle {
    /// The loop re-tests its condition on its own: `continue 'l`.
    Head,
    /// Work remains before the next iteration: `break 'l_body`.
    BodyLabel,
}

/// A place, ready to be read from or written to.
struct LoweredPlace {
    /// Statements that must run before `access` is used.
    setup: TokenStream,
    /// A Rust place expression that may be evaluated more than once.
    access: TokenStream,
    /// Set when the place is a bit-field, in which case `access` is the record
    /// holding it and the bits are reached through the generated accessors.
    bits: Option<BitAccess>,
    /// Set when the object may not be aligned the way its type asks.
    ///
    /// C reaches such an object through a packed member or a pointer cast, and
    /// says nothing about the load; Rust makes `*p` on an underaligned `p`
    /// undefined behaviour, which a debug build turns into an abort. Such a
    /// place is read and written through `read_unaligned` and
    /// `write_unaligned` instead. See [`Codegen::place_align`].
    unaligned: bool,
    /// Set when the object is `_Atomic`: reading it is a sequentially
    /// consistent load and writing it a sequentially consistent store, both
    /// through `AtomicX::from_ptr` over its address (C11 6.5.2.4, 6.5.16).
    ///
    /// The [class](AtomicClass) says which atomic, and the [`Ty`] is the
    /// object's own type with the `_Atomic` taken off — what the value the
    /// load produces is converted to.
    atomic: Option<(AtomicClass, Ty)>,
}

impl LoweredPlace {
    /// An ordinary place, whose access is the value.
    fn plain(setup: TokenStream, access: TokenStream) -> Self {
        Self {
            setup,
            access,
            bits: None,
            unaligned: false,
            atomic: None,
        }
    }
}

/// What a read-modify-write of an `_Atomic` place does to it.
enum PlaceRmw<'a> {
    /// `place op= value`, in the type `compute`.
    Compound {
        /// The operator.
        op: BinOp,
        /// The right operand, already converted for `compute`.
        value: &'a Expr,
        /// The type the operation is carried out in.
        compute: Ty,
    },
    /// `++place` or `--place`.
    Step {
        /// Whether this decrements.
        dec: bool,
    },
}

/// Which value such a read-modify-write leaves behind.
#[derive(Clone, Copy, PartialEq, Eq)]
enum RmwValue {
    /// None: it was written as a statement.
    None,
    /// The value from before the update, which is what `x++` is.
    Old,
    /// The value after it, which is what `++x` and `x += v` are.
    New,
}

/// The atomic operation a compound assignment operator performs, where there
/// is one.
fn rmw_of_binop(op: BinOp) -> Option<ir::AtomicRmw> {
    Some(match op {
        BinOp::Add => ir::AtomicRmw::Add,
        BinOp::Sub => ir::AtomicRmw::Sub,
        BinOp::BitAnd => ir::AtomicRmw::And,
        BinOp::BitOr => ir::AtomicRmw::Or,
        BinOp::BitXor => ir::AtomicRmw::Xor,
        _ => return None,
    })
}

/// The accessors a bit-field place is read and written through.
struct BitAccess {
    getter: Ident,
    setter: Ident,
}

/// The unsigned word a bit-field's bytes are gathered into, and what its
/// accessors need to know about it.
///
/// See [`Codegen::bit_field_window`]; the word is `u64` for every bit-field
/// standard C allows and `u128` for the wide ones GNU's `__int128` makes
/// possible.
struct BitWindow {
    /// The expression that reads the overlapping bytes into the word.
    read: TokenStream,
    /// The field's bit offset inside the word.
    shift: u32,
    /// The field's bits, in place, inside the word.
    mask: u128,
    /// The word's Rust type: `u64` or `::core::primitive::u128`.
    word: TokenStream,
    /// Its width in bits, which is what a mask and a sign extension are
    /// written against.
    word_bits: u32,
}

/// Where the pristine argument list of the function being generated lives.
#[derive(Clone, Copy, PartialEq, Eq)]
enum VaSource {
    /// There is none: the function takes no variable arguments.
    None,
    /// The synthetic `...` parameter of a variadic definition.
    Ellipsis,
    /// The function's own `va_list` parameter.
    Param(ir::ObjectId),
    /// The function's own `va_list *` parameter, whose *pointee* is the list.
    PtrParam(ir::ObjectId),
}

struct Codegen<'a> {
    program: &'a Program,
    map: &'a SourceMap,
    options: &'a Options,
    continue_styles: HashMap<LoopId, ContinueStyle>,
    /// The Rust names of the locals of the function being generated, wherever
    /// they differ from the C ones: in [CFG mode](crate::cfg), where every
    /// local of the function shares one scope, and for a local whose name
    /// would shadow a file-scope item (see [`PRELUDE_PATTERNS`]).
    local_names: HashMap<ir::ObjectId, String>,
    /// The hidden pointer the function being generated reaches each enclosing
    /// object through, when it is a [lifted nested function](ir::EnvParam).
    ///
    /// It is what a call from inside one passes on: an object this function
    /// does not own itself arrives as a pointer, and the callee wants the same
    /// pointer rather than the address of a local that is not there.
    env: HashMap<ir::ObjectId, ir::ObjectId>,
    /// The names a `let` binding or a parameter must not use.
    reserved: HashSet<String>,
    /// The Rust spelling of every C name the unit gives to something; see
    /// [`Names`].
    names: Names,
    va_source: VaSource,
    ret_ty: Ty,
    /// Whether the function being generated is a [state
    /// machine](crate::cfg), in which case every local is already bound at the
    /// top and a definition is an assignment.
    in_cfg: bool,
    /// Whether the function being generated is [safe](Function::is_safe), so
    /// that its body has no `unsafe` block around it.
    in_safe: bool,
    temporaries: u32,
    /// Set the first time a `__int128` reaches the output, which is what
    /// decides whether the [data-model check](Codegen::data_model_check) has
    /// anything to say about `i128`'s alignment.
    ///
    /// A [`Cell`] because [`Codegen::ty`] takes `&self`; the check is built
    /// after every item, so it sees the final answer.
    uses_int128: Cell<bool>,
    /// Set the first time a complex type reaches the output, for the same
    /// reason and by the same route as [`Codegen::uses_int128`]: only a unit
    /// that has one asserts the layout of `Complex<f32>` and `Complex<f64>`.
    uses_complex: Cell<bool>,
    /// Whether anything in the unit needs the `cleanup` drop guard item.
    uses_cleanup: Cell<bool>,
    /// Whether any function of the unit opens with the bump arena variable
    /// length arrays and `alloca` allocate from, so that the unit needs the
    /// arena's type; see [`Codegen::arena_items`].
    uses_arena: Cell<bool>,
    /// Where the unit's first variable length array or `alloca` was written,
    /// which is what the arena's items are spanned with: in a `#![no_std]`
    /// crate that did not say `#pragma cinrs no_std`, `rustc`'s "cannot find
    /// `std`" then points at the C that needed it.
    arena_span: Cell<Option<Span>>,
    /// In a [CFG-mode](crate::cfg) function, the slot of the function's array
    /// of arena marks each variable length array's hidden frame object is;
    /// see [`Codegen::vla_def`].
    vla_slots: HashMap<ir::ObjectId, usize>,
    /// Every [x86 intrinsic](crate::x86) whose *address* the unit took, in the
    /// order it first did.
    ///
    /// An intrinsic is not a symbol, and `core::arch`'s function has the Rust
    /// ABI, so its address is not an `unsafe extern "C" fn` pointer and cannot
    /// be made into one. What C asks for is a function of that signature that
    /// does what the intrinsic does, which is a shim — and GCC's own headers
    /// are exactly that, `__always_inline__` functions whose address is
    /// therefore takeable. One is generated per intrinsic, private to the
    /// unit's module, and the same `FuncId` used twice shares it.
    address_taken: RefCell<Vec<ir::FuncId>>,
    /// The number of every label a `&&label` took the address of — the value
    /// of its address — over the whole unit.
    ///
    /// A [`ir::LabelId`] is unique across the translation unit, which is what
    /// makes one map enough: a block-scope `static void *table[] = { &&a };`
    /// becomes an item at module level and is generated before any function
    /// body, so the number cannot be looked up in the function being emitted.
    /// See [`Cfg::labels`].
    label_states: HashMap<ir::LabelId, u32>,
    /// The Rust label each [region](ir::Region) of the function being
    /// generated carries: the C label's name, unless Rust cannot spell it as a
    /// label or this module already gives that name to a loop.
    region_names: HashMap<ir::LabelId, String>,
    /// The regions the statement being generated stands inside, which is what
    /// tells a `goto` whether it leaves one or restarts it.
    region_kinds: HashMap<ir::LabelId, ir::RegionKind>,
    /// The Rust label each loop [the relooper](crate::reloop) recovered
    /// carries: the C label its head stands at, or `'rN`.
    loop_names: HashMap<u32, String>,
    /// The labelled blocks and loops open around the shape being generated,
    /// outermost first.
    shape_scopes: Vec<ShapeScope>,
    /// Numbers the labelled blocks a sequence of shapes needs, so that each
    /// has a name of its own within the function.
    shape_labels: u32,
    /// The labels a jump really used: one nothing named is left out, braces
    /// and all.
    used_labels: HashSet<u32>,
}

/// A labelled block or loop the shape being generated stands inside.
///
/// Together they are how a jump the graph still holds is emitted: the
/// innermost scope whose [exit](reloop::Exit) names the target block says
/// whether it is a `break` or a `continue`, and of what.
#[derive(Clone)]
struct ShapeScope {
    /// Identifies the label in [`Codegen::used_labels`]. A loop's is
    /// [`u32::MAX`], which is never looked up: a loop always carries its name.
    id: u32,
    /// The name, without the tick.
    name: String,
    /// Whether arriving at `exit` is `continue` rather than `break`.
    repeats: bool,
    /// Where the jump arrives.
    exit: reloop::Exit,
}

impl<'a> Codegen<'a> {
    fn new(program: &'a Program, map: &'a SourceMap, options: &'a Options) -> Self {
        let mut reserved: HashSet<String> =
            PRELUDE_PATTERNS.iter().map(|s| (*s).to_owned()).collect();
        // A `static mut` and a `const` are both in the value namespace, so a
        // binding of the same name is `E0530` rather than a shadow.
        for var in &program.statics {
            if let Some(item_name) = program.object(var.object).storage.item_name() {
                reserved.insert(item_name.to_owned());
            }
        }
        for constant in &program.enum_constants {
            reserved.insert(constant.rust_name.clone());
        }
        let mut label_states = HashMap::new();
        for func in &program.functions {
            if let Some(ir::Body::Cfg(cfg)) = &func.body {
                label_states.extend(cfg.labels.iter().map(|(id, number)| (*id, *number)));
            }
        }
        Self {
            program,
            map,
            options,
            continue_styles: HashMap::new(),
            local_names: HashMap::new(),
            env: HashMap::new(),
            reserved,
            names: Names::new(program),
            va_source: VaSource::None,
            ret_ty: Ty::Void,
            in_cfg: false,
            in_safe: false,
            temporaries: 0,
            uses_int128: Cell::new(false),
            uses_complex: Cell::new(false),
            uses_cleanup: Cell::new(false),
            uses_arena: Cell::new(false),
            arena_span: Cell::new(None),
            vla_slots: HashMap::new(),
            address_taken: RefCell::new(Vec::new()),
            label_states,
            region_names: HashMap::new(),
            region_kinds: HashMap::new(),
            loop_names: HashMap::new(),
            shape_scopes: Vec::new(),
            shape_labels: 0,
            used_labels: HashSet::new(),
        }
    }

    /// Whether `va_list` appears anywhere in a type.
    fn uses_va_list(&self, ty: Ty) -> bool {
        match ty {
            Ty::VaList => true,
            Ty::Pointer(id) => self.uses_va_list(self.program.types.pointer_type(id).pointee),
            Ty::Array(id) => self.uses_va_list(self.program.types.array_type(id).elem),
            Ty::Func(id) => {
                let func = self.program.types.func_type(id);
                self.uses_va_list(func.ret) || func.params.iter().any(|ty| self.uses_va_list(*ty))
            }
            _ => false,
        }
    }

    fn sp(&self, range: SourceRange) -> Span {
        self.map.span(range)
    }

    /// The Rust identifier a C name is generated as; see [`Names`].
    ///
    /// Every name the generator writes goes through here, so that one C name
    /// reads as one Rust name wherever it appears — as an item, as a member,
    /// as a designator, in `offsetof`, in a bit-field accessor.
    fn c_ident(&self, name: &str, span: Span) -> Ident {
        self.names.ident(name, span)
    }

    /// The name of the companion type a record whose flexible array member
    /// holds `len` elements is generated under; see [`Codegen::flexible_items`].
    fn flexible_ident(&self, rust_name: &str, len: u64, span: Span) -> Ident {
        Ident::new(
            &format!("__cinrs_{}_{len}", self.names.spelling(rust_name)),
            span,
        )
    }

    /// The name of the item an externally linked **object** is declared under;
    /// see [`Program::extern_object_name`] for why an object and not a
    /// function.
    ///
    /// The symbol itself is what `#[link_name]` says; this is only the Rust
    /// side of it, and it is built out of the spelling every other C name is
    /// given so that two symbols never end up under one item.
    fn extern_object_ident(&self, symbol: &str, span: Span) -> Ident {
        let spelling = self.names.spelling(symbol);
        Ident::new(&self.program.extern_object_name(&spelling), span)
    }

    /// A name no C identifier can collide with.
    ///
    /// `Span::mixed_site()` gives the identifier this crate's own hygiene, so
    /// even a C variable spelled `__cinrs_tmp0` refers to something else.
    fn temporary(&mut self) -> Ident {
        let name = format!("__cinrs_tmp{}", self.temporaries);
        self.temporaries += 1;
        Ident::new(&name, Span::mixed_site())
    }

    /// A temporary whose *position* is the C it came from, while it still
    /// resolves as though it had been written at the macro's definition site.
    ///
    /// `Span::mixed_site` carries both a hygiene context and a position, and
    /// the position it carries is the whole invocation. That is invisible until
    /// a diagnostic is about an expression built out of such a name — `p[i]`
    /// becomes a temporary holding `p.offset(i)` and the place
    /// `(*__cinrs_tmp0)`, whose span `rustc` widens to cover the name as well
    /// and therefore to the macro call as a whole. `resolved_at` keeps
    /// the hygiene and takes the position from the C instead, which is where
    /// the caret belongs: a dereference a *safe* function may not do is
    /// reported on the `p[i]` that asked for it.
    fn temporary_at(&mut self, span: Span) -> Ident {
        let name = format!("__cinrs_tmp{}", self.temporaries);
        self.temporaries += 1;
        Ident::new(&name, span.resolved_at(Span::mixed_site()))
    }

    /// Builds a Rust label such as `'l0`.
    fn label(&self, name: &str, span: Span) -> TokenStream {
        let mut tick = Punct::new('\'', Spacing::Joint);
        tick.set_span(span);
        let mut out = TokenStream::new();
        out.extend([
            TokenTree::Punct(tick),
            TokenTree::Ident(Ident::new(name, span)),
        ]);
        out
    }

    // -- types --------------------------------------------------------------

    /// The Rust type a C type maps to, always fully qualified.
    fn ty(&self, ty: Ty, span: Span) -> TokenStream {
        let name = match ty {
            // Only reachable on the error path, where a `compile_error!` is
            // already going out; the unit type keeps the stub items parseable.
            Ty::Void | Ty::Error => return quote_spanned! {span=> () },
            // `typedef _Bool bool;` is what every C23 compatibility header
            // writes, so the name has to be the qualified one or the alias
            // this unit generates for it is `pub type bool = bool;`.
            Ty::Bool => return primitive_ty("bool", span),
            Ty::Char => "c_char",
            Ty::SChar => "c_schar",
            Ty::UChar => "c_uchar",
            Ty::Short => "c_short",
            Ty::UShort => "c_ushort",
            Ty::Int => "c_int",
            Ty::UInt => "c_uint",
            Ty::Long => "c_long",
            Ty::ULong => "c_ulong",
            Ty::LongLong => "c_longlong",
            Ty::ULongLong => "c_ulonglong",
            // `core::ffi` has no alias for these: `__int128` is not a C type
            // the standard knows, and Rust's own `i128` has had its ABI since
            // 1.77. The `core::primitive` path rather than the bare name,
            // because `typedef unsigned __int128 u128;` is how real C spells
            // it and `pub type u128 = u128;` is a cycle.
            Ty::Int128 => {
                self.uses_int128.set(true);
                return primitive_ty("i128", span);
            }
            Ty::UInt128 => {
                self.uses_int128.set(true);
                return primitive_ty("u128", span);
            }
            Ty::Float => "c_float",
            Ty::Double => "c_double",
            // `core::ffi` has nothing for these, and there is nothing it could
            // have: a complex value is a pair, and the pair the ecosystem
            // already agrees on is `num_complex::Complex`, which the runtime
            // re-exports. See [`Codegen::rt_path`].
            Ty::ComplexFloat | Ty::ComplexDouble => {
                self.uses_complex.set(true);
                let component =
                    primitive_ty(if ty == Ty::ComplexFloat { "f32" } else { "f64" }, span);
                let rt = self.rt_path(span);
                return quote_spanned! {span=> #rt::Complex<#component> };
            }
            // The lifetime is elided: `VaList` only ever appears as the type of
            // a parameter or of a local, where elision does the right thing.
            Ty::VaList => "VaList",
            // `core::arch`'s own `__m128i` and friends: the same name, the
            // same size, the same alignment and the same calling convention
            // as C's, which is what makes the intrinsics a name-level mapping
            // rather than a translation. The module is `x86_64` or `x86`
            // depending on the [target model](crate::target); a unit that
            // reaches one of these types on any other architecture never got
            // past the `#error` in the bundled header.
            Ty::Vector(vec) => {
                let module = self.arch_module(span);
                let name = Ident::new(vec.name(), span);
                return quote_spanned! {span=> ::core::arch::#module::#name };
            }
            Ty::Pointer(id) => {
                let pointer = self.program.types.pointer_type(id);
                if let Ty::Func(func) = pointer.pointee {
                    // C's function pointers can be null, and Rust's cannot;
                    // `Option` is how the two are reconciled, and it has the
                    // same representation.
                    let signature = self.fn_ty(func, span);
                    return quote_spanned! {span=> ::core::option::Option<#signature> };
                }
                let pointee = self.pointee_ty(pointer.pointee, span);
                return if pointer.konst {
                    quote_spanned! {span=> *const #pointee }
                } else {
                    quote_spanned! {span=> *mut #pointee }
                };
            }
            Ty::Array(id) => {
                // A variably modified array's object *is* a pointer to its
                // first element: the elements themselves are one bump off the
                // function's arena, however many dimensions there are, and nothing in the
                // generated code ever names the array as a value. See
                // [`ir::VlaDef`].
                if self.program.types.is_vm(ty) {
                    let step = self.ty(self.program.types.vm_step_ty(ty), span);
                    return quote_spanned! {span=> *mut #step };
                }
                let array = self.program.types.array_type(id);
                let elem = self.ty(array.elem, span);
                let len = usize_literal(array.len, span);
                let inner = quote_spanned! {span=> #elem ; #len };
                return bracketed(inner, span);
            }
            Ty::Func(id) => return self.fn_ty(id, span),
            Ty::Record(id) => {
                let name = self.c_ident(&self.program.types.record(id).rust_name, span);
                return quote_spanned! {span=> #name };
            }
            Ty::Enum(id) => {
                let name = self.c_ident(&self.program.types.enum_def(id).rust_name, span);
                return quote_spanned! {span=> #name };
            }
            // An `_Atomic T` object *is* a `T` in the generated Rust: the
            // atomicity is in how it is reached — `AtomicX::from_ptr` over its
            // address — and not in what it holds. Where the two differ is
            // alignment, which the layout code takes from the atomic type and
            // the generated item carries as `#[repr(C, align(N))]`.
            Ty::Atomic(id) => {
                let inner = self.program.types.atomic_inner(id);
                return self.ty(inner, span);
            }
        };
        let ident = Ident::new(name, span);
        quote_spanned! {span=> ::core::ffi::#ident }
    }

    /// The `core::arch` submodule the x86 intrinsics and vector types live in:
    /// `x86_64` for a 64-bit target and `x86` for a 32-bit one.
    ///
    /// The two modules have the same names in them — `core::arch::x86` has
    /// `_mm_add_epi32` too — so only the path differs between the targets.
    /// Anything else is unreachable: the bundled `<immintrin.h>` is an
    /// `#error` on a target that is neither.
    fn arch_module(&self, span: Span) -> Ident {
        let name = if self.options.target.arch == crate::target::Arch::X86 {
            "x86"
        } else {
            "x86_64"
        };
        Ident::new(name, span)
    }

    /// The path of the runtime module the generated code calls: `::cinrs::rt`,
    /// or whatever `#pragma cinrs crate` said instead of `::cinrs`.
    ///
    /// It is the one thing an expansion names outside `core` (and outside the
    /// `alloc`/`std` a variable length array needs), and it is named in full
    /// because the expansion lives in a module of its own where nothing is in
    /// scope. See [`ir::DEFAULT_CRATE_PATH`].
    fn rt_path(&self, span: Span) -> TokenStream {
        let path = TokenStream::from_str(&self.program.crate_path)
            .unwrap_or_else(|_| TokenStream::from_str(ir::DEFAULT_CRATE_PATH).expect("valid"));
        let path = respan(path, span);
        quote_spanned! {span=> #path::rt }
    }

    /// A function of `cinrs_rt::complex`, by name.
    fn rt_complex(&self, name: &str, span: Span) -> TokenStream {
        let rt = self.rt_path(span);
        let ident = Ident::new(name, span);
        quote_spanned! {span=> #rt::complex::#ident }
    }

    /// The suffix the runtime spells a complex type's component width with.
    fn complex_suffix(ty: Ty) -> &'static str {
        if ty.complex_component() == Ty::Float {
            "f32"
        } else {
            "f64"
        }
    }

    /// `unsafe extern "C" fn(…) -> R`, the type a function pointer wraps.
    ///
    /// A function type with no prototype has no parameters to write, so it
    /// comes out as `unsafe extern "C" fn() -> R`; what a call through one
    /// really passes is written at the call site instead. See
    /// [`Codegen::call`].
    fn fn_ty(&self, id: ir::FuncTyId, span: Span) -> TokenStream {
        let func = self.program.types.func_type(id).clone();
        self.fn_ptr_ty(&func.params, func.variadic, func.ret, span)
    }

    // -- the heap the emulated automatic storage comes from -------------------

    /// The crate the `Vec`s behind the bump arena of variable length arrays
    /// and `alloca` come from.
    ///
    /// Everything else the expansion generates is `core`-only; the arena needs
    /// an allocator, which is `std` in an ordinary crate and `alloc` in one
    /// that said `#pragma cinrs no_std` (and therefore wrote
    /// `extern crate alloc;` itself, since a procedural macro cannot add one).
    fn alloc_crate(&self, span: Span) -> Ident {
        let name = if self.program.no_std { "alloc" } else { "std" };
        Ident::new(name, span)
    }

    /// `::std::vec::Vec<T>`.
    fn vec_ty(&self, elem: TokenStream, span: Span) -> TokenStream {
        let krate = self.alloc_crate(span);
        quote_spanned! {span=> ::#krate::vec::Vec<#elem> }
    }

    /// `::std::vec::Vec::new()`.
    fn vec_new(&self, span: Span) -> TokenStream {
        let krate = self.alloc_crate(span);
        quote_spanned! {span=> ::#krate::vec::Vec::new() }
    }

    /// `::std::vec::from_elem(value, len)`, which is what `vec![value; len]`
    /// expands to; a procedural macro is better off naming the function.
    fn vec_of(&self, value: TokenStream, len: TokenStream, span: Span) -> TokenStream {
        let krate = self.alloc_crate(span);
        quote_spanned! {span=> ::#krate::vec::from_elem(#value, #len) }
    }

    /// The Rust type a *pointee* maps to.
    ///
    /// `void *` is the one place where C's `void` is not Rust's `()`: it stands
    /// for "some object of unknown type", which is exactly what
    /// [`core::ffi::c_void`] is for. Its size is one byte, so the `void *`
    /// arithmetic GCC allows keeps working.
    fn pointee_ty(&self, ty: Ty, span: Span) -> TokenStream {
        if ty.is_void() {
            let ident = Ident::new("c_void", span);
            return quote_spanned! {span=> ::core::ffi::#ident };
        }
        // A pointer to a variably modified type points at what is left under
        // the variable dimensions — `double (*)[m]` is a `*mut c_double` —
        // and every offset through it is scaled by the bound at run time. See
        // [`Codegen::vm_scale`].
        if self.program.types.is_vm(ty) {
            return self.ty(self.program.types.vm_step_ty(ty), span);
        }
        self.ty(ty, span)
    }

    // -- items --------------------------------------------------------------

    /// The `struct`, `union`, `enum` and `typedef` items of the unit.
    fn type_items(&mut self) -> TokenStream {
        let mut out = self.align_wrapper_items();
        for record in self.program.types.records() {
            if !record.emit {
                continue;
            }
            out.extend(self.record_item(record));
        }
        out.extend(self.flexible_items());
        for def in self.program.types.enums() {
            if !def.emit {
                continue;
            }
            let span = self.sp(def.range);
            let name = self.c_ident(&def.rust_name, span);
            let int = self.ty(Ty::Int, span);
            // C says an enumerated type is compatible with an implementation
            // defined integer type; every ABI this targets picks `int`.
            out.extend(quote_spanned! {span=> pub type #name = #int; });
        }
        for constant in &self.program.enum_constants {
            let span = self.sp(constant.range);
            let name = self.c_ident(&constant.rust_name, span);
            let ty = self.ty(constant.ty, span);
            let value = bare_int_literal(constant.value, constant.ty, span);
            out.extend(quote_spanned! {span=> pub const #name: #ty = #value; });
        }
        for typedef in &self.program.typedefs {
            // No alias is generated for `va_list`, which is what the
            // `typedef` in <stdarg.h> writes. `core::ffi::VaList` carries the
            // lifetime of the frame it reads, so `pub type va_list = VaList;`
            // does not even parse — and nothing needs the alias, since every
            // generated signature names the type directly.
            if self.uses_va_list(typedef.ty) {
                continue;
            }
            let span = self.sp(typedef.range);
            let name = self.c_ident(&typedef.rust_name, span);
            let ty = self.ty(typedef.ty, span);
            out.extend(quote_spanned! {span=> pub type #name = #ty; });
        }
        out
    }

    /// The `#[repr(C, align(N))]` wrappers the unit's over-aligned objects are
    /// generated inside, one per distinct alignment.
    ///
    /// Rust can over-align a *type* and nothing else, so an object an
    /// `_Alignas(64)` made stricter than its type becomes
    ///
    /// ```text
    /// #[repr(C, align(64))] #[derive(Copy, Clone)]
    /// pub struct __cinrs_align_64<T>(pub T);
    ///
    /// let mut buf: __cinrs_align_64<[c_char; 256]> = __cinrs_align_64(…);
    /// ```
    ///
    /// and every access to `buf` goes through `buf.0` — which is also how Rust
    /// code that reaches such an object reads it. The C type is unchanged:
    /// `sizeof buf` is the array's size, and only the binding knows about the
    /// wrapper. See [`ir::Object::align`].
    fn align_wrapper_items(&self) -> TokenStream {
        let mut wanted: Vec<(u64, SourceRange)> = self
            .program
            .objects
            .iter()
            .filter_map(|object| Some((object.align?, object.range)))
            .collect();
        wanted.sort_by_key(|(align, _)| *align);
        wanted.dedup_by_key(|(align, _)| *align);
        let mut out = TokenStream::new();
        for (align, range) in wanted {
            let span = self.sp(range);
            let name = align_wrapper_ident(align, span);
            let literal = Literal::u64_unsuffixed(align);
            out.extend(quote_spanned! {span=>
                #[repr(C, align(#literal))]
                #[derive(Copy, Clone)]
                pub struct #name<T>(pub T);
            });
        }
        out
    }

    /// The companion types an object with a filled-in flexible array member
    /// needs, one per distinct (record, length).
    ///
    /// C99 forbids initialising such a member because the object would have to
    /// be larger than its type; GNU C allows it for an object with static
    /// storage duration, and this is where that extra room comes from:
    ///
    /// ```text
    /// #[repr(C)] pub struct __cinrs_W_3 { pub n: c_int, pub data: [c_int; 3] }
    /// pub static mut w: __cinrs_W_3 = __cinrs_W_3 { n: 3, data: [1, 2, 3] };
    /// ```
    ///
    /// The leading layout is the record's own — the same Rust fields, in the
    /// same order — so `(&raw mut w).cast::<W>()` is a pointer to a `W`, which
    /// is what every use of the object goes through. `sizeof w` is still
    /// `sizeof(struct W)`, as it is in GCC. See [`ir::Object::flexible_len`].
    fn flexible_items(&self) -> TokenStream {
        let mut wanted: Vec<(ir::RecordId, u64)> = self
            .program
            .objects
            .iter()
            .filter_map(|object| match (object.flexible_len, object.ty) {
                (Some(len), Ty::Record(record)) => Some((record, len)),
                _ => None,
            })
            .collect();
        wanted.sort_unstable_by_key(|(record, len)| (record.0, *len));
        wanted.dedup_by_key(|(record, len)| (record.0, *len));
        let mut out = TokenStream::new();
        for (record, len) in wanted {
            let def = self.program.types.record(record);
            let span = self.sp(def.range);
            let name = self.flexible_ident(&def.rust_name, len, span);
            out.extend(self.record_body(def, &name, Some(len)));
        }
        out
    }

    fn record_item(&self, record: &ir::RecordDef) -> TokenStream {
        let span = self.sp(record.range);
        let name = self.c_ident(&record.rust_name, span);
        let item = self.record_body(record, &name, None);
        let accessors = self.bit_field_accessors(record, span);
        quote_spanned! {span=> #item #accessors }
    }

    /// The `struct` (or `union`) item itself, under `name`.
    ///
    /// `tail` sizes the [flexible array member](ir::Field::flexible), which is
    /// what makes a [companion type](Codegen::flexible_items) differ from the
    /// record it stands for; `None` is the record as C declared it, whose
    /// member is the `[T; 0]` the type says it is.
    fn record_body(&self, record: &ir::RecordDef, name: &Ident, tail: Option<u64>) -> TokenStream {
        let span = self.sp(record.range);
        // `Copy` is what makes a C struct behave like one: assigning it,
        // passing it and returning it all copy the bytes.
        let derives = match (record.align, record.packed) {
            // `__attribute__((packed))` and `#pragma pack(N)` are exactly
            // Rust's own `packed(N)`: every field's alignment is capped at N,
            // and so is the record's.
            (_, Some(1)) => quote_spanned! {span=> #[repr(C, packed)] #[derive(Copy, Clone)] },
            (_, Some(pack)) => {
                let pack = usize_literal(pack, span);
                quote_spanned! {span=>
                    #[repr(C, packed(#pack))] #[derive(Copy, Clone)]
                }
            }
            // `_Alignas` or `aligned(N)` on a member, or a bit-field whose
            // type is stricter than any field the item really has, is honoured
            // by raising the *record's* alignment; sema has already placed the
            // members where that leaves them.
            (Some(align), None) => {
                let align = usize_literal(align, span);
                quote_spanned! {span=>
                    #[repr(C, align(#align))] #[derive(Copy, Clone)]
                }
            }
            (None, None) => quote_spanned! {span=> #[repr(C)] #[derive(Copy, Clone)] },
        };
        let byte = primitive_ty("u8", span);
        if !record.complete {
            // A tag that is never completed can still be pointed at. An empty
            // body is the closest Rust has to C's incomplete type.
            return quote_spanned! {span=>
                #derives pub struct #name { _incomplete: [#byte; 0] }
            };
        }
        let mut fields = TokenStream::new();
        for rust_field in &record.rust_fields {
            match rust_field {
                ir::RustField::Member(index) => {
                    let field = &record.fields[*index];
                    let fspan = self.sp(field.range);
                    let fname = self.c_ident(&field.name, fspan);
                    let fty = match (tail, field.flexible) {
                        (Some(len), true) => {
                            let elem = self
                                .program
                                .types
                                .elem(field.ty)
                                .expect("a flexible member is an array");
                            let elem = self.ty(elem, fspan);
                            let len = usize_literal(len, fspan);
                            bracketed(quote_spanned! {fspan=> #elem ; #len }, fspan)
                        }
                        _ => self.ty(field.ty, fspan),
                    };
                    // Members are `pub` so Rust code can build and read the
                    // value.
                    fields.extend(quote_spanned! {fspan=> pub #fname: #fty, });
                }
                ir::RustField::Bits { name, bytes, .. } | ir::RustField::Pad { name, bytes } => {
                    let fname = Ident::new(name, span);
                    let len = usize_literal(*bytes, span);
                    fields.extend(quote_spanned! {span=> pub #fname: [#byte; #len], });
                }
                ir::RustField::Align { name, align } => {
                    let fname = Ident::new(name, span);
                    let unit = unsigned_rust_ty((*align * 8) as u32, span);
                    fields.extend(quote_spanned! {span=> pub #fname: [#unit; 0], });
                }
            }
        }
        if record.rust_fields.is_empty() && record.kind == RecordKind::Union {
            // GCC gives an empty `union` a size of zero, and so does an empty
            // Rust `struct`; a Rust `union` has to have at least one field, so
            // this is the one place the two kinds are generated differently.
            fields.extend(quote_spanned! {span=> pub __cinrs_empty: [#byte; 0], });
        }
        let body = braced(fields, span);
        match record.kind {
            RecordKind::Struct => quote_spanned! {span=> #derives pub struct #name #body },
            RecordKind::Union => quote_spanned! {span=> #derives pub union #name #body },
        }
    }

    /// The `impl` block holding one getter and one setter per named bit-field.
    ///
    /// The bits are not a field, so this is the only way to reach them — from
    /// the generated code and from Rust alike. Everything is written out
    /// inline: no helper type, no runtime, nothing to look up.
    fn bit_field_accessors(&self, record: &ir::RecordDef, span: Span) -> TokenStream {
        let mut methods = TokenStream::new();
        for field in &record.fields {
            let Some(bits) = &field.bits else {
                continue;
            };
            let fspan = self.sp(field.range);
            methods.extend(self.bit_field_getter(record, field, bits, fspan));
            methods.extend(self.bit_field_setter(record, field, bits, fspan));
        }
        if methods.is_empty() {
            return TokenStream::new();
        }
        let name = self.c_ident(&record.rust_name, span);
        quote_spanned! {span=> impl #name { #methods } }
    }

    /// Reads the bytes a bit-field overlaps into one unsigned integer.
    ///
    /// The unit rule keeps a field inside one object of its own type, so eight
    /// bytes are enough for every type standard C allows a bit-field to have.
    /// GNU's `__int128` is the one that can ask for more — `unsigned __int128
    /// x : 70` overlaps nine or ten bytes — and the window widens to `u128`
    /// there. The word type and its width come back with the tokens, since the
    /// getter and the setter have to spell them too.
    fn bit_field_window(&self, bits: &ir::BitField, span: Span) -> BitWindow {
        let storage = Ident::new(&bits.storage, span);
        let start = bits.offset_in_storage();
        let first = start / 8;
        let shift = (start % 8) as u32;
        let count = (shift + bits.width).div_ceil(8);
        let word_bits: u32 = if shift + bits.width > 64 { 128 } else { 64 };
        let word = window_ty(word_bits, false, span);
        let mut read = TokenStream::new();
        for step in 0..count {
            let index = usize_literal(first + u64::from(step), span);
            let byte = if count == 1 {
                quote_spanned! {span=> self.#storage[#index] as #word }
            } else {
                quote_spanned! {span=> (self.#storage[#index] as #word) }
            };
            read.extend(if step == 0 {
                byte
            } else {
                let by = usize_literal(u64::from(step) * 8, span);
                quote_spanned! {span=> | (#byte << #by) }
            });
        }
        // The mask of the field's bits inside that window; the window was
        // chosen so that `shift + width` fits in it, which makes the shift fit
        // too.
        let mask = mask_of(shift + bits.width, word_bits) & !mask_of(shift, word_bits);
        BitWindow {
            read,
            shift,
            mask,
            word,
            word_bits,
        }
    }

    fn bit_field_getter(
        &self,
        record: &ir::RecordDef,
        field: &ir::Field,
        bits: &ir::BitField,
        span: Span,
    ) -> TokenStream {
        let BitWindow {
            read,
            shift,
            word,
            word_bits,
            ..
        } = self.bit_field_window(bits, span);
        let ty = self.ty(field.ty, span);
        let name = self.c_ident(&bits.getter, span);
        let mask = word_literal(mask_of(bits.width, word_bits), word_bits, span);
        let shifted = if shift == 0 {
            quote_spanned! {span=> raw & #mask }
        } else {
            let by = usize_literal(u64::from(shift), span);
            quote_spanned! {span=> (raw >> #by) & #mask }
        };
        let value = if field.ty.is_bool() {
            quote_spanned! {span=> value != 0 }
        } else if !bits.signed || bits.width == word_bits {
            quote_spanned! {span=> value as #ty }
        } else {
            // Sign extension: shift the field's top bit up to the sign bit of
            // the window's signed counterpart and let the arithmetic shift
            // bring it back down.
            let signed = window_ty(word_bits, true, span);
            let by = usize_literal(u64::from(word_bits - bits.width), span);
            quote_spanned! {span=> (((value << #by) as #signed) >> #by) as #ty }
        };
        let body = self.accessor_body(
            record,
            quote_spanned! {span=>
                let raw: #word = #read;
                let value: #word = #shifted;
                #value
            },
            span,
        );
        quote_spanned! {span=>
            #[inline]
            pub fn #name(&self) -> #ty { #body }
        }
    }

    fn bit_field_setter(
        &self,
        record: &ir::RecordDef,
        field: &ir::Field,
        bits: &ir::BitField,
        span: Span,
    ) -> TokenStream {
        let BitWindow {
            read,
            shift,
            mask,
            word,
            word_bits,
        } = self.bit_field_window(bits, span);
        let ty = self.ty(field.ty, span);
        let name = self.c_ident(&bits.setter, span);
        let storage = Ident::new(&bits.storage, span);
        let value = Ident::new("value", span);
        let field_mask = word_literal(mask, word_bits, span);
        let keep = word_literal(!mask & mask_of(word_bits, word_bits), word_bits, span);
        let shifted = if shift == 0 {
            quote_spanned! {span=> (#value as #word) & #field_mask }
        } else {
            let by = usize_literal(u64::from(shift), span);
            quote_spanned! {span=> ((#value as #word) << #by) & #field_mask }
        };
        let start = bits.offset_in_storage();
        let first = start / 8;
        let count = (shift + bits.width).div_ceil(8);
        let mut writes = TokenStream::new();
        let byte = primitive_ty("u8", span);
        for step in 0..count {
            let index = usize_literal(first + u64::from(step), span);
            if step == 0 {
                writes.extend(quote_spanned! {span=> self.#storage[#index] = raw as #byte; });
            } else {
                let by = usize_literal(u64::from(step) * 8, span);
                writes.extend(
                    quote_spanned! {span=> self.#storage[#index] = (raw >> #by) as #byte; },
                );
            }
        }
        let body = self.accessor_body(
            record,
            quote_spanned! {span=>
                let bits: #word = #shifted;
                let raw: #word = #read;
                let raw: #word = (raw & #keep) | bits;
                #writes
            },
            span,
        );
        quote_spanned! {span=>
            #[inline]
            pub fn #name(&mut self, #value: #ty) { #body }
        }
    }

