syn-canon 0.5.2

//! Original module environments and borrowed function declarations.

use alloc::boxed::Box;
use alloc::collections::btree_map::Entry;
use alloc::collections::{BTreeMap, BTreeSet};
use alloc::string::String;
use alloc::string::ToString;
use alloc::vec;
use alloc::vec::Vec;

use core::cell::Cell;

use quote::ToTokens;
use syn::visit::Visit;

use crate::alpha;
use crate::form::{self, OWNER_PARAMETER};
use crate::scope::{Binding, BindingId, Frame, Ns, Origin, PrimTy, ident_name, unraw};

/// The original owner of a function.
#[derive(Clone, PartialEq, Eq, Debug)]
pub enum Owner {
    /// A free function.
    Free,
    /// A method of an inherent impl.
    Inherent(String),
    /// A default body of a trait.
    Trait(String),
}

impl Owner {
    /// The resolved owner name or an injective encoding of its type.
    fn self_name(&self, seed: &Frame) -> Option<String> {
        let text = match self {
            Self::Free => return None,
            Self::Inherent(text) | Self::Trait(text) => text,
        };
        if let Some(binding) = seed.lookup(Ns::Type, unraw(text)) {
            return Some(binding.canon.clone());
        }
        Some(mangle_owner(text))
    }
}

/// A valid-identifier spelling of `text`, injective over its bytes.
fn mangle_owner(text: &str) -> String {
    const HEX: [char; 16] = [
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f',
    ];
    let mut out = String::from("__dejadoc_self");
    for byte in text.bytes() {
        out.push(HEX[usize::from(byte >> 4)]);
        out.push(HEX[usize::from(byte & 0x0f)]);
    }
    out
}

/// The private id of an indexed module, file-based or inline.
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
struct ModuleId(usize);

/// One indexed module: its seed frame and its own glob uncertainty.
#[expect(
    clippy::struct_excessive_bools,
    reason = "primitive, derive, observer, helper and prelude facts of a module are independent"
)]
struct Module {
    seed: Frame,
    prim_fallback: bool,
    /// Every std derive, formatting macro and crate root the module sees names the std item.
    std_names: bool,
    observed: bool,
    helper_blocked: bool,
    /// No attribute on the module, an ancestor or the crate root removes the standard prelude.
    prelude: bool,
    /// Every `use` of the module imports from `std`, `core` or `alloc`.
    standard_imports: bool,
}

/// One original declaration and its module environment.
#[expect(
    clippy::struct_excessive_bools,
    reason = "primitive, observer and helper facts of a declaration are independent"
)]
struct Decl<'a> {
    module: ModuleId,
    /// The original declaration attributes.
    attrs: &'a [syn::Attribute],
    owner: Owner,
    prim_fallback: bool,
    observed: bool,
    /// An enclosing inline module, impl or trait carries an attribute macro reading the function.
    enclosing_observed: bool,
    helper_module: Vec<String>,
    helper_blocked: bool,
    /// The original enclosing impl or trait generics.
    generics: Option<*const syn::Generics>,
}

/// The source context, one entry per module file and inline module.
pub struct SourceContext<'a> {
    modules: Vec<Module>,
    decls: BTreeMap<(*const syn::Signature, *const syn::Block), Decl<'a>>,
    /// Generic frames indexed by their original declarations.
    gen_frames: BTreeMap<*const syn::Generics, Frame>,
    /// Complete helper summaries shared by every borrowed declaration.
    helpers: crate::helpers::HelperTable,
    /// No trait of the crate declares a method named `to_string` or `to_owned`.
    owned_methods: bool,
    /// No trait of the crate declares a method named `clone`.
    clone_method: bool,
    /// No trait of the crate declares a collection method the collection readings rely on.
    collection_method: bool,
}

/// A function view selected from a source context.
pub struct FunctionView<'a> {
    ctx: &'a SourceContext<'a>,
    entry: &'a Decl<'a>,
    sig: &'a syn::Signature,
    block: &'a syn::Block,
}

impl FunctionView<'_> {
    /// The original declaration's canonical form in its module environment.
    #[must_use]
    pub fn canonicalize(&self) -> crate::CanonicalForm {
        let seed = self.ctx.seed(self.entry.module);
        let generics = self.entry.generics.map(|key| self.ctx.gen_frame(key));
        let self_owner = self.entry.owner.self_name(seed);
        let mut sig = self.sig.clone();
        if !matches!(self.entry.owner, Owner::Free) {
            sig.ident = syn::Ident::new("__dejadoc_method", sig.ident.span());
        }
        let mut item = syn::ItemFn {
            attrs: self
                .entry
                .attrs
                .iter()
                .filter(|attribute| live(&attribute.meta))
                .cloned()
                .collect(),
            vis: syn::Visibility::Inherited,
            modifiers: syn::FnModifiers::default(),
            sig,
            block: Box::new(self.block.clone()),
        };
        let helpers = transfer_helpers(&self.ctx.helpers, self.sig, self.block, &mut item);
        let file = syn::File {
            shebang: None,
            frontmatter: None,
            attrs: Vec::new(),
            items: vec![syn::Item::Fn(item)],
        };
        form::canonical_file_with_options(
            file,
            true,
            alpha::Options {
                seed: Some(seed),
                generics,
                prim_fallback: self.entry.prim_fallback,
                observed: self.entry.observed,
                enclosing_observed: self.entry.enclosing_observed,
                std_names: self.ctx.modules[self.entry.module.0].std_names,
                prelude: self.ctx.modules[self.entry.module.0].prelude,
                owned_methods: self.ctx.owned_methods,
                clone_free: self.ctx.modules[self.entry.module.0].prelude
                    && self.ctx.modules[self.entry.module.0].standard_imports
                    && self.ctx.clone_method,
                collection_free: self.ctx.modules[self.entry.module.0].prelude
                    && self.ctx.modules[self.entry.module.0].standard_imports
                    && self.ctx.collection_method,
                self_owner,
                helper_table: Some(&self.ctx.helpers),
                helper_module: &self.entry.helper_module,
                helper_blocked: self.entry.helper_blocked,
                helper_syntax: Some(helpers),
            },
        )
    }
}