    /// Wraps an accessor body in `unsafe` where reading the storage needs it,
    /// which is exactly when the record is a `union`.
    fn accessor_body(&self, record: &ir::RecordDef, body: TokenStream, span: Span) -> TokenStream {
        if record.kind == RecordKind::Union {
            let block = braced(body, span);
            return quote_spanned! {span=> unsafe #block };
        }
        body
    }

    /// `const _: () = { assert!(…); };` — the assumptions the data model made,
    /// checked against the target the expansion is really compiled for.
    ///
    /// Everything this crate computes at expansion time — `sizeof`, member
    /// offsets, bit-field storage, the type of an integer constant, the value
    /// of an `#if` — comes out of a [`TargetModel`](crate::TargetModel) that
    /// was chosen from `CINRS_TARGET`, from `#pragma cinrs target`, or (with
    /// neither) from the *host*. Any of the three may be the wrong one, and a
    /// wrong one would leave every one of those answers quietly wrong. So the
    /// expansion states them: `long` is this many bytes, a pointer is that
    /// many, plain `char` is signed, `double` is aligned so. The generated
    /// code uses the `core::ffi` aliases, which follow the *target*, so a
    /// mismatch is a failed assertion at the caret of the C rather than a
    /// program that computes the wrong thing — and the message names both the
    /// model that was used and the knob that chose it.
    ///
    /// `__int128`'s alignment is included only where the unit has one: it is
    /// the one scalar whose alignment is not fixed by its width, and a unit
    /// that never mentions it must not be refused over it.
    fn data_model_check(&self) -> TokenStream {
        let span = self.map.span(SourceRange::at(0));
        let target = &self.options.target;
        // Every message ends with this, so that a failure says what to change
        // rather than only what went wrong.
        let chosen = format!(
            "Translated for {}; set CINRS_TARGET from a build script \
             (cargo:rustc-env=CINRS_TARGET=$TARGET) or write #pragma cinrs target.",
            target.describe(&self.options.target_source)
        );
        let mut body = TokenStream::new();
        let mut width = |ty: TokenStream, bits: u32, what: &str| {
            let bytes = usize_literal(u64::from(bits).div_ceil(8), span);
            let message = message_literal(
                &format!(
                    "cinrs: {what} is {} bytes in the data model this unit was translated for, \
                     and is not on this target. {chosen}",
                    bits.div_ceil(8)
                ),
                span,
            );
            body.extend(quote_spanned! {span=>
                assert!(::core::mem::size_of::<#ty>() == #bytes, #message);
            });
        };
        width(
            quote_spanned! {span=> ::core::ffi::c_short },
            target.short_bits,
            "'short'",
        );
        width(
            quote_spanned! {span=> ::core::ffi::c_int },
            target.int_bits,
            "'int'",
        );
        width(
            quote_spanned! {span=> ::core::ffi::c_long },
            target.long_bits,
            "'long'",
        );
        width(
            quote_spanned! {span=> ::core::ffi::c_longlong },
            target.long_long_bits,
            "'long long'",
        );
        width(
            quote_spanned! {span=> *const ::core::ffi::c_void },
            target.ptr_bits,
            "a pointer",
        );
        // Plain `char`'s signedness decides what `'\xff'` is worth and how a
        // `char` widens, so it is an assumption like any other. `c_char` is an
        // alias for `i8` or `u8`, and only the unsigned one has a zero minimum.
        let (test, said) = if target.char_signed {
            (
                quote_spanned! {span=> ::core::ffi::c_char::MIN != 0 },
                "signed",
            )
        } else {
            (
                quote_spanned! {span=> ::core::ffi::c_char::MIN == 0 },
                "unsigned",
            )
        };
        let message = message_literal(
            &format!(
                "cinrs: plain 'char' is {said} in the data model this unit was translated \
                 for, and is not on this target. {chosen}"
            ),
            span,
        );
        body.extend(quote_spanned! {span=> assert!(#test, #message); });
        // The one property two targets of the same *data model* differ on: the
        // i386 System V ABI aligns `long long` and `double` to four bytes and
        // the Microsoft one to eight, and every member offset the front end
        // computed followed whichever this model says. `c_longlong` and
        // `c_double` are the aliases, so the assertion follows the target.
        let align = usize_literal(target.max_scalar_align.min(8), span);
        let message = message_literal(
            &format!(
                "cinrs: 'long long' and 'double' are {}-byte aligned in the data model this \
                 unit was translated for, and are not on this target — so every 'sizeof' and \
                 member offset in it would be wrong. {chosen}",
                target.max_scalar_align.min(8)
            ),
            span,
        );
        body.extend(quote_spanned! {span=>
            assert!(
                ::core::mem::align_of::<::core::ffi::c_longlong>() == #align
                    && ::core::mem::align_of::<::core::ffi::c_double>() == #align,
                #message
            );
        });
        if self.uses_int128.get() {
            let align = usize_literal(target.int128_align, span);
            let message = message_literal(
                &format!(
                    "cinrs: '__int128' is {}-byte aligned in the data model this unit was \
                     translated for, and is not on this target. {chosen}",
                    target.int128_align
                ),
                span,
            );
            let i128 = primitive_ty("i128", span);
            body.extend(quote_spanned! {span=>
                assert!(::core::mem::align_of::<#i128>() == #align, #message);
            });
        }
        if self.uses_complex.get() {
            // A complex type *is* two of its component type side by side —
            // that is the whole reason the runtime uses a `#[repr(C)]` pair —
            // and every `sizeof`, member offset and array stride in the unit
            // was computed from that. A `Complex<f64>` that is not sixteen
            // bytes would make all of them wrong, so the unit says so.
            for (ty, name) in [
                (Ty::ComplexFloat, "'float _Complex'"),
                (Ty::ComplexDouble, "'double _Complex'"),
            ] {
                let layout = self
                    .program
                    .types
                    .size_align(ty, target)
                    .expect("a complex type has a layout");
                let rust = self.ty(ty, span);
                let size = usize_literal(layout.size, span);
                let align = usize_literal(layout.align, span);
                let message = message_literal(
                    &format!(
                        "cinrs: {name} is {} bytes and {}-byte aligned in the data model this \
                         unit was translated for, and is not on this target. {chosen}",
                        layout.size, layout.align
                    ),
                    span,
                );
                body.extend(quote_spanned! {span=>
                    assert!(
                        ::core::mem::size_of::<#rust>() == #size
                            && ::core::mem::align_of::<#rust>() == #align,
                        #message
                    );
                });
            }
        }
        let block = braced(body, span);
        quote_spanned! {span=> const _: () = #block; }
    }

    /// The `extern` block declaring everything the unit does not define, with
    /// the libraries the unit links beside it — one empty block each, never an
    /// attribute on this one; [`Codegen::link_blocks`] is why.
    ///
    /// A **function** is declared under its own C name — `pub fn crc32`, not a
    /// hidden one — so that the glob re-export carries it out of the unit's
    /// module and `#include <zlib.h>` is all Rust needs to call it. An
    /// **object** is not, because a glob-imported `static` changes what a `let`
    /// of the same name means; [`Program::extern_object_name`] is that rule and
    /// its reason.
    fn extern_block(&mut self) -> TokenStream {
        if !self.program.has_externs() {
            return TokenStream::new();
        }
        let span = self.map.span(SourceRange::at(0));
        let mut items = TokenStream::new();
        // The symbols this block links by, which is what decides whether it
        // needs a library of its own; see [`LEGACY_STDIO`]. Collected here
        // rather than asked of the program a second time, so that what the
        // attribute answers for and what the block declares cannot drift apart.
        let mut symbols: Vec<&str> = Vec::new();
        for id in &self.program.externs {
            let object = self.program.object(*id);
            let Storage::Extern { item_name } = &object.storage else {
                continue;
            };
            let ospan = self.sp(object.range);
            let rust_name = self.extern_object_ident(item_name, ospan);
            let ty = self.ty(object.ty, ospan);
            // An `__asm__("symbol")` label renames the declaration, which is
            // exactly what `#[link_name]` already says.
            let symbol = object.asm_label.as_deref().unwrap_or(item_name);
            symbols.push(symbol);
            let link = link_name(symbol, ospan);
            items.extend(quote_spanned! {ospan=> #link pub static mut #rust_name: #ty; });
        }
        for func in &self.program.functions {
            if !func.is_extern() {
                continue;
            }
            // An [x86 intrinsic](crate::x86) has no symbol: a call to it is
            // generated as `::core::arch::x86_64::<name>`, and declaring one
            // here would put a `pub fn _mm_add_ps` into the unit's module —
            // which the facade glob re-exports, so it would clash with a
            // `use core::arch::x86_64::*` in the user's own code and link
            // against nothing if it were ever called. The bundled
            // `<immintrin.h>` declares eight hundred of them; **none** of
            // them reaches the generated Rust.
            if func.intrinsic.is_some() {
                continue;
            }
            let fspan = self.sp(func.range);
            // The C name, through the same mapping every other name goes
            // through: a keyword becomes `r#yield`, `a$b` becomes
            // `a_dollar_b`, and [`Names`] has already made it unique within
            // the unit. It is what a call in this unit names too; see
            // [`Codegen::function_path`].
            let rust_name = self.c_ident(func.item_name(), fspan);
            let params = self.extern_params(func, fspan);
            let ret = if func.sig.ret.is_void() {
                TokenStream::new()
            } else {
                let ty = self.ty(func.sig.ret, fspan);
                quote_spanned! {fspan=> -> #ty }
            };
            // An `__asm__("symbol")` label is the program's own answer and wins;
            // failing that, a name the Microsoft library exports differently is
            // linked by the name it really has. See [`MSVC_RENAMED`].
            let symbol = match func.asm_label.as_deref() {
                Some(label) => label,
                None => self.msvc_symbol(&func.name),
            };
            symbols.push(symbol);
            let link = link_name(symbol, fspan);
            items.extend(quote_spanned! {fspan=> #link pub fn #rust_name(#params) #ret; });
        }
        let links = self.link_blocks(&symbols, span);
        quote_spanned! {span=> #links unsafe extern "C" { #items } }
    }

    /// The libraries this unit links: one `#[link(name = "…")] unsafe extern "C"
    /// {}` — the ordinary Rust idiom for "also link this" — for every
    /// `#pragma cinrs link` the unit wrote, and for the library an MSVC target
    /// needs to resolve the `printf` family.
    ///
    /// Nothing here is needed for the C library itself, which the Rust runtime
    /// already links; the pragma is for the program that calls into something
    /// else, and [`LEGACY_STDIO`] is the one library cinrs asks for on its own
    /// initiative. `symbols` is what the declarations in the block next to these
    /// link by, which is what decides the second of those.
    ///
    /// A library named by both — `#pragma cinrs link "legacy_stdio_definitions"`
    /// written out by hand — is emitted once.
    ///
    /// # Why an empty block of its own, and not an attribute on the declarations
    ///
    /// Because `#[link(name = "…")]` says two things, and cinrs means only the
    /// first. It puts the library on the link line, and it also makes `rustc`
    /// reach every `static` declared *in that very block* through a `dllimport`
    /// on a Windows target — `native_library` is asked per foreign item, so a
    /// sibling block is untouched. A `dllimport` is right for an object that
    /// lives in another image and silently wrong for one that lives in this one:
    /// with the attribute on the declarations, a unit that declared
    /// `extern int counter;` while another exported unit of the same crate
    /// defined it read rubbish — 7887437 for 17, measured on Windows — and
    /// `lld-link` said `LNK4217: locally defined symbol imported`.
    ///
    /// For the `printf` rule that was cinrs's own doing: a unit that includes
    /// `<stdio.h>` never asked for a library. The pragma is the program's own
    /// statement, but it is no better placed to decide, because in C adding a
    /// `.lib` to a link line implies `__declspec(dllimport)` on nothing, and
    /// cinrs has no way to write that per declaration. So no generated
    /// declaration is a `dllimport`, which is C's own default, and a program that
    /// wants a DLL's *data* export — the one case the import library exposes as
    /// `__imp_name` alone — names that pointer itself:
    ///
    /// ```c
    /// #pragma cinrs link "gdi32"
    /// extern unsigned long *batch_limit __asm__("__imp_GdiBatchLimit");
    /// ```
    ///
    /// That reads the DLL's value; the same symbol declared as a plain object is
    /// `lld-link: error: undefined symbol: GdiBatchLimit` — loud, at link time,
    /// rather than a wrong value. With the attribute on the declarations even the
    /// pointer was out of reach: `rustc` asked for its `__imp_` in turn, and
    /// `lld-link` said `undefined symbol: __declspec(dllimport)
    /// __imp_GdiBatchLimit`. Every sentence here was measured on
    /// `x86_64-pc-windows-msvc`; `doc/cross-compilation.md` keeps the numbers and
    /// `tests/declared_names.rs` the cases.
    fn link_blocks(&self, symbols: &[&str], span: Span) -> TokenStream {
        let mut libraries: Vec<&str> = Vec::new();
        for name in &self.program.link_libraries {
            if !libraries.contains(&name.as_str()) {
                libraries.push(name);
            }
        }
        for library in self.legacy_stdio_libraries(symbols) {
            if !libraries.contains(&library) {
                libraries.push(library);
            }
        }
        let mut out = TokenStream::new();
        for name in libraries {
            let mut literal = Literal::string(name);
            literal.set_span(span);
            out.extend(quote_spanned! {span=> #[link(name = #literal)] unsafe extern "C" {} });
        }
        out
    }

    /// The symbol a **declaration** of the C function `name` links by: the name
    /// itself, or, on an MSVC target, what the Microsoft C runtime exports it
    /// as. See [`MSVC_RENAMED`] for the table and for how each row was read out
    /// of a real import library.
    ///
    /// Only a declaration: a function this unit *defines* is its own symbol, and
    /// a C program that defines `time` has defined `time`.
    fn msvc_symbol<'name>(&self, name: &'name str) -> &'name str {
        if !self.options.target.is_msvc() {
            return name;
        }
        for (c_name, symbol) in MSVC_RENAMED {
            if *c_name == name {
                return symbol;
            }
        }
        name
    }

    /// The libraries the `printf` and `scanf` declarations among `symbols` have
    /// to be linked against, in the order [`LEGACY_STDIO`] lists them.
    ///
    /// Empty on every target but an MSVC one: everywhere else the platform's
    /// library exports those functions as ordinary symbols and there is nothing
    /// to ask for. See [`LEGACY_STDIO`] for why the Microsoft library differs.
    fn legacy_stdio_libraries(&self, symbols: &[&str]) -> Vec<&'static str> {
        if !self.options.target.is_msvc() {
            return Vec::new();
        }
        let mut out: Vec<&'static str> = Vec::new();
        for (symbol, library) in LEGACY_STDIO {
            if symbols.contains(symbol) && !out.contains(library) {
                out.push(library);
            }
        }
        out
    }

    fn extern_params(&self, func: &Function, span: Span) -> TokenStream {
        let mut params = TokenStream::new();
        for (index, ty) in func.sig.params.iter().enumerate() {
            if index > 0 {
                params.extend(quote_spanned! {span=> , });
            }
            let ty = self.ty(*ty, span);
            match func.param_names.get(index).and_then(|n| n.as_ref()) {
                Some(name) => {
                    let name = self.c_ident(name, span);
                    params.extend(quote_spanned! {span=> #name: #ty });
                }
                None => params.extend(quote_spanned! {span=> _: #ty }),
            }
        }
        if func.sig.variadic {
            if !func.sig.params.is_empty() {
                params.extend(quote_spanned! {span=> , });
            }
            params.extend(quote_spanned! {span=> ... });
        }
        params
    }

    fn static_item(&mut self, var: &ir::StaticVar) -> TokenStream {
        let object = self.program.object(var.object);
        let span = self.sp(object.range);
        if object.storage.is_thread_local() {
            return self.thread_local_item(var);
        }
        let Storage::Static {
            item_name,
            exported,
        } = &object.storage
        else {
            return TokenStream::new();
        };
        let name = self.c_ident(item_name, span);
        let ty = self.binding_ty(var.object, self.storage_ty(var.object, span), span);
        let init = self.static_init(&var.init, object.ty, span);
        let init = self.binding_init(var.object, init, span);
        let (vis, export) = if *exported {
            let export = if self.program.export {
                let symbol = object.asm_label.as_deref().unwrap_or(&object.name);
                export_attr(symbol, &name, span)
            } else {
                TokenStream::new()
            };
            (quote_spanned! {span=> pub }, export)
        } else {
            (TokenStream::new(), TokenStream::new())
        };
        let section = match &object.section {
            Some(section) => {
                let mut literal = Literal::string(section);
                literal.set_span(span);
                quote_spanned! {span=> #[unsafe(link_section = #literal)] }
            }
            None => TokenStream::new(),
        };
        // `static mut` rather than a cell: C code assigns to globals from
        // anywhere, and reading or writing one directly (never taking a
        // reference) is what keeps edition 2024's `static_mut_refs` quiet.
        quote_spanned! {span=>
            #export
            #section
            #vis static mut #name: #ty = #init;
        }
    }

    /// `std::thread_local! { static X: UnsafeCell<T> = const { … }; }` — the
    /// item a `_Thread_local` object becomes.
    ///
    /// C's thread-local object has static storage duration and one instance
    /// per thread, and `thread_local!` is exactly that. The cell is what makes
    /// the object *mutable*: `with` hands out a `&UnsafeCell<T>`, and the
    /// `*mut T` inside it is valid for as long as the thread's copy is, which
    /// is the lifetime C promises the address of such an object.
    ///
    /// The initialiser goes inside a `const` block wherever it can — that is
    /// the form with no lazy-initialisation flag and no destructor to register
    /// — and directly otherwise. Only one thing keeps it out: an initialiser
    /// that mentions the address of another item, which a `const` may not
    /// refer to (`E0013`).
    fn thread_local_item(&mut self, var: &ir::StaticVar) -> TokenStream {
        let object = self.program.object(var.object);
        let span = self.sp(object.range);
        let Storage::ThreadLocal {
            item_name,
            exported,
        } = &object.storage
        else {
            return TokenStream::new();
        };
        if self.program.no_std {
            // `sema::check_pragmas` has already said that a thread-local object
            // needs `std`; emitting `::std::thread_local!` anyway would add
            // `rustc`'s own "cannot find `std`" on top of it.
            return TokenStream::new();
        }
        let name = self.c_ident(item_name, span);
        let ty = self.binding_ty(var.object, self.storage_ty(var.object, span), span);
        let init = self.static_init(&var.init, object.ty, span);
        let init = self.binding_init(var.object, init, span);
        let vis = if *exported {
            quote_spanned! {span=> pub }
        } else {
            TokenStream::new()
        };
        let cell = quote_spanned! {span=> ::core::cell::UnsafeCell<#ty> };
        let value = quote_spanned! {span=> ::core::cell::UnsafeCell::new(#init) };
        let value = if self.const_initialisable(&var.init) {
            quote_spanned! {span=> const { #value } }
        } else {
            value
        };
        // The lint exemptions are the unit module's own (see
        // [`crate::in_module`]), and a `thread_local!` is expanded inside it
        // like anything else, so the `static` it generates inherits them.
        quote_spanned! {span=>
            ::std::thread_local! {
                #vis static #name: #cell = #value;
            }
        }
    }

    /// Whether an initialiser may go inside a `const { … }` block.
    ///
    /// A Rust constant may not refer to a `static` (`E0013`), which rules out
    /// exactly the initialisers whose value is the address of another item: a
    /// pointer to a file-scope object, a function pointer, and the `static`
    /// that holds the characters of a wide string literal. A *narrow* literal
    /// is a byte string whose `as_ptr` is const, and every arithmetic constant
    /// is fine.
    ///
    /// This is what decides between `thread_local!`'s two forms; see
    /// [`Codegen::thread_local_item`].
    fn const_initialisable(&self, expr: &Expr) -> bool {
        match &expr.kind {
            ExprKind::FuncAddr(_) => false,
            ExprKind::AddrOf(place) | ExprKind::Load(place) => match &place.kind {
                PlaceKind::Str(id) => {
                    let elem = self.program.string(*id).elem;
                    elem.size_bytes(&self.options.target) == 1
                }
                _ => !rooted_in_static(place, self.program),
            },
            ExprKind::Cast(inner) => self.const_initialisable(inner),
            ExprKind::PtrOffset { ptr, .. } => self.const_initialisable(ptr),
            ExprKind::RecordLit { fields, .. } => {
                fields.iter().all(|f| self.const_initialisable(f))
            }
            ExprKind::UnionLit { value, .. } => self.const_initialisable(value),
            ExprKind::ArrayLit(items) => items.iter().all(|i| self.const_initialisable(i)),
            ExprKind::ArrayRepeat { value, .. } => self.const_initialisable(value),
            _ => true,
        }
    }

    /// The initialiser of a `static mut`, wrapped in `unsafe` when it needs to
    /// be (`mem::zeroed`, or the address of another `static mut`).
    fn static_init(&mut self, expr: &Expr, ty: Ty, span: Span) -> TokenStream {
        let tokens = self.expr_at(expr, ty);
        if needs_unsafe(&self.program.types, expr) {
            let block = braced(tokens, span);
            return quote_spanned! {span=> unsafe #block };
        }
        tokens
    }

    fn signature(&mut self, func: &Function) -> TokenStream {
        let span = self.sp(func.range);
        let name = self.c_ident(func.item_name(), span);
        let mut params = TokenStream::new();
        // A lifted nested function takes the objects it uses from the
        // enclosing frame as pointers, in front of everything the program
        // wrote; see [`ir::EnvParam`].
        for entry in &func.env {
            let object = self.program.object(entry.param);
            let pspan = self.sp(object.range);
            let pname = self.object_ident(entry.param, pspan);
            let pty = self.ty(object.ty, pspan);
            params.extend(quote_spanned! {pspan=> #pname: #pty , });
        }
        // A definition whose parameters did not check out may have fewer than
        // the signature says; either way the list still has to have the right
        // shape, so it falls back to names of our own.
        let named = func.params.len() == func.sig.params.len();
        for (index, ty) in func.sig.params.iter().enumerate() {
            if index > 0 {
                params.extend(quote_spanned! {span=> , });
            }
            if named {
                let id = func.params[index];
                let object = self.program.object(id);
                let pspan = self.sp(object.range);
                let pname = self.object_ident(id, pspan);
                let pty = self.ty(*ty, pspan);
                params.extend(quote_spanned! {pspan=> mut #pname: #pty });
            } else {
                let pname = Ident::new(&format!("__cinrs_arg{index}"), Span::mixed_site());
                let pty = self.ty(*ty, span);
                params.extend(quote_spanned! {span=> #pname: #pty });
            }
        }
        if func.sig.variadic {
            if !func.sig.params.is_empty() {
                params.extend(quote_spanned! {span=> , });
            }
            // The variable part of the argument list arrives as one more
            // parameter. It is never advanced: `va_start` and every `va_list`
            // local copy it, so it stays the list as the caller left it.
            let name = self.va_ident();
            params.extend(quote_spanned! {span=> #name: ... });
        }
        let ret = if func.sig.ret.is_void() {
            TokenStream::new()
        } else {
            let ty = self.ty(func.sig.ret, span);
            quote_spanned! {span=> -> #ty }
        };
        // A definition is a Rust item rather than a C symbol unless the unit
        // asked for real symbols, so an `__asm__("name")` label on one only
        // means something there — and there it names the symbol the item
        // takes, which is what `#[unsafe(export_name)]` says. On a *declaration*
        // the label is always honoured, through the `extern` block's
        // `#[link_name]`.
        let exported = !func.is_static && self.program.export;
        let vis = if func.is_static {
            TokenStream::new()
        } else {
            quote_spanned! {span=> pub }
        };
        let export = if exported {
            let symbol = func.asm_label.as_deref().unwrap_or(&func.name);
            export_attr(symbol, &name, span)
        } else {
            TokenStream::new()
        };
        // `#[inline]` is ignored on an exported function, and saying so is
        // `rustc`'s job rather than the user's to read: leave it out.
        //
        // `always_inline` on a function with a target feature — from
        // `__attribute__((target))` or `#pragma GCC target`, both of which
        // are in `target_features` — is `#[inline]`: rustc refuses
        // `#[inline(always)]` together with `#[target_feature]` ("cannot use
        // `#[inline(always)]` with `#[target_feature]`", rust-lang/rust#145574),
        // and BLAKE3's SIMD files are exactly that pairing, an `INLINE` macro
        // of `static inline __attribute__((always_inline))` under `#pragma GCC
        // target("sse4.1")`. The hint is then only a hint, and LLVM does not
        // always take it — so where it can, the helper drops the
        // `#[target_feature]` instead and keeps `#[inline(always)]`; see
        // [`Function::inline_helper`] for when, and why that is sound.
        let helper = func.inline_helper();
        let inline = match func.inline_hint {
            Some(_) if exported => TokenStream::new(),
            _ if helper => quote_spanned! {span=> #[inline(always)] },
            Some(ir::InlineHint::Always) if !func.target_features.is_empty() => {
                quote_spanned! {span=> #[inline] }
            }
            Some(ir::InlineHint::Always) => quote_spanned! {span=> #[inline(always)] },
            Some(ir::InlineHint::Never) => quote_spanned! {span=> #[inline(never)] },
            None if func.is_inline && !exported => quote_spanned! {span=> #[inline] },
            None => TokenStream::new(),
        };
        let cold = if func.cold {
            quote_spanned! {span=> #[cold] }
        } else {
            TokenStream::new()
        };
        let deprecated = match &func.deprecated {
            Some(Some(message)) => {
                let mut literal = Literal::string(message);
                literal.set_span(span);
                quote_spanned! {span=> #[deprecated(note = #literal)] }
            }
            Some(None) => quote_spanned! {span=> #[deprecated] },
            None => TokenStream::new(),
        };
        let section = match &func.section {
            Some(section) => {
                let mut literal = Literal::string(section);
                literal.set_span(span);
                quote_spanned! {span=> #[unsafe(link_section = #literal)] }
            }
            None => TokenStream::new(),
        };
        // `__attribute__((target("avx2")))`, which is the same promise
        // `#[target_feature(enable = "avx2")]` makes: the body may use that
        // instruction set, and a caller has to have checked that the
        // processor has it — with `__builtin_cpu_supports`, as in C.
        let target_feature = if helper {
            TokenStream::new()
        } else {
            self.target_feature_attrs(func, span)
        };
        // Everything is `extern "C"`, so that its address is a C function
        // pointer, except that helper, whose address nothing takes and which
        // could not pass a vector through the C ABI without the feature.
        let abi = if helper {
            TokenStream::new()
        } else {
            quote_spanned! {span=> extern "C" }
        };
        // A function the unit asked to be safe is generated without `unsafe`,
        // and its body without the `unsafe` block, so that `rustc` checks every
        // operation in it; see [`crate::sema::check_safe`] for what that
        // catches and what it refuses outright.
        let unsafety = if func.is_safe() {
            TokenStream::new()
        } else {
            quote_spanned! {span=> unsafe }
        };
        quote_spanned! {span=>
            #export
            #inline
            #cold
            #deprecated
            #section
            #target_feature
            #vis #unsafety #abi fn #name(#params) #ret
        }
    }

    /// One `#[target_feature(enable = "…")]` per instruction set the function
    /// asked for.
    ///
    /// One attribute per name rather than one attribute with several `enable`s
    /// — both are legal, and this reads as the list of `-m` switches it came
    /// from.
    fn target_feature_attrs(&self, func: &Function, span: Span) -> TokenStream {
        let mut out = TokenStream::new();
        for feature in &func.target_features {
            let mut literal = Literal::string(feature);
            literal.set_span(span);
            out.extend(quote_spanned! {span=> #[target_feature(enable = #literal)] });
        }
        out
    }

    /// The `static` that puts a `constructor` or `destructor` function into the
    /// table the runtime walks before `main` (or after it).
    ///
    /// ELF has `.init_array` and `.fini_array`, and Mach-O has
    /// `__DATA,__mod_init_func` and `__mod_term_func`; nothing else this crate
    /// can name has such a table, so a program that asks for one elsewhere is
    /// told so rather than quietly built without it.
    fn init_array_item(&mut self, func: &Function, kind: ir::InitKind) -> TokenStream {
        let span = self.sp(func.range);
        let name = self.c_ident(func.item_name(), span);
        let signature = self.function_pointer_ty(func, span);
        let item = Ident::new(
            &format!(
                "__CINRS_INIT_{:08x}_{}",
                self.program.unit_id as u32,
                func.item_name()
            ),
            span,
        );
        let elf = match kind {
            ir::InitKind::Constructor => ".init_array",
            ir::InitKind::Destructor => ".fini_array",
        };
        let apple = match kind {
            ir::InitKind::Constructor => "__DATA,__mod_init_func",
            ir::InitKind::Destructor => "__DATA,__mod_term_func",
        };
        let mut elf_literal = Literal::string(elf);
        elf_literal.set_span(span);
        let mut apple_literal = Literal::string(apple);
        apple_literal.set_span(span);
        // The section name is the one thing about this that is not portable,
        // so it is chosen at compile time rather than assumed.
        quote_spanned! {span=>
            #[used]
            #[cfg_attr(target_vendor = "apple", unsafe(link_section = #apple_literal))]
            #[cfg_attr(not(target_vendor = "apple"), unsafe(link_section = #elf_literal))]
            static #item: #signature = #name;
        }
    }

    /// The one check a unit with a `constructor` or a `destructor` carries.
    ///
    /// A procedural macro is compiled for the *host*, so it cannot know which
    /// target the code it generates is for; the check therefore has to be part
    /// of the expansion. It is emitted once per unit rather than once per
    /// function, since it says the same thing either way.
    fn init_array_guard(&self) -> TokenStream {
        let span = self.map.span(SourceRange::at(0));
        quote_spanned! {span=>
            const _: () = {
                #[cfg(not(any(target_os = "linux", target_os = "android",
                              target_os = "freebsd", target_os = "netbsd",
                              target_os = "openbsd", target_os = "dragonfly",
                              target_vendor = "apple")))]
                ::core::compile_error!(
                    "'constructor' and 'destructor' need a target whose runtime walks an initialiser table (ELF or Mach-O)"
                );
            };
        }
    }

    fn function_item(&mut self, func: &Function) -> TokenStream {
        let span = self.sp(func.range);
        let Some(body) = &func.body else {
            return TokenStream::new();
        };
        self.enter_function(func);
        let signature = self.signature(func);
        let body = match body {
            Body::Structured(stmts) => self.stmts(stmts),
            Body::Cfg(cfg) => self.cfg_body(cfg, span),
        };
        // `alloca`'s memory belongs to the function, not to the block the call
        // was written in, so the arena is opened here and dropped by whichever
        // `return` runs; a variable length array only borrows space from it
        // until its block ends.
        let arena = if func.uses_arena {
            self.uses_arena.set(true);
            let name = self.arena_ident(span);
            let ty = arena_ty();
            quote_spanned! {span=> let #name = #ty::new(); }
        } else {
            TokenStream::new()
        };
        // One `unsafe` block around the whole body: in edition 2024 the body of
        // an `unsafe fn` is not itself an unsafe block any more. A safe
        // function is exactly the one that does not get it — that block is what
        // would stop `rustc` checking the translation.
        if func.is_safe() {
            return quote_spanned! {span=>
                #signature { #arena #body }
            };
        }
        quote_spanned! {span=>
            #signature {
                unsafe { #arena #body }
            }
        }
    }

    /// The name of the bump arena a function that declares a variable length
    /// array or calls `alloca` opens with, in this crate's own hygiene, for a
    /// use of it written at `span`.
    fn arena_ident(&self, span: Span) -> Ident {
        if self.arena_span.get().is_none() {
            self.arena_span.set(Some(span));
        }
        Ident::new("__cinrs_vla", Span::mixed_site())
    }

    /// The bump arena's type and its frame guard: one pair of private items
    /// per unit that has a variable length array or an `alloca` anywhere.
    ///
    /// C gives both kinds of object automatic storage — a compiler bumps the
    /// stack pointer — and the emulation is the same idea on the heap, per
    /// function call:
    ///
    /// * the storage is a list of chunks that never move, each a `Vec<u128>`
    ///   so that its base is 16-byte aligned, and the position is a chunk
    ///   index and a byte offset in it. No chunk is allocated until the first
    ///   allocation, and the arena, dropped by the `return`, frees them all;
    /// * `alloc` pads the position up to the alignment — by address, so that
    ///   `aligned(64)` works in a 16-byte-aligned chunk — bumps it, and zeroes
    ///   what it hands out: C leaves the array indeterminate, but reading
    ///   uninitialised bytes through a raw pointer is undefined in Rust as
    ///   well, and zeroed memory is what the translation always gave. Only
    ///   when the current chunk is full does it go to the next one, reusing it
    ///   if it is big enough and otherwise dropping every chunk after the
    ///   current one — nothing above the position is live — for a new one of
    ///   at least twice the size;
    /// * a variable length array's lifetime is a frame: the position before it
    ///   was allocated, which the frame's `Drop` at the end of the block moves
    ///   the arena back down to. The position never goes below `floor`, which
    ///   `alloca` raises past each allocation it makes: its memory lives until
    ///   the function returns, even out of a block whose array is given back —
    ///   GCC's own rule;
    /// * a [CFG-mode](crate::cfg) function has no blocks to drop frames at, so
    ///   it keeps one mark per variable length array in an array, and
    ///   `redefine` moves the arena back down to a declaration's previous mark
    ///   when the declaration is reached again, forgetting every mark taken
    ///   after it: those arrays were declared later on the path that got here,
    ///   so jumping back over this declaration ended their lifetimes too.
    ///
    /// Everything is in [`Cell`]s so that any number of frames can hold a
    /// shared borrow of the arena at once. The methods are safe: the only
    /// memory they touch is the arena's own.
    fn arena_items(&self, span: Span) -> TokenStream {
        let arena = arena_ty();
        let frame = Ident::new("__cinrs_vla_frame", Span::mixed_site());
        let t = Ident::new("T", Span::mixed_site());
        let usize_ty = primitive_ty("usize", span);
        let u128_ty = primitive_ty("u128", span);
        let u8_ty = primitive_ty("u8", span);
        let chunk_ty = self.vec_ty(u128_ty.clone(), span);
        let chunks_ty = self.vec_ty(chunk_ty, span);
        let empty = self.vec_new(span);
        let new_chunk = self.vec_of(
            quote_spanned! {span=> 0 },
            quote_spanned! {span=> size.div_ceil(::core::mem::size_of::<#u128_ty>()) },
            span,
        );
        let void = self.pointee_ty(Ty::Void, span);
        quote_spanned! {span=>
            #[allow(
                unknown_lints,
                elided_lifetimes_in_paths,
                missing_debug_implementations,
                single_use_lifetimes,
                unused_qualifications,
                clippy::pedantic,
                clippy::nursery
            )]
            struct #arena {
                chunks: ::core::cell::UnsafeCell<#chunks_ty>,
                cur: ::core::cell::Cell<#usize_ty>,
                top: ::core::cell::Cell<#usize_ty>,
                floor: ::core::cell::Cell<(#usize_ty, #usize_ty)>,
            }
            #[allow(
                unknown_lints,
                elided_lifetimes_in_paths,
                single_use_lifetimes,
                unused_qualifications,
                clippy::pedantic,
                clippy::nursery
            )]
            impl #arena {
                #[inline(always)]
                fn new() -> Self {
                    Self {
                        chunks: ::core::cell::UnsafeCell::new(#empty),
                        cur: ::core::cell::Cell::new(0),
                        top: ::core::cell::Cell::new(0),
                        floor: ::core::cell::Cell::new((0, 0)),
                    }
                }
                #[inline(always)]
                fn mark(&self) -> (#usize_ty, #usize_ty) {
                    (self.cur.get(), self.top.get())
                }
                #[inline(always)]
                fn frame(&self) -> #frame<'_> {
                    #frame(self, self.mark())
                }
                /// Moves the position back down to `mark`, but never below
                /// `floor` and never up.
                #[inline(always)]
                fn release(&self, mark: (#usize_ty, #usize_ty)) {
                    let floor = self.floor.get();
                    let to = if mark > floor { mark } else { floor };
                    if to < self.mark() {
                        self.cur.set(to.0);
                        self.top.set(to.1);
                    }
                }
                #[inline(always)]
                fn bytes(&self, bytes: #usize_ty, align: #usize_ty) -> *mut #u8_ty {
                    // SAFETY: nothing else borrows the list of chunks while
                    // this runs, and the memory written is inside a chunk.
                    unsafe {
                        let chunks = &mut *self.chunks.get();
                        let top = self.top.get();
                        if let ::core::option::Option::Some(chunk) = chunks.get_mut(self.cur.get()) {
                            let base = chunk.as_mut_ptr().cast::<#u8_ty>();
                            let size = chunk.len() * ::core::mem::size_of::<#u128_ty>();
                            let start = top + (base.addr().wrapping_add(top).wrapping_neg() & (align - 1));
                            if start <= size && bytes <= size - start {
                                self.top.set(start + bytes);
                                let first = base.add(start);
                                ::core::ptr::write_bytes(first, 0, bytes);
                                return first;
                            }
                        }
                    }
                    self.grow(bytes, align)
                }
                #[cold]
                #[inline(never)]
                fn grow(&self, bytes: #usize_ty, align: #usize_ty) -> *mut #u8_ty {
                    let need = bytes
                        .checked_add(align)
                        .expect("a variable length array or alloca is larger than the address space");
                    {
                        // SAFETY: as in `bytes`; the chunks dropped are above
                        // the position, where nothing is live.
                        let chunks = unsafe { &mut *self.chunks.get() };
                        let next = if chunks.is_empty() { 0 } else { self.cur.get() + 1 };
                        let unit = ::core::mem::size_of::<#u128_ty>();
                        let fits = match chunks.get(next) {
                            ::core::option::Option::Some(chunk) => chunk.len() * unit >= need,
                            ::core::option::Option::None => false,
                        };
                        if !fits {
                            let last = match chunks.last() {
                                ::core::option::Option::Some(chunk) => chunk.len() * unit,
                                ::core::option::Option::None => 0,
                            };
                            chunks.truncate(next);
                            let size = need.max(last.saturating_mul(2)).max(4096);
                            chunks.push(#new_chunk);
                        }
                        self.cur.set(next);
                        self.top.set(0);
                    }
                    self.bytes(bytes, align)
                }
                /// `count` zeroed elements of `T`.
                #[inline(always)]
                fn alloc<#t>(&self, count: #usize_ty) -> *mut #t {
                    self.alloc_aligned::<#t>(count, 1)
                }
                /// `count` zeroed elements of `T`, the first at a multiple of
                /// `align` if that is stricter than `T`'s own alignment.
                #[inline(always)]
                fn alloc_aligned<#t>(&self, count: #usize_ty, align: #usize_ty) -> *mut #t {
                    let bytes = count
                        .checked_mul(::core::mem::size_of::<#t>())
                        .expect("a variable length array is larger than the address space");
                    let natural = ::core::mem::align_of::<#t>();
                    self.bytes(bytes, if align > natural { align } else { natural })
                        .cast::<#t>()
                }
                /// `size` zeroed bytes, 16-byte aligned, that no frame gives
                /// back: they live until the arena is dropped.
                #[inline(always)]
                fn alloca(&self, size: #usize_ty) -> *mut #void {
                    let first = self.bytes(size, 16);
                    self.floor.set(self.mark());
                    first.cast::<#void>()
                }
                /// A CFG-mode variable length array's declaration reached:
                /// gives back what its previous pass took, and everything
                /// allocated after that, and records where this one starts.
                #[inline]
                fn redefine(
                    &self,
                    marks: &mut [::core::option::Option<(#usize_ty, #usize_ty)>],
                    slot: #usize_ty,
                ) {
                    if let ::core::option::Option::Some(mine) = marks[slot] {
                        // No let chain: the user's crate may be on an
                        // edition before 2024.
                        for mark in marks.iter_mut() {
                            if let ::core::option::Option::Some(other) = *mark {
                                if other >= mine {
                                    *mark = ::core::option::Option::None;
                                }
                            }
                        }
                        self.release(mine);
                    }
                    marks[slot] = ::core::option::Option::Some(self.mark());
                }
            }
            #[allow(
                unknown_lints,
                elided_lifetimes_in_paths,
                missing_debug_implementations,
                single_use_lifetimes,
                clippy::pedantic,
                clippy::nursery
            )]
            struct #frame<'a>(&'a #arena, (#usize_ty, #usize_ty));
            #[allow(
                unknown_lints,
                elided_lifetimes_in_paths,
                single_use_lifetimes,
                clippy::pedantic,
                clippy::nursery
            )]
            impl ::core::ops::Drop for #frame<'_> {
                #[inline(always)]
                fn drop(&mut self) {
                    self.0.release(self.1);
                }
            }
        }
    }

    fn stub_item(&mut self, func: &Function) -> TokenStream {
        let span = self.sp(func.range);
        self.enter_function(func);
        let signature = self.signature(func);
        if func.is_safe() {
            return quote_spanned! {span=>
                #signature { ::core::unreachable!() }
            };
        }
        quote_spanned! {span=>
            #signature {
                unsafe { ::core::unreachable!() }
            }
        }
    }

    /// Resets the per-function state before a body is generated.
    fn enter_function(&mut self, func: &Function) {
        self.ret_ty = func.sig.ret;
        self.in_cfg = matches!(func.body, Some(Body::Cfg(_)));
        self.in_safe = func.is_safe();
        self.temporaries = 0;
        self.continue_styles.clear();
        self.local_names.clear();
        self.region_names.clear();
        self.region_kinds.clear();
        self.loop_names.clear();
        self.shape_scopes.clear();
        self.used_labels.clear();
        self.shape_labels = 0;
        self.vla_slots.clear();
        if let Some(Body::Structured(stmts)) = &func.body {
            self.name_regions(stmts);
        }
        if let Some(Body::Cfg(cfg)) = &func.body
            && let Some(plan) = &cfg.shape
        {
            let mut taken = HashSet::new();
            self.name_loops(&plan.body, &mut taken);
        }
        self.env = func
            .env
            .iter()
            .map(|entry| (entry.owner, entry.param))
            .collect();

        // Every object this function binds with a `let` or a parameter, so
        // that one that would shadow a file-scope item can be renamed apart
        // and the rename can be checked against the others. Sema's list is
        // what makes a local declared inside a statement expression — which no
        // walk over the *statements* would reach — part of it.
        let mut bound: Vec<ir::ObjectId> = func.env.iter().map(|entry| entry.param).collect();
        bound.extend(func.params.iter().copied());
        if let Some(Body::Cfg(cfg)) = &func.body {
            for local in &cfg.locals {
                self.local_names
                    .insert(local.object, local.rust_name.clone());
            }
        }
        bound.extend(func.locals.iter().copied());
        self.rename_shadowing(&bound);

        self.va_source = if func.sig.variadic {
            VaSource::Ellipsis
        } else {
            // A `va_list *` parameter points at the caller's list, which is
            // what a `va_list` local of this function starts out as — the same
            // thing a `va_list` parameter is, one indirection further out.
            func.params
                .iter()
                .find_map(|id| {
                    let ty = self.program.object(*id).ty;
                    if ty.is_va_list() {
                        return Some(VaSource::Param(*id));
                    }
                    let pointee = self.program.types.pointee(ty)?;
                    pointee.is_va_list().then_some(VaSource::PtrParam(*id))
                })
                .unwrap_or(VaSource::None)
        };
    }

    /// The name a local or parameter would be generated under before the
    /// shadowing check.
    fn plain_local_name(&self, id: ir::ObjectId) -> String {
        match self.local_names.get(&id) {
            Some(name) => name.clone(),
            None => self.program.object(id).name.clone(),
        }
    }

    /// Renames the bindings of the current function whose names would shadow a
    /// file-scope item.
    ///
    /// The new name is the C one with `_1`, `_2`, … appended, the same shape
    /// the [CFG lowering](crate::cfg) uses when it hoists two locals of the
    /// same name into one scope, and it is checked against the function's
    /// other bindings so that a rename never captures one of them. The check
    /// is against their Rust *spellings* (see [`Names`]), which is what two
    /// bindings collide in.
    fn rename_shadowing(&mut self, bound: &[ir::ObjectId]) {
        if self.reserved.is_empty() {
            return;
        }
        let mut used: HashSet<String> = bound
            .iter()
            .map(|id| {
                self.names
                    .spelling(&self.plain_local_name(*id))
                    .into_owned()
            })
            .collect();
        for id in bound {
            let base = self.plain_local_name(*id);
            if !self.reserved.contains(&base) {
                continue;
            }
            let name = (1u32..)
                .map(|n| format!("{base}_{n}"))
                .find(|c| {
                    !used.contains(self.names.spelling(c).as_ref()) && !self.reserved.contains(c)
                })
                .expect("the sequence of candidates is unbounded");
            used.insert(self.names.spelling(&name).into_owned());
            self.local_names.insert(*id, name);
        }
    }

    /// The name of the synthetic `...` parameter.
    ///
    /// Mixed-site hygiene keeps it distinct from a C variable of the same
    /// name, however unlikely one is.
    fn va_ident(&self) -> Ident {
        Ident::new("__cinrs_va", Span::mixed_site())
    }

    /// A fresh copy of the argument list the function was called with.
    fn va_pristine(&mut self, span: Span) -> TokenStream {
        match self.va_source {
            VaSource::Param(id) => {
                let name = self.object_ident(id, span);
                quote_spanned! {span=> #name.clone() }
            }
            VaSource::PtrParam(id) => {
                let name = self.object_ident(id, span);
                quote_spanned! {span=> (*#name).clone() }
            }
            // `VaSource::None` cannot reach codegen: sema refuses a `va_list`
            // that has nothing to copy.
            _ => {
                let name = self.va_ident();
                quote_spanned! {span=> #name.clone() }
            }
        }
    }

    /// The alignment an object's binding has to be wrapped in, if any.
    ///
    /// Rust has no way to over-align a binding, so an object an `_Alignas` or
    /// an `aligned` made stricter than its type is generated inside a
    /// one-field wrapper that carries the alignment; see
    /// [`Codegen::align_wrapper_items`] and [`ir::Object::align`].
    fn object_align(&self, id: ir::ObjectId) -> Option<u64> {
        self.program.object(id).align
    }

    /// The type an object's binding is declared with.
    fn binding_ty(&self, id: ir::ObjectId, ty: TokenStream, span: Span) -> TokenStream {
        match self.object_align(id) {
            Some(align) => {
                let wrapper = align_wrapper_ident(align, span);
                quote_spanned! {span=> #wrapper<#ty> }
            }
            None => ty,
        }
    }

    /// The value an object's binding is initialised with.
    fn binding_init(&self, id: ir::ObjectId, init: TokenStream, span: Span) -> TokenStream {
        match self.object_align(id) {
            Some(align) => {
                let wrapper = align_wrapper_ident(align, span);
                quote_spanned! {span=> #wrapper(#init) }
            }
            None => init,
        }
    }

    /// The type an object's *storage* has, which is its own except for the
    /// [companion](ir::Object::flexible_len) a filled-in flexible array member
    /// needs.
    fn storage_ty(&self, id: ir::ObjectId, span: Span) -> TokenStream {
        let object = self.program.object(id);
        match (object.flexible_len, object.ty) {
            (Some(len), Ty::Record(record)) => {
                let name =
                    self.flexible_ident(&self.program.types.record(record).rust_name, len, span);
                quote_spanned! {span=> #name }
            }
            _ => self.ty(object.ty, span),
        }
    }

    /// The place expression naming an object, reaching through the alignment
    /// wrapper and the flexible-array companion when the binding has them.
    fn object_access(&self, id: ir::ObjectId, span: Span) -> TokenStream {
        let name = self.object_ident(id, span);
        self.through_storage(id, quote_spanned! {span=> #name }, span)
    }

    /// Reaches the C object inside the storage its binding really has.
    ///
    /// Two wrappers can sit in between, and they compose: the
    /// [alignment](ir::Object::align) wrapper's one field, and the
    /// [flexible-array companion](ir::Object::flexible_len), whose leading
    /// layout is the record's and whose address is therefore a pointer to it.
    fn through_storage(&self, id: ir::ObjectId, base: TokenStream, span: Span) -> TokenStream {
        let object = self.program.object(id);
        let mut access = base;
        if object.align.is_some() {
            let field = Literal::usize_unsuffixed(0);
            access = quote_spanned! {span=> #access.#field };
        }
        if object.flexible_len.is_some() {
            let ty = self.ty(object.ty, span);
            access = parenthesize(
                quote_spanned! {span=> *(&raw mut #access).cast::<#ty>() },
                span,
            );
        }
        access
    }

    /// Reading a `va_list` copies it: Rust's is not `Copy`, and C says a list
    /// passed on is indeterminate afterwards anyway.
    ///
    /// A `va_list` place is an object of its own or the `*p` of a `va_list *`
    /// — nothing may keep one as a member — so the only setup there can be is
    /// the temporary that pointer goes into. Out of line for the same reason
    /// [`Codegen::label_address`] is.
    #[inline(never)]
    fn va_list_load(&mut self, place: &Place, span: Span) -> Value {
        let lowered = self.place(place, false);
        let access = lowered.access;
        let value = Value::new(quote_spanned! {span=> #access.clone() }, prec::CALL);
        if lowered.setup.is_empty() {
            return value;
        }
        let setup = lowered.setup;
        let tokens = value.at(prec::LOWEST, span);
        Value::new(quote_spanned! {span=> { #setup #tokens } }, prec::BLOCK)
    }

    /// GNU's `&&label`: the number the label was given among those of its
    /// function whose address is taken — from 1, so never a null pointer —
    /// cast to the pointer type the expression has. See [`crate::cfg`].
    ///
    /// Out of line — like [`Codegen::label_difference`] — because
    /// [`Codegen::expr_value`] recurses once per operator and every arm's
    /// locals are part of its frame; see
    /// `codegen_of_deeply_nested_input_fits_in_a_small_stack`.
    #[inline(never)]
    fn label_address(&self, id: ir::LabelId, ty: Ty, span: Span) -> Value {
        let state = self.label_states.get(&id).copied().unwrap_or(0);
        let mut literal = Literal::usize_suffixed(state as usize);
        literal.set_span(span);
        let target = self.ty(ty, span);
        Value::new(quote_spanned! {span=> #literal as #target }, prec::CAST).type_end(true)
    }

    /// GNU's `&&a - &&b`, folded on the two labels' numbers.
    #[inline(never)]
    fn label_difference(&self, lhs: &Expr, rhs: &Expr, ty: Ty, span: Span) -> Value {
        let left = self.label_state_literal(lhs, span);
        let right = self.label_state_literal(rhs, span);
        let target = self.ty(ty, span);
        Value::new(
            quote_spanned! {span=> (#left - #right) as #target },
            prec::CAST,
        )
        .type_end(true)
    }

    /// The number a [label address](ir::ExprKind::LabelAddr) stands for, as an
    /// `isize` literal.
    fn label_state_literal(&self, expr: &Expr, span: Span) -> TokenStream {
        let id = label_state(expr).expect("a label address");
        let state = self.label_states.get(&id).copied().unwrap_or(0);
        let mut literal = Literal::isize_suffixed(state as isize);
        literal.set_span(span);
        quote_spanned! {span=> #literal }
    }

    /// The Rust name an object is generated under.
    fn object_ident(&self, id: ir::ObjectId, span: Span) -> Ident {
        let object = self.program.object(id);
        match &object.storage {
            Storage::Automatic => match self.local_names.get(&id) {
                Some(name) => self.c_ident(name, span),
                None => self.c_ident(&object.name, span),
            },
            // A `static mut` is used as a place, never referenced, so
            // edition 2024's `static_mut_refs` lint has nothing to say.
            Storage::Static { item_name, .. } | Storage::ThreadLocal { item_name, .. } => {
                self.c_ident(item_name, span)
            }
            Storage::Extern { item_name } => self.extern_object_ident(item_name, span),
        }
    }

    // -- the control-flow-graph form ----------------------------------------

    /// Emits a [CFG](crate::cfg) body.
    ///
    /// Every local is bound at the top — Rust has no way to jump over a `let`
    /// — and then either the structured shapes [the relooper](crate::reloop)
    /// recovered, or, for a graph it gave up on, the state machine.
    fn cfg_body(&mut self, cfg: &Cfg, span: Span) -> TokenStream {
        let mut out = self.cfg_locals(cfg);
        match &cfg.shape {
            Some(plan) => {
                out.extend(self.shape_seq(cfg, &plan.body, &reloop::Exit::nowhere(), span))
            }
            None => out.extend(self.state_machine(cfg, span)),
        }
        out
    }

    /// The `let` bindings every local of a graph-lowered function gets.
    fn cfg_locals(&mut self, cfg: &Cfg) -> TokenStream {
        let mut out = TokenStream::new();
        let mut slots = 0;
        let mut slots_span = None;
        for local in &cfg.locals {
            let object = self.program.object(local.object);
            let ospan = self.sp(object.range);
            // The hidden frame of a variable length array is a slot of one
            // array of arena marks, which starts out empty; the declaration
            // fills it where it was written, first giving back whatever a
            // previous pass over it took. See [`Codegen::arena_items`].
            if object.vla_storage {
                self.vla_slots.insert(local.object, slots);
                slots += 1;
                slots_span.get_or_insert(ospan);
                continue;
            }
            let name = self.object_ident(local.object, ospan);
            let (ty, init) = if object.ty.is_va_list() {
                // A `va_list` has no zero value; it starts out as a copy of
                // the list the function was called with, exactly as it does
                // when the declaration stays where it was written.
                (self.ty(object.ty, ospan), self.va_pristine(ospan))
            } else {
                (
                    self.ty(object.ty, ospan),
                    self.zero_tokens(object.ty, ospan),
                )
            };
            let ty = self.binding_ty(local.object, ty, ospan);
            let init = self.binding_init(local.object, init, ospan);
            out.extend(quote_spanned! {ospan=> let mut #name: #ty = #init; });
        }
        if let Some(span) = slots_span {
            let marks = vla_marks_ident();
            let usize_ty = primitive_ty("usize", span);
            let len = Literal::usize_unsuffixed(slots);
            out.extend(quote_spanned! {span=>
                let mut #marks: [::core::option::Option<(#usize_ty, #usize_ty)>; #len] =
                    [::core::option::Option::None; #len];
            });
        }
        out
    }

    /// The graph as a state machine, for one [the relooper](crate::reloop) gave
    /// up on: shapes nested deeper than `rustc` parses.
    ///
    /// Every arm of the `match` ends in `continue 'cfg` or in a `return`, so
    /// the loop never finishes and the function needs no value after it.
    fn state_machine(&mut self, cfg: &Cfg, span: Span) -> TokenStream {
        let state = self.state_ident();
        let label = self.cfg_label();
        let mut arms = TokenStream::new();
        for (index, block) in cfg.blocks.iter().enumerate() {
            let bspan = self.block_span(block, span);
            let pattern = state_literal(index, bspan);
            let body = self.block_tokens(block, bspan);
            arms.extend(quote_spanned! {bspan=> #pattern => { #body } });
        }
        arms.extend(quote_spanned! {span=> _ => ::core::unreachable!(), });
        let u32_ty = primitive_ty("u32", span);
        quote_spanned! {span=>
            let mut #state: #u32_ty = 0;
            #label: loop {
                match #state { #arms }
            }
        }
    }

    /// The state variable, in this crate's own hygiene.
    fn state_ident(&self) -> Ident {
        Ident::new("__cinrs_state", Span::mixed_site())
    }

    fn cfg_label(&self) -> TokenStream {
        self.label("cfg", Span::mixed_site())
    }

    /// Where a block's tokens are attributed to: the first thing in it that
    /// came from the C source.
    fn block_span(&self, block: &BasicBlock, fallback: Span) -> Span {
        if let Some(range) = block.stmts.iter().find_map(|s| self.stmt_range(s)) {
            return self.sp(range);
        }
        match &block.term {
            Terminator::Jump { range, .. }
            | Terminator::Switch { range, .. }
            | Terminator::Return { range, .. } => self.sp(*range),
            Terminator::Branch { cond, .. } => self.sp(cond.range),
            Terminator::Unreachable | Terminator::InvalidTarget => fallback,
        }
    }

    fn block_tokens(&mut self, block: &BasicBlock, span: Span) -> TokenStream {
        let mut out = self.stmts(&block.stmts);
        let label = self.cfg_label();
        match &block.term {
            Terminator::Jump { target, range } => {
                let jump = self.enter_block(*target, self.sp(*range));
                out.extend(quote_spanned! {span=> #jump continue #label; });
            }
            Terminator::Branch {
                cond,
                then_blk,
                else_blk,
            } => {
                let cspan = self.sp(cond.range);
                let test = self.condition(cond).at_condition(cspan);
                let then_tokens = self.enter_block(*then_blk, cspan);
                let else_tokens = self.enter_block(*else_blk, cspan);
                out.extend(quote_spanned! {cspan=>
                    if #test { #then_tokens } else { #else_tokens }
                    continue #label;
                });
            }
            Terminator::Switch {
                value,
                cases,
                default,
                range,
            } => {
                let sspan = self.sp(*range);
                let scrutinee = self.expr(value).at(prec::UNARY, sspan);
                let mut arms = TokenStream::new();
                for (target, values) in group_cases(cases) {
                    let mut pattern = TokenStream::new();
                    for (index, case) in values.iter().enumerate() {
                        if index > 0 {
                            pattern.extend(quote_spanned! {sspan=> | });
                        }
                        pattern.extend(case_pattern(*case, value.ty, sspan));
                    }
                    let enter = self.enter_block(target, sspan);
                    arms.extend(quote_spanned! {sspan=> #pattern => { #enter } });
                }
                let enter = self.enter_block(*default, sspan);
                arms.extend(quote_spanned! {sspan=> _ => { #enter } });
                out.extend(quote_spanned! {sspan=>
                    match #scrutinee { #arms }
                    continue #label;
                });
            }
            Terminator::Return { value, range } => {
                let rspan = self.sp(*range);
                match value {
                    Some(value) => {
                        let ret = self.ret_ty;
                        let tokens = self.expr_at(value, ret);
                        out.extend(quote_spanned! {rspan=> return #tokens; });
                    }
                    None => out.extend(quote_spanned! {rspan=> return; }),
                }
            }
            Terminator::Unreachable => {
                out.extend(quote_spanned! {span=> ::core::unreachable!(); });
            }
            Terminator::InvalidTarget => out.extend(invalid_target(span)),
        }
        out
    }

    /// The assignment that moves the state machine to `target`.
    fn enter_block(&self, target: BlockId, span: Span) -> TokenStream {
        let state = self.state_ident();
        let value = state_literal(target.0 as usize, span);
        quote_spanned! {span=> #state = #value; }
    }

    // -- the structured form of the graph ------------------------------------

    /// Names the loops [the relooper](crate::reloop) recovered, after the C
    /// labels their heads stand at.
    ///
    /// The same rules as [`Codegen::name_regions`]: a name Rust cannot spell
    /// as a label, or one this module gives its own loops and blocks, is
    /// replaced by `rN`, and a name two loops would share gets an underscore.
    fn name_loops(&mut self, seq: &reloop::Seq, taken: &mut HashSet<String>) {
        for shape in seq {
            match shape {
                reloop::Shape::Loop { id, name, body, .. } => {
                    let mut candidate = match name {
                        Some(name) => rust_spelling(name).into_owned(),
                        None => String::new(),
                    };
                    if !is_label_name(&candidate) {
                        candidate = format!("r{id}");
                    }
                    while taken.contains(&candidate) {
                        candidate.push('_');
                    }
                    taken.insert(candidate.clone());
                    self.loop_names.insert(*id, candidate);
                    self.name_loops(body, taken);
                }
                reloop::Shape::Simple { arms, .. } | reloop::Shape::Dispatch { arms, .. } => {
                    for arm in arms {
                        self.name_loops(&arm.body, taken);
                    }
                }
            }
        }
    }

    /// A run of shapes, one after another.
    ///
    /// Everything before a shape stands inside a labelled block that ends
    /// where that shape begins, so that a jump forwards is `break` of it —
    /// the same shape [`regions`](crate::regions) gives an outward `goto`.
    /// The nesting runs the other way round from the order they are emitted
    /// in: the block for the *second* shape is the innermost one, so that
    /// breaking it lands on the second shape and breaking any of the others
    /// skips past what is in between.
    fn shape_seq(
        &mut self,
        cfg: &Cfg,
        seq: &reloop::Seq,
        fall: &reloop::Exit,
        span: Span,
    ) -> TokenStream {
        let Some(first) = seq.first() else {
            return TokenStream::new();
        };
        let exits: Vec<reloop::Exit> = seq.iter().map(reloop::Shape::exit).collect();
        // An irreducible region's state variable is bound before anything that
        // could jump into it, which is the head of the run it stands in.
        let mut out = TokenStream::new();
        for shape in seq {
            if let reloop::Shape::Loop {
                state: Some(state), ..
            } = shape
            {
                let name = entry_ident(*state);
                let u32_ty = primitive_ty("u32", span);
                out.extend(quote_spanned! {span=> let mut #name: #u32_ty = 0; });
            }
        }
        let ids: Vec<u32> = (1..seq.len())
            .map(|_| {
                let id = self.shape_labels;
                self.shape_labels += 1;
                id
            })
            .collect();
        for (index, id) in ids.iter().enumerate().rev() {
            self.shape_scopes.push(ShapeScope {
                id: *id,
                name: format!("b{id}"),
                repeats: false,
                exit: exits[index + 1].clone(),
            });
        }
        let mut code = self.shape(cfg, first, exits.get(1).unwrap_or(fall), span);
        for (index, shape) in seq.iter().enumerate().skip(1) {
            let scope = self.shape_scopes.pop().expect("one scope per shape");
            if self.used_labels.contains(&scope.id) {
                let label = self.label(&scope.name, span);
                code = quote_spanned! {span=> #label: { #code } };
            }
            code.extend(self.shape(cfg, shape, exits.get(index + 1).unwrap_or(fall), span));
        }
        out.extend(code);
        out
    }

    /// One shape.
    fn shape(
        &mut self,
        cfg: &Cfg,
        shape: &reloop::Shape,
        fall: &reloop::Exit,
        span: Span,
    ) -> TokenStream {
        match shape {
            reloop::Shape::Simple { block, arms } => {
                self.shape_simple(cfg, *block, arms, fall, span)
            }
            reloop::Shape::Loop {
                id,
                entries,
                state,
                body,
                ..
            } => {
                let name = self
                    .loop_names
                    .get(id)
                    .cloned()
                    .unwrap_or_else(|| format!("r{id}"));
                let bspan = entries.first().map_or(span, |entry| {
                    self.block_span(&cfg.blocks[entry.index()], span)
                });
                let head = reloop::Exit {
                    targets: entries.clone(),
                    state: *state,
                };
                // Leaving the loop is `break`, going round again `continue`.
                self.shape_scopes.push(ShapeScope {
                    id: u32::MAX,
                    name: name.clone(),
                    repeats: false,
                    exit: fall.clone(),
                });
                self.shape_scopes.push(ShapeScope {
                    id: u32::MAX,
                    name: name.clone(),
                    repeats: true,
                    exit: head.clone(),
                });
                let body = self.shape_seq(cfg, body, &head, bspan);
                self.shape_scopes.pop();
                self.shape_scopes.pop();
                let label = self.label(&name, bspan);
                quote_spanned! {bspan=> #label: loop { #body } }
            }
            reloop::Shape::Dispatch { state, arms } => {
                self.shape_dispatch(cfg, *state, arms, fall, span)
            }
        }
    }

    /// The head of an irreducible region: which entry this turn runs.
    fn shape_dispatch(
        &mut self,
        cfg: &Cfg,
        state: u32,
        arms: &[reloop::Arm],
        fall: &reloop::Exit,
        span: Span,
    ) -> TokenStream {
        let Some((last, rest)) = arms.split_last() else {
            return TokenStream::new();
        };
        if rest.is_empty() {
            return self.shape_seq(cfg, &last.body, fall, span);
        }
        let name = entry_ident(state);
        // Two heads is the common irreducible region, and two heads is an
        // `if`: a `match` of `0 =>` and `_ =>` says nothing more.
        if rest.len() == 1 {
            let first = self.shape_seq(cfg, &rest[0].body, fall, span);
            let second = self.shape_seq(cfg, &last.body, fall, span);
            let zero = state_literal(0, span);
            return quote_spanned! {span=>
                if #name == #zero { #first } else { #second }
            };
        }
        let mut cases = TokenStream::new();
        for (index, arm) in rest.iter().enumerate() {
            let body = self.shape_seq(cfg, &arm.body, fall, span);
            let pattern = state_literal(index, span);
            cases.extend(quote_spanned! {span=> #pattern => { #body } });
        }
        let body = self.shape_seq(cfg, &last.body, fall, span);
        cases.extend(quote_spanned! {span=> _ => { #body } });
        quote_spanned! {span=> match #name { #cases } }
    }

    /// A block, and the branch its terminator becomes.
    fn shape_simple(
        &mut self,
        cfg: &Cfg,
        block: BlockId,
        arms: &[reloop::Arm],
        fall: &reloop::Exit,
        span: Span,
    ) -> TokenStream {
        let basic = &cfg.blocks[block.index()];
        let span = self.block_span(basic, span);
        let mut out = self.stmts(&basic.stmts);
        match &basic.term {
            Terminator::Jump { target, range } => {
                let jspan = self.sp(*range);
                out.extend(self.enter_shape(cfg, arms, *target, fall, jspan));
            }
            Terminator::Branch {
                cond,
                then_blk,
                else_blk,
            } => out.extend(self.shape_branch(cfg, arms, cond, *then_blk, *else_blk, fall)),
            Terminator::Switch {
                value,
                cases,
                default,
                range,
            } => {
                let sspan = self.sp(*range);
                out.extend(self.shape_switch(cfg, arms, value, cases, *default, fall, sspan));
            }
            Terminator::Return { value, range } => {
                let rspan = self.sp(*range);
                match value {
                    Some(value) => {
                        let ret = self.ret_ty;
                        let tokens = self.expr_at(value, ret);
                        out.extend(quote_spanned! {rspan=> return #tokens; });
                    }
                    None => out.extend(quote_spanned! {rspan=> return; }),
                }
            }
            Terminator::Unreachable => {
                out.extend(quote_spanned! {span=> ::core::unreachable!(); });
            }
            Terminator::InvalidTarget => out.extend(invalid_target(span)),
        }
        out
    }

    /// A two-way branch: `if`, with the blocks only it reaches in its arms.
    fn shape_branch(
        &mut self,
        cfg: &Cfg,
        arms: &[reloop::Arm],
        cond: &Expr,
        then_blk: BlockId,
        else_blk: BlockId,
        fall: &reloop::Exit,
    ) -> TokenStream {
        let span = self.sp(cond.range);
        // Both edges lead to the same block: the condition is still evaluated,
        // for what it does rather than for what it says.
        if then_blk == else_blk {
            let effects = self.expr_stmt(cond);
            let jump = self.enter_shape(cfg, arms, then_blk, fall, span);
            return quote_spanned! {span=> #effects #jump };
        }
        let then_tokens = self.enter_shape(cfg, arms, then_blk, fall, span);
        let else_tokens = self.enter_shape(cfg, arms, else_blk, fall, span);
        if then_tokens.is_empty() && else_tokens.is_empty() {
            return self.expr_stmt(cond);
        }
        if else_tokens.is_empty() {
            let test = self.condition(cond).at_condition(span);
            return quote_spanned! {span=> if #test { #then_tokens } };
        }
        if then_tokens.is_empty() {
            // No empty arm: the test is inverted rather than left standing
            // with nothing in it.
            let test = self.condition(cond).at(prec::UNARY, span);
            return quote_spanned! {span=> if !#test { #else_tokens } };
        }
        let test = self.condition(cond).at_condition(span);
        quote_spanned! {span=> if #test { #then_tokens } else { #else_tokens } }
    }

    /// A `switch`: one `match`, with the case bodies in its arms and
    /// fallthrough as the code that follows it.
    #[allow(clippy::too_many_arguments)]
    fn shape_switch(
        &mut self,
        cfg: &Cfg,
        arms: &[reloop::Arm],
        value: &Expr,
        cases: &[(ir::CaseRange, BlockId)],
        default: BlockId,
        fall: &reloop::Exit,
        span: Span,
    ) -> TokenStream {
        let groups = group_cases(cases);
        let mut lowered: Vec<(Vec<ir::CaseRange>, TokenStream)> = Vec::new();
        for (target, values) in groups {
            let tokens = self.enter_shape(cfg, arms, target, fall, span);
            lowered.push((values, tokens));
        }
        let fallback = self.enter_shape(cfg, arms, default, fall, span);
        // One arm is no dispatch at all: every value goes the same way, and
        // all the `match` would do is evaluate the controlling expression.
        if lowered.iter().all(|(_, tokens)| tokens.is_empty()) && fallback.is_empty() {
            return self.expr_stmt(value);
        }
        if lowered.is_empty() {
            let effects = self.expr_stmt(value);
            return quote_spanned! {span=> #effects #fallback };
        }
        let scrutinee = self.expr(value).at(prec::UNARY, span);
        let mut out = TokenStream::new();
        for (values, tokens) in lowered {
            let mut pattern = TokenStream::new();
            for (index, case) in values.iter().enumerate() {
                if index > 0 {
                    pattern.extend(quote_spanned! {span=> | });
                }
                pattern.extend(case_pattern(*case, value.ty, span));
            }
            out.extend(quote_spanned! {span=> #pattern => { #tokens } });
        }
        out.extend(quote_spanned! {span=> _ => { #fallback } });
        quote_spanned! {span=> match #scrutinee { #out } }
    }

    /// What one edge of a terminator becomes: the shapes only it reaches,
    /// emitted here, or a jump to where they stand.
    fn enter_shape(
        &mut self,
        cfg: &Cfg,
        arms: &[reloop::Arm],
        target: BlockId,
        fall: &reloop::Exit,
        span: Span,
    ) -> TokenStream {
        match arms.iter().find(|arm| arm.entry == target) {
            Some(arm) => self.shape_seq(cfg, &arm.body, fall, span),
            None => self.goto_shape(target, fall, span),
        }
    }

    /// The jump that reaches `target` from here.
    ///
    /// Falling out of the construct being generated is free; anything else is
    /// a `break` or a `continue` of the innermost labelled block or loop that
    /// arrives there. [`reloop::plan`] has checked that one of the two always
    /// applies.
    fn goto_shape(&mut self, target: BlockId, fall: &reloop::Exit, span: Span) -> TokenStream {
        if let Some(index) = fall.index_of(target) {
            return entry_assignment(fall, index, span);
        }
        for depth in (0..self.shape_scopes.len()).rev() {
            let Some(index) = self.shape_scopes[depth].exit.index_of(target) else {
                continue;
            };
            let scope = self.shape_scopes[depth].clone();
            self.used_labels.insert(scope.id);
            let label = self.label(&scope.name, span);
            let set = entry_assignment(&scope.exit, index, span);
            return if scope.repeats {
                quote_spanned! {span=> #set continue #label; }
            } else {
                quote_spanned! {span=> #set break #label; }
            };
        }
        unreachable!("the shapes were checked before they were generated")
    }

    // -- statements ---------------------------------------------------------

    fn stmts(&mut self, stmts: &[Stmt]) -> TokenStream {
        let mut out = TokenStream::new();
        for stmt in stmts {
            out.extend(self.stmt(stmt));
        }
        out
    }

    /// Emits a statement as a braced block, reusing the braces C already wrote
    /// when it wrote a compound statement.
    fn block_of(&mut self, stmt: &Stmt, span: Span) -> TokenStream {
        match stmt {
            Stmt::Block(items) => {
                let items = self.stmts(items);
                braced(items, span)
            }
            other => {
                let tokens = self.stmt(other);
                braced(tokens, span)
            }
        }
    }

    fn stmt(&mut self, stmt: &Stmt) -> TokenStream {
        match stmt {
            Stmt::Nop => TokenStream::new(),
            Stmt::Asm(asm) => self.asm_stmt(asm),
            Stmt::Expr(expr) => self.expr_stmt(expr),
            Stmt::Let { object, init, .. } => {
                let id = *object;
                let name = self.object_ident(id, self.sp(self.program.object(id).range));
                let object = self.program.object(id);
                let span = self.sp(object.range);
                let object_ty = object.ty;
                let ty = self.binding_ty(id, self.ty(object_ty, span), span);
                let init = self.expr_at(init, object_ty);
                let init = self.binding_init(id, init, span);
                quote_spanned! {span=> let mut #name: #ty = #init; }
            }
            Stmt::Vla(def) => self.vla_def(def),
            Stmt::Cleanup(def) => self.cleanup_def(def),
            Stmt::Block(items) => {
                let span = self.stmts_span(items);
                let items = self.stmts(items);
                braced(items, span)
            }
            Stmt::If {
                cond,
                then_branch,
                else_branch,
            } => {
                let span = self.sp(cond.range);
                let cond_tokens = self.condition(cond).at_condition(span);
                let then_tokens = self.block_of(then_branch, span);
                let else_tokens = match else_branch {
                    Some(branch) => {
                        let tokens = self.block_of(branch, span);
                        quote_spanned! {span=> else #tokens }
                    }
                    None => TokenStream::new(),
                };
                quote_spanned! {span=> if #cond_tokens #then_tokens #else_tokens }
            }
            Stmt::While {
                id,
                cond,
                body,
                range,
            } => {
                let span = self.sp(*range);
                self.continue_styles.insert(*id, ContinueStyle::Head);
                let label = self.loop_label(*id, span);
                let body_tokens = self.block_of(body, span);
                // `while (1)` becomes `loop`, not `while true`: only a `loop`
                // tells Rust the statement never finishes, which is what a
                // function ending in one relies on.
                if ir::is_always_true(cond) {
                    return quote_spanned! {span=> #label: loop #body_tokens };
                }
                let cond_tokens = self.condition(cond).at_condition(span);
                quote_spanned! {span=> #label: while #cond_tokens #body_tokens }
            }
            Stmt::DoWhile {
                id,
                body,
                cond,
                range,
            } => {
                let span = self.sp(*range);
                self.continue_styles.insert(*id, ContinueStyle::BodyLabel);
                let label = self.loop_label(*id, span);
                let body_label = self.loop_body_label(*id, span);
                let body_tokens = self.block_of(body, span);
                let test = if ir::is_always_true(cond) {
                    TokenStream::new()
                } else {
                    let cond_tokens = self.condition(cond).at(prec::LOWEST, span);
                    quote_spanned! {span=> if !(#cond_tokens) { break #label; } }
                };
                quote_spanned! {span=>
                    #label: loop {
                        #body_label: #body_tokens
                        #test
                    }
                }
            }
            Stmt::For {
                id,
                init,
                cond,
                step,
                body,
                range,
            } => {
                let span = self.sp(*range);
                self.continue_styles.insert(*id, ContinueStyle::BodyLabel);
                let label = self.loop_label(*id, span);
                let body_label = self.loop_body_label(*id, span);
                let init_tokens = self.stmts(init);
                let test = match cond {
                    Some(cond) if !ir::is_always_true(cond) => {
                        let tokens = self.condition(cond).at(prec::LOWEST, span);
                        quote_spanned! {span=> if !(#tokens) { break #label; } }
                    }
                    _ => TokenStream::new(),
                };
                let body_tokens = self.block_of(body, span);
                let step_tokens = match step {
                    Some(step) => self.expr_stmt(step),
                    None => TokenStream::new(),
                };
                // The whole loop is wrapped so that a C99 declaration in the
                // init clause stays scoped to the loop, as C says it is.
                quote_spanned! {span=>
                    {
                        #init_tokens
                        #label: loop {
                            #test
                            #body_label: #body_tokens
                            #step_tokens
                        }
                    }
                }
            }
            Stmt::Switch(switch) => self.switch(switch),
            Stmt::Region(region) => self.region(region),
            // The CFG lowering consumes these, and a label the structured mode
            // kept is where a region ends rather than anything of its own: only
            // the statement under it is left.
            Stmt::Label { body, .. } | Stmt::Case { body, .. } => self.stmt(body),
            // A `goto` left in a structured body leaves the region its label
            // stands for; see [`crate::regions`]. Anything else is consumed by
            // the CFG lowering, which is the only mode it is generated in.
            Stmt::Goto { id, range } => {
                let span = self.sp(*range);
                match self.region_kinds.get(id).copied() {
                    Some(kind) => {
                        let label = self.region_label(*id, span);
                        match kind {
                            ir::RegionKind::Block => quote_spanned! {span=> break #label; },
                            ir::RegionKind::Loop => quote_spanned! {span=> continue #label; },
                        }
                    }
                    None => TokenStream::new(),
                }
            }
            Stmt::GotoPtr { .. } => TokenStream::new(),
            Stmt::SwitchTree(switch) => self.stmt(&switch.body),
            Stmt::Break { target, range } => {
                let span = self.sp(*range);
                let label = match target {
                    BreakTarget::Loop(id) => self.loop_label(*id, span),
                    BreakTarget::Switch(id) => self.switch_label(*id, span),
                };
                quote_spanned! {span=> break #label; }
            }
            Stmt::Continue { id, range } => {
                let span = self.sp(*range);
                match self.continue_styles.get(id) {
                    Some(ContinueStyle::BodyLabel) => {
                        let label = self.loop_body_label(*id, span);
                        quote_spanned! {span=> break #label; }
                    }
                    _ => {
                        let label = self.loop_label(*id, span);
                        quote_spanned! {span=> continue #label; }
                    }
                }
            }
            Stmt::Return { value, range } => {
                let span = self.sp(*range);
                match value {
                    Some(value) => {
                        let ret = self.ret_ty;
                        let tokens = self.expr_at(value, ret);
                        quote_spanned! {span=> return #tokens; }
                    }
                    None => quote_spanned! {span=> return; },
                }
            }
        }
    }

    /// A variable length array's definition, `T a[n];`.
    ///
    /// Two bindings after the bounds' own: the frame of the function's bump
    /// arena, whose `Drop` at the end of the block — on every way out of it —
    /// gives the space back and is the object's lifetime; and the object
    /// itself, a pointer to the first of the zeroed elements the arena bumps
    /// off. The frame stands first, so Rust drops it after anything declared
    /// with the array. See [`Codegen::arena_items`].
    ///
    /// In [CFG mode](crate::cfg) there are no blocks to drop a frame at, and
    /// the pointer is already bound at the top of the function — Rust has no
    /// way to jump over a `let` — so what is written here is the arena's
    /// `redefine` of this array's slot in the function's array of marks, which
    /// gives back a previous pass's space before taking the new mark, and an
    /// assignment.
    fn vla_def(&mut self, def: &ir::VlaDef) -> TokenStream {
        let span = self.sp(def.range);
        let object = self.program.object(def.object);
        // Whatever is left under the variable dimensions: one allocation
        // holds the whole object, however many of them there are.
        let elem = self.program.types.vm_step_ty(object.ty);
        let name = self.object_ident(def.object, span);
        let count = self.expr(&def.count).at(prec::CAST, span);
        let elem_ty = self.ty(elem, span);
        let usize_ty = primitive_ty("usize", span);
        let arena = self.arena_ident(span);
        // A requested alignment stricter than the element's pads the arena's
        // position up to a multiple of it before the array starts.
        let first = match def.align {
            Some(align) => {
                let align = Literal::usize_unsuffixed(align as usize);
                quote_spanned! {span=>
                    #arena.alloc_aligned::<#elem_ty>(#count as #usize_ty, #align)
                }
            }
            None => quote_spanned! {span=> #arena.alloc::<#elem_ty>(#count as #usize_ty) },
        };
        if self.in_cfg {
            let marks = vla_marks_ident();
            let slot = Literal::usize_unsuffixed(self.vla_slots[&def.storage]);
            return quote_spanned! {span=>
                #arena.redefine(&mut #marks, #slot);
                #name = #first;
            };
        }
        let frame = self.object_ident(def.storage, span);
        quote_spanned! {span=>
            let #frame = #arena.frame();
            let mut #name: *mut #elem_ty = #first;
        }
    }

    /// A `cleanup` attribute's drop guard, in the structured lowering.
    ///
    /// The binding stands right after the object's own, so Rust drops it
    /// first — and drops it on every way out of the block, which is exactly
    /// what GCC promises. See [`ir::CleanupDef`].
    fn cleanup_def(&mut self, def: &ir::CleanupDef) -> TokenStream {
        let span = self.sp(def.range);
        let guard = cleanup_guard_ty();
        // One binding per object, in this crate's own hygiene: the C program
        // cannot name it, and two guards in one block cannot collide.
        let name = Ident::new(
            &format!("__cinrs_cleanup{}", def.object.0),
            Span::mixed_site(),
        );
        let object = self.program.object(def.object);
        let place = ir::Place {
            kind: PlaceKind::Object(def.object),
            ty: object.ty,
            is_const: object.is_const,
            range: def.range,
        };
        let address = self
            .address_of(&place, def.param, span)
            .at(prec::LOWEST, span);
        let function = self.func_pointer(def.func, span);
        self.uses_cleanup.set(true);
        quote_spanned! {span=>
            let #name = #guard(#address, #function);
        }
    }

    /// `struct __cinrs_cleanup<P, R>(P, unsafe extern "C" fn(P) -> R);` and
    /// its `Drop`.
    ///
    /// One item per unit — each unit is a module of its own, so two of them in
    /// one Rust module do not collide — generated only when something asks for
    /// it. It is generic over the *pointer* the function takes rather than
    /// over the object's type, so that a `void *`-taking cleanup (the
    /// `_cleanup_free_` idiom) needs nothing special, and over the return
    /// type, which GCC ignores.
    fn cleanup_guard_item(&self, span: Span) -> TokenStream {
        let name = cleanup_guard_ty();
        let p = Ident::new("P", Span::mixed_site());
        let r = Ident::new("R", Span::mixed_site());
        quote_spanned! {span=>
            struct #name<#p: ::core::marker::Copy, #r>(#p, unsafe extern "C" fn(#p) -> #r);
            impl<#p: ::core::marker::Copy, #r> ::core::ops::Drop for #name<#p, #r> {
                fn drop(&mut self) {
                    unsafe {
                        (self.1)(self.0);
                    }
                }
            }
        }
    }

    /// `Some(f as unsafe extern "C" fn(…) -> R)`: a function name used as a
    /// value.
    ///
    /// A method of its own rather than an arm of [`Codegen::expr`]'s match
    /// because that function is the recursion this module is bounded by, and in
    /// an unoptimised build every temporary of every arm has a stack slot of
    /// its own; see `expand.rs`'s `codegen_of_deeply_nested_input_fits_in_a_
    /// small_stack`.
    fn func_addr(&self, id: ir::FuncId, span: Span) -> Value {
        let function = self.program.function(id);
        let name = if function.intrinsic.is_some() {
            self.intrinsic_shim_name(id, span)
        } else {
            self.function_path(function, span)
        };
        let signature = self.function_pointer_ty(function, span);
        Value::new(
            quote_spanned! {span=> ::core::option::Option::Some(#name as #signature) },
            prec::CALL,
        )
    }

    /// The name of the shim that stands for an x86 intrinsic whose address was
    /// taken, remembering that the unit needs it.
    ///
    /// `Span::mixed_site()` is the same hygiene the cleanup guard type uses: it
    /// puts the name out of reach of anything the C declares, so no
    /// `__cinrs_intrinsic_…` a program writes for itself can collide with it.
    fn intrinsic_shim_name(&self, id: ir::FuncId, span: Span) -> TokenStream {
        let function = self.program.function(id);
        {
            let mut wanted = self.address_taken.borrow_mut();
            if !wanted.contains(&id) {
                wanted.push(id);
            }
        }
        let name = Ident::new(
            &format!("__cinrs_intrinsic_{}", function.name),
            Span::mixed_site(),
        );
        quote_spanned! {span=> #name }
    }

    /// One `unsafe extern "C" fn` per intrinsic whose address the unit took,
    /// forwarding to `core::arch`.
    ///
    /// The shim carries the intrinsic's own `#[target_feature]`, which is what
    /// GCC's `__always_inline__` header function carries too: without it a
    /// 256-bit vector could not be passed by value through the C ABI at all,
    /// and rustc refuses the definition. It is private to the unit's module —
    /// `pub` would put a name into the glob re-export that no C declared — and
    /// `#[inline]` so that the indirect call through it costs nothing when the
    /// optimiser can see the target. `#[inline(always)]` is deliberately *not*
    /// used: rustc refuses it together with `#[target_feature]`.
    fn intrinsic_shim_items(&self) -> TokenStream {
        let mut out = TokenStream::new();
        for id in self.address_taken.borrow().iter() {
            let function = self.program.function(*id);
            let Some(intr) = function.intrinsic else {
                continue;
            };
            let span = self.sp(function.range);
            let name = Ident::new(
                &format!("__cinrs_intrinsic_{}", function.name),
                Span::mixed_site(),
            );
            let mut params = TokenStream::new();
            let mut args = TokenStream::new();
            for (index, ty) in function.sig.params.iter().enumerate() {
                if index > 0 {
                    params.extend(quote_spanned! {span=> , });
                    args.extend(quote_spanned! {span=> , });
                }
                let pname = Ident::new(&format!("__cinrs_arg{index}"), Span::mixed_site());
                let pty = self.ty(*ty, span);
                params.extend(quote_spanned! {span=> #pname: #pty });
                // The same cast a direct call's pointer argument gets.
                let cast = self.intrinsic_pointer_arg(*ty, span).unwrap_or_default();
                args.extend(quote_spanned! {span=> #pname #cast });
            }
            let ret = if function.sig.ret.is_void() {
                TokenStream::new()
            } else {
                let ty = self.ty(function.sig.ret, span);
                quote_spanned! {span=> -> #ty }
            };
            let feature = if intr.feature.is_empty() {
                TokenStream::new()
            } else {
                let mut literal = Literal::string(intr.feature);
                literal.set_span(span);
                quote_spanned! {span=> #[target_feature(enable = #literal)] }
            };
            let module = self.arch_module(span);
            let call = Ident::new(intr.name, span);
            out.extend(quote_spanned! {span=>
                #[inline]
                #feature
                unsafe extern "C" fn #name(#params) #ret {
                    unsafe { ::core::arch::#module::#call(#args) }
                }
            });
        }
        out
    }

    /// `f as unsafe extern "C" fn(…) -> R`, the function a drop guard holds.
    fn func_pointer(&self, id: ir::FuncId, span: Span) -> TokenStream {
        let function = self.program.function(id);
        let name = self.function_path(function, span);
        let signature = self.function_pointer_ty(function, span);
        quote_spanned! {span=> #name as #signature }
    }

    /// A span standing for a run of statements.
    fn stmts_span(&self, stmts: &[Stmt]) -> Span {
        stmts
            .iter()
            .find_map(|s| self.stmt_range(s))
            .map_or_else(Span::call_site, |range| self.sp(range))
    }

    /// An inline assembly statement: `::core::arch::asm!`, with the operands
    /// sema mapped (see `sema/asm.rs`).
    ///
    /// ```text
    /// {
    ///     let p = …;                          // each output place's setup, once
    ///     ::core::arch::asm!("addl {o1:e}, {o0:e}",
    ///         o0 = inout(reg) (*p).f,         // the place itself
    ///         o1 = in(reg) b,
    ///         out("rcx") _,
    ///         options(att_syntax));
    /// }
    /// ```
    ///
    /// An output is written straight into its place whenever that place is an
    /// ordinary Rust place expression: a [lowered place](LoweredPlace)'s
    /// access may be evaluated more than once and its setup — where the side
    /// effects of `a[i++]` or `f()->x` happen — runs exactly once, before the
    /// statement. Only a place that may be underaligned goes through a typed
    /// temporary, read with `read_unaligned` (for `+`) and stored back with
    /// `write_unaligned` after the statement: `asm!` would otherwise write it
    /// with an aligned store. Bit-fields and `_Atomic` objects never get here;
    /// sema refuses both.
    ///
    /// Inputs are emitted with [`Codegen::expr`], never bare: an `asm!`
    /// operand has no expected type, so an unsuffixed literal would be an
    /// `i32` whatever the C operand was, and the register would be read at the
    /// wrong width.
    fn asm_stmt(&mut self, asm: &ir::AsmStmt) -> TokenStream {
        let span = self.sp(asm.range);
        let mut setup = TokenStream::new();
        let mut store_back = TokenStream::new();
        let mut operands = TokenStream::new();
        for (index, operand) in asm.operands.iter().enumerate() {
            let op_span = self.sp(operand.range);
            let head = asm_operand_head(operand, op_span);
            let value = match &operand.kind {
                ir::AsmOperandKind::In(expr) => self.expr(expr).at(prec::LOWEST, op_span),
                ir::AsmOperandKind::Scratch(expr) => {
                    let value = self.expr(expr).at(prec::LOWEST, op_span);
                    quote_spanned! {op_span=> #value => _ }
                }
                ir::AsmOperandKind::Const(value) => asm_const_literal(*value, op_span),
                ir::AsmOperandKind::Out { place, .. } => {
                    self.asm_output(place, None, index, &mut setup, &mut store_back)
                }
                ir::AsmOperandKind::InOut { input, output } => {
                    let input = input
                        .as_ref()
                        .map(|expr| self.expr(expr).at(prec::LOWEST, op_span));
                    let read_place = input.is_none();
                    let target = self.asm_output(
                        output,
                        Some(read_place),
                        index,
                        &mut setup,
                        &mut store_back,
                    );
                    match input {
                        Some(input) => quote_spanned! {op_span=> #input => #target },
                        None => target,
                    }
                }
            };
            operands.extend(quote_spanned! {op_span=> #head #value, });
        }
        for clobber in &asm.clobbers {
            let name = Literal::string(clobber);
            operands.extend(quote_spanned! {span=> out(#name) _, });
        }
        let mut template = Literal::string(&asm.template);
        template.set_span(span);
        let call = quote_spanned! {span=>
            ::core::arch::asm!(#template, #operands options(att_syntax));
        };
        if setup.is_empty() && store_back.is_empty() {
            return call;
        }
        quote_spanned! {span=> { #setup #call #store_back } }
    }

    /// Where an `asm` output goes: the place itself, or — for a place that
    /// may be underaligned — a temporary that is stored back afterwards.
    ///
    /// `read` is `None` for a pure output, and for an in/out operand says
    /// whether its input is the place's own value (GCC's `+`).
    fn asm_output(
        &mut self,
        place: &Place,
        read: Option<bool>,
        index: usize,
        setup: &mut TokenStream,
        store_back: &mut TokenStream,
    ) -> TokenStream {
        let span = self.sp(place.range);
        let lowered = self.place(place, true);
        setup.extend(lowered.setup.clone());
        if !lowered.unaligned {
            let access = &lowered.access;
            return quote_spanned! {span=> #access };
        }
        let temp = Ident::new(&format!("__cinrs_asm{index}"), span);
        let ty = self.ty(place.ty, span);
        let access = &lowered.access;
        // A pure output is initialised by the statement itself, exactly once.
        setup.extend(if read == Some(true) {
            quote_spanned! {span=> let mut #temp: #ty = (&raw const #access).read_unaligned(); }
        } else {
            quote_spanned! {span=> let #temp: #ty; }
        });
        store_back.extend(self.write(&lowered, quote_spanned! {span=> #temp }, span));
        quote_spanned! {span=> #temp }
    }

    fn stmt_range(&self, stmt: &Stmt) -> Option<SourceRange> {
        Some(match stmt {
            Stmt::Expr(expr) => expr.range,
            Stmt::Let { object, .. } => self.program.object(*object).range,
            Stmt::Vla(def) => def.range,
            Stmt::Cleanup(def) => def.range,
            Stmt::If { cond, .. } => cond.range,
            Stmt::While { range, .. }
            | Stmt::DoWhile { range, .. }
            | Stmt::For { range, .. }
            | Stmt::Break { range, .. }
            | Stmt::Continue { range, .. }
            | Stmt::Return { range, .. }
            | Stmt::Label { range, .. }
            | Stmt::Case { range, .. }
            | Stmt::Goto { range, .. }
            | Stmt::GotoPtr { range, .. } => *range,
            Stmt::Switch(switch) => switch.range,
            Stmt::SwitchTree(switch) => switch.range,
            Stmt::Region(region) => region.range,
            Stmt::Asm(asm) => asm.range,
            Stmt::Block(items) => return items.iter().find_map(|s| self.stmt_range(s)),
            Stmt::Nop => return None,
        })
    }

    fn loop_label(&self, id: LoopId, span: Span) -> TokenStream {
        self.label(&format!("l{}", id.0), span)
    }

    fn loop_body_label(&self, id: LoopId, span: Span) -> TokenStream {
        self.label(&format!("l{}_body", id.0), span)
    }

    fn switch_label(&self, id: ir::SwitchId, span: Span) -> TokenStream {
        self.label(&format!("sw{}", id.0), span)
    }

    fn switch_case_label(&self, id: ir::SwitchId, index: usize, span: Span) -> TokenStream {
        self.label(&format!("sw{}_c{index}", id.0), span)
    }

    /// The Rust label a [region](ir::Region) carries.
    fn region_label(&self, id: ir::LabelId, span: Span) -> TokenStream {
        match self.region_names.get(&id) {
            Some(name) => self.label(name, span),
            // Only a `goto` outside every region for its label, which
            // [`crate::regions`] does not leave behind.
            None => self.label(&format!("cinrs_label{}", id.0), span),
        }
    }

    /// A labelled block or a labelled loop, and the statements inside it.
    ///
    /// The two shapes are what an outward `goto` becomes — see
    /// [`crate::regions`]. A loop ends with a `break` of its own so that
    /// falling off the end of the region leaves it; one whose body cannot
    /// reach its end has none, which is what lets a function ending in it need
    /// no `return`.
    fn region(&mut self, region: &ir::Region) -> TokenStream {
        let span = self.sp(region.range);
        let label = self.region_label(region.label, span);
        let previous = self.region_kinds.insert(region.label, region.kind);
        let body = self.stmts(&region.body);
        match previous {
            Some(kind) => self.region_kinds.insert(region.label, kind),
            None => self.region_kinds.remove(&region.label),
        };
        match region.kind {
            ir::RegionKind::Block => quote_spanned! {span=> #label: { #body } },
            ir::RegionKind::Loop if region.falls_out => quote_spanned! {span=>
                #label: loop { #body break #label; }
            },
            ir::RegionKind::Loop => quote_spanned! {span=> #label: loop { #body } },
        }
    }

    /// Names the regions of a structured body, after the C labels they stand
    /// for.
    ///
    /// A name that Rust cannot spell as a label — a keyword, or one of the
    /// shapes this module gives a loop or a `switch` — gets the label's number
    /// appended, which no generated label can collide with; one it cannot
    /// spell at all, such as an extended identifier, gives up its own name.
    /// The block and the loop a label with jumps of both kinds gets are named
    /// once: they never overlap, and `break` and `continue` say which is meant.
    fn name_regions(&mut self, stmts: &[Stmt]) {
        let mut taken: HashSet<String> = HashSet::new();
        let mut found = Vec::new();
        collect_regions(stmts, &mut found);
        for (id, name) in found {
            if self.region_names.contains_key(&id) {
                continue;
            }
            let mut candidate = rust_spelling(&name).into_owned();
            if !is_label_name(&candidate) {
                candidate = format!("{candidate}_{}", id.0);
            }
            if !is_label_name(&candidate) {
                candidate = format!("cinrs_label{}", id.0);
            }
            while taken.contains(&candidate) {
                candidate.push('_');
            }
            taken.insert(candidate.clone());
            self.region_names.insert(id, candidate);
        }
    }

    /// Builds the labelled-block chain described in the [module docs](self).
    fn switch(&mut self, switch: &Switch) -> TokenStream {
        let span = self.sp(switch.range);
        let scrutinee_ty = switch.scrutinee.ty;
        let scrutinee = self.expr(&switch.scrutinee).at(prec::UNARY, span);

        let mut arms = TokenStream::new();
        for (index, group) in switch.groups.iter().enumerate() {
            if group.values.is_empty() {
                continue;
            }
            let label = self.switch_case_label(switch.id, index, span);
            let mut pattern = TokenStream::new();
            for (i, value) in group.values.iter().enumerate() {
                if i > 0 {
                    pattern.extend(quote_spanned! {span=> | });
                }
                // A pattern takes its type from the scrutinee, so the bare
                // literal is both correct and the most readable form.
                pattern.extend(case_pattern(*value, scrutinee_ty, span));
            }
            arms.extend(quote_spanned! {span=> #pattern => break #label, });
        }
        let fallback = match switch.default_group {
            Some(index) => self.switch_case_label(switch.id, index, span),
            None => self.switch_label(switch.id, span),
        };
        arms.extend(quote_spanned! {span=> _ => break #fallback, });

        // Anything before the first label can never be reached, but it is still
        // part of the program and may declare things later groups use.
        let prelude = self.stmts(&switch.prelude);
        let mut inner = quote_spanned! {span=> match #scrutinee { #arms } #prelude };
        for (index, group) in switch.groups.iter().enumerate() {
            let label = self.switch_case_label(switch.id, index, span);
            let body = self.stmts(&group.body);
            inner = quote_spanned! {span=> #label: { #inner } #body };
        }

        let mut hoisted = TokenStream::new();
        for id in &switch.hoisted {
            let object = self.program.object(*id);
            let ospan = self.sp(object.range);
            let name = self.object_ident(*id, ospan);
            let ty = self.binding_ty(*id, self.ty(object.ty, ospan), ospan);
            let zero = self.zero_tokens(object.ty, ospan);
            let zero = self.binding_init(*id, zero, ospan);
            hoisted.extend(quote_spanned! {ospan=> let mut #name: #ty = #zero; });
        }

        let label = self.switch_label(switch.id, span);
        // The extra block keeps the hoisted declarations from shadowing
        // anything the enclosing block declared under the same name.
        quote_spanned! {span=>
            {
                #hoisted
                #label: { #inner }
            }
        }
    }

    /// Emits an expression evaluated for its side effects.
    fn expr_stmt(&mut self, expr: &Expr) -> TokenStream {
        let span = self.sp(expr.range);
        match &expr.kind {
            ExprKind::Assign { place, value } => {
                let lowered = self.place(place, true);
                let value = self.expr_at(value, self.program.types.unatomic(place.ty));
                let store = self.write(&lowered, value, span);
                let setup = &lowered.setup;
                quote_spanned! {span=> #setup #store }
            }
            ExprKind::CompoundAssign {
                place,
                op,
                value,
                compute,
            } => {
                let lowered = self.place(place, true);
                if lowered.atomic.is_some() {
                    let kind = PlaceRmw::Compound {
                        op: *op,
                        value,
                        compute: *compute,
                    };
                    return self.atomic_place_rmw(&lowered, kind, RmwValue::None, span);
                }
                let (hoist, rhs) = self.compound_rhs(value);
                let current = self.read(&lowered, span);
                let updated = self.compound_value(current, place.ty, *op, value, rhs, *compute);
                let updated = updated.at(prec::LOWEST, span);
                let store = self.write(&lowered, updated, span);
                let setup = &lowered.setup;
                quote_spanned! {span=> #setup #hoist #store }
            }
            ExprKind::IncDec { place, dec, .. } => {
                let lowered = self.place(place, true);
                if lowered.atomic.is_some() {
                    let kind = PlaceRmw::Step { dec: *dec };
                    return self.atomic_place_rmw(&lowered, kind, RmwValue::None, span);
                }
                let current = self.read(&lowered, span);
                let next = self.step_value(current, place.ty, *dec, span);
                let store = self.write(&lowered, next, span);
                let setup = &lowered.setup;
                quote_spanned! {span=> #setup #store }
            }
            ExprKind::Call { .. } => {
                // A call to a `_Noreturn` function does not come back, and
                // Rust has to be told: the call's own type is whatever the
                // function was declared to return, so a function that ends in
                // `exit(1);` would otherwise be missing its value. The
                // `unreachable!()` is a panic rather than
                // `unreachable_unchecked`, because it is *this crate* putting
                // it there.
                let diverges = ir::expr_never_returns(expr, &self.program.functions);
                let tokens = self.expr(expr).at(prec::LOWEST, span);
                if diverges {
                    quote_spanned! {span=> #tokens; ::core::unreachable!(); }
                } else {
                    quote_spanned! {span=> #tokens; }
                }
            }
            ExprKind::Unreachable => {
                quote_spanned! {span=> ::core::hint::unreachable_unchecked(); }
            }
            // A store, a `clear` and a fence have no value at all, in C or in
            // the Rust they become; the `let _ =` the fallback below would
            // wrap them in says nothing.
            ExprKind::Atomic(_) if expr.ty.is_void() => {
                let tokens = self.expr(expr).at(prec::LOWEST, span);
                quote_spanned! {span=> #tokens; }
            }
            ExprKind::Comma { .. } => {
                // A chain of comma operators is a flat sequence of statements
                // — and one stack frame per operand if it is walked
                // recursively, which a four-thousand-character logical source
                // line cannot afford. See [`Codegen::binary_chain`].
                let mut out = TokenStream::new();
                for operand in comma_operands(expr) {
                    out.extend(self.expr_stmt(operand));
                }
                out
            }
            ExprKind::Cond { .. } => {
                // A chain of them is `if … else if … else …`, built without
                // recursing down the chain; see [`Codegen::cond_chain`].
                let mut spine = Vec::new();
                let mut node = expr;
                while let ExprKind::Cond {
                    cond,
                    then_expr,
                    else_expr,
                } = &node.kind
                {
                    let span = self.sp(node.range);
                    let cond_tokens = self.condition(cond).at_condition(span);
                    let then_tokens = self.expr_stmt(then_expr);
                    spine.push((cond_tokens, then_tokens, span));
                    node = else_expr;
                }
                let mut tokens = self.expr_stmt(node);
                while let Some((cond_tokens, then_tokens, span)) = spine.pop() {
                    tokens = quote_spanned! {span=>
                        if #cond_tokens { #then_tokens } else { #tokens }
                    };
                }
                tokens
            }
            // `(void)x;` evaluates and discards, which is what the fall-through
            // below does anyway; unwrapping keeps the output tidy.
            ExprKind::Cast(inner) if expr.ty.is_void() => self.expr_stmt(inner),
            // `va_end(ap);` is a statement that does nothing at all.
            ExprKind::VaEnd => TokenStream::new(),
            _ => {
                let tokens = self.expr(expr).at(prec::LOWEST, span);
                quote_spanned! {span=> let _ = #tokens; }
            }
        }
    }

    // -- expressions --------------------------------------------------------

    /// Emits an expression whose type the surrounding context already fixes.
    ///
    /// A constant then needs no `as`, which is the difference between
    /// `let mut i: c_int = 0;` and `let mut i: c_int = 0 as c_int;`. It is the
    /// *only* place a bare literal is emitted from, and the reason it is safe
    /// is that every caller writes the tokens somewhere the type is already
    /// stated — the annotation of a `let`, a place being assigned to, a
    /// parameter, a field, the return type of the function.
    ///
    /// [`Codegen::expr`] has no such promise, so nothing it produces may
    /// depend on inference; that is what the conditional below is about.
    fn expr_at(&mut self, expr: &Expr, expected: Ty) -> TokenStream {
        let span = self.sp(expr.range);
        if expr.ty == expected {
            match &expr.kind {
                ExprKind::Int(value) => return bare_int_literal(*value, expected, span),
                ExprKind::Float(value) if value.is_finite() => {
                    return bare_float_literal(*value, span);
                }
                // The arms of a conditional may stay bare here, because
                // whatever fixes this expression's type fixes theirs — unless
                // the type is `void`, where the arms have no common type and
                // [`Codegen::expr`] emits each of them as a statement.
                ExprKind::Cond { .. } if !expected.is_void() => {
                    return self.cond_chain_at(expr, expected);
                }
                _ => {}
            }
        }
        self.expr(expr).at(prec::LOWEST, span)
    }

    fn expr(&mut self, expr: &Expr) -> Value {
        let value = self.expr_value(expr);
        // A chain of binary operators reduces each of its own nodes; see
        // `Codegen::binary_chain`.
        if matches!(expr.kind, ExprKind::Binary { .. }) {
            return value;
        }
        self.reduce_bits(value, expr)
    }

    /// Reduces a computed value to the precision the expression is evaluated
    /// in, which is narrower than its type only for a wide bit-field; see
    /// [`ir::Expr::bits`].
    ///
    /// Only an operator whose result can leave the field's range needs it: a
    /// bitwise `&`, `|` or `^` of two forty-bit values is a forty-bit value
    /// already, and so is a quotient, a remainder or a right shift.
    fn reduce_bits(&mut self, value: Value, expr: &Expr) -> Value {
        let Some(bits) = expr.bits else {
            return value;
        };
        let width = expr.ty.bits(&self.options.target);
        let overflows = match &expr.kind {
            ExprKind::Binary { op, .. } => {
                matches!(op, BinOp::Add | BinOp::Sub | BinOp::Mul | BinOp::Shl)
            }
            ExprKind::Neg(_) | ExprKind::BitNot(_) => true,
            _ => false,
        };
        if !overflows || !expr.ty.is_integer() || bits == 0 || bits >= width {
            return value;
        }
        let span = self.sp(expr.range);
        if expr.ty.is_signed(&self.options.target) {
            // Sign extension: the two shifts are what a narrower signed type
            // does to a value that has just overflowed out of it.
            let shift = Literal::u32_unsuffixed(width - bits);
            let tokens = value.at(prec::CALL, span);
            return Value::new(
                quote_spanned! {span=> #tokens.wrapping_shl(#shift).wrapping_shr(#shift) },
                prec::CALL,
            );
        }
        let mask = bare_int_literal(((1u128 << bits) - 1) as i128, expr.ty, span);
        let tokens = value.at(prec::BIT_AND, span);
        Value::new(quote_spanned! {span=> #tokens & #mask }, prec::BIT_AND)
    }

    fn expr_value(&mut self, expr: &Expr) -> Value {
        let span = self.sp(expr.range);
        match &expr.kind {
            ExprKind::Int(value) => self.int_literal(*value, expr.ty, span),
            ExprKind::Float(value) => self.float_literal(*value, expr.ty, span),
            ExprKind::Zeroed => Value::new(self.zero_tokens(expr.ty, span), zero_prec(expr.ty)),
            // Reading a `va_list` copies it: Rust's is not `Copy`, and C says
            // a list passed on is indeterminate afterwards anyway.
            ExprKind::Load(place) if place.ty.is_va_list() => self.va_list_load(place, span),
            ExprKind::Load(place) => {
                let lowered = self.place(place, false);
                let value = self.read(&lowered, span);
                if lowered.setup.is_empty() {
                    value
                } else {
                    let setup = &lowered.setup;
                    let tokens = value.at(prec::LOWEST, span);
                    Value::new(quote_spanned! {span=> { #setup #tokens } }, prec::BLOCK)
                }
            }
            ExprKind::AddrOf(place) => self.address_of(place, expr.ty, span),
            ExprKind::FuncAddr(id) => self.func_addr(*id, span),
            // GNU's `&&label`. The value is the label's number, cast to the
            // pointer type the expression has — which is what `goto *`'s
            // dispatch matches on, and what lets a dispatch table be an
            // ordinary array of `void *`.
            ExprKind::LabelAddr(id) => self.label_address(*id, expr.ty, span),
            ExprKind::Assign { .. } => self.assign_chain(expr),
            ExprKind::CompoundAssign {
                place,
                op,
                value,
                compute,
            } => {
                let lowered = self.place(place, true);
                if lowered.atomic.is_some() {
                    let kind = PlaceRmw::Compound {
                        op: *op,
                        value,
                        compute: *compute,
                    };
                    let tokens = self.atomic_place_rmw(&lowered, kind, RmwValue::New, span);
                    return Value::new(tokens, prec::BLOCK);
                }
                let (hoist, rhs) = self.compound_rhs(value);
                let current = self.read(&lowered, span);
                let updated = self.compound_value(current, place.ty, *op, value, rhs, *compute);
                let updated = updated.at(prec::LOWEST, span);
                let store = self.write(&lowered, updated, span);
                let read = self.read(&lowered, span).at(prec::LOWEST, span);
                let setup = &lowered.setup;
                Value::new(
                    quote_spanned! {span=> { #setup #hoist #store #read } },
                    prec::BLOCK,
                )
            }
            ExprKind::IncDec {
                place,
                dec,
                postfix,
            } => {
                let lowered = self.place(place, true);
                if lowered.atomic.is_some() {
                    let want = if *postfix {
                        RmwValue::Old
                    } else {
                        RmwValue::New
                    };
                    let tokens =
                        self.atomic_place_rmw(&lowered, PlaceRmw::Step { dec: *dec }, want, span);
                    return Value::new(tokens, prec::BLOCK);
                }
                let current = self.read(&lowered, span);
                let next = self.step_value(current, place.ty, *dec, span);
                let store = self.write(&lowered, next, span);
                let read = self.read(&lowered, span).at(prec::LOWEST, span);
                let setup = &lowered.setup;
                if *postfix {
                    let tmp = self.temporary();
                    Value::new(
                        quote_spanned! {span=>
                            { #setup let #tmp = #read; #store #tmp }
                        },
                        prec::BLOCK,
                    )
                } else {
                    Value::new(
                        quote_spanned! {span=> { #setup #store #read } },
                        prec::BLOCK,
                    )
                }
            }
            // The three complex shapes are one call apiece: this function is
            // the one code generation recurses through, and every local in it
            // costs a slice of the stack `rustc` gives macro expansion.
            ExprKind::ComplexOf { re, im } => self.complex_literal(expr.ty, re, im, span),
            ExprKind::Neg(operand) if expr.ty.is_complex() => self.complex_neg(operand, span),
            ExprKind::BitNot(operand) if expr.ty.is_complex() => {
                self.complex_conj(operand, expr.ty, span)
            }
            ExprKind::Neg(operand) => {
                let value = self.expr(operand);
                if expr.ty.is_floating() {
                    let ends_with_type = value.ends_with_type;
                    let tokens = value.at(prec::UNARY, span);
                    Value::new(quote_spanned! {span=> -#tokens }, prec::UNARY)
                        .type_end(ends_with_type)
                } else {
                    // Signed overflow is undefined in C and a panic in Rust;
                    // wrapping is the predictable choice, and it is what
                    // unsigned arithmetic requires anyway.
                    let tokens = value.at(prec::CALL, span);
                    Value::new(quote_spanned! {span=> #tokens.wrapping_neg() }, prec::CALL)
                }
            }
            ExprKind::BitNot(operand) => {
                let value = self.expr(operand);
                let ends_with_type = value.ends_with_type;
                let tokens = value.at(prec::UNARY, span);
                Value::new(quote_spanned! {span=> !#tokens }, prec::UNARY).type_end(ends_with_type)
            }
            ExprKind::Binary { .. } => self.binary_chain(expr),
            ExprKind::PtrOffset { ptr, index, sub } => {
                let pointee = self.program.types.pointee(ptr.ty).unwrap_or(Ty::Void);
                let base = self.expr(ptr).at(prec::CALL, span);
                let offset = self.scaled_offset(pointee, index, *sub, span);
                Value::new(quote_spanned! {span=> #base.offset(#offset) }, prec::CALL)
            }
            // GNU's label difference, `&&a - &&b`. Both operands are state
            // numbers rather than addresses, so the subtraction is done on the
            // numbers: `offset_from` would want two pointers into one object,
            // and a `static` table of such differences — which is the whole
            // idiom — needs an expression a `const` can fold.
            ExprKind::PtrDiff { lhs, rhs }
                if label_state(lhs).is_some() && label_state(rhs).is_some() =>
            {
                self.label_difference(lhs, rhs, expr.ty, span)
            }
            ExprKind::PtrDiff { lhs, rhs } => {
                let pointee = self.program.types.pointee(lhs.ty).unwrap_or(Ty::Void);
                let scale = self.vm_scale(pointee, span);
                let left = self.expr(lhs).at(prec::CALL, span);
                let right = self.expr(rhs).at(prec::LOWEST, span);
                let target = self.ty(expr.ty, span);
                // The difference the generated pointers give is in step-type
                // elements; C's is in whole ones.
                let difference = match scale {
                    None => quote_spanned! {span=> #left.offset_from(#right) },
                    Some(scale) => {
                        quote_spanned! {span=> (#left.offset_from(#right) / (#scale)) }
                    }
                };
                Value::new(quote_spanned! {span=> #difference as #target }, prec::CAST)
                    .type_end(true)
            }
            ExprKind::Compare { .. } | ExprKind::Logical { .. } => {
                // C's comparisons and logical operators produce an `int`.
                let condition = self.condition(expr).at(prec::LOWEST, span);
                let ty = self.ty(Ty::Int, span);
                let parens = parenthesize(condition, span);
                Value::new(quote_spanned! {span=> #parens as #ty }, prec::CAST).type_end(true)
            }
            // `(void)x` and a `void`-typed conditional have no value at all:
            // Rust's `()` is not something `as` produces, so each of them is
            // emitted as the statement it is, wrapped in a block whose own
            // value is `()`. A conditional gets here when it is written where
            // a value is expected — the left operand of a comma is a
            // statement, but the right one is not.
            ExprKind::Cast(inner) if expr.ty.is_void() => {
                let tokens = self.expr_stmt(inner);
                Value::new(quote_spanned! {span=> { #tokens } }, prec::BLOCK)
            }
            ExprKind::Cond { .. } if expr.ty.is_void() => {
                let tokens = self.expr_stmt(expr);
                Value::new(quote_spanned! {span=> { #tokens } }, prec::BLOCK)
            }
            ExprKind::Cast(inner) => {
                let from = inner.ty;
                let value = self.expr(inner);
                self.cast(value, from, expr.ty, span)
            }
            // Both arms are emitted as expressions of their own rather than at
            // the conditional's type: an `if` whose arms are two bare literals
            // is `{integer}`, which Rust either resolves to `i32` — wrong
            // wherever C said `long` — or refuses to resolve at all, as it
            // does for the receiver of `wrapping_mul` (`E0689`). The bare form
            // is still used from [`Codegen::expr_at`], where the context says
            // what the type is.
            ExprKind::Cond { .. } => self.cond_chain(expr),
            ExprKind::Comma { lhs, rhs } => {
                let lhs = self.expr_stmt(lhs);
                let rhs = self.expr(rhs).at(prec::LOWEST, span);
                Value::new(quote_spanned! {span=> { #lhs #rhs } }, prec::BLOCK)
            }
            ExprKind::Call { callee, args } => self.call(callee, args, span),
            ExprKind::RecordLit { record, fields } => self.record_literal(*record, fields, span),
            ExprKind::UnionLit {
                record,
                index,
                value,
            } => self.union_literal(*record, *index, value, span),
            ExprKind::ArrayLit(items) => {
                let elem = self.program.types.elem(expr.ty).unwrap_or(Ty::Int);
                let mut tokens = TokenStream::new();
                for (index, item) in items.iter().enumerate() {
                    if index > 0 {
                        tokens.extend(quote_spanned! {span=> , });
                    }
                    tokens.extend(self.expr_at(item, elem));
                }
                Value::atom(bracketed(tokens, span))
            }
            ExprKind::ArrayRepeat { value, len } => {
                let elem = self.program.types.elem(expr.ty).unwrap_or(value.ty);
                let tokens = self.expr_at(value, elem);
                let len = usize_literal(*len, span);
                Value::atom(bracketed(quote_spanned! {span=> #tokens ; #len }, span))
            }
            // GNU's `a ?: b`. The value is held in a temporary so that the
            // operand is evaluated exactly once, which is the whole point.
            ExprKind::CondDefault { value, else_expr } => {
                let ty = expr.ty;
                let first = self.expr_at(value, ty);
                let other = self.expr_at(else_expr, ty);
                let tmp = self.temporary();
                let target = self.ty(ty, span);
                let test = if ty.is_pointer() {
                    quote_spanned! {span=> !#tmp.is_null() }
                } else {
                    let zero = self.zero_tokens(ty, span);
                    quote_spanned! {span=> #tmp != #zero }
                };
                Value::new(
                    quote_spanned! {span=>
                        { let #tmp: #target = #first; if #test { #tmp } else { #other } }
                    },
                    prec::BLOCK,
                )
            }
            // GNU's statement expression, which is what a Rust block is.
            ExprKind::StmtExpr { stmts, value } => {
                let body = self.stmts(stmts);
                let tail = match value {
                    Some(value) => {
                        let ty = expr.ty;
                        self.expr_at(value, ty)
                    }
                    None => TokenStream::new(),
                };
                Value::new(quote_spanned! {span=> { #body #tail } }, prec::BLOCK)
            }
            ExprKind::Builtin { op, args } => self.builtin(*op, args, span),
            ExprKind::Atomic(atomic) => self.atomic(atomic, span),
            ExprKind::VaListPristine => Value::new(self.va_pristine(span), prec::CALL),
            ExprKind::VaArg { ap, record } => self.va_arg(ap, record.as_deref(), expr.ty, span),
            // `va_end` is nothing: the list ends when its value is dropped.
            ExprKind::VaEnd => Value::atom(quote_spanned! {span=> () }),
            // C23's `unreachable()`. C says reaching it is undefined, and
            // saying so to Rust is what lets the optimiser use the promise —
            // this is the one place the expansion trusts the C program with
            // undefined behaviour, because the program asked for it by name.
            ExprKind::Unreachable => Value::new(
                quote_spanned! {span=> ::core::hint::unreachable_unchecked() },
                prec::CALL,
            ),
        }
    }

    // -- complex ------------------------------------------------------------

    /// `::cinrs::rt::Complex::<f64>::new(re, im)`.
    ///
    /// A call rather than a struct literal, and with the component type spelled
    /// out: `Complex { … }` at the start of an `if` condition would be read as
    /// the condition's block, and a bare `0.0` part would infer `f64` where the
    /// type is `float _Complex`. `Complex::new` is a `const fn`, so this is
    /// also what a `static` initialiser holds.
    fn complex_new(&self, ty: Ty, re: TokenStream, im: TokenStream, span: Span) -> Value {
        self.uses_complex.set(true);
        let rt = self.rt_path(span);
        let component = primitive_ty(if ty == Ty::ComplexFloat { "f32" } else { "f64" }, span);
        Value::new(
            quote_spanned! {span=> #rt::Complex::<#component>::new(#re, #im) },
            prec::CALL,
        )
    }

    /// `__builtin_complex(re, im)`, an imaginary constant, and a folded
    /// complex constant: the two parts, side by side.
    fn complex_literal(&mut self, ty: Ty, re: &Expr, im: &Expr, span: Span) -> Value {
        let re = self.expr(re).at(prec::LOWEST, span);
        let im = self.expr(im).at(prec::LOWEST, span);
        self.complex_new(ty, re, im, span)
    }

    /// `-z`, which is `num_complex`'s own `Neg` — both parts negated exactly.
    fn complex_neg(&mut self, operand: &Expr, span: Span) -> Value {
        let tokens = self.expr(operand).at(prec::UNARY, span);
        Value::new(quote_spanned! {span=> -#tokens }, prec::UNARY)
    }

    /// GNU's `~z` and `__builtin_conj(z)`: the conjugate.
    fn complex_conj(&mut self, operand: &Expr, ty: Ty, span: Span) -> Value {
        let tokens = self.expr(operand).at(prec::LOWEST, span);
        let conj = self.rt_complex(&format!("conj_{}", Self::complex_suffix(ty)), span);
        Value::new(quote_spanned! {span=> #conj(#tokens) }, prec::CALL)
    }

    /// `+`, `-`, `*` or `/` with at least one complex operand.
    ///
    /// Each operand arrives with the C type its tokens have, because that is
    /// what decides which runtime function is called: C computes a *real*
    /// operand componentwise rather than widening it — see `cinrs_rt::complex`
    /// for what that changes and why. Two complex operands add and subtract
    /// through `num_complex`'s own operators, which are exactly componentwise,
    /// and multiply and divide through Annex G.
    fn complex_binary(
        &mut self,
        op: BinOp,
        (lhs, lhs_ty): (Value, Ty),
        (rhs, rhs_ty): (Value, Ty),
        ty: Ty,
        span: Span,
    ) -> Value {
        self.uses_complex.set(true);
        let suffix = Self::complex_suffix(ty);
        let both = lhs_ty.is_complex() && rhs_ty.is_complex();
        if both && matches!(op, BinOp::Add | BinOp::Sub) {
            let (level, tokens) = match op {
                BinOp::Add => (prec::SUM, quote_spanned! {span=> + }),
                _ => (prec::SUM, quote_spanned! {span=> - }),
            };
            let mut out = lhs.at(level, span);
            let rhs = rhs.at(level + 1, span);
            out.extend(quote_spanned! {span=> #tokens #rhs });
            return Value::new(out, level);
        }
        // `<what>_<suffix>`, with the name saying which operand is the real
        // one: `mul_real` is `z * x` and `real_mul` is `x * z`.
        let name = match (op, lhs_ty.is_complex(), rhs_ty.is_complex()) {
            (BinOp::Add, true, false) => "add_real",
            (BinOp::Add, false, true) => "real_add",
            (BinOp::Sub, true, false) => "sub_real",
            (BinOp::Sub, false, true) => "real_sub",
            (BinOp::Mul, true, true) => "mul",
            (BinOp::Mul, true, false) => "mul_real",
            (BinOp::Mul, false, true) => "real_mul",
            (BinOp::Div, true, true) => "div",
            (BinOp::Div, true, false) => "div_real",
            (BinOp::Div, false, true) => "real_div",
            // Sema refuses every other operator on a complex operand, and one
            // of the two always is complex.
            _ => unreachable!("'{}' does not reach complex code generation", op.as_str()),
        };
        let func = self.rt_complex(&format!("{name}_{suffix}"), span);
        let lhs = lhs.at(prec::LOWEST, span);
        let rhs = rhs.at(prec::LOWEST, span);
        Value::new(quote_spanned! {span=> #func(#lhs, #rhs) }, prec::CALL)
    }

    /// A conversion with a complex type on one side or the other
    /// (C99 6.3.1.6, 6.3.1.7).
    fn complex_cast(&mut self, value: Value, from: Ty, to: Ty, span: Span) -> Value {
        self.uses_complex.set(true);
        if from.is_complex() && to.is_complex() {
            let name = if to == Ty::ComplexDouble {
                "widen_f32"
            } else {
                "narrow_f64"
            };
            let func = self.rt_complex(name, span);
            let tokens = value.at(prec::LOWEST, span);
            return Value::new(quote_spanned! {span=> #func(#tokens) }, prec::CALL);
        }
        if to.is_complex() {
            // A real value becomes a complex one with a zero imaginary part.
            let component = to.complex_component();
            let re = self
                .cast(value, from, component, span)
                .at(prec::LOWEST, span);
            let zero = bare_float_literal(0.0, span);
            return self.complex_new(to, re, zero, span);
        }
        // Converting a complex value to a real type discards the imaginary
        // part — except for `_Bool`, which asks whether *either* part is
        // non-zero (C99 6.3.1.2).
        if to.is_bool() {
            let func = self.rt_complex(&format!("nonzero_{}", Self::complex_suffix(from)), span);
            let tokens = value.at(prec::LOWEST, span);
            return Value::new(quote_spanned! {span=> #func(#tokens) }, prec::CALL);
        }
        let tokens = value.at(prec::CALL, span);
        let real = Value::new(quote_spanned! {span=> #tokens.re }, prec::CALL);
        self.cast(real, from.complex_component(), to, span)
    }

    /// `z == w` and `z != w` (C99 6.5.9p3: equal exactly when both parts are).
    ///
    /// Rust's own `==` would give the same answer — the runtime's complex type
    /// derives `PartialEq` — and it is *not* used, because `PartialEq::eq`
    /// takes `&self`: a reference to a field of a `#[repr(packed)]` record is
    /// `E0793`, and a packed `__complex__ float` member compared against a
    /// constant is exactly `gcc.c-torture/execute/20020227-1`. The runtime's
    /// by-value equality asks for a *copy* of each operand, which a packed
    /// field will give.
    ///
    /// Sema converts both operands to one complex type before this, so a
    /// mixed real/complex comparison never arrives here.
    fn complex_equality(&mut self, op: CmpOp, lhs: &Expr, rhs: &Expr, span: Span) -> Value {
        self.uses_complex.set(true);
        let name = match op {
            CmpOp::Eq => "eq",
            CmpOp::Ne => "ne",
            // 6.5.8p2 gives the relational operators real operands only, and
            // sema has already said so.
            other => unreachable!("{other:?} does not reach a complex comparison"),
        };
        let func = self.rt_complex(&format!("{name}_{}", Self::complex_suffix(lhs.ty)), span);
        let (lhs, rhs) = self.operands(lhs, rhs, BinOp::BitOr);
        let lhs = lhs.at(prec::LOWEST, span);
        let rhs = rhs.at(prec::LOWEST, span);
        Value::new(quote_spanned! {span=> #func(#lhs, #rhs) }, prec::CALL)
    }

    /// `z != 0` as Rust's `bool`: what an `if`, a `while` and `!z` ask of a
    /// complex value.
    fn complex_condition(&mut self, expr: &Expr, span: Span) -> Value {
        self.uses_complex.set(true);
        let func = self.rt_complex(&format!("nonzero_{}", Self::complex_suffix(expr.ty)), span);
        let tokens = self.expr(expr).at(prec::LOWEST, span);
        Value::new(quote_spanned! {span=> #func(#tokens) }, prec::CALL)
    }

    /// A `struct` value: one expression per member, in declaration order.
    ///
    /// Without bit-fields this is a plain Rust struct literal. With them the
    /// members that share a storage field have to be *packed* into it: every
    /// constant one is folded into the `[u8; K]` there and then, which is what
    /// lets a `static` — where nothing may run — hold a bit-field at all, and
    /// what makes the byte pattern visible in the expansion. A member whose
    /// value is not constant is stored afterwards through its setter, so the
    /// literal becomes a block.
    fn record_literal(&mut self, record: ir::RecordId, fields: &[Expr], span: Span) -> Value {
        let def = self.program.types.record(record).clone();
        // An initialised flexible array member makes the value the *companion*
        // type's rather than the record's: the tail is as long as the
        // initialiser, and the member's value carries that length.
        let tail = def
            .fields
            .iter()
            .position(|field| field.flexible)
            .and_then(|index| {
                Some((
                    index,
                    array_len(&self.program.types, fields.get(index)?.ty)?,
                ))
            })
            .filter(|(_, len)| *len > 0);
        let name = match tail {
            Some((_, len)) => self.flexible_ident(&def.rust_name, len, span),
            None => self.c_ident(&def.rust_name, span),
        };
        let mut packed: HashMap<String, Vec<u8>> = HashMap::new();
        let mut dynamic: Vec<usize> = Vec::new();
        for rust_field in &def.rust_fields {
            if let ir::RustField::Bits { name, bytes, .. } = rust_field {
                packed.insert(name.clone(), vec![0u8; *bytes as usize]);
            }
        }
        for (index, field) in def.fields.iter().enumerate() {
            let Some(bits) = &field.bits else { continue };
            match fields.get(index).and_then(constant_bits) {
                Some(value) => {
                    if let Some(storage) = packed.get_mut(&bits.storage) {
                        pack_bits(storage, bits, value);
                    }
                }
                None => dynamic.push(index),
            }
        }

        let mut items = TokenStream::new();
        for rust_field in &def.rust_fields {
            match rust_field {
                ir::RustField::Member(index) => {
                    let field = &def.fields[*index];
                    let fname = self.c_ident(&field.name, span);
                    // The flexible member's value is longer than its own type,
                    // and the companion's field is what it fills.
                    let value = &fields[*index];
                    let want = match tail {
                        Some((flexible, _)) if flexible == *index => value.ty,
                        _ => field.ty,
                    };
                    let tokens = self.expr_at(value, want);
                    items.extend(quote_spanned! {span=> #fname: #tokens, });
                }
                ir::RustField::Bits { name, .. } => {
                    let fname = Ident::new(name, span);
                    let value = byte_array(&packed[name], span);
                    items.extend(quote_spanned! {span=> #fname: #value, });
                }
                ir::RustField::Pad { name, bytes } => {
                    let fname = Ident::new(name, span);
                    let len = usize_literal(*bytes, span);
                    items.extend(quote_spanned! {span=> #fname: [0; #len], });
                }
                ir::RustField::Align { name, .. } => {
                    let fname = Ident::new(name, span);
                    items.extend(quote_spanned! {span=> #fname: [], });
                }
            }
        }
        let body = braced(items, span);
        let literal = quote_spanned! {span=> #name #body };
        if dynamic.is_empty() {
            return Value::new(literal, prec::ATOM);
        }
        // The members that are not constants are stored through their setters,
        // in declaration order, which is one of the orders C allows.
        let tmp = self.temporary();
        let ty = self.ty(Ty::Record(record), span);
        let mut stores = TokenStream::new();
        for index in dynamic {
            let field = &def.fields[index];
            let bits = field.bits.as_ref().expect("only bit-fields are deferred");
            let setter = self.c_ident(&bits.setter, span);
            let value = self.expr_at(&fields[index], field.ty);
            stores.extend(quote_spanned! {span=> #tmp.#setter(#value); });
        }
        Value::new(
            quote_spanned! {span=>
                { let mut #tmp: #ty = #literal; #stores #tmp }
            },
            prec::BLOCK,
        )
    }

    /// A `union` value, which initialises exactly one member.
    fn union_literal(
        &mut self,
        record: ir::RecordId,
        index: usize,
        value: &Expr,
        span: Span,
    ) -> Value {
        let def = self.program.types.record(record).clone();
        let name = self.c_ident(&def.rust_name, span);
        let field = &def.fields[index];
        let Some(bits) = &field.bits else {
            let fname = self.c_ident(&field.name, span);
            let tokens = self.expr_at(value, field.ty);
            let body = braced(quote_spanned! {span=> #fname: #tokens }, span);
            return Value::new(quote_spanned! {span=> #name #body }, prec::ATOM);
        };
        let bytes = def
            .rust_fields
            .iter()
            .find_map(|rust_field| match rust_field {
                ir::RustField::Bits { name, bytes, .. } if *name == bits.storage => Some(*bytes),
                _ => None,
            })
            .unwrap_or(0);
        let mut packed = vec![0u8; bytes as usize];
        let constant = constant_bits(value);
        if let Some(constant) = constant {
            pack_bits(&mut packed, bits, constant);
        }
        let storage = Ident::new(&bits.storage, span);
        let array = byte_array(&packed, span);
        let body = braced(quote_spanned! {span=> #storage: #array }, span);
        let literal = quote_spanned! {span=> #name #body };
        if constant.is_some() {
            return Value::new(literal, prec::ATOM);
        }
        let tmp = self.temporary();
        let ty = self.ty(Ty::Record(record), span);
        let setter = self.c_ident(&bits.setter, span);
        let value = self.expr_at(value, field.ty);
        Value::new(
            quote_spanned! {span=>
                { let mut #tmp: #ty = #literal; #tmp.#setter(#value); #tmp }
            },
            prec::BLOCK,
        )
    }

    /// One of the builtins that becomes a fixed piece of Rust.
    ///
    /// The bit-manipulation ones are the integer methods of the same name, on
    /// the *unsigned* type of the operand's width — C's are defined on
    /// unsigned values and Rust's `leading_zeros` counts the same way. The
    /// overflow ones do the arithmetic in `i128` and ask whether the value
    /// survives the round trip through the type it is stored in, which is
    /// exactly "compute in infinite precision, then convert".
    fn builtin(&mut self, op: ir::BuiltinOp, args: &[Expr], span: Span) -> Value {
        use ir::BuiltinOp;
        let int = self.ty(Ty::Int, span);
        match op {
            BuiltinOp::ComplexProj => {
                let ty = args[0].ty;
                let func = self.rt_complex(&format!("proj_{}", Self::complex_suffix(ty)), span);
                let value = self.expr(&args[0]).at(prec::LOWEST, span);
                Value::new(quote_spanned! {span=> #func(#value) }, prec::CALL)
            }
            BuiltinOp::Discard => {
                let mut out = TokenStream::new();
                for arg in args {
                    out.extend(self.expr_stmt(arg));
                }
                Value::new(quote_spanned! {span=> { #out } }, prec::BLOCK)
            }
            // A hint, so dropping it is always correct, and it is dropped
            // where emitting it would change what compiles: a safe function
            // has no `unsafe` block for the call to sit in, and only x86 and
            // AArch64 have a stable way to say it.
            BuiltinOp::Prefetch(packed) => {
                let (write, locality) = ir::prefetch_hint(packed);
                let arch = self.options.target.arch;
                let pointer = self.expr(&args[0]).at(prec::LOWEST, span);
                let tmp = self.temporary();
                let prefetch = match arch {
                    _ if self.in_safe => TokenStream::new(),
                    // `_mm_prefetch` has no form for writing without
                    // `prefetchw`, which is a feature of its own, so `rw` is
                    // not passed on. The hints are GCC's: locality 3 is
                    // `prefetcht0`, 0 is `prefetchnta`. The `cfg` is for a
                    // 32-bit target without SSE (`i586`), where there is
                    // nothing to call; x86-64 always has SSE.
                    crate::target::Arch::X86 | crate::target::Arch::X86_64 => {
                        let module = self.arch_module(span);
                        let hint = Ident::new(
                            ["_MM_HINT_NTA", "_MM_HINT_T2", "_MM_HINT_T1", "_MM_HINT_T0"]
                                [usize::from(locality)],
                            span,
                        );
                        let sse = if arch == crate::target::Arch::X86 {
                            quote_spanned! {span=> #[cfg(target_feature = "sse")] }
                        } else {
                            TokenStream::new()
                        };
                        quote_spanned! {span=>
                            #sse
                            ::core::arch::#module::_mm_prefetch::<{ ::core::arch::#module::#hint }>(
                                #tmp.cast::<i8>()
                            );
                        }
                    }
                    // GCC's choice of `prfm` operation for each locality.
                    crate::target::Arch::Aarch64 => {
                        let kind = if write { "pst" } else { "pld" };
                        let level = ["l1strm", "l3keep", "l2keep", "l1keep"][usize::from(locality)];
                        let mut template = Literal::string(&format!("prfm {kind}{level}, [{{0}}]"));
                        template.set_span(span);
                        quote_spanned! {span=>
                            ::core::arch::asm!(
                                #template,
                                in(reg) #tmp,
                                options(nostack, preserves_flags, readonly)
                            );
                        }
                    }
                    _ => TokenStream::new(),
                };
                Value::new(
                    quote_spanned! {span=> { let #tmp = #pointer; #prefetch } },
                    prec::BLOCK,
                )
            }
            // `__builtin_cpu_supports("avx2")` asks the processor, exactly as
            // it does in C, and the answer is an `int` because that is what
            // GCC's builtin returns. Several features for one GCC name — `abm`
            // is LZCNT and POPCNT — are all of them or nothing.
            BuiltinOp::CpuSupports(row) => {
                let mut test = TokenStream::new();
                for (index, feature) in crate::x86::detect_features(row).iter().enumerate() {
                    if index > 0 {
                        test.extend(quote_spanned! {span=> && });
                    }
                    let mut literal = Literal::string(feature);
                    literal.set_span(span);
                    // The root path, not `std::arch`'s: `is_x86_feature_detected`
                    // has been a `std` root macro since 1.27, which is older
                    // than anything this crate supports.
                    test.extend(quote_spanned! {span=> ::std::is_x86_feature_detected!(#literal) });
                }
                Value::new(quote_spanned! {span=> ((#test) as #int) }, prec::LOWEST)
            }
            // Sixteen-byte aligned bytes off the function's arena, which no
            // variable length array's frame gives back: they live until the
            // function returns. See [`Codegen::arena_items`].
            BuiltinOp::Alloca => {
                let arena = self.arena_ident(span);
                let size = self.expr(&args[0]).at(prec::CAST, span);
                let usize_ty = primitive_ty("usize", span);
                Value::new(
                    quote_spanned! {span=> #arena.alloca(#size as #usize_ty) },
                    prec::CALL,
                )
            }
            BuiltinOp::Bswap => {
                let operand = args[0].ty;
                let value = self.unsigned_operand(&args[0], span);
                let target = self.ty(operand, span);
                Value::new(
                    quote_spanned! {span=> #value.swap_bytes() as #target },
                    prec::CAST,
                )
                .type_end(true)
            }
            BuiltinOp::Popcount => {
                let value = self.unsigned_operand(&args[0], span);
                Value::new(
                    quote_spanned! {span=> #value.count_ones() as #int },
                    prec::CAST,
                )
                .type_end(true)
            }
            BuiltinOp::Parity => {
                let value = self.unsigned_operand(&args[0], span);
                Value::new(
                    quote_spanned! {span=> (#value.count_ones() & 1) as #int },
                    prec::CAST,
                )
                .type_end(true)
            }
            BuiltinOp::Clz => {
                let value = self.unsigned_operand(&args[0], span);
                Value::new(
                    quote_spanned! {span=> #value.leading_zeros() as #int },
                    prec::CAST,
                )
                .type_end(true)
            }
            BuiltinOp::Ctz => {
                let value = self.unsigned_operand(&args[0], span);
                Value::new(
                    quote_spanned! {span=> #value.trailing_zeros() as #int },
                    prec::CAST,
                )
                .type_end(true)
            }
            BuiltinOp::Ffs => {
                let value = self.unsigned_operand(&args[0], span);
                let tmp = self.temporary();
                Value::new(
                    quote_spanned! {span=>
                        { let #tmp = #value;
                          if #tmp == 0 { 0 } else { #tmp.trailing_zeros() as #int + 1 } }
                    },
                    prec::BLOCK,
                )
            }
            // The number of leading bits that repeat the sign bit, not
            // counting the sign bit itself — which is what `leading_zeros` of
            // the value XORed with itself shifted left gives.
            BuiltinOp::Clrsb => {
                let width = args[0].ty.bits(&self.options.target);
                let signed = signed_rust_ty(width, span);
                let value = self.expr(&args[0]).at(prec::CAST, span);
                let tmp = self.temporary();
                let bits = usize_literal(u64::from(width), span);
                Value::new(
                    quote_spanned! {span=>
                        { let #tmp = #value as #signed;
                          ((#tmp ^ (#tmp << 1)).leading_zeros() as #int)
                              .min(#bits as #int - 1) }
                    },
                    prec::BLOCK,
                )
            }
            BuiltinOp::Overflow(bin) | BuiltinOp::OverflowP(bin) => {
                let store = matches!(op, BuiltinOp::Overflow(_));
                // The third operand carries the result type: the pointee of
                // the pointer the value is stored through, or the type of the
                // expression the `_p` forms only ask about.
                let result_ty = if store {
                    self.program.types.pointee(args[2].ty).unwrap_or(Ty::Int)
                } else {
                    args[2].ty
                };
                self.overflow_builtin(bin, args, store, result_ty, span)
            }
            BuiltinOp::Fabs => {
                let bits = self.float_bits_of(&args[0], span);
                let (float_ty, mask) = float_bit_ty(args[0].ty, span);
                Value::new(
                    quote_spanned! {span=> <#float_ty>::from_bits(#bits & #mask) },
                    prec::CALL,
                )
            }
            BuiltinOp::Copysign => {
                let magnitude = self.float_bits_of(&args[0], span);
                let sign = self.float_bits_of(&args[1], span);
                let (float_ty, mask) = float_bit_ty(args[0].ty, span);
                Value::new(
                    quote_spanned! {span=>
                        <#float_ty>::from_bits((#magnitude & #mask) | (#sign & !#mask))
                    },
                    prec::CALL,
                )
            }
            BuiltinOp::FloatOrder(order) => self.float_order(order, args, span),
            BuiltinOp::FloatClass(class) => self.float_class(class, &args[0], span),
            BuiltinOp::Fpclassify => {
                let value = self.expr(&args[5]).at(prec::CALL, span);
                let arms = ["Nan", "Infinite", "Normal", "Subnormal", "Zero"]
                    .iter()
                    .zip(args)
                    .map(|(name, answer)| {
                        let variant = Ident::new(name, span);
                        let answer = self.expr_at(answer, Ty::Int);
                        quote_spanned! {span=>
                            ::core::num::FpCategory::#variant => #answer,
                        }
                    })
                    .collect::<TokenStream>();
                Value::new(
                    quote_spanned! {span=> match #value.classify() { #arms } },
                    prec::BLOCK,
                )
            }
        }
    }

    /// A floating operand as the unsigned integer of its own width.
    fn float_bits_of(&mut self, arg: &Expr, span: Span) -> TokenStream {
        let value = self.expr(arg).at(prec::CALL, span);
        parenthesize(quote_spanned! {span=> #value.to_bits() }, span)
    }

    /// `__builtin_isgreater` and its relatives.
    ///
    /// Rust's floating comparisons are the quiet ones, which is exactly what
    /// C99 7.12.14 asks these for; the operands go into temporaries because
    /// two of the six mention each of them twice.
    fn float_order(&mut self, order: ir::FloatOrder, args: &[Expr], span: Span) -> Value {
        use ir::FloatOrder;
        let int = self.ty(Ty::Int, span);
        let lhs = self.expr(&args[0]).at(prec::LOWEST, span);
        let rhs = self.expr(&args[1]).at(prec::LOWEST, span);
        let (a, b) = (self.temporary(), self.temporary());
        let test = match order {
            FloatOrder::Greater => quote_spanned! {span=> #a > #b },
            FloatOrder::GreaterEqual => quote_spanned! {span=> #a >= #b },
            FloatOrder::Less => quote_spanned! {span=> #a < #b },
            FloatOrder::LessEqual => quote_spanned! {span=> #a <= #b },
            FloatOrder::LessGreater => quote_spanned! {span=> #a < #b || #a > #b },
            FloatOrder::Unordered => quote_spanned! {span=> #a.is_nan() || #b.is_nan() },
        };
        Value::new(
            quote_spanned! {span=>
                { let #a = #lhs; let #b = #rhs; (#test) as #int }
            },
            prec::BLOCK,
        )
    }

    /// `__builtin_isnan` and its relatives.
    ///
    /// The predicates are `core`'s own, which are pure inspections of the bit
    /// pattern and so need nothing of the maths library; `issignaling` is the
    /// one that has no method, and is a NaN whose leading mantissa bit — the
    /// quiet bit — is clear.
    fn float_class(&mut self, class: ir::FloatClass, arg: &Expr, span: Span) -> Value {
        use ir::FloatClass;
        let int = self.ty(Ty::Int, span);
        let method = |name: &str| Ident::new(name, span);
        let test = match class {
            FloatClass::IsNan => Some(method("is_nan")),
            FloatClass::IsInf => Some(method("is_infinite")),
            FloatClass::IsFinite => Some(method("is_finite")),
            FloatClass::IsNormal => Some(method("is_normal")),
            FloatClass::SignBit => Some(method("is_sign_negative")),
            FloatClass::IsInfSign | FloatClass::IsSignaling => None,
        };
        if let Some(test) = test {
            let value = self.expr(arg).at(prec::CALL, span);
            return Value::new(quote_spanned! {span=> #value.#test() as #int }, prec::CAST)
                .type_end(true);
        }
        let tmp = self.temporary();
        let value = self.expr(arg).at(prec::LOWEST, span);
        if class == FloatClass::IsInfSign {
            return Value::new(
                quote_spanned! {span=>
                    { let #tmp = #value;
                      if #tmp.is_infinite() {
                          if #tmp.is_sign_negative() { -1 as #int } else { 1 as #int }
                      } else { 0 as #int } }
                },
                prec::BLOCK,
            );
        }
        // A signalling NaN is a NaN with the quiet bit clear: bit 51 of a
        // `double` and bit 22 of a `float`.
        let quiet = quiet_bit_literal(arg.ty, span);
        Value::new(
            quote_spanned! {span=>
                { let #tmp = #value;
                  (#tmp.is_nan() && (#tmp.to_bits() & #quiet) == 0) as #int }
            },
            prec::BLOCK,
        )
    }

    // -- atomics ------------------------------------------------------------

    /// `::core::sync::atomic::AtomicU32`, or `AtomicPtr<c_void>`.
    fn atomic_path(&self, class: AtomicClass, span: Span) -> TokenStream {
        if matches!(class, AtomicClass::Ptr | AtomicClass::FnPtr) {
            let void = self.pointee_ty(Ty::Void, span);
            return quote_spanned! {span=>
                ::core::sync::atomic::AtomicPtr::<#void>
            };
        }
        let name = Ident::new(class.rust_name(), span);
        quote_spanned! {span=> ::core::sync::atomic::#name }
    }

    /// The Rust type the atomic holds, which is what its `from_ptr` points at.
    ///
    /// Every pointer goes through one `AtomicPtr<c_void>`: all object pointers
    /// have the same representation, and the value is cast back to the C type
    /// it came from as it comes out. A function pointer goes through the same
    /// one, transmuted rather than cast.
    fn atomic_repr_ty(&self, class: AtomicClass, span: Span) -> TokenStream {
        if matches!(class, AtomicClass::Ptr | AtomicClass::FnPtr) {
            let void = self.pointee_ty(Ty::Void, span);
            return quote_spanned! {span=> *mut #void };
        }
        primitive_ty(class.repr_name(), span)
    }

    /// `AtomicU32::from_ptr(p as *mut u32)`, the `&AtomicU32` everything else
    /// is a method call on.
    ///
    /// `from_ptr` is safe to build here for the reason C gives: the object is
    /// properly aligned for its own type, and the atomic's alignment is that
    /// type's size, which is what [`ir::Types::size_align`] gives an
    /// `_Atomic` and what sema checks before it accepts a pointer to a plain
    /// one.
    fn atomic_ref(&self, class: AtomicClass, ptr: TokenStream, span: Span) -> TokenStream {
        let path = self.atomic_path(class, span);
        let repr = self.atomic_repr_ty(class, span);
        quote_spanned! {span=> #path::from_ptr(#ptr as *mut #repr) }
    }

    /// `::core::sync::atomic::Ordering::SeqCst`.
    fn ordering(&self, order: ir::MemOrder, span: Span) -> TokenStream {
        let name = Ident::new(order.rust_name(), span);
        quote_spanned! {span=> ::core::sync::atomic::Ordering::#name }
    }

    /// The value an atomic yields, as the C type the object has.
    fn repr_to_value(&self, class: AtomicClass, ty: Ty, value: TokenStream, span: Span) -> Value {
        match class {
            AtomicClass::Bool => Value::atom(value),
            AtomicClass::Float { bytes } => {
                let float = primitive_ty(if bytes == 4 { "f32" } else { "f64" }, span);
                Value::new(
                    quote_spanned! {span=> <#float>::from_bits(#value) },
                    prec::CALL,
                )
            }
            AtomicClass::Int { .. } | AtomicClass::Ptr => {
                let target = self.ty(ty, span);
                Value::new(quote_spanned! {span=> #value as #target }, prec::CAST).type_end(true)
            }
            // `*mut c_void` to `Option<unsafe extern "C" fn(…)>`: `as` does
            // not go that way, and the two have the same size and the same
            // null, so the conversion is one `transmute` with both types
            // written out — an inferred one here would be a bug waiting to
            // happen.
            AtomicClass::FnPtr => {
                let repr = self.atomic_repr_ty(class, span);
                let target = self.ty(ty, span);
                Value::new(
                    quote_spanned! {span=>
                        ::core::mem::transmute::<#repr, #target>(#value)
                    },
                    prec::CALL,
                )
            }
        }
    }

    /// A C value, as the Rust primitive the atomic holds. `ty` is the C type
    /// the value has, which only the function-pointer class needs.
    fn value_to_repr(&self, class: AtomicClass, ty: Ty, value: Value, span: Span) -> TokenStream {
        match class {
            AtomicClass::Bool => value.at(prec::LOWEST, span),
            AtomicClass::Float { bytes } => {
                let float = primitive_ty(if bytes == 4 { "f32" } else { "f64" }, span);
                let value = value.at(prec::LOWEST, span);
                quote_spanned! {span=> <#float>::to_bits(#value) }
            }
            AtomicClass::Int { .. } | AtomicClass::Ptr => {
                let repr = self.atomic_repr_ty(class, span);
                let value = value.at(prec::CAST, span);
                quote_spanned! {span=> #value as #repr }
            }
            // The other half of [`Codegen::repr_to_value`]'s transmute.
            AtomicClass::FnPtr => {
                let repr = self.atomic_repr_ty(class, span);
                let source = self.ty(ty, span);
                let value = value.at(prec::LOWEST, span);
                quote_spanned! {span=>
                    ::core::mem::transmute::<#source, #repr>(#value)
                }
            }
        }
    }

    /// `match … { Ok(v) | Err(v) => v }`, which is how the old value is taken
    /// out of a `fetch_update` that never says no.
    fn either_way(&mut self, result: TokenStream, span: Span) -> TokenStream {
        let value = self.temporary();
        quote_spanned! {span=>
            match #result {
                ::core::result::Result::Ok(#value) | ::core::result::Result::Err(#value) => #value,
            }
        }
    }

    /// The compare-exchange loop an operation Rust has no method for becomes,
    /// whose value is the **old** one.
    ///
    /// `updated` is the new value written in terms of `current`, and is
    /// recomputed on every attempt, which is exactly what C's "read, modify,
    /// write, atomically" comes to when the processor has no single
    /// instruction for it — a `fetch_nand`, a `*=` on an atomic object, a
    /// pointer that moves by elements.
    ///
    /// Written out rather than left to `Atomic::fetch_update`: that method is
    /// being renamed to `try_update`, and the old name is deprecated on newer
    /// toolchains while the new one does not exist on the oldest this crate
    /// supports. The loop is what it does anyway.
    fn atomic_cas_loop(
        &mut self,
        object: &TokenStream,
        current: &Ident,
        updated: TokenStream,
        order: ir::MemOrder,
        span: Span,
    ) -> TokenStream {
        let success = self.ordering(order, span);
        let failure = self.ordering(order.failure_order(), span);
        let slot = self.temporary();
        let fresh = self.temporary();
        let seen = self.temporary();
        quote_spanned! {span=>
            {
                let #slot = #object;
                let mut #current = #slot.load(#failure);
                loop {
                    let #fresh = #updated;
                    match #slot.compare_exchange_weak(#current, #fresh, #success, #failure) {
                        ::core::result::Result::Ok(_) => break #current,
                        ::core::result::Result::Err(#seen) => #current = #seen,
                    }
                }
            }
        }
    }

    /// One of the atomic builtins; see [`ir::AtomicExpr`].
    fn atomic(&mut self, atomic: &ir::AtomicExpr, span: Span) -> Value {
        let class = atomic.class;
        let success = self.ordering(atomic.success, span);
        if let ir::AtomicOp::Fence { signal } = atomic.op {
            // C11 7.17.4p2 makes a relaxed fence a no-op, and Rust's `fence`
            // panics on one rather than saying so.
            if atomic.success == ir::MemOrder::Relaxed {
                return Value::atom(quote_spanned! {span=> () });
            }
            let name = Ident::new(if signal { "compiler_fence" } else { "fence" }, span);
            return Value::new(
                quote_spanned! {span=> ::core::sync::atomic::#name(#success) },
                prec::CALL,
            );
        }
        let ptr = match &atomic.ptr {
            Some(ptr) => self.expr(ptr).at(prec::CAST, span),
            None => return Value::atom(quote_spanned! {span=> () }),
        };
        let object = self.atomic_ref(class, ptr, span);
        let ty = atomic.value_ty;
        match atomic.op {
            ir::AtomicOp::Fence { .. } => unreachable!("handled above"),
            ir::AtomicOp::Load => self.repr_to_value(
                class,
                ty,
                quote_spanned! {span=> #object.load(#success) },
                span,
            ),
            ir::AtomicOp::Store => {
                let value = self.atomic_operand(atomic, span);
                Value::new(
                    quote_spanned! {span=> #object.store(#value, #success) },
                    prec::CALL,
                )
            }
            ir::AtomicOp::Exchange => {
                let value = self.atomic_operand(atomic, span);
                self.repr_to_value(
                    class,
                    ty,
                    quote_spanned! {span=> #object.swap(#value, #success) },
                    span,
                )
            }
            ir::AtomicOp::Clear => Value::new(
                quote_spanned! {span=> #object.store(0, #success) },
                prec::CALL,
            ),
            // GCC sets the byte to `__GCC_ATOMIC_TEST_AND_SET_TRUEVAL`, which
            // is 1, and answers whether it was already set.
            ir::AtomicOp::TestAndSet => Value::new(
                quote_spanned! {span=> #object.swap(1, #success) != 0 },
                prec::CMP,
            ),
            ir::AtomicOp::CompareExchange { weak } => {
                self.compare_exchange(atomic, object, weak, span)
            }
            ir::AtomicOp::SyncCompareSwap { value_is_old } => {
                self.sync_compare_swap(atomic, object, value_is_old, span)
            }
            ir::AtomicOp::Rmw { op, returns_new } => {
                self.atomic_rmw(atomic, object, op, returns_new, span)
            }
        }
    }

    /// The value operand of an atomic builtin, as the atomic's own type.
    fn atomic_operand(&mut self, atomic: &ir::AtomicExpr, span: Span) -> TokenStream {
        let Some(value) = &atomic.value else {
            return quote_spanned! {span=> () };
        };
        let value = self.expr(value);
        self.value_to_repr(atomic.class, atomic.value_ty, value, span)
    }

    /// `__atomic_compare_exchange_n`, which writes the value it observed back
    /// through `expected` when it fails and answers whether it succeeded.
    fn compare_exchange(
        &mut self,
        atomic: &ir::AtomicExpr,
        object: TokenStream,
        weak: bool,
        span: Span,
    ) -> Value {
        let class = atomic.class;
        let ty = atomic.value_ty;
        let expected = match &atomic.expected {
            Some(expected) => self.expr(expected).at(prec::CALL, span),
            None => return Value::atom(quote_spanned! {span=> false }),
        };
        let desired = self.atomic_operand(atomic, span);
        let slot = self.temporary();
        let seen = self.temporary();
        let current = self.value_to_repr(
            class,
            ty,
            Value::atom(quote_spanned! {span=> *#slot }),
            span,
        );
        let method = Ident::new(
            if weak {
                "compare_exchange_weak"
            } else {
                "compare_exchange"
            },
            span,
        );
        let success = self.ordering(atomic.success, span);
        let failure = self.ordering(atomic.failure, span);
        let observed = self
            .repr_to_value(class, ty, quote_spanned! {span=> #seen }, span)
            .at(prec::LOWEST, span);
        Value::new(
            quote_spanned! {span=>
                {
                    let #slot = #expected;
                    match #object.#method(#current, #desired, #success, #failure) {
                        ::core::result::Result::Ok(_) => true,
                        ::core::result::Result::Err(#seen) => {
                            *#slot = #observed;
                            false
                        }
                    }
                }
            },
            prec::BLOCK,
        )
    }

    /// `__sync_bool_compare_and_swap` and `__sync_val_compare_and_swap`, whose
    /// expected value is a value and which write nothing back.
    fn sync_compare_swap(
        &mut self,
        atomic: &ir::AtomicExpr,
        object: TokenStream,
        value_is_old: bool,
        span: Span,
    ) -> Value {
        let class = atomic.class;
        let ty = atomic.value_ty;
        let expected = match &atomic.expected {
            Some(expected) => {
                let value = self.expr(expected);
                self.value_to_repr(class, ty, value, span)
            }
            None => return Value::atom(quote_spanned! {span=> false }),
        };
        let desired = self.atomic_operand(atomic, span);
        let seq = self.ordering(ir::MemOrder::SeqCst, span);
        let call = quote_spanned! {span=>
            #object.compare_exchange(#expected, #desired, #seq, #seq)
        };
        if !value_is_old {
            return Value::new(quote_spanned! {span=> #call.is_ok() }, prec::CALL);
        }
        let old = self.either_way(call, span);
        self.repr_to_value(class, ty, parenthesize(old, span), span)
    }

    /// The `fetch_add` family, and the compare-exchange loops the ones Rust
    /// has no method for turn into.
    fn atomic_rmw(
        &mut self,
        atomic: &ir::AtomicExpr,
        object: TokenStream,
        op: ir::AtomicRmw,
        returns_new: bool,
        span: Span,
    ) -> Value {
        let class = atomic.class;
        let ty = atomic.value_ty;
        let success = self.ordering(atomic.success, span);
        let operand = self.temporary();
        let old = self.temporary();
        // A pointer moves by *bytes*: the scaling C11 wants for
        // `atomic_fetch_add` is already in the operand, put there by sema.
        if class == AtomicClass::Ptr {
            let delta = match &atomic.value {
                Some(value) => self.expr(value).at(prec::CAST, span),
                None => quote_spanned! {span=> 0 },
            };
            let isize_ty = primitive_ty("isize", span);
            let signed = if op == ir::AtomicRmw::Sub {
                quote_spanned! {span=> -(#delta as #isize_ty) }
            } else {
                quote_spanned! {span=> #delta as #isize_ty }
            };
            let step = self.temporary();
            let updated = self.atomic_cas_loop(
                &object,
                &step,
                quote_spanned! {span=> #step.wrapping_byte_offset(#operand) },
                atomic.success,
                span,
            );
            let tail = if returns_new {
                quote_spanned! {span=> #old.wrapping_byte_offset(#operand) }
            } else {
                quote_spanned! {span=> #old }
            };
            let tail = self
                .repr_to_value(class, ty, tail, span)
                .at(prec::LOWEST, span);
            return Value::new(
                quote_spanned! {span=>
                    {
                        let #operand: #isize_ty = #signed;
                        let #old = #updated;
                        #tail
                    }
                },
                prec::BLOCK,
            );
        }
        let value = self.atomic_operand(atomic, span);
        let repr = self.atomic_repr_ty(class, span);
        // `fetch_nand` exists for `AtomicBool` and for nothing else, so an
        // integer nand is the compare-exchange loop Rust would have written.
        let update = match op.rust_method() {
            Some(method) if op != ir::AtomicRmw::Nand || class == AtomicClass::Bool => {
                let method = Ident::new(method, span);
                quote_spanned! {span=> #object.#method(#operand, #success) }
            }
            _ if class == AtomicClass::Bool => {
                let method = Ident::new("fetch_nand", span);
                quote_spanned! {span=> #object.#method(#operand, #success) }
            }
            _ => {
                let current = self.temporary();
                self.atomic_cas_loop(
                    &object,
                    &current,
                    quote_spanned! {span=> !(#current & #operand) },
                    atomic.success,
                    span,
                )
            }
        };
        let combined = self.atomic_combine(op, &old, &operand, span);
        let tail = if returns_new {
            combined
        } else {
            quote_spanned! {span=> #old }
        };
        let tail = self
            .repr_to_value(class, ty, tail, span)
            .at(prec::LOWEST, span);
        Value::new(
            quote_spanned! {span=>
                {
                    let #operand: #repr = #value;
                    let #old = #update;
                    #tail
                }
            },
            prec::BLOCK,
        )
    }

    /// The read-modify-write an `x += v`, `x++` or `--x` on an `_Atomic`
    /// object performs.
    ///
    /// C11 6.5.16.2p3 and 6.5.2.4p2 make each of them *one* atomic
    /// read-modify-write, not a load and a store, so none of them may go
    /// through [`Codegen::read`] and [`Codegen::write`]. The five operators an
    /// atomic has a method for become that method; everything else — `*=`,
    /// `<<=`, a floating object, a pointer that moves by elements — becomes
    /// the `fetch_update` loop the method would have been.
    ///
    /// The right operand is always evaluated into a temporary first: the
    /// update is written twice when the value of the expression is the new
    /// one, and C evaluates it once.
    fn atomic_place_rmw(
        &mut self,
        lowered: &LoweredPlace,
        kind: PlaceRmw<'_>,
        want: RmwValue,
        span: Span,
    ) -> TokenStream {
        let (class, ty) = lowered.atomic.expect("an atomic place");
        let object = self.atomic_object_of(lowered, span);
        let setup = &lowered.setup;
        let operand = self.temporary();
        let old = self.temporary();
        let success = self.ordering(ir::MemOrder::SeqCst, span);
        // The one shape that is a plain `fetch_*`: an integer object whose
        // operator is one of the five, computed in the object's own type, so
        // that no widening happens between the read and the write.
        let method = match (class, &kind) {
            (
                AtomicClass::Int { .. },
                PlaceRmw::Compound {
                    op,
                    compute,
                    value: _,
                },
            ) if *compute == ty => rmw_of_binop(*op),
            (AtomicClass::Int { .. }, PlaceRmw::Step { dec }) => Some(if *dec {
                ir::AtomicRmw::Sub
            } else {
                ir::AtomicRmw::Add
            }),
            _ => None,
        };
        if let Some(op) = method.filter(|op| op.rust_method().is_some()) {
            let repr = self.atomic_repr_ty(class, span);
            let value = match &kind {
                PlaceRmw::Compound { value, compute, .. } => {
                    let tokens = self.expr_at(value, *compute);
                    self.value_to_repr(class, ty, Value::new(tokens, prec::LOWEST), span)
                }
                PlaceRmw::Step { .. } => quote_spanned! {span=> 1 },
            };
            let name = Ident::new(op.rust_method().expect("filtered"), span);
            let tail = self.atomic_rmw_tail((class, ty), op, &old, &operand, want, span);
            return quote_spanned! {span=>
                { #setup
                  let #operand: #repr = #value;
                  let #old = #object.#name(#operand, #success);
                  #tail }
            };
        }
        // The general form: a compare-exchange loop over the very expression
        // an ordinary compound assignment would have stored.
        let hoisted = match &kind {
            PlaceRmw::Compound { value, compute, .. } => {
                let tokens = self.expr_at(value, *compute);
                let rhs_ty = self.ty(*compute, span);
                Some(quote_spanned! {span=> let #operand: #rhs_ty = #tokens; })
            }
            PlaceRmw::Step { .. } => None,
        };
        let param = self.temporary();
        let current = self.repr_to_value(class, ty, quote_spanned! {span=> #param }, span);
        let updated = self.apply_place_rmw(&kind, current, ty, &operand, span);
        let updated = self.value_to_repr(class, ty, updated, span);
        let loop_result =
            self.atomic_cas_loop(&object, &param, updated, ir::MemOrder::SeqCst, span);
        let tail = match want {
            RmwValue::None => TokenStream::new(),
            RmwValue::Old => self
                .repr_to_value(class, ty, quote_spanned! {span=> #old }, span)
                .at(prec::LOWEST, span),
            RmwValue::New => {
                let previous = self.repr_to_value(class, ty, quote_spanned! {span=> #old }, span);
                let value = self.apply_place_rmw(&kind, previous, ty, &operand, span);
                value.at(prec::LOWEST, span)
            }
        };
        quote_spanned! {span=>
            { #setup #hoisted
              let #old = #loop_result;
              #tail }
        }
    }

    /// The new value of an atomic place, from the old one.
    fn apply_place_rmw(
        &mut self,
        kind: &PlaceRmw<'_>,
        current: Value,
        ty: Ty,
        operand: &Ident,
        span: Span,
    ) -> Value {
        match kind {
            PlaceRmw::Compound { op, value, compute } => {
                let rhs = Value::atom(quote_spanned! {span=> #operand });
                self.compound_value(current, ty, *op, value, Some(rhs), *compute)
            }
            PlaceRmw::Step { dec } => {
                Value::new(self.step_value(current, ty, *dec, span), prec::LOWEST)
            }
        }
    }

    /// The value a `fetch_*` on a place ends with: nothing, the old value or
    /// the new one.
    ///
    /// `atomic` is the place's [class](AtomicClass) and the C type behind it,
    /// which is what the value the atomic returned is converted back to.
    fn atomic_rmw_tail(
        &mut self,
        atomic: (AtomicClass, Ty),
        op: ir::AtomicRmw,
        old: &Ident,
        operand: &Ident,
        want: RmwValue,
        span: Span,
    ) -> TokenStream {
        let (class, ty) = atomic;
        let value = match want {
            RmwValue::None => return TokenStream::new(),
            RmwValue::Old => quote_spanned! {span=> #old },
            RmwValue::New => self.atomic_combine(op, old, operand, span),
        };
        self.repr_to_value(class, ty, value, span)
            .at(prec::LOWEST, span)
    }

    /// The new value a `…_fetch` form answers with, computed from the old one
    /// the atomic returned and the operand.
    fn atomic_combine(
        &self,
        op: ir::AtomicRmw,
        old: &Ident,
        operand: &Ident,
        span: Span,
    ) -> TokenStream {
        match op {
            // Wrapping, because C's atomic arithmetic is modular even for the
            // signed types — the atomic instruction has no other behaviour.
            ir::AtomicRmw::Add => quote_spanned! {span=> #old.wrapping_add(#operand) },
            ir::AtomicRmw::Sub => quote_spanned! {span=> #old.wrapping_sub(#operand) },
            ir::AtomicRmw::And => quote_spanned! {span=> (#old & #operand) },
            ir::AtomicRmw::Or => quote_spanned! {span=> (#old | #operand) },
            ir::AtomicRmw::Xor => quote_spanned! {span=> (#old ^ #operand) },
            ir::AtomicRmw::Nand => quote_spanned! {span=> !(#old & #operand) },
        }
    }

    /// The operand of a bit-manipulation builtin, as the unsigned integer of
    /// its own width.
    fn unsigned_operand(&mut self, arg: &Expr, span: Span) -> TokenStream {
        let width = arg.ty.bits(&self.options.target);
        let target = unsigned_rust_ty(width, span);
        let value = self.expr(arg).at(prec::CAST, span);
        parenthesize(quote_spanned! {span=> #value as #target }, span)
    }

    /// `__builtin_add_overflow(a, b, &r)` and its relatives.
    ///
    /// The arithmetic happens in an `i128`, which is what "infinite precision"
    /// comes to while both operands are at most 64 bits wide — sema refuses a
    /// wider one. The *result* type may still be 128 bits, and that is the one
    /// thing that changes how the answer is checked: the general test asks
    /// whether the narrowed value converts back to what infinite precision
    /// gave, and a 128-bit conversion is a reinterpretation that always does.
    /// A signed 128-bit result therefore never overflows, and an unsigned one
    /// overflows exactly when the exact answer was negative.
    fn overflow_builtin(
        &mut self,
        op: BinOp,
        args: &[Expr],
        store: bool,
        result_ty: Ty,
        span: Span,
    ) -> Value {
        let method = match op {
            BinOp::Add => "wrapping_add",
            BinOp::Sub => "wrapping_sub",
            _ => "wrapping_mul",
        };
        let method = Ident::new(method, span);
        let checked = match op {
            BinOp::Add => "checked_add",
            BinOp::Sub => "checked_sub",
            _ => "checked_mul",
        };
        let checked = Ident::new(checked, span);
        let lhs = self.expr(&args[0]).at(prec::CAST, span);
        let rhs = self.expr(&args[1]).at(prec::CAST, span);
        let target = self.ty(result_ty, span);
        let a = self.temporary();
        let b = self.temporary();
        let wide = self.temporary();
        let narrow = self.temporary();
        // Pathed, because `typedef __int128 i128;` is a name a C unit may take.
        let i128 = primitive_ty("i128", span);
        let compute = quote_spanned! {span=>
            let #a: #i128 = #lhs as #i128;
            let #b: #i128 = #rhs as #i128;
            let #wide: #i128 = #a.#method(#b);
            let #narrow: #target = #wide as #target;
        };
        // The value overflows exactly when the wrapped result no longer equals
        // what infinite precision gave — and `i128` itself can only overflow
        // on a multiplication, where the answer is certainly out of range.
        let fits = match result_ty {
            // Both 128-bit conversions are reinterpretations, so the general
            // test below is vacuous there; what is left is the sign.
            Ty::Int128 => quote_spanned! {span=> false },
            Ty::UInt128 => quote_spanned! {span=> #wide < 0 },
            _ => quote_spanned! {span=> (#narrow as #i128) != #wide },
        };
        let flag = quote_spanned! {span=>
            #a.#checked(#b).is_none() || #fits
        };
        if !store {
            // The `_p` forms still evaluate their third operand.
            let third = self.expr_stmt(&args[2]);
            return Value::new(
                quote_spanned! {span=> { #third #compute #flag } },
                prec::BLOCK,
            );
        }
        let place = self.expr(&args[2]).at(prec::CALL, span);
        let out = self.temporary();
        Value::new(
            quote_spanned! {span=>
                { #compute let #out = #place; *#out = #narrow; #flag }
            },
            prec::BLOCK,
        )
    }

    /// `va_arg(ap, T)`: reads the next argument and advances the list.
    ///
    /// `VaArgSafe` — the bound `next_arg` needs — is implemented for the
    /// primitives the `core::ffi` aliases stand for, so the C type can be
    /// asked for by name. A function pointer is the exception: `Option<fn>` is
    /// not one of them, so the argument is read as a `void *` and transmuted,
    /// which is what it is.
    ///
    /// A `struct` or a `union` is not a primitive at all, and is rebuilt from
    /// the registers the ABI passed it in — one `next_arg` per
    /// [eightbyte](ir::Eightbyte), which `sema` has already classified. The
    /// words are gathered into a `[u64; N]`, whose bytes are exactly the
    /// object's, and read back out of it: `read_unaligned` rather than a
    /// `transmute`, because the record may be *shorter* than the words that
    /// carried it — `struct { char x[13]; }` arrives in two of them — and
    /// because `[u64; N]` is eight-byte aligned while a record holding an
    /// `__int128` wants sixteen.
    fn va_arg(
        &mut self,
        ap: &Place,
        record: Option<&[ir::Eightbyte]>,
        ty: Ty,
        span: Span,
    ) -> Value {
        let access = self.place(ap, true).access;
        if let Some(classes) = record {
            let target = self.ty(ty, span);
            let u64_ty = primitive_ty("u64", span);
            let f64_ty = primitive_ty("f64", span);
            let words = classes.iter().map(|class| match class {
                ir::Eightbyte::Int => quote_spanned! {span=> #access.next_arg::<#u64_ty>() },
                ir::Eightbyte::Sse => {
                    quote_spanned! {span=> #access.next_arg::<#f64_ty>().to_bits() }
                }
                // The ABI passes an eightbyte nothing reaches in no register,
                // so there is nothing to read and nothing the record can see.
                ir::Eightbyte::None => quote_spanned! {span=> 0 },
            });
            let count = Literal::usize_unsuffixed(classes.len());
            let value = self.temporary();
            return Value::new(
                quote_spanned! {span=>
                    {
                        let #value: [#u64_ty; #count] = [#(#words),*];
                        ::core::ptr::read_unaligned(#value.as_ptr().cast::<#target>())
                    }
                },
                prec::BLOCK,
            );
        }
        if self.program.types.is_func_pointer(ty) {
            let target = self.ty(ty, span);
            let void = self.pointee_ty(Ty::Void, span);
            return Value::new(
                quote_spanned! {span=>
                    ::core::mem::transmute::<*mut #void, #target>(
                        #access.next_arg::<*mut #void>()
                    )
                },
                prec::CALL,
            );
        }
        let target = self.ty(ty, span);
        Value::new(
            quote_spanned! {span=> #access.next_arg::<#target>() },
            prec::CALL,
        )
    }

    /// The argument of `offset`, which is always an `isize`.
    /// How far one element of a variably modified pointee is, in units of the
    /// [step type](ir::Types::vm_step_ty) the generated pointer points at.
    ///
    /// `double (*p)[m]` is a `*mut c_double` in the expansion, so `p + 1` has
    /// to move by `m` of them, and `double (*p)[n][3]` by `n` of the `[f64; 3]`
    /// it points at. Everything C says about such a pointer follows from
    /// scaling every offset by this product; `None` is the ordinary case,
    /// where Rust's own pointer arithmetic already has the right stride.
    fn vm_scale(&self, pointee: Ty, span: Span) -> Option<TokenStream> {
        if !self.program.types.is_vm(pointee) {
            return None;
        }
        let isize_ty = primitive_ty("isize", span);
        let mut product: Option<TokenStream> = None;
        for dim in self.program.types.vm_dims(pointee).iter().rev() {
            let factor = match dim {
                ir::VmDim::Fixed(len) => {
                    let literal = Literal::isize_unsuffixed(*len as isize);
                    quote_spanned! {span=> #literal }
                }
                ir::VmDim::Len(id) => {
                    let name = self.object_ident(*id, span);
                    quote_spanned! {span=> #name as #isize_ty }
                }
                // Sema refuses every expression that would need a bound it
                // never evaluated, so nothing reaches here.
                ir::VmDim::Unknown => quote_spanned! {span=> 1 },
            };
            product = Some(match product {
                None => factor,
                Some(left) => quote_spanned! {span=> (#left).wrapping_mul(#factor) },
            });
        }
        product
    }

    /// An offset in elements, scaled for a [variably
    /// modified](Codegen::vm_scale) pointee.
    fn scaled_offset(&mut self, pointee: Ty, index: &Expr, sub: bool, span: Span) -> TokenStream {
        let Some(scale) = self.vm_scale(pointee, span) else {
            return self.offset_argument(index, sub, span);
        };
        let isize_ty = primitive_ty("isize", span);
        // A constant subscript comes out of `offset_argument` as a bare
        // literal, whose type `wrapping_mul` would have nothing to infer from.
        let offset = match &index.kind {
            ExprKind::Int(value) => {
                let value = if sub { -*value } else { *value };
                let literal = int_literal_token(value, span);
                quote_spanned! {span=> (#literal as #isize_ty) }
            }
            _ => {
                let inner = self.offset_argument(index, sub, span);
                quote_spanned! {span=> (#inner) }
            }
        };
        quote_spanned! {span=> #offset.wrapping_mul(#scale) }
    }

    fn offset_argument(&mut self, index: &Expr, sub: bool, span: Span) -> TokenStream {
        if let ExprKind::Int(value) = &index.kind {
            let value = if sub { value.wrapping_neg() } else { *value };
            let literal = int_literal_token(value, span);
            // A small literal is left bare, and Rust infers the `isize`
            // `offset` wants. A larger one carries a suffix of its own —
            // `u64`, or `i128` — which is then the wrong type rather than an
            // open one, so it is converted. (`int a[2]; a[1L << 40]` is
            // undefined in C, and this is what `as` makes of it.)
            if value.unsigned_abs() > UNSUFFIXED_LIMIT as u128 {
                let isize_ty = primitive_ty("isize", span);
                return quote_spanned! {span=> (#literal as #isize_ty) };
            }
            return literal;
        }
        let tokens = self.expr(index).at(prec::CAST, span);
        let isize_ty = primitive_ty("isize", span);
        if sub {
            quote_spanned! {span=> -(#tokens as #isize_ty) }
        } else {
            quote_spanned! {span=> #tokens as #isize_ty }
        }
    }

    fn call(&mut self, callee: &Callee, args: &[Expr], span: Span) -> Value {
        // An [x86 intrinsic](crate::x86) is not a symbol: the call is the
        // function of the same name in `core::arch`, with the immediate
        // operands moved into a turbofish.
        if let Callee::Direct(id) = callee
            && let Some(intr) = self.program.function(*id).intrinsic
        {
            return self.intrinsic_call(intr, *id, args, span);
        }
        let sig = self.callee_signature(callee);
        // A call through a function type with *no prototype* passes as many
        // arguments as it was given, each with the default argument promotions
        // applied, and the callee reads them as though the prototype had said
        // so (C99 6.5.2.2p6). Rust has no such call, so the reinterpretation is
        // written out: the callee is transmuted to the signature the promoted
        // arguments make, and that signature is called.
        //
        // It is the same contract C's own ABI relies on — the program is
        // defined only if the function really does take parameters of those
        // types, and undefined otherwise — and on every ABI this crate targets
        // a function pointer transmuted this way is the same address. Sema has
        // already applied the promotions, so `arg.ty` is the parameter type to
        // write.
        //
        // The argument *types* are compared as well as their number, because a
        // declaration with no prototype may be completed by a definition that
        // has one after the call was written: `int *h(); … h(j(), n); … int
        // *h(unsigned, int) { … }` — `execute/pr103209` — leaves the call
        // holding arguments the definition's parameters do not have. The call
        // was checked against the type in scope where it stands, which is the
        // one with no prototype, so the reinterpretation is what C says
        // happens there too. Where the prototype *was* in scope, sema has
        // already converted every argument to its parameter's type and the
        // comparison is an equality that holds.
        let reinterpreted = !sig.variadic
            && (args.len() != sig.params.len()
                || args.iter().zip(&sig.params).any(|(arg, param)| {
                    arg.ty != *param && !arg.ty.is_error() && !param.is_error()
                }));
        let promoted: Vec<Ty> = if reinterpreted {
            args.iter().map(|arg| arg.ty).collect()
        } else {
            Vec::new()
        };
        let params = if reinterpreted {
            &promoted
        } else {
            &sig.params
        };

        let mut target = match callee {
            Callee::Direct(id) => {
                let function = self.program.function(*id);
                let path = self.function_path(function, span);
                if reinterpreted {
                    // A function *item* is not a function pointer, so the `as`
                    // coercion has to be written before it can be transmuted.
                    let source = self.function_pointer_ty(function, span);
                    parenthesize(quote_spanned! {span=> #path as #source }, span)
                } else {
                    path
                }
            }
            Callee::Indirect(expr) => {
                let value = self.expr(expr).at(prec::CALL, span);
                // C's function pointers may be null and Rust's may not, so the
                // `Option` has to come off before the call.
                parenthesize(
                    quote_spanned! {span=> #value.expect("null function pointer") },
                    span,
                )
            }
        };
        if reinterpreted {
            let source = self.fn_ptr_ty(&sig.params, sig.variadic, sig.ret, span);
            let wanted = self.fn_ptr_ty(params, false, sig.ret, span);
            target = parenthesize(
                quote_spanned! {span=>
                    ::core::mem::transmute::<#source, #wanted>(#target)
                },
                span,
            );
        }

        // A lifted nested function's hidden arguments come first, and one
        // written argument after them needs the comma the loop would only put
        // between two of its own.
        let mut tokens = self.env_arguments(callee, span);
        let hidden = !tokens.is_empty();
        for (index, arg) in args.iter().enumerate() {
            if index > 0 || hidden {
                tokens.extend(quote_spanned! {span=> , });
            }
            match params.get(index) {
                // A parameter's type is what the argument is written at, so a
                // constant needs no `as`.
                Some(expected) => tokens.extend(self.expr_at(arg, *expected)),
                // An argument matched by `...` has no parameter to take its
                // type from, and Rust gives an unsuffixed literal in that
                // position `i32` (or `f64`), whatever C says it is: `%ld` with
                // a bare `-1` would read four bytes of an eight-byte
                // argument. The type has to be written out.
                None => {
                    let arg_span = self.sp(arg.range);
                    tokens.extend(self.expr(arg).at(prec::LOWEST, arg_span));
                }
            }
        }
        let call = parenthesize(tokens, span);
        Value::new(quote_spanned! {span=> #target #call }, prec::CALL)
    }

    /// A call to an x86 intrinsic: `::core::arch::x86_64::_mm_slli_epi32::<{
    /// 3i32 }>(v)`.
    ///
    /// Everything about it lines up by construction — the bundled header's
    /// prototype was generated from the very signature `rustc` will check the
    /// call against — so there is none of the reinterpreting an ordinary call
    /// may need. Two things are not a plain transliteration:
    ///
    /// * the operands Intel requires to be integer constant expressions are
    ///   `const` generics in `core::arch`, so they move out of the argument
    ///   list into a turbofish. Sema has already folded each one to an
    ///   [`ExprKind::Int`] and reported the ones it could not (see
    ///   [`crate::x86::Intrinsic::imm`]);
    /// * the literal carries the `const` parameter's own type, and is wrapped
    ///   in a block, so that a negative value is still a const argument Rust
    ///   parses;
    /// * a pointer argument is cast with `as *const _` or `as *mut _` (see
    ///   [`Codegen::intrinsic_pointer_arg`]), because the header spells a
    ///   memory operand `void *` where GCC's does and `core::arch` types it.
    fn intrinsic_call(
        &mut self,
        intr: &'static crate::x86::Intrinsic,
        id: ir::FuncId,
        args: &[Expr],
        span: Span,
    ) -> Value {
        let module = self.arch_module(span);
        let name = Ident::new(intr.name, span);
        let mut generics = TokenStream::new();
        for (index, imm) in intr.imm.iter().enumerate() {
            if index > 0 {
                generics.extend(quote_spanned! {span=> , });
            }
            let value = match args.get(usize::from(imm.index)).map(|arg| &arg.kind) {
                Some(ExprKind::Int(value)) => *value,
                // Unreachable: sema folds every immediate and reports the ones
                // it cannot, and a unit with an error generates stubs rather
                // than this. A zero keeps the expansion parseable.
                _ => 0,
            };
            let literal = suffixed_int_literal(value, imm.rust_ty, span);
            generics.extend(quote_spanned! {span=> { #literal } });
        }
        let turbofish = if generics.is_empty() {
            TokenStream::new()
        } else {
            quote_spanned! {span=> ::<#generics> }
        };

        let mut tokens = TokenStream::new();
        let mut written = 0usize;
        for (index, arg) in args.iter().enumerate() {
            if intr.immediate_at(index).is_some() {
                continue;
            }
            if written > 0 {
                tokens.extend(quote_spanned! {span=> , });
            }
            written += 1;
            let param = self.program.function(id).sig.params.get(index).copied();
            match param.and_then(|ty| self.intrinsic_pointer_arg(ty, span)) {
                Some(cast) => {
                    let value = self.expr(arg).at(prec::CAST, span);
                    tokens.extend(quote_spanned! {span=> #value #cast });
                }
                None => tokens.extend(self.expr_at(arg, arg.ty)),
            }
        }
        let call = parenthesize(tokens, span);
        Value::new(
            quote_spanned! {span=> ::core::arch::#module::#name #turbofish #call },
            prec::CALL,
        )
    }

    /// The cast an intrinsic's pointer argument gets: `as *const _` when the
    /// C parameter points at `const`, `as *mut _` otherwise, and `None` for a
    /// parameter that is not a pointer.
    ///
    /// The header declares a memory operand `void *` wherever GCC's does, so
    /// that `_mm512_loadu_si512(p)` takes an `int *` as it does in GCC, while
    /// `core::arch` types it (`*const __m512i`); rustc infers the `_` from
    /// that. Every pointer parameter gets the cast, typed or not — for a
    /// typed one it is the identity, and one rule is simpler to state.
    fn intrinsic_pointer_arg(&self, param: Ty, span: Span) -> Option<TokenStream> {
        let Ty::Pointer(pointer) = param else {
            return None;
        };
        Some(if self.program.types.pointer_type(pointer).konst {
            quote_spanned! {span=> as *const _ }
        } else {
            quote_spanned! {span=> as *mut _ }
        })
    }

    /// The hidden arguments a call to a lifted nested function opens with.
    ///
    /// Each one is the address of the object the callee wants: the enclosing
    /// function passes `&raw mut x` for a local of its own, and a function
    /// that was itself passed the pointer passes that on. Nothing else has a
    /// hidden argument, so this is empty for every ordinary call.
    fn env_arguments(&self, callee: &Callee, span: Span) -> TokenStream {
        let Callee::Direct(id) = callee else {
            return TokenStream::new();
        };
        let mut tokens = TokenStream::new();
        for (index, entry) in self.program.function(*id).env.iter().enumerate() {
            if index > 0 {
                tokens.extend(quote_spanned! {span=> , });
            }
            match self.env.get(&entry.owner) {
                // The caller was handed the pointer itself; it passes it on.
                Some(param) => {
                    let name = self.object_ident(*param, span);
                    tokens.extend(quote_spanned! {span=> #name });
                }
                // The object is the caller's own.
                None => {
                    let object = self.program.object(entry.owner);
                    let name = self.object_access(entry.owner, span);
                    tokens.extend(if object.is_const {
                        quote_spanned! {span=> &raw const #name }
                    } else {
                        quote_spanned! {span=> &raw mut #name }
                    });
                }
            }
        }
        tokens
    }

    /// The signature a call goes through: the callee's own for a direct call,
    /// and the pointed-to function type for an indirect one.
    fn callee_signature(&self, callee: &Callee) -> ir::Signature {
        match callee {
            Callee::Direct(id) => self.program.function(*id).sig.clone(),
            Callee::Indirect(expr) => match self.program.types.pointee(expr.ty) {
                Some(Ty::Func(id)) => {
                    let func = self.program.types.func_type(id);
                    ir::Signature {
                        ret: func.ret,
                        params: func.params.clone(),
                        variadic: func.variadic,
                        prototyped: func.prototyped,
                    }
                }
                // Only reachable on the error path, where a `compile_error!` is
                // already going out.
                _ => ir::Signature {
                    ret: Ty::Void,
                    params: Vec::new(),
                    variadic: false,
                    prototyped: true,
                },
            },
        }
    }

    /// `unsafe extern "C" fn(P…) -> R`, written out from its pieces.
    fn fn_ptr_ty(&self, params: &[Ty], variadic: bool, ret: Ty, span: Span) -> TokenStream {
        let mut list = TokenStream::new();
        for (index, param) in params.iter().enumerate() {
            if index > 0 {
                list.extend(quote_spanned! {span=> , });
            }
            list.extend(self.ty(*param, span));
        }
        if variadic {
            if !params.is_empty() {
                list.extend(quote_spanned! {span=> , });
            }
            list.extend(quote_spanned! {span=> ... });
        }
        let list = parenthesize(list, span);
        let ret = if ret.is_void() {
            TokenStream::new()
        } else {
            let ty = self.ty(ret, span);
            quote_spanned! {span=> -> #ty }
        };
        quote_spanned! {span=> unsafe extern "C" fn #list #ret }
    }

    /// The path a call to `function` uses.
    ///
    /// One name whether the unit defines the function or only declares it: the
    /// item in the `extern` block carries the C name as well (see
    /// [`Codegen::extern_block`]), and a lifted nested function carries the name
    /// it was lifted under.
    fn function_path(&self, function: &Function, span: Span) -> TokenStream {
        let name = self.c_ident(function.item_name(), span);
        quote_spanned! {span=> #name }
    }

    /// `unsafe extern "C" fn(…) -> R` for a named function, which is what its
    /// address has to be cast to.
    fn function_pointer_ty(&self, function: &Function, span: Span) -> TokenStream {
        let sig = &function.sig;
        self.fn_ptr_ty(&sig.params, sig.variadic, sig.ret, span)
    }

    /// Emits an expression as a Rust `bool`, the way C tests a scalar against
    /// zero.
    fn condition(&mut self, expr: &Expr) -> Value {
        let span = self.sp(expr.range);
        match &expr.kind {
            ExprKind::Compare { op, lhs, rhs } => {
                if let Some(value) = self.null_test(*op, lhs, rhs, span) {
                    return value;
                }
                if lhs.ty.is_complex() && rhs.ty.is_complex() {
                    return self.complex_equality(*op, lhs, rhs, span);
                }
                let (lhs, rhs) = self.operands(lhs, rhs, BinOp::BitOr);
                let left_min = if *op == CmpOp::Lt && lhs.ends_with_type {
                    prec::CAST + 1
                } else {
                    prec::CMP + 1
                };
                let ends_with_type = rhs.ends_with_type;
                let lhs = lhs.at(left_min, span);
                let rhs = rhs.at(prec::CMP + 1, span);
                let op = cmp_tokens(*op, span);
                Value::new(quote_spanned! {span=> #lhs #op #rhs }, prec::CMP)
                    .type_end(ends_with_type)
            }
            ExprKind::Logical { .. } => self.logical_chain(expr),
            ExprKind::Int(value) => {
                let ident = Ident::new(if *value != 0 { "true" } else { "false" }, span);
                Value::atom(quote_spanned! {span=> #ident })
            }
            _ if expr.ty.is_bool() => self.expr(expr),
            _ if expr.ty.is_complex() => self.complex_condition(expr, span),
            _ if expr.ty.is_pointer() => self.not_null(expr, span),
            _ => {
                let ty = expr.ty;
                let value = self.expr(expr).at(prec::CMP + 1, span);
                let zero = self.zero_tokens(ty, span);
                Value::new(quote_spanned! {span=> #value != #zero }, prec::CMP)
            }
        }
    }

    /// `p != NULL` reads better as `!p.is_null()`, and that is also the only
    /// form that works for a function pointer.
    fn null_test(&mut self, op: CmpOp, lhs: &Expr, rhs: &Expr, span: Span) -> Option<Value> {
        if !matches!(op, CmpOp::Eq | CmpOp::Ne) {
            return None;
        }
        let (pointer, _) = match (&lhs.kind, &rhs.kind) {
            (ExprKind::Zeroed, _) if rhs.ty.is_pointer() => (rhs, lhs),
            (_, ExprKind::Zeroed) if lhs.ty.is_pointer() => (lhs, rhs),
            _ => return None,
        };
        let test = self.is_null(pointer, span);
        if op == CmpOp::Eq {
            return Some(test);
        }
        let tokens = test.at(prec::UNARY, span);
        Some(Value::new(quote_spanned! {span=> !#tokens }, prec::UNARY))
    }

    /// `p.is_null()`, or `{ p }.is_some()` for a function pointer.
    fn is_null(&mut self, expr: &Expr, span: Span) -> Value {
        if self.program.types.is_func_pointer(expr.ty) {
            let tokens = self.copied_receiver(expr, span);
            return Value::new(quote_spanned! {span=> #tokens.is_none() }, prec::CALL);
        }
        let tokens = self.expr(expr).at(prec::CALL, span);
        Value::new(quote_spanned! {span=> #tokens.is_null() }, prec::CALL)
    }

    fn not_null(&mut self, expr: &Expr, span: Span) -> Value {
        if self.program.types.is_func_pointer(expr.ty) {
            let tokens = self.copied_receiver(expr, span);
            return Value::new(quote_spanned! {span=> #tokens.is_some() }, prec::CALL);
        }
        let tokens = self.expr(expr).at(prec::CALL, span);
        Value::new(quote_spanned! {span=> !#tokens.is_null() }, prec::UNARY)
    }

    /// A receiver for a method that takes `&self`.
    ///
    /// `Option::is_some` borrows, and borrowing a `static mut` is an error in
    /// edition 2024; a block copies the value out first. Only a place that
    /// really is a `static mut` needs it, so an ordinary local keeps reading
    /// as one.
    fn copied_receiver(&mut self, expr: &Expr, span: Span) -> TokenStream {
        if self.reads_a_static(expr) {
            let tokens = self.expr(expr).at(prec::LOWEST, span);
            return braced(tokens, span);
        }
        self.expr(expr).at(prec::CALL, span)
    }

    /// Whether an expression reads an object with static storage duration,
    /// which is what a shared reference may not be taken to.
    fn reads_a_static(&self, expr: &Expr) -> bool {
        let ExprKind::Load(place) = &expr.kind else {
            return false;
        };
        let mut place = place;
        loop {
            match &place.kind {
                PlaceKind::Object(id) => {
                    let storage = &self.program.object(*id).storage;
                    // A thread-local object is a `*mut T` out of its cell, so
                    // it is behind a raw pointer like anything else below.
                    return !matches!(storage, Storage::Automatic) && !storage.is_thread_local();
                }
                PlaceKind::Field { base, .. } => place = base,
                // Anything reached through a pointer is behind a raw pointer
                // already, so no reference to the static itself is created.
                _ => return false,
            }
        }
    }

    /// Emits the operands of a binary operation.
    ///
    /// Both operands already have the result type, so a constant one can be
    /// left as a bare literal and take its Rust type from the other side —
    /// `n == 0` rather than `n == 0 as ::core::ffi::c_int`. That only works
    /// while the other side actually has a type to give, and never for the
    /// receiver of a method call, where a bare integer literal followed by a
    /// `.` would not survive being printed back out as text.
    fn operands(&mut self, lhs: &Expr, rhs: &Expr, op: BinOp) -> (Value, Value) {
        self.operands_with(None, lhs, rhs, op)
    }

    /// [`Codegen::operands`] with the left operand possibly already emitted.
    ///
    /// `folded` is what [`Codegen::binary_chain`] has built so far. It is only
    /// ever a binary operation, and [`constant_of`] answers `None` for one, so
    /// the bare-literal reasoning below is unaffected by it.
    fn operands_with(
        &mut self,
        folded: Option<Value>,
        lhs: &Expr,
        rhs: &Expr,
        op: BinOp,
    ) -> (Value, Value) {
        let uses_method = matches!(
            op,
            BinOp::Add | BinOp::Sub | BinOp::Mul | BinOp::Shl | BinOp::Shr
        );
        let lhs_constant = constant_of(lhs);
        let rhs_constant = constant_of(rhs);

        // At most one side may be bare, and it is never the left one where a
        // method call follows.
        let lhs_bare = lhs_constant.is_some() && !uses_method && rhs_constant.is_none();
        let lhs_value = match (folded, lhs_constant) {
            (Some(value), _) => value,
            (None, Some(value)) if lhs_bare => self.bare_value(value, lhs.ty, self.sp(lhs.range)),
            (None, _) => self.expr(lhs),
        };
        let rhs_value = match rhs_constant {
            // The shift amount is converted to `u32` whatever its C type is,
            // so a constant there never needs the other operand's help — and
            // it is reduced to *that* `u32` here rather than left as the C
            // value. Every count a program may legitimately write is under
            // the width and so unchanged; the ones that are not are undefined
            // in C, and writing them out as they stand is what `rustc`
            // refuses. A negative one is `-64 as u32`, which it reads as the
            // negation of a `u32` (`E0600`, `execute/pr98681`) even
            // parenthesised, and one above `u32::MAX` is a literal out of
            // range. `wrapping_shl` masks the count by the width, exactly as
            // the hardware does.
            Some(ConstValue::Int(value)) if op.is_shift() => self.bare_value(
                ConstValue::Int(i128::from(value as u32)),
                Ty::UInt,
                self.sp(rhs.range),
            ),
            Some(value) if op.is_shift() => self.bare_value(value, rhs.ty, self.sp(rhs.range)),
            Some(value) if !lhs_bare => self.bare_value(value, rhs.ty, self.sp(rhs.range)),
            _ => self.expr(rhs),
        };
        (lhs_value, rhs_value)
    }

    /// A constant emitted without its `as`, for a context that will supply the
    /// type.
    fn bare_value(&mut self, value: ConstValue, ty: Ty, span: Span) -> Value {
        match value {
            ConstValue::Int(v) => {
                let tokens = bare_int_literal(v, ty, span);
                let mut out = Value::new(tokens, if v < 0 { prec::UNARY } else { prec::ATOM });
                out.bare_integer = v >= 0 && !ty.is_bool();
                out
            }
            ConstValue::Float(v) => Value::new(
                bare_float_literal(v, span),
                if v.is_sign_negative() {
                    prec::UNARY
                } else {
                    prec::ATOM
                },
            ),
            // A complex constant is never *bare*: it has to name its own type,
            // and [`constant_of`] answers `None` for one so that nothing asks.
            ConstValue::Complex(re, im) => {
                let re = bare_float_literal(re, span);
                let im = bare_float_literal(im, span);
                self.complex_new(ty, re, im, span)
            }
        }
    }

    /// A chain of conditional operators, folded without recursing down it.
    ///
    /// `a ? b : c ? d : e` is right-associative, so unlike the chains
    /// [`Codegen::binary_chain`] handles this one really is nesting: what
    /// comes out is `if … { … } else { if … }`, one level per operator, and
    /// that is as it should be. What must not be one level per operator is
    /// the *walk*, which runs on the eight mebibytes `rustc` gives macro
    /// expansion.
    ///
    /// The condition and the `then` arm of each level are emitted on the way
    /// down and the tree is built on the way back up, which is the order the
    /// recursive walk had — so the tokens, and the numbering of any
    /// temporaries in them, are exactly what it produced.
    fn cond_chain(&mut self, expr: &Expr) -> Value {
        let mut spine = Vec::new();
        let mut node = expr;
        while let ExprKind::Cond {
            cond,
            then_expr,
            else_expr,
        } = &node.kind
        {
            let span = self.sp(node.range);
            let cond_tokens = self.condition(cond).at_condition(span);
            let then_tokens = self.expr(then_expr).at(prec::LOWEST, span);
            spine.push((cond_tokens, then_tokens, span));
            node = else_expr;
        }
        // The innermost `else` is parenthesised against the span of the
        // conditional it belongs to, which is the innermost one on the spine.
        let inner = spine
            .last()
            .map_or_else(|| self.sp(expr.range), |(_, _, span)| *span);
        let mut tokens = self.expr(node).at(prec::LOWEST, inner);
        while let Some((cond_tokens, then_tokens, span)) = spine.pop() {
            tokens = quote_spanned! {span=>
                if #cond_tokens { #then_tokens } else { #tokens }
            };
        }
        Value::new(tokens, prec::BLOCK)
    }

    /// [`Codegen::cond_chain`] where the surrounding context fixes the type.
    ///
    /// The chain ends wherever a level's own type is no longer `expected`,
    /// which is where [`Codegen::expr_at`] would have stopped treating the
    /// arms as bare anyway.
    fn cond_chain_at(&mut self, expr: &Expr, expected: Ty) -> TokenStream {
        let mut spine = Vec::new();
        let mut node = expr;
        while let ExprKind::Cond {
            cond,
            then_expr,
            else_expr,
        } = &node.kind
        {
            if node.ty != expected {
                break;
            }
            let span = self.sp(node.range);
            let cond_tokens = self.condition(cond).at_condition(span);
            let then_tokens = self.expr_at(then_expr, expected);
            spine.push((cond_tokens, then_tokens, span));
            node = else_expr;
        }
        let mut tokens = self.expr_at(node, expected);
        while let Some((cond_tokens, then_tokens, span)) = spine.pop() {
            tokens = quote_spanned! {span=>
                if #cond_tokens { #then_tokens } else { #tokens }
            };
        }
        tokens
    }

    /// A chain of assignments, folded without recursing down it.
    ///
    /// `a = b = c` is right-associative, so this is nesting in the same way
    /// [`Codegen::cond_chain`] is, and the same bargain applies: the shape of
    /// the output is unchanged and only the walk is flattened. Each place is
    /// lowered on the way down — which is where the recursive walk lowered it,
    /// and therefore where it took any temporary it needed — and the blocks
    /// are built on the way back up, innermost first, exactly as the
    /// recursion unwound.
    fn assign_chain(&mut self, expr: &Expr) -> Value {
        let mut spine = Vec::new();
        let mut node = expr;
        while let ExprKind::Assign { place, value } = &node.kind {
            let span = self.sp(node.range);
            let lowered = self.place(place, true);
            spine.push((lowered, self.program.types.unatomic(place.ty), span));
            node = value;
        }
        let (_, innermost, _) = spine
            .last()
            .expect("assign_chain is only entered on an assignment");
        let mut tokens = self.expr_at(node, *innermost);
        while let Some((lowered, ty, span)) = spine.pop() {
            // The value of an assignment to an *atomic* object is the value
            // stored and not what the object holds afterwards: another thread
            // may have changed it already, and reading it back would be a
            // second atomic operation C never asked for.
            if lowered.atomic.is_some() {
                let tmp = self.temporary();
                let target = self.ty(ty, span);
                let store = self.write(&lowered, quote_spanned! {span=> #tmp }, span);
                let setup = &lowered.setup;
                tokens = quote_spanned! {span=>
                    { #setup let #tmp: #target = #tokens; #store #tmp }
                };
                continue;
            }
            let store = self.write(&lowered, tokens, span);
            // The value of an assignment is the value stored, which for a
            // place is exactly what reading it back gives — including for a
            // bit-field, where reading back is what truncates.
            let read = self.read(&lowered, span).at(prec::LOWEST, span);
            let setup = &lowered.setup;
            tokens = quote_spanned! {span=> { #setup #store #read } };
        }
        Value::new(tokens, prec::BLOCK)
    }

    /// A chain of binary operators, folded without recursing down it.
    ///
    /// `a + b + c + …` is left-associative, so the tree it leaves behind is
    /// one node per operand with the whole of the rest hanging off its left.
    /// Walking that recursively is one stack frame per operand — and code
    /// generation runs on the caller's thread, which is the eight mebibytes
    /// `rustc` gives macro expansion, where a chain of about eight hundred is
    /// a "fatal runtime error: stack overflow" with no diagnostic at all.
    ///
    /// C23 5.2.5.2p1 asks every implementation to accept a logical source
    /// line of 4095 characters, which is well past that, so the spine is
    /// collected into a vector and folded back up in a loop: one frame,
    /// however long the chain. The tokens that come out are the same ones the
    /// recursive walk produced, and in the same order — `a.wrapping_add(b)
    /// .wrapping_add(c)` is a receiver chain, which is *flat*, so neither is
    /// the output deeply nested.
    ///
    /// Nesting — `a + (b + (c + …))`, which is one level of parentheses per
    /// operand — still costs a frame per level, and is what
    /// `parse::MAX_RECURSION_DEPTH` bounds.
    fn binary_chain(&mut self, expr: &Expr) -> Value {
        let mut spine = vec![expr];
        let mut node = expr;
        while let ExprKind::Binary { lhs, .. } = &node.kind {
            node = lhs;
            if matches!(node.kind, ExprKind::Binary { .. }) {
                spine.push(node);
            }
        }
        let mut folded = None;
        while let Some(node) = spine.pop() {
            let ExprKind::Binary { op, lhs, rhs } = &node.kind else {
                unreachable!("the spine holds binary operations only");
            };
            let span = self.sp(node.range);
            let (lhs_value, rhs_value) = self.operands_with(folded, lhs, rhs, *op);
            let value = if node.ty.is_complex() {
                self.complex_binary(*op, (lhs_value, lhs.ty), (rhs_value, rhs.ty), node.ty, span)
            } else {
                self.binary(*op, lhs_value, rhs_value, node.ty, span)
            };
            folded = Some(self.reduce_bits(value, node));
        }
        folded.expect("the chain has at least the node it started from")
    }

    /// A chain of `&&` or `||`, folded without recursing down it.
    ///
    /// The same shape and the same reason as [`Codegen::binary_chain`]; these
    /// live on the `bool` side of code generation, so the fold is over
    /// [`Codegen::condition`] rather than over `expr`.
    fn logical_chain(&mut self, expr: &Expr) -> Value {
        let mut spine = vec![expr];
        let mut node = expr;
        while let ExprKind::Logical { lhs, .. } = &node.kind {
            node = lhs;
            if matches!(node.kind, ExprKind::Logical { .. }) {
                spine.push(node);
            }
        }
        // `node` is now the innermost left operand, which is not itself a
        // logical operator; emitting it first keeps the order the recursive
        // walk had, which is source order.
        let mut folded = self.condition(node);
        while let Some(node) = spine.pop() {
            let ExprKind::Logical { op, rhs, .. } = &node.kind else {
                unreachable!("the spine holds logical operations only");
            };
            let span = self.sp(node.range);
            let (level, tokens) = match op {
                LogicalOp::And => (prec::AND, quote_spanned! {span=> && }),
                LogicalOp::Or => (prec::OR, quote_spanned! {span=> || }),
            };
            let mut out = folded.at(level, span);
            let rhs = self.condition(rhs);
            let ends_with_type = rhs.ends_with_type;
            let rhs = rhs.at(level + 1, span);
            out.extend(quote_spanned! {span=> #tokens #rhs });
            folded = Value::new(out, level).type_end(ends_with_type);
        }
        folded
    }

    fn binary(&mut self, op: BinOp, lhs: Value, rhs: Value, ty: Ty, span: Span) -> Value {
        if ty.is_floating() {
            let (level, tokens) = match op {
                BinOp::Add => (prec::SUM, quote_spanned! {span=> + }),
                BinOp::Sub => (prec::SUM, quote_spanned! {span=> - }),
                BinOp::Mul => (prec::PRODUCT, quote_spanned! {span=> * }),
                BinOp::Div => (prec::PRODUCT, quote_spanned! {span=> / }),
                // Sema rejects every other operator on floating operands.
                _ => (prec::PRODUCT, quote_spanned! {span=> % }),
            };
            let ends_with_type = rhs.ends_with_type;
            let mut out = lhs.at(level, span);
            let rhs = rhs.at(level + 1, span);
            // Appended in place rather than built into a fresh stream around
            // the left operand: a chain of a thousand would otherwise copy
            // the whole of it a thousand times over. See
            // [`Codegen::binary_chain`].
            out.extend(quote_spanned! {span=> #tokens #rhs });
            return Value::new(out, level).type_end(ends_with_type);
        }

        // `+`, `-`, `*` and the shifts go through the wrapping methods:
        // unsigned wrap-around is defined in C, and while signed overflow is
        // undefined, wrapping is far more predictable than a panic in the
        // middle of translated code.
        let method = match op {
            BinOp::Add => Some("wrapping_add"),
            BinOp::Sub => Some("wrapping_sub"),
            BinOp::Mul => Some("wrapping_mul"),
            BinOp::Shl => Some("wrapping_shl"),
            BinOp::Shr => Some("wrapping_shr"),
            _ => None,
        };
        if let Some(method) = method {
            let mut receiver = lhs.at(prec::CALL, span);
            let method = Ident::new(method, span);
            let argument = if op.is_shift() && !rhs.bare_integer {
                // `wrapping_shl` takes the shift amount as a `u32` whatever the
                // shifted type is; a bare literal simply is one already.
                let amount = rhs.at(prec::CAST, span);
                // Belt and braces: nothing that reaches here opens with a
                // unary minus — a constant count was reduced to its `u32` in
                // [`Codegen::operands_with`] and `-x` is written
                // `x.wrapping_neg()` — and if one ever did, `rustc` would read
                // `-e as u32` as the negation of a `u32` and refuse it
                // (`E0600`) however it is bracketed by precedence.
                let amount = if starts_with_minus(&amount) {
                    parenthesize(amount, span)
                } else {
                    amount
                };
                let u32_ty = primitive_ty("u32", span);
                quote_spanned! {span=> #amount as #u32_ty }
            } else {
                rhs.at(prec::LOWEST, span)
            };
            let args = parenthesize(argument, span);
            receiver.extend(quote_spanned! {span=> .#method #args });
            return Value::new(receiver, prec::CALL);
        }

        // `/` and `%` stay plain: Rust truncates towards zero and takes the
        // sign of the dividend, exactly as C99 does. Both panic where C is
        // undefined (division by zero, and `INT_MIN / -1`).
        let (level, tokens) = match op {
            BinOp::Div => (prec::PRODUCT, quote_spanned! {span=> / }),
            BinOp::Rem => (prec::PRODUCT, quote_spanned! {span=> % }),
            BinOp::BitAnd => (prec::BIT_AND, quote_spanned! {span=> & }),
            BinOp::BitXor => (prec::BIT_XOR, quote_spanned! {span=> ^ }),
            BinOp::BitOr => (prec::BIT_OR, quote_spanned! {span=> | }),
            _ => unreachable!("every other operator was handled above"),
        };
        let ends_with_type = rhs.ends_with_type;
        let mut out = lhs.at(level, span);
        let rhs = rhs.at(level + 1, span);
        out.extend(quote_spanned! {span=> #tokens #rhs });
        Value::new(out, level).type_end(ends_with_type)
    }

    /// The value `place op= value` stores.
    ///
    /// `hoisted` is the right operand when it was evaluated ahead of the read;
    /// see [`Codegen::compound_rhs`].
    fn compound_value(
        &mut self,
        current: Value,
        place_ty: Ty,
        op: BinOp,
        value: &Expr,
        hoisted: Option<Value>,
        compute: Ty,
    ) -> Value {
        let span = self.sp(value.range);
        if place_ty.is_pointer() {
            // `p += n` moves by elements, not by bytes.
            let access = current.at(prec::CALL, span);
            let offset = match hoisted {
                Some(index) => self.offset_of_value(index, op == BinOp::Sub, span),
                None => self.offset_argument(value, op == BinOp::Sub, span),
            };
            let pointee = self.program.types.pointee(place_ty).unwrap_or(Ty::Void);
            let offset = match self.vm_scale(pointee, span) {
                None => offset,
                Some(scale) => quote_spanned! {span=> (#offset).wrapping_mul(#scale) },
            };
            return Value::new(quote_spanned! {span=> #access.offset(#offset) }, prec::CALL);
        }
        // A complex computation keeps a real operand real on *both* sides:
        // sema left the right one alone, and the left one — the place — is
        // widened only as far as the common real type when it is real too. See
        // [`Codegen::complex_binary`].
        if compute.is_complex() {
            let lhs_ty = if place_ty.is_complex() {
                compute
            } else {
                compute.complex_component()
            };
            let current = self.cast(current, place_ty, lhs_ty, span);
            let rhs = self.compound_operand(value, hoisted, span);
            let result = self.complex_binary(op, (current, lhs_ty), (rhs, value.ty), compute, span);
            return self.cast(result, compute, place_ty, span);
        }
        let current = self.cast(current, place_ty, compute, span);
        let rhs = self.compound_operand(value, hoisted, span);
        let result = self.binary(op, current, rhs, compute, span);
        self.cast(result, compute, place_ty, span)
    }

    /// The right operand of a compound assignment, already hoisted or not.
    ///
    /// The left operand is a place and therefore always typed, so a constant
    /// right operand can stay a bare literal.
    fn compound_operand(&mut self, value: &Expr, hoisted: Option<Value>, span: Span) -> Value {
        match hoisted {
            Some(rhs) => rhs,
            None => match constant_of(value) {
                Some(constant) => self.bare_value(constant, value.ty, span),
                None => self.expr(value),
            },
        }
    }

    /// Evaluates the right operand of a compound assignment ahead of the read,
    /// when C says it happens either wholly before it or wholly after it.
    ///
    /// `E1 op= E2` is a read, an operation and a write, and C11 6.5.16.2p3
    /// makes the three of them *one* evaluation with respect to an
    /// indeterminately sequenced function call. Writing them out as
    /// `E1 = E1 op E2` would read `E1`, call whatever `E2` calls, and only then
    /// store — which is the one order the standard rules out, and which GCC's
    /// `pr58943` is about. Binding `E2` to a temporary first restores it: the
    /// place is computed, then the call happens, then the read-modify-write.
    ///
    /// Only an operand that can call something needs it; everything else is
    /// left where it was written, because a temporary per `i += 1` would be
    /// noise.
    fn compound_rhs(&mut self, value: &Expr) -> (TokenStream, Option<Value>) {
        if !ir::calls_a_function(value) {
            return (TokenStream::new(), None);
        }
        let span = self.sp(value.range);
        let tokens = self.expr(value).at(prec::LOWEST, span);
        let tmp = self.temporary();
        (
            quote_spanned! {span=> let #tmp = #tokens; },
            Some(Value::atom(quote_spanned! {span=> #tmp })),
        )
    }

    /// The `offset` argument for an index that has already been emitted.
    fn offset_of_value(&mut self, index: Value, sub: bool, span: Span) -> TokenStream {
        let tokens = index.at(prec::CAST, span);
        let isize_ty = primitive_ty("isize", span);
        if sub {
            quote_spanned! {span=> -(#tokens as #isize_ty) }
        } else {
            quote_spanned! {span=> #tokens as #isize_ty }
        }
    }

    /// The value `++place` or `--place` stores.
    fn step_value(&mut self, current: Value, ty: Ty, dec: bool, span: Span) -> TokenStream {
        if ty.is_pointer() {
            let access = current.at(prec::CALL, span);
            let pointee = self.program.types.pointee(ty).unwrap_or(Ty::Void);
            let one = match (self.vm_scale(pointee, span), dec) {
                (None, true) => quote_spanned! {span=> -1 },
                (None, false) => quote_spanned! {span=> 1 },
                (Some(scale), true) => quote_spanned! {span=> -(#scale) },
                (Some(scale), false) => scale,
            };
            return quote_spanned! {span=> #access.offset(#one) };
        }
        if ty.is_floating() {
            let access = current.at(prec::SUM, span);
            let one = Literal::f64_unsuffixed(1.0);
            let op = if dec {
                quote_spanned! {span=> - }
            } else {
                quote_spanned! {span=> + }
            };
            return quote_spanned! {span=> #access #op #one };
        }
        if ty.is_complex() {
            // GCC's `z++` adds one to the *real* part and leaves the
            // imaginary one alone, which is the componentwise `z + 1`.
            let name = if dec { "sub_real" } else { "add_real" };
            let func = self.rt_complex(&format!("{name}_{}", Self::complex_suffix(ty)), span);
            let access = current.at(prec::LOWEST, span);
            let one = Literal::f64_unsuffixed(1.0);
            return quote_spanned! {span=> #func(#access, #one) };
        }
        if ty.is_bool() {
            // `b++` is `b = b + 1 != 0`, which is `true` for `++` and the
            // negation of `b` for `--`.
            let access = current.at(prec::CAST, span);
            let int = self.ty(Ty::Int, span);
            let method = Ident::new(if dec { "wrapping_sub" } else { "wrapping_add" }, span);
            return quote_spanned! {span=> (#access as #int).#method(1) != 0 };
        }
        let access = current.at(prec::CALL, span);
        let method = Ident::new(if dec { "wrapping_sub" } else { "wrapping_add" }, span);
        quote_spanned! {span=> #access.#method(1) }
    }

    /// Emits a conversion between two scalar types.
    fn cast(&mut self, value: Value, from: Ty, to: Ty, span: Span) -> Value {
        if from == to {
            return value;
        }
        if from.is_complex() || to.is_complex() {
            return self.complex_cast(value, from, to, span);
        }
        let types = &self.program.types;
        let from_fn = types.is_func_pointer(from);
        let to_fn = types.is_func_pointer(to);
        if to.is_bool() {
            if from.is_pointer() {
                // Handled by the caller for the common shapes; this is the
                // explicit `(_Bool)p`.
                let tokens = value.at(prec::CALL, span);
                return if from_fn {
                    let braced = braced(tokens, span);
                    Value::new(quote_spanned! {span=> #braced.is_some() }, prec::CALL)
                } else {
                    Value::new(quote_spanned! {span=> !#tokens.is_null() }, prec::UNARY)
                };
            }
            // Converting to `_Bool` yields 0 or 1, which is what a comparison
            // against zero gives.
            let zero = self.zero_tokens(from, span);
            let tokens = value.at(prec::CMP + 1, span);
            return Value::new(quote_spanned! {span=> #tokens != #zero }, prec::CMP);
        }
        if from_fn || to_fn {
            // Rust has no `as` between an `Option<fn>` and anything else, so a
            // transmute is the honest translation of what C's cast does — but
            // only between two things of the same size. An integer of any
            // other width goes through `usize`, which is what C's own
            // implementation-defined conversion between a pointer and an
            // integer amounts to.
            let target = self.ty(to, span);
            let source = self.ty(from, span);
            // Two C function types the generated Rust cannot tell apart need
            // no transmute at all: `void (*)()` and `void (*)(void)` are
            // distinct types in C and one `Option<unsafe extern "C" fn()>`
            // here. Writing the transmute anyway would be a no-op that still
            // has to be inside an `unsafe` block, which a `static` initialiser
            // then has to grow.
            if from_fn && to_fn && source.to_string() == target.to_string() {
                return value;
            }
            let usize_ty = primitive_ty("usize", span);
            if to_fn && !from.is_pointer() {
                let tokens = value.at(prec::CAST, span);
                return Value::new(
                    quote_spanned! {span=>
                        ::core::mem::transmute::<#usize_ty, #target>(#tokens as #usize_ty)
                    },
                    prec::CALL,
                );
            }
            if from_fn && !to.is_pointer() {
                let tokens = value.at(prec::LOWEST, span);
                return Value::new(
                    quote_spanned! {span=>
                        ::core::mem::transmute::<#source, #usize_ty>(#tokens) as #target
                    },
                    prec::CAST,
                )
                .type_end(true);
            }
            let tokens = value.at(prec::LOWEST, span);
            return Value::new(
                quote_spanned! {span=>
                    ::core::mem::transmute::<#source, #target>(#tokens)
                },
                prec::CALL,
            );
        }
        let target = self.ty(to, span);
        if from.is_pointer() && to.is_integer() {
            // Rust only casts a pointer to `usize`; the rest is an ordinary
            // integer conversion.
            let tokens = value.at(prec::CAST, span);
            let usize_ty = primitive_ty("usize", span);
            return Value::new(
                quote_spanned! {span=> #tokens as #usize_ty as #target },
                prec::CAST,
            )
            .type_end(true);
        }
        if from.is_bool() && to.is_floating() {
            // Rust has no `bool as f64`; C's `_Bool` to floating conversion
            // goes through the integer value.
            let int = self.ty(Ty::Int, span);
            let tokens = value.at(prec::CAST, span);
            return Value::new(
                quote_spanned! {span=> #tokens as #int as #target },
                prec::CAST,
            )
            .type_end(true);
        }
        let tokens = value.at(prec::CAST, span);
        Value::new(quote_spanned! {span=> #tokens as #target }, prec::CAST).type_end(true)
    }

    // -- places -------------------------------------------------------------

    /// A place, ready to be read from or written to.
    ///
    /// `mutable` says whether the place is about to be assigned to or have its
    /// address taken, which is what decides whether a `const` pointer under it
    /// has to lose the qualifier: Rust refuses `&raw mut (*p).f` when `p` is a
    /// `*const T`, while reading through one is fine.
    fn place(&mut self, place: &Place, mutable: bool) -> LoweredPlace {
        // Even *reading* an atomic object needs a `*mut` to it:
        // `AtomicX::from_ptr` takes one, and a `const _Atomic int *` would
        // otherwise reach `&raw mut *p` through a `*const` pointer, which
        // Rust refuses. What C promises is enough for the cast — every object
        // this crate generates lives in writable storage.
        let atomic = matches!(place.ty, Ty::Atomic(_));
        let mut lowered = self.place_access(place, mutable || atomic);
        if lowered.bits.is_none() && self.place_underaligned(place) {
            lowered.unaligned = true;
        }
        if let Ty::Atomic(id) = place.ty {
            let inner = self.program.types.atomic_inner(id);
            lowered.atomic = ir::atomic_class(&self.program.types, inner, &self.options.target)
                .map(|c| (c, inner));
            // There is no unaligned atomic: the alignment of an `_Atomic` type
            // is its size, and sema refuses the declarations that could not
            // have it.
            if lowered.atomic.is_some() {
                lowered.unaligned = false;
            }
        }
        lowered
    }

    /// Whether the place has to be reached through an unaligned load or store.
    ///
    /// Two shapes need one. A `*p` or a `p[i]` whose pointer this crate itself
    /// built out of a packed member is underaligned, and so is a member read
    /// through a pointer to a record that is: `(*(T *) &buf).f`, where `buf` is
    /// a `char` array, is a member access Rust would compile to an aligned load
    /// of a byte-aligned address. A member of a *packed* record whose own
    /// address is fine needs nothing — Rust knows the layout of the item it was
    /// given and reads such a member unaligned by itself.
    fn place_underaligned(&self, place: &Place) -> bool {
        match &place.kind {
            PlaceKind::Deref(_) | PlaceKind::Index { .. } => {
                self.place_align(place) < self.type_align(place.ty)
            }
            PlaceKind::Field { base, .. } => self.place_align(base) < self.type_align(base.ty),
            // A part of a complex object reached through a packed member is
            // no better aligned than the object is.
            PlaceKind::ComplexPart { .. } => self.place_align(place) < self.type_align(place.ty),
            _ => false,
        }
    }

    /// The alignment code generation can count on for a place's address.
    ///
    /// Everything C promises about an object's alignment holds for a place that
    /// names one, so the interesting half is what this crate itself can see
    /// through: the address of a member sits at its offset from a base whose
    /// alignment is known, and a cast in between changes nothing. Anything else
    /// — a pointer out of a variable, a parameter, a call — is taken at C's
    /// word and assumed to point at something its type is aligned for.
    fn place_align(&self, place: &Place) -> u64 {
        match &place.kind {
            PlaceKind::Deref(ptr) => self.pointer_align(ptr),
            PlaceKind::Index { base, .. } => {
                // Every element of an array sits at a multiple of the element
                // size from the first, and a size is always a multiple of the
                // alignment, so the elements are no worse aligned than the
                // element type asks and no better than the array is.
                self.pointer_align(base).min(self.type_align(place.ty))
            }
            PlaceKind::Field {
                base,
                record,
                index,
            } => {
                let base_align = self.place_align(base);
                let offset = self.program.types.record(*record).fields[*index].offset;
                if offset == 0 {
                    base_align
                } else {
                    base_align.min(1 << offset.trailing_zeros())
                }
            }
            // The real part sits at the object's own address and the imaginary
            // one exactly one component later, which is a power of two.
            PlaceKind::ComplexPart { base, imag } => {
                let base_align = self.place_align(base);
                if *imag {
                    base_align.min(place.ty.size_bytes(&self.options.target).max(1))
                } else {
                    base_align
                }
            }
            // An `_Alignas` on the declaration is a promise about the object
            // that the wrapper really keeps.
            PlaceKind::Object(id) => self
                .object_align(*id)
                .unwrap_or(1)
                .max(self.type_align(place.ty)),
            _ => self.type_align(place.ty),
        }
    }

    /// The alignment of what a pointer expression points at.
    fn pointer_align(&self, ptr: &Expr) -> u64 {
        match &ptr.kind {
            ExprKind::AddrOf(place) => self.place_align(place),
            // A pointer cast moves no bytes, and neither does the decay of an
            // array to its first element.
            ExprKind::Cast(inner) if inner.ty.is_pointer() || inner.ty.is_array() => {
                let from = self.pointer_align(inner);
                // ...but a cast *to* a pointer to an under-aligned `typedef`
                // takes back the promise: `*(const xxh_unalign64 *) p`.
                match &ptr.ty {
                    Ty::Pointer(id) => match self.program.types.pointer_type(*id).align {
                        Some(align) => from.min(align),
                        None => from,
                    },
                    _ => from,
                }
            }
            ExprKind::PtrOffset { ptr, .. } => self
                .pointer_align(ptr)
                .min(self.pointee_align(ptr.ty).unwrap_or(1)),
            _ => self.pointee_align(ptr.ty).unwrap_or(u64::MAX),
        }
    }

    /// The alignment of the type a pointer or array type addresses.
    fn pointee_align(&self, ty: Ty) -> Option<u64> {
        // A pointer to an under-aligned `typedef` (`xxh_unalign64 *`) promises
        // only the `typedef`'s alignment; see `ir::PointerType::align`.
        if let Ty::Pointer(id) = ty
            && let Some(align) = self.program.types.pointer_type(id).align
        {
            return Some(align);
        }
        let pointee = self.program.types.pointee(ty)?;
        (!pointee.is_void() && !pointee.is_func()).then(|| self.type_align(pointee))
    }

    /// The alignment the *generated Rust type* has, which is what a `*p` in the
    /// expansion is checked against.
    fn type_align(&self, ty: Ty) -> u64 {
        match ty {
            // A packed record's item is one byte aligned however strict C says
            // the record is; see `ir::RecordDef::rust_align`.
            Ty::Record(id) => self.program.types.record(id).rust_align,
            Ty::Array(id) => self.type_align(self.program.types.array_type(id).elem),
            _ => self
                .program
                .types
                .size_align(ty, &self.options.target)
                .map_or(1, |layout| layout.align),
        }
    }

    fn place_access(&mut self, place: &Place, mutable: bool) -> LoweredPlace {
        let span = self.sp(place.range);
        match &place.kind {
            PlaceKind::Object(id) if self.program.object(*id).storage.is_thread_local() => {
                // A thread-local object is reached through the `*mut T` inside
                // its cell, which is valid for as long as this thread's copy
                // of the object is — exactly the lifetime C gives it. The
                // pointer is taken once and the place is a dereference of it,
                // so an expression that reads the object twice still calls
                // `with` once.
                let name = self.object_ident(*id, span);
                let tmp = self.temporary_at(span);
                let cell = Ident::new("__cinrs_cell", Span::mixed_site());
                // The cell holds whatever wrappers the storage carries, and
                // the object is reached through them exactly as it is for
                // every other storage class.
                let object = parenthesize(quote_spanned! {span=> *#tmp }, span);
                let object = self.through_storage(*id, object, span);
                LoweredPlace::plain(
                    quote_spanned! {span=>
                        let #tmp = #name.with(|#cell| ::core::cell::UnsafeCell::get(#cell));
                    },
                    object,
                )
            }
            PlaceKind::Object(id) => {
                let access = self.object_access(*id, span);
                LoweredPlace::plain(TokenStream::new(), access)
            }
            PlaceKind::Deref(ptr) => self.deref_place(ptr, place.ty, mutable, span),
            PlaceKind::Index { base, index } => {
                let pointer = self.pointer_operand(base, place.ty, mutable, span);
                let offset = self.scaled_offset(place.ty, index, false, span);
                let tmp = self.temporary_at(span);
                LoweredPlace::plain(
                    quote_spanned! {span=> let #tmp = #pointer.offset(#offset); },
                    parenthesize(quote_spanned! {span=> *#tmp }, span),
                )
            }
            PlaceKind::Field {
                base,
                record,
                index,
            } => {
                let field = self.program.types.record(*record).fields[*index].clone();
                let lowered = self.place(base, mutable);
                let access = lowered.access;
                let Some(bits) = &field.bits else {
                    let name = self.c_ident(&field.name, span);
                    return LoweredPlace::plain(
                        lowered.setup,
                        quote_spanned! {span=> #access.#name },
                    );
                };
                // The accessors take `&self` and `&mut self`, and a reference
                // to a `static mut` is exactly what edition 2024 refuses; the
                // raw pointer keeps the item out of the expression.
                let access = if rooted_in_static(base, self.program) {
                    parenthesize(quote_spanned! {span=> *(&raw mut #access) }, span)
                } else {
                    access
                };
                LoweredPlace {
                    setup: lowered.setup,
                    access,
                    bits: Some(BitAccess {
                        getter: self.c_ident(&bits.getter, span),
                        setter: self.c_ident(&bits.setter, span),
                    }),
                    unaligned: false,
                    atomic: None,
                }
            }
            // `__real__ z` and `__imag__ z` are the two fields of the runtime's
            // `Complex`, which is why they are assignable: the place is a Rust
            // place expression like any member access.
            PlaceKind::ComplexPart { base, imag } => {
                let lowered = self.place(base, mutable);
                let access = lowered.access;
                let field = Ident::new(if *imag { "im" } else { "re" }, span);
                LoweredPlace::plain(lowered.setup, quote_spanned! {span=> #access.#field })
            }
            PlaceKind::Str(id) => {
                let pointer = self.string_pointer(*id, !mutable, span);
                let tmp = self.temporary_at(span);
                LoweredPlace::plain(
                    quote_spanned! {span=> let #tmp = #pointer; },
                    parenthesize(quote_spanned! {span=> *#tmp }, span),
                )
            }
            PlaceKind::Temporary(expr) => {
                let ty = expr.ty;
                let value = self.expr_at(expr, ty);
                let tmp = self.temporary();
                LoweredPlace::plain(
                    quote_spanned! {span=> let mut #tmp = #value; },
                    quote_spanned! {span=> #tmp },
                )
            }
            // The binding itself was made at the top of the enclosing block —
            // C gives the object that lifetime, and it is what lets `&(T){…}`
            // outlive the expression. What happens *here* is the
            // initialisation, so that side effects in it happen where the
            // literal was written and a literal in a loop is rebuilt on every
            // iteration.
            PlaceKind::CompoundLiteral { object, init } => {
                let name = self.object_ident(*object, span);
                let value = self.expr_at(init, place.ty);
                LoweredPlace::plain(
                    quote_spanned! {span=> #name = #value; },
                    quote_spanned! {span=> #name },
                )
            }
        }
    }

    /// Reads a lowered place.
    fn read(&self, place: &LoweredPlace, span: Span) -> Value {
        let access = &place.access;
        if let Some((class, ty)) = place.atomic {
            let object = self.atomic_object_of(place, span);
            let order = self.ordering(ir::MemOrder::SeqCst, span);
            return self.repr_to_value(
                class,
                ty,
                quote_spanned! {span=> #object.load(#order) },
                span,
            );
        }
        match &place.bits {
            Some(bits) => {
                let getter = &bits.getter;
                Value::new(quote_spanned! {span=> #access.#getter() }, prec::CALL)
            }
            None if place.unaligned => Value::new(
                quote_spanned! {span=> (&raw const #access).read_unaligned() },
                prec::CALL,
            ),
            None => Value::atom(access.clone()),
        }
    }

    /// The statement that stores `value` into a lowered place.
    fn write(&self, place: &LoweredPlace, value: TokenStream, span: Span) -> TokenStream {
        let access = &place.access;
        if let Some((class, ty)) = place.atomic {
            let object = self.atomic_object_of(place, span);
            let order = self.ordering(ir::MemOrder::SeqCst, span);
            let value = self.value_to_repr(class, ty, Value::new(value, prec::LOWEST), span);
            return quote_spanned! {span=> #object.store(#value, #order); };
        }
        match &place.bits {
            Some(bits) => {
                let setter = &bits.setter;
                quote_spanned! {span=> #access.#setter(#value); }
            }
            None if place.unaligned => {
                quote_spanned! {span=> (&raw mut #access).write_unaligned(#value); }
            }
            None => quote_spanned! {span=> #access = #value; },
        }
    }

    /// The `&AtomicX` an `_Atomic` place is reached through.
    ///
    /// A raw pointer to the object rather than a reference to it: the object
    /// is a plain `static mut` or `let mut` of the underlying type, and its
    /// address is what `from_ptr` wants.
    fn atomic_object_of(&self, place: &LoweredPlace, span: Span) -> TokenStream {
        let access = &place.access;
        let class = place.atomic.expect("an atomic place").0;
        self.atomic_ref(class, quote_spanned! {span=> (&raw mut #access) }, span)
    }

    /// `*p` as a place.
    fn deref_place(&mut self, ptr: &Expr, pointee: Ty, mutable: bool, span: Span) -> LoweredPlace {
        let simple =
            matches!(&ptr.kind, ExprKind::Load(p) if matches!(p.kind, PlaceKind::Object(_)));
        if simple {
            // A variable holding the pointer can be dereferenced as often as
            // needed, so no temporary is called for.
            let tokens = self.pointer_operand(ptr, pointee, mutable, span);
            return LoweredPlace::plain(
                TokenStream::new(),
                parenthesize(quote_spanned! {span=> *#tokens }, span),
            );
        }
        let value = self.pointer_operand(ptr, pointee, mutable, span);
        let tmp = self.temporary_at(span);
        LoweredPlace::plain(
            quote_spanned! {span=> let #tmp = #value; },
            parenthesize(quote_spanned! {span=> *#tmp }, span),
        )
    }

    /// The pointer a place is built on.
    ///
    /// Writing through a `*const T` is not allowed even in `unsafe` Rust, and
    /// C's own `const` checking has already happened in sema, so a place that
    /// is about to be written to (or have its address taken) drops the
    /// qualifier here rather than at every use.
    fn pointer_operand(
        &mut self,
        ptr: &Expr,
        pointee: Ty,
        mutable: bool,
        span: Span,
    ) -> TokenStream {
        if mutable && self.program.types.points_to_const(ptr.ty) {
            let target = self.pointee_ty(pointee, span);
            let tokens = self.expr(ptr).at(prec::CAST, span);
            return parenthesize(quote_spanned! {span=> #tokens as *mut #target }, span);
        }
        self.expr(ptr).at(prec::CALL, span)
    }

    /// The address of a place, as a pointer of type `want`.
    fn address_of(&mut self, place: &Place, want: Ty, span: Span) -> Value {
        // A variably modified object's binding already *is* the address of its
        // first element, so both the decay `a` and the array pointer `&a` are
        // that binding — the second one only differs in its C type.
        if let PlaceKind::Object(id) = &place.kind
            && self.program.types.is_vm(place.ty)
        {
            let name = self.object_ident(*id, span);
            let value = Value::atom(quote_spanned! {span=> #name });
            let elem = self.program.types.elem(place.ty);
            let natural = self.program.types.pointee(want) == elem
                && !self.program.types.points_to_const(want);
            if natural {
                return value;
            }
            let target = self.ty(want, span);
            let tokens = value.at(prec::CAST, span);
            return Value::new(quote_spanned! {span=> #tokens as #target }, prec::CAST)
                .type_end(true);
        }
        // `&*p` is `p`, and `&a[i]` is `a + i`; saying so keeps the output
        // free of pointless round trips through a place.
        match &place.kind {
            PlaceKind::Deref(ptr) => {
                let from = ptr.ty;
                let value = self.expr(ptr);
                return self.pointer_cast(value, from, want, span);
            }
            PlaceKind::Index { base, index } => {
                let from = base.ty;
                let pointer = self.expr(base).at(prec::CALL, span);
                let offset = self.scaled_offset(place.ty, index, false, span);
                let value = Value::new(
                    quote_spanned! {span=> #pointer.offset(#offset) },
                    prec::CALL,
                );
                return self.pointer_cast(value, from, want, span);
            }
            PlaceKind::Str(id) => {
                let konst = self.program.types.points_to_const(want);
                let tokens = self.string_pointer(*id, konst, span);
                return Value::new(tokens, prec::CALL);
            }
            _ => {}
        }
        let lowered = self.place(place, true);
        let access = lowered.access;
        let address = quote_spanned! {span=> &raw mut #access };
        let natural = Value::new(parenthesize(address, span), prec::ATOM);
        let value = self.array_or_pointer_cast(natural, place.ty, want, span);
        if lowered.setup.is_empty() {
            return value;
        }
        let setup = lowered.setup;
        let tokens = value.at(prec::LOWEST, span);
        Value::new(quote_spanned! {span=> { #setup #tokens } }, prec::BLOCK)
    }

    /// Adjusts `&raw mut place` to the pointer type the expression wants.
    fn array_or_pointer_cast(&mut self, value: Value, from: Ty, want: Ty, span: Span) -> Value {
        if let Ty::Array(id) = from {
            // The address of an array is a pointer to the array; decaying it
            // to a pointer to the first element is a `cast`, which — unlike
            // `as` — cannot silently change anything else.
            let array = self.program.types.array_type(id);
            let wanted_pointee = self.program.types.pointee(want);
            if wanted_pointee == Some(array.elem) {
                let elem = self.pointee_ty(array.elem, span);
                let tokens = value.at(prec::CALL, span);
                let cast = Value::new(quote_spanned! {span=> #tokens.cast::<#elem>() }, prec::CALL);
                return self.constify(cast, want, span);
            }
        }
        let natural_mut = matches!(self.program.types.pointee(want), Some(pointee) if pointee == from)
            && !self.program.types.points_to_const(want);
        if natural_mut {
            return value;
        }
        let target = self.ty(want, span);
        let tokens = value.at(prec::CAST, span);
        Value::new(quote_spanned! {span=> #tokens as #target }, prec::CAST).type_end(true)
    }

    /// Adds the `as *const T` that a `*mut T` needs to become a `*const T`.
    fn constify(&mut self, value: Value, want: Ty, span: Span) -> Value {
        if !self.program.types.points_to_const(want) {
            return value;
        }
        let target = self.ty(want, span);
        let tokens = value.at(prec::CAST, span);
        Value::new(quote_spanned! {span=> #tokens as #target }, prec::CAST).type_end(true)
    }

    /// Casts a pointer value to the pointer type wanted.
    fn pointer_cast(&mut self, value: Value, from: Ty, want: Ty, span: Span) -> Value {
        if from == want {
            return value;
        }
        let target = self.ty(want, span);
        let tokens = value.at(prec::CAST, span);
        Value::new(quote_spanned! {span=> #tokens as #target }, prec::CAST).type_end(true)
    }

    /// The pointer a string literal decays to.
    fn string_pointer(&mut self, id: ir::StrId, konst: bool, span: Span) -> TokenStream {
        let data = self.program.string(id);
        let element = data.elem;
        if element.size_bytes(&self.options.target) > 1 {
            // A `wchar_t`, `char16_t` or `char32_t` literal needs real storage
            // of that type; a `static` in the enclosing block is the only
            // thing with a long enough lifetime.
            let elem = self.ty(element, span);
            let mut items = TokenStream::new();
            for value in data.values.iter().chain(std::iter::once(&0)) {
                let literal =
                    int_literal_token(element.wrap(i128::from(*value), &self.options.target), span);
                items.extend(quote_spanned! {span=> #literal, });
            }
            let len = usize_literal(data.len_with_nul(), span);
            let name = Ident::new("__CINRS_WIDE", Span::mixed_site());
            let array = bracketed(items, span);
            let ty = bracketed(quote_spanned! {span=> #elem ; #len }, span);
            let pointer = if konst {
                quote_spanned! {span=> (&raw const #name).cast::<#elem>() }
            } else {
                quote_spanned! {span=> (&raw const #name).cast::<#elem>().cast_mut() }
            };
            return quote_spanned! {span=>
                { static #name: #ty = #array; #pointer }
            };
        }
        // A narrow or `u8"…"` literal is a Rust byte string, whose elements
        // are the same bytes whether C calls them `char` or `char8_t`.
        let mut literal = Literal::byte_string(&nul_terminated(&data.values));
        literal.set_span(span);
        let elem = self.ty(element, span);
        if konst {
            quote_spanned! {span=> #literal.as_ptr().cast::<#elem>() }
        } else {
            quote_spanned! {span=> #literal.as_ptr().cast::<#elem>().cast_mut() }
        }
    }

    // -- literals -----------------------------------------------------------

    fn int_literal(&self, value: i128, ty: Ty, span: Span) -> Value {
        if ty.is_bool() {
            return Value::atom(bare_int_literal(value, ty, span));
        }
        if ty == Ty::UInt128 {
            // The constant is the bit pattern; the literal has to spell the
            // `u128` it stands for rather than the `i128` those bits read as.
            let literal = u128_literal_token(value as u128, span);
            let target = self.ty(ty, span);
            return Value::new(quote_spanned! {span=> #literal as #target }, prec::CAST)
                .type_end(true);
        }
        let literal = int_literal_token(value, span);
        let target = self.ty(ty, span);
        Value::new(quote_spanned! {span=> #literal as #target }, prec::CAST).type_end(true)
    }

    fn float_literal(&self, value: f64, ty: Ty, span: Span) -> Value {
        if !value.is_finite() {
            return Value::new(self.non_finite_literal(value, ty, span), prec::CAST).type_end(true);
        }
        let literal = float_literal_token(value, span);
        let target = self.ty(ty, span);
        Value::new(quote_spanned! {span=> #literal as #target }, prec::CAST).type_end(true)
    }

    /// An infinity or a NaN, which no Rust literal can spell.
    ///
    /// A NaN that is not the default quiet one — `__builtin_nan("0x123")`, and
    /// the negative NaN `-__builtin_nan("")` is — is written out bit for bit.
    /// `f64::NAN` is *one* NaN, and a payload and a sign are part of the value
    /// a program asked for; `as` between the two widths is free to lose both.
    fn non_finite_literal(&self, value: f64, ty: Ty, span: Span) -> TokenStream {
        let target = self.ty(ty, span);
        let f64_ty = primitive_ty("f64", span);
        if value.is_nan() {
            let bits = value.to_bits();
            if bits == f64::NAN.to_bits() {
                return quote_spanned! {span=> <#f64_ty>::NAN as #target };
            }
            if ty == Ty::Float {
                let f32_ty = primitive_ty("f32", span);
                let literal =
                    unsigned_hex_literal(u64::from(ir::narrow_nan_bits(bits)), "u32", span);
                return quote_spanned! {span=> <#f32_ty>::from_bits(#literal) };
            }
            let literal = unsigned_hex_literal(bits, "u64", span);
            return quote_spanned! {span=> <#f64_ty>::from_bits(#literal) as #target };
        }
        if value.is_sign_negative() {
            quote_spanned! {span=> -<#f64_ty>::INFINITY as #target }
        } else {
            quote_spanned! {span=> <#f64_ty>::INFINITY as #target }
        }
    }

    /// The all-bits-zero value of a type.
    fn zero_tokens(&self, ty: Ty, span: Span) -> TokenStream {
        match ty {
            // The zero of an `_Atomic T` is the zero of `T`: the object is
            // generated as a plain `T`, and initialising it is a plain write.
            Ty::Atomic(id) => self.zero_tokens(self.program.types.atomic_inner(id), span),
            _ if ty.is_complex() => {
                let zero = bare_float_literal(0.0, span);
                self.complex_new(ty, zero.clone(), zero, span)
                    .at(prec::LOWEST, span)
            }
            _ if ty.is_floating() => bare_float_literal(0.0, span),
            _ if ty.is_integer() => bare_int_literal(0, ty, span),
            // A variably modified array is generated as a pointer, and the
            // only thing that ever asks for its zero is the hoisting a [CFG
            // body](crate::cfg) does before the declaration is reached.
            Ty::Array(_) if self.program.types.is_vm(ty) => {
                let step = self.ty(self.program.types.vm_step_ty(ty), span);
                quote_spanned! {span=> ::core::ptr::null_mut::<#step>() }
            }
            Ty::Pointer(id) => {
                let pointer = self.program.types.pointer_type(id);
                if let Ty::Func(func) = pointer.pointee {
                    // The signature is written out rather than left to
                    // inference: a null function pointer is often the whole
                    // expression — `((void (*)(void))0)()` — and a bare
                    // `Option::None` there is `E0282`.
                    let signature = self.fn_ty(func, span);
                    return quote_spanned! {span=>
                        ::core::option::Option::<#signature>::None
                    };
                }
                let pointee = self.pointee_ty(pointer.pointee, span);
                if pointer.konst {
                    quote_spanned! {span=> ::core::ptr::null::<#pointee>() }
                } else {
                    quote_spanned! {span=> ::core::ptr::null_mut::<#pointee>() }
                }
            }
            other => {
                // A zeroed aggregate: `mem::zeroed` is a `const fn`, so this
                // works in a `static` initialiser as well as in a body.
                let target = self.ty(other, span);
                quote_spanned! {span=> ::core::mem::zeroed::<#target>() }
            }
        }
    }
}

/// The value a bit-field's initialiser folds to, if it folds at all.
///
/// Sema has already reduced a constant to a bare `Int` node, and the implicit
/// zero every unmentioned member gets is either that or [`ExprKind::Zeroed`].
fn constant_bits(expr: &Expr) -> Option<i128> {
    match &expr.kind {
        ExprKind::Int(value) => Some(*value),
        ExprKind::Zeroed if expr.ty.is_integer() => Some(0),
        _ => None,
    }
}

/// Writes the low `width` bits of `value` into a run's storage bytes.
fn pack_bits(storage: &mut [u8], bits: &ir::BitField, value: i128) {
    let start = bits.offset_in_storage();
    for bit in 0..u64::from(bits.width) {
        if (value as u128) >> bit & 1 == 0 {
            continue;
        }
        let at = start + bit;
        if let Some(byte) = storage.get_mut((at / 8) as usize) {
            *byte |= 1 << (at % 8);
        }
    }
}

/// `[0x1f, 0x00, …]`, the initialiser of a storage field.
fn byte_array(bytes: &[u8], span: Span) -> TokenStream {
    if bytes.iter().all(|byte| *byte == 0) {
        let len = usize_literal(bytes.len() as u64, span);
        return bracketed(quote_spanned! {span=> 0; #len }, span);
    }
    let mut items = TokenStream::new();
    for byte in bytes {
        let value = hex_literal(u64::from(*byte), span);
        items.extend(quote_spanned! {span=> #value, });
    }
    bracketed(items, span)
}

/// Whether a place ultimately names an object with static storage duration.
///
/// The bit-field accessors borrow, and edition 2024 refuses a reference to a
/// `static mut`; a place rooted in one is reached through `&raw mut` instead.
/// Anything behind a pointer is already a raw dereference, so it needs nothing
/// — a thread-local object included, since its place is the dereference of the
/// pointer out of its cell.
///
/// It is also what says whether an initialiser refers to an item, which a Rust
/// `const` may not; see [`Codegen::const_initialisable`].
fn rooted_in_static(place: &Place, program: &Program) -> bool {
    let mut place = place;
    loop {
        match &place.kind {
            PlaceKind::Object(id) => {
                let storage = &program.object(*id).storage;
                return !matches!(storage, Storage::Automatic) && !storage.is_thread_local();
            }
            PlaceKind::Field { base, .. } => place = base,
            _ => return false,
        }
    }
}

/// The Rust unsigned integer of a given width, which is what the bit-counting
/// builtins are defined on.
fn unsigned_rust_ty(width: u32, span: Span) -> TokenStream {
    primitive_ty(
        match width {
            0..=8 => "u8",
            9..=16 => "u16",
            17..=32 => "u32",
            _ => "u64",
        },
        span,
    )
}

/// The signed counterpart, for `__builtin_clrsb`.
fn signed_rust_ty(width: u32, span: Span) -> TokenStream {
    primitive_ty(
        match width {
            0..=8 => "i8",
            9..=16 => "i16",
            17..=32 => "i32",
            _ => "i64",
        },
        span,
    )
}

/// A mask of `width` low bits, inside a word of `word_bits`.
fn mask_of(width: u32, word_bits: u32) -> u128 {
    let width = width.min(word_bits);
    if width >= 128 {
        u128::MAX
    } else {
        (1u128 << width) - 1
    }
}

/// A mask literal, written in hexadecimal at the width of its word.
///
/// A `u128` mask carries its suffix: a bare hexadecimal literal above
/// `u64::MAX` would be out of range for whatever `u64` the surrounding
/// annotation asked for, and the annotation is what makes the narrow case
/// readable.
fn word_literal(value: u128, word_bits: u32, span: Span) -> TokenStream {
    if word_bits <= 64 {
        return hex_literal(value as u64, span);
    }
    let mut literal = Literal::from_str(&format!("0x{value:x}u128"))
        .unwrap_or_else(|_| Literal::u128_suffixed(value));
    literal.set_span(span);
    TokenStream::from(TokenTree::Literal(literal))
}

/// A `u64` literal written in hexadecimal, which is how a mask reads.
fn hex_literal(value: u64, span: Span) -> TokenStream {
    let mut literal = Literal::from_str(&format!("0x{value:x}"))
        .unwrap_or_else(|_| Literal::u64_unsuffixed(value));
    literal.set_span(span);
    TokenStream::from(TokenTree::Literal(literal))
}

/// The variable that says which entry of an irreducible region control is
/// going to, in this crate's own hygiene.
///
/// One per region rather than one per function; see [`crate::reloop`].
fn entry_ident(state: u32) -> Ident {
    Ident::new(&format!("__cinrs_entry{state}"), Span::mixed_site())
}

/// The store that says which entry of a dispatch is meant, or nothing at all
/// when the jump has only one place to arrive.
fn entry_assignment(exit: &reloop::Exit, index: usize, span: Span) -> TokenStream {
    let Some(state) = exit.state else {
        return TokenStream::new();
    };
    let name = entry_ident(state);
    let value = state_literal(index, span);
    quote_spanned! {span=> #name = #value; }
}

/// A state number, which is a `u32` because the state variable is.
/// The `default` of a computed `goto`'s dispatch: a value that is no label's
/// number, which is undefined behaviour in C.
///
/// A build with debug assertions — the user's, since `cfg!` is expanded in
/// their crate — panics, which catches the bug; any other build assumes it
/// away, as GCC does, which is what lets the `match` be a bare jump table.
fn invalid_target(span: Span) -> TokenStream {
    quote_spanned! {span=>
        if ::core::cfg!(debug_assertions) {
            ::core::unreachable!()
        } else {
            unsafe { ::core::hint::unreachable_unchecked() }
        }
    }
}

fn state_literal(value: usize, span: Span) -> TokenStream {
    let mut literal = Literal::u32_unsuffixed(value as u32);
    literal.set_span(span);
    TokenStream::from(TokenTree::Literal(literal))
}

/// Groups a switch's cases by the block they enter, keeping source order.
///
/// `case 0: case 1:` reaches the same block through two labels, and one arm
/// with an or-pattern is how that should read.
fn group_cases(cases: &[(ir::CaseRange, BlockId)]) -> Vec<(BlockId, Vec<ir::CaseRange>)> {
    let mut out: Vec<(BlockId, Vec<ir::CaseRange>)> = Vec::new();
    for (value, target) in cases {
        match out.iter_mut().find(|(block, _)| block == target) {
            Some((_, values)) => values.push(*value),
            None => out.push((*target, vec![*value])),
        }
    }
    out
}

/// The Rust pattern one `case` label matches: a literal, or a range.
fn case_pattern(value: ir::CaseRange, ty: Ty, span: Span) -> TokenStream {
    let low = bare_int_literal(value.low, ty, span);
    if value.is_single() {
        return low;
    }
    let high = bare_int_literal(value.high, ty, span);
    quote_spanned! {span=> #low ..= #high }
}

/// The precedence of the tokens [`Codegen::zero_tokens`] produces.
fn zero_prec(ty: Ty) -> u8 {
    if ty.is_arithmetic() {
        prec::ATOM
    } else {
        prec::CALL
    }
}

/// `#[link_name = "…"]`, which points a renamed declaration back at its symbol.
fn link_name(symbol: &str, span: Span) -> TokenStream {
    let mut literal = Literal::string(symbol);
    literal.set_span(span);
    quote_spanned! {span=> #[link_name = #literal] }
}

/// The attribute that gives a definition the C symbol `symbol`, for a unit
/// that asked for `#pragma cinrs export`.
///
/// `#[unsafe(no_mangle)]` is the edition-2024 spelling and is accepted in every
/// edition since 1.82, so one expansion works wherever it is written. It says
/// "the name of the item is the symbol", which is not quite always true here:
/// a C name that is a Rust keyword becomes `r#match`, and the five names that
/// cannot even be raw grow an underscore, so those go through `export_name`
/// instead and say the symbol outright.
fn export_attr(symbol: &str, item: &Ident, span: Span) -> TokenStream {
    if item.to_string().trim_start_matches("r#") == symbol {
        return quote_spanned! {span=> #[unsafe(no_mangle)] };
    }
    let mut literal = Literal::string(symbol);
    literal.set_span(span);
    quote_spanned! {span=> #[unsafe(export_name = #literal)] }
}

/// Whether a static initialiser has to be wrapped in `unsafe`.
///
/// `types` is the arena, because one of the answers depends on it: a cast to
/// or from a function pointer is written out as a `transmute`, and that is an
/// unsafe call wherever it stands. `frob f[] = { abort };` with `typedef void
/// (*frob)();` is the shape — `execute/921110-1`.
fn needs_unsafe(types: &ir::Types, expr: &Expr) -> bool {
    let recurse = |inner| needs_unsafe(types, inner);
    match &expr.kind {
        // `mem::zeroed` is unsafe, and so is naming a `static mut`.
        ExprKind::Zeroed => !expr.ty.is_scalar(),
        // A string literal's address is safe to take; anything else with static
        // storage duration is a `static mut`.
        ExprKind::AddrOf(place) => !matches!(place.kind, PlaceKind::Str(_)),
        ExprKind::Cast(inner) => {
            let transmuted = types.is_func_pointer(expr.ty) || types.is_func_pointer(inner.ty);
            transmuted || recurse(inner)
        }
        // `<*mut T>::offset` is an unsafe call however safe its operand is:
        // `static const char *p = "foo" + 1;` — `execute/pr53084` — is the
        // address of a string literal, which is safe to take, plus one.
        ExprKind::PtrOffset { .. } => true,
        ExprKind::ComplexOf { re, im } => recurse(re) || recurse(im),
        ExprKind::RecordLit { fields, .. } => fields.iter().any(recurse),
        ExprKind::UnionLit { value, .. } => recurse(value),
        ExprKind::ArrayLit(items) => items.iter().any(recurse),
        ExprKind::ArrayRepeat { value, .. } => recurse(value),
        _ => false,
    }
}

/// The bytes of a narrow string literal, with its terminating NUL.
fn nul_terminated(values: &[u32]) -> Vec<u8> {
    let mut bytes: Vec<u8> = values.iter().map(|v| *v as u8).collect();
    bytes.push(0);
    bytes
}

// ---------------------------------------------------------------------------
// literal tokens
// ---------------------------------------------------------------------------

/// The largest magnitude an unsuffixed literal is safe to have: Rust infers
/// `i32` for a literal with no other constraint.
const UNSUFFIXED_LIMIT: i128 = i32::MAX as i128;

/// A `u128` literal, always suffixed: nothing else can spell a value above
/// `i128::MAX`.
fn u128_literal_token(value: u128, span: Span) -> TokenStream {
    let mut literal = Literal::u128_suffixed(value);
    literal.set_span(span);
    TokenStream::from(TokenTree::Literal(literal))
}

/// A literal for `value`, with a Rust suffix only when inference needs one.
fn int_literal_token(value: i128, span: Span) -> TokenStream {
    if value == i128::MIN {
        // `-(2^127)` has no positive magnitude an `i128` can hold. Rust reads
        // the negation of the out-of-range literal as exactly this value,
        // which is how `i128::MIN` is written in Rust source too.
        let mut literal = Literal::from_str("170141183460469231731687303715884105728i128")
            .expect("a decimal literal followed by a suffix is a token");
        literal.set_span(span);
        return quote_spanned! {span=> -#literal };
    }
    let magnitude = value.unsigned_abs();
    let mut literal = if magnitude <= UNSUFFIXED_LIMIT as u128 {
        Literal::u128_unsuffixed(magnitude)
    } else if value >= 0 {
        if magnitude <= u32::MAX as u128 {
            Literal::u32_suffixed(magnitude as u32)
        } else if magnitude <= u64::MAX as u128 {
            Literal::u64_suffixed(magnitude as u64)
        } else {
            Literal::u128_suffixed(magnitude)
        }
    } else if magnitude <= i64::MAX as u128 {
        Literal::i64_suffixed(magnitude as i64)
    } else {
        Literal::i128_suffixed(magnitude as i128)
    };
    literal.set_span(span);
    if value < 0 {
        quote_spanned! {span=> -#literal }
    } else {
        TokenStream::from(TokenTree::Literal(literal))
    }
}

/// A literal for `value` with no suffix at all, for a context that already
/// fixes its type.
fn bare_int_literal(value: i128, ty: Ty, span: Span) -> TokenStream {
    if ty.is_bool() {
        let ident = Ident::new(if value != 0 { "true" } else { "false" }, span);
        return quote_spanned! {span=> #ident };
    }
    if ty == Ty::UInt128 {
        // The value is carried as a bit pattern; `-1` in a `u128` context is
        // not what the constant means.
        let mut literal = Literal::u128_unsuffixed(value as u128);
        literal.set_span(span);
        return TokenStream::from(TokenTree::Literal(literal));
    }
    let mut literal = Literal::u128_unsuffixed(value.unsigned_abs());
    literal.set_span(span);
    if value < 0 {
        quote_spanned! {span=> -#literal }
    } else {
        TokenStream::from(TokenTree::Literal(literal))
    }
}

/// The integer a bit-field's bytes are gathered into: `u64`/`i64` for a
/// window of 64 bits and the 128-bit primitives for a wider one.
///
/// All four take the [`core::primitive`] path, `typedef unsigned long long
/// u64;` being every bit as ordinary in C as `typedef unsigned __int128 u128;`.
fn window_ty(word_bits: u32, signed: bool, span: Span) -> TokenStream {
    match (word_bits, signed) {
        (128, false) => primitive_ty("u128", span),
        (128, true) => primitive_ty("i128", span),
        (_, false) => primitive_ty("u64", span),
        (_, true) => primitive_ty("i64", span),
    }
}

/// `::core::primitive::u64` and every other primitive this code generator
/// writes.
///
/// **Every** bare primitive name goes through here, and none is ever written
/// as a bare identifier, because each of them is a name a C `typedef` can
/// take. `typedef _Bool bool;` is in the C23 compatibility header of half the
/// world's C — and in gcc.c-torture's `execute/20030714-1` — while `typedef
/// unsigned int u32;`, `typedef unsigned long usize;` and `typedef long long
/// i64;` are how a great deal of embedded C spells its types. The generated
/// item is then `pub type bool = bool;`, which is a cycle (`E0391`), and even
/// where it is not a cycle the alias shadows the primitive for the rest of the
/// module — so the padding of a `struct`, a bit-field accessor's window and a
/// pointer difference would all silently take the C type instead.
///
/// The [`core::primitive`] module exists for exactly this, and the leading
/// `::core` keeps it working in a crate that has renamed its own `core`.
fn primitive_ty(name: &str, span: Span) -> TokenStream {
    let ident = Ident::new(name, span);
    quote_spanned! {span=> ::core::primitive::#ident }
}

/// Stamps every token of a stream with one span.
///
/// Only the crate path `#pragma cinrs crate` gives needs it: everything else
/// the generator emits is built token by token from a span it already has,
/// while that one is *parsed* out of a string and so arrives with call-site
/// spans that would send `rustc`'s complaint about a bad path to the wrong
/// place.
fn respan(tokens: TokenStream, span: Span) -> TokenStream {
    tokens
        .into_iter()
        .map(|tree| {
            let mut tree = match tree {
                TokenTree::Group(group) => {
                    TokenTree::Group(Group::new(group.delimiter(), respan(group.stream(), span)))
                }
                other => other,
            };
            tree.set_span(span);
            tree
        })
        .collect()
}

/// A string literal for an `assert!` message, which a `const` context needs to
/// be a literal rather than anything formatted.
fn message_literal(text: &str, span: Span) -> TokenStream {
    let mut literal = Literal::string(text);
    literal.set_span(span);
    TokenStream::from(TokenTree::Literal(literal))
}

/// An array length, which Rust counts in `usize`.
fn usize_literal(value: u64, span: Span) -> TokenStream {
    let mut literal = Literal::usize_unsuffixed(value as usize);
    literal.set_span(span);
    TokenStream::from(TokenTree::Literal(literal))
}

/// An integer literal with the Rust suffix `ty` names: `3i32`, `7u32`.
///
/// It is what an x86 intrinsic's immediate operand becomes, where the type of
/// `core::arch`'s `const` parameter has to be written out because a const
/// argument is not inferred from the parameter.
/// The head of an `asm!` operand, up to its expression: `o0 = lateout(reg)`,
/// `inout("eax")`, `o2 = const`.
fn asm_operand_head(operand: &ir::AsmOperand, span: Span) -> TokenStream {
    let dir = match &operand.kind {
        ir::AsmOperandKind::In(_) => "in",
        ir::AsmOperandKind::Out { late: true, .. } => "lateout",
        ir::AsmOperandKind::Out { late: false, .. } => "out",
        ir::AsmOperandKind::InOut { .. } | ir::AsmOperandKind::Scratch(_) => "inout",
        ir::AsmOperandKind::Const(_) => "const",
    };
    let dir = Ident::new(dir, span);
    let reg = match (&operand.kind, operand.reg) {
        (ir::AsmOperandKind::Const(_), _) => TokenStream::new(),
        (_, ir::AsmReg::Class(class)) => {
            let class = Ident::new(class, span);
            quote_spanned! {span=> (#class) }
        }
        (_, ir::AsmReg::Explicit(name)) => {
            let mut name = Literal::string(name);
            name.set_span(span);
            quote_spanned! {span=> (#name) }
        }
    };
    match &operand.name {
        Some(name) => {
            let name = Ident::new(name, span);
            quote_spanned! {span=> #name = #dir #reg }
        }
        None => quote_spanned! {span=> #dir #reg },
    }
}

/// The value of an `asm!` `const` operand: an `i64` literal, or a `u64` one
/// for a value only that holds, so that no inference default narrows it.
fn asm_const_literal(value: i128, span: Span) -> TokenStream {
    let mut literal = if i64::try_from(value).is_ok() {
        Literal::i64_suffixed(value as i64)
    } else {
        Literal::u64_suffixed(value as u64)
    };
    literal.set_span(span);
    TokenStream::from(TokenTree::Literal(literal))
}

fn suffixed_int_literal(value: i128, ty: &str, span: Span) -> TokenStream {
    let mut literal = match ty {
        // An unsigned `const` parameter takes the bit pattern the C constant
        // was reduced to, which is what a mask written as `-1` means.
        "u32" => Literal::u32_suffixed(value as u32),
        "u64" => Literal::u64_suffixed(value as u64),
        "i64" => Literal::i64_suffixed(value as i64),
        // `i32` is what every immediate operand in `core::arch` is.
        _ => Literal::i32_suffixed(value as i32),
    };
    literal.set_span(span);
    TokenStream::from(TokenTree::Literal(literal))
}

/// The Rust floating type of a C floating type, and the mask that clears the
/// sign bit of its bit pattern.
///
/// `long double` is `double` here, so only the two widths exist.
fn float_bit_ty(ty: Ty, span: Span) -> (TokenStream, TokenStream) {
    if ty == Ty::Float {
        return (
            primitive_ty("f32", span),
            unsigned_hex_literal(0x7fff_ffff, "u32", span),
        );
    }
    (
        primitive_ty("f64", span),
        unsigned_hex_literal(0x7fff_ffff_ffff_ffff, "u64", span),
    )
}

/// The quiet bit of a floating type: the leading bit of the mantissa, which is
/// set in a quiet NaN and clear in a signalling one.
fn quiet_bit_literal(ty: Ty, span: Span) -> TokenStream {
    if ty == Ty::Float {
        return unsigned_hex_literal(1 << 22, "u32", span);
    }
    unsigned_hex_literal(1 << 51, "u64", span)
}

/// A hexadecimal literal with an explicit unsigned suffix.
fn unsigned_hex_literal(value: u64, suffix: &str, span: Span) -> TokenStream {
    let mut literal = Literal::from_str(&format!("0x{value:x}{suffix}"))
        .expect("a hexadecimal literal followed by a suffix is a token");
    literal.set_span(span);
    TokenStream::from(TokenTree::Literal(literal))
}

fn float_literal_token(value: f64, span: Span) -> TokenStream {
    let mut literal = Literal::f64_unsuffixed(value.abs());
    literal.set_span(span);
    if value.is_sign_negative() {
        quote_spanned! {span=> -#literal }
    } else {
        TokenStream::from(TokenTree::Literal(literal))
    }
}

fn bare_float_literal(value: f64, span: Span) -> TokenStream {
    float_literal_token(value, span)
}

/// The constant an expression is, if it is one.
///
/// Sema folds conversions of constants, so a constant is always a bare `Int`
/// or `Float` node rather than a cast wrapping one.
/// The operands of a chain of comma operators, in source order.
///
/// `a, b, c` is `Comma(Comma(a, b), c)`, so this walks down the left spine
/// into a vector and hands it back the right way round. Iterating rather than
/// recursing is what lets a logical source line hold the 4095 characters C23
/// 5.2.5.2p1 asks for; see [`Codegen::binary_chain`].
fn comma_operands(expr: &Expr) -> Vec<&Expr> {
    let mut out = Vec::new();
    let mut node = expr;
    while let ExprKind::Comma { lhs, rhs } = &node.kind {
        out.push(&**rhs);
        node = lhs;
    }
    out.push(node);
    out.reverse();
    out
}

fn constant_of(expr: &Expr) -> Option<ConstValue> {
    match &expr.kind {
        ExprKind::Int(value) => Some(ConstValue::Int(*value)),
        ExprKind::Float(value) if value.is_finite() => Some(ConstValue::Float(*value)),
        _ => None,
    }
}

fn cmp_tokens(op: CmpOp, span: Span) -> TokenStream {
    match op {
        CmpOp::Lt => quote_spanned! {span=> < },
        CmpOp::Gt => quote_spanned! {span=> > },
        CmpOp::Le => quote_spanned! {span=> <= },
        CmpOp::Ge => quote_spanned! {span=> >= },
        CmpOp::Eq => quote_spanned! {span=> == },
        CmpOp::Ne => quote_spanned! {span=> != },
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    /// What a unit spelling `names` gives each of them, as `(C, Rust)` pairs
    /// of everything that does not keep its own name.
    fn spellings(names: &[&str]) -> Vec<(String, String)> {
        let mut pairs: Vec<(String, String)> = unique_spellings(names.iter().copied())
            .into_iter()
            .collect();
        pairs.sort();
        pairs
    }

    /// Asserts that `names` are spelled as `expected` says, and that the unit
    /// has no two names with one spelling.
    fn assert_spellings(names: &[&str], expected: &[(&str, &str)]) {
        let renamed = unique_spellings(names.iter().copied());
        let want: Vec<(String, String)> = expected
            .iter()
            .map(|(c, rust)| ((*c).to_owned(), (*rust).to_owned()))
            .collect();
        assert_eq!(spellings(names), want);
        let mut seen: HashMap<String, &str> = HashMap::new();
        for name in names {
            let spelling = match renamed.get(*name) {
                Some(unique) => unique.clone(),
                None => plain_spelling(name),
            };
            if let Some(other) = seen.insert(spelling.clone(), name) {
                assert_eq!(
                    other, *name,
                    "{other} and {name} are both spelled {spelling}"
                );
            }
        }
    }

    #[test]
    fn a_name_rust_can_spell_keeps_it() {
        // Nothing at all is renamed in the program that has no collision,
        // keywords included: `match` is `r#match`, which is a token of its
        // own.
        assert_spellings(&["counter", "match", "loop", "café"], &[]);
    }

    #[test]
    fn the_five_names_that_cannot_be_raw_grow_an_underscore() {
        assert_spellings(
            &["self", "Self", "super", "crate", "_"],
            &[
                ("Self", "Self_"),
                ("_", "__"),
                ("crate", "crate_"),
                ("self", "self_"),
                ("super", "super_"),
            ],
        );
    }

    #[test]
    fn a_dollar_is_spelled_out() {
        assert_spellings(
            &["a$b", "$", "x$"],
            &[
                ("$", "_dollar_"),
                ("a$b", "a_dollar_b"),
                ("x$", "x_dollar_"),
            ],
        );
    }

    #[test]
    fn a_spelling_the_program_already_uses_grows_another_underscore() {
        // The collision the whole mechanism exists for: `self` cannot be
        // `self_`, because the program has a `self_` of its own.
        assert_spellings(&["self", "self_"], &[("self", "self__")]);
        assert_spellings(&["self", "self_", "self__"], &[("self", "self___")]);
        assert_spellings(&["a$b", "a_dollar_b"], &[("a$b", "a_dollar_b_")]);
    }

    #[test]
    fn the_answer_does_not_depend_on_the_order_the_names_arrive_in() {
        let forwards = spellings(&["self", "self_", "crate", "crate_", "a$b", "a_dollar_b"]);
        let backwards = spellings(&["a_dollar_b", "a$b", "crate_", "crate", "self_", "self"]);
        assert_eq!(forwards, backwards);
    }

    #[test]
    fn a_name_is_spelled_the_same_way_however_often_it_is_collected() {
        // Every name space is collected into one list, so a tag, a member and
        // a local all spelled `self` arrive several times over.
        assert_spellings(&["self", "self", "self_", "self"], &[("self", "self__")]);
    }
}