fn transfer_helpers(
    helpers: &crate::helpers::HelperTable,
    signature: &syn::Signature,
    body: &syn::Block,
    cloned: &mut syn::ItemFn,
) -> crate::helpers::HelperSyntax {
    crate::helpers::HelperSyntax::cloned_function(helpers, signature, body, cloned)
}

impl<'a> SourceContext<'a> {
    /// The shared seed of the module a declaration was indexed under.
    fn seed(&self, module: ModuleId) -> &Frame {
        &self.modules[module.0].seed
    }

    /// The shared frame of an impl or trait's generics.
    fn gen_frame(&self, key: *const syn::Generics) -> &Frame {
        &self.gen_frames[&key]
    }

    /// Index every module's top-level seed and every function it declares,
    /// inline modules under their own path, seed, and glob uncertainty.
    pub fn new<I>(files: I) -> Self
    where
        I: IntoIterator<Item = (&'a [String], &'a syn::File)>,
    {
        let files: Vec<(&'a [String], &'a syn::File)> = files.into_iter().collect();
        let mut scopes: ModuleMap<'a> = BTreeMap::new();
        for (path, file) in &files {
            declare_module(&mut scopes, (*path).to_vec(), ModuleScope::File(file));
        }
        for (path, file) in &files {
            register_inline(&mut scopes, path, &file.items);
        }
        let ancestry = module_ancestry(&scopes);
        // A `macro_rules!` definition reaches past its module, and a `#[macro_use] extern crate`
        // reaches every module.
        let crate_rebound = files
            .iter()
            .any(|(_, file)| crate::drift::file_rebound(file) != 0);
        let blocked: BTreeSet<Vec<String>> = ancestry
            .iter()
            .filter(|(_, flags)| flags.blocked)
            .map(|(path, _)| path.clone())
            .collect();
        let mut module_ids: ModuleIds = BTreeMap::new();
        let mut modules: Vec<Module> = Vec::new();
        for ((path, declarations), ancestry) in scopes.iter().zip(ancestry.values()) {
            let ids = declarations
                .iter()
                .map(|scope| {
                    let prim_fallback = scope_primitives_known(scope.items());
                    let id = ModuleId(modules.len());
                    modules.push(Module {
                        seed: seed_frame(&scopes, path, &scope, prim_fallback),
                        prim_fallback,
                        std_names: !crate_rebound && std_names_kept(&scopes, path, &scope),
                        observed: ancestry.observed,
                        helper_blocked: ancestry.blocked,
                        prelude: !ancestry.bare,
                        standard_imports: standard_imports(scope.items()),
                    });
                    (scope.key(), id)
                })
                .collect();
            module_ids.insert(path.clone(), ids);
        }
        let helpers = crate::helpers::module_table(&crate::helpers::Scopes::new(scopes, blocked));
        for module in &mut modules {
            for binding in module.seed.bindings_mut(Ns::Value).values_mut() {
                binding.helper = helpers.by_canon(&binding.canon);
            }
        }
        let mut decls: BTreeMap<(*const syn::Signature, *const syn::Block), Decl<'a>> =
            BTreeMap::new();
        let mut gen_frames: BTreeMap<*const syn::Generics, Frame> = BTreeMap::new();
        for (path, file) in &files {
            index_scope(
                &mut decls,
                &module_ids,
                &modules,
                path,
                ModuleScope::File(file),
                false,
                false,
                false,
                &mut gen_frames,
            );
        }
        let owned_methods = !files
            .iter()
            .any(|(_, file)| declares_trait_method(&file.items, &["to_string", "to_owned"]));
        let clone_method = !files
            .iter()
            .any(|(_, file)| declares_trait_method(&file.items, &["clone"]));
        let collection_method = !files
            .iter()
            .any(|(_, file)| declares_trait_method(&file.items, &alpha::COLLECTION_METHODS));
        Self {
            modules,
            decls,
            gen_frames,
            helpers,
            owned_methods,
            clone_method,
            collection_method,
        }
    }

    /// The view of the exact declaration its signature and block pointers
    /// index, `None` when the pair does not select one.
    #[must_use]
    pub fn function<'s>(
        &'s self,
        sig: &'s syn::Signature,
        block: &'s syn::Block,
    ) -> Option<FunctionView<'s>> {
        let key = (
            core::ptr::from_ref::<syn::Signature>(sig),
            core::ptr::from_ref::<syn::Block>(block),
        );
        let entry = self.decls.get(&key)?;
        Some(FunctionView {
            ctx: self,
            entry,
            sig,
            block,
        })
    }
}

/// Whether a module imports no wildcard names.
fn glob_free(items: &[syn::Item]) -> bool {
    !items
        .iter()
        .any(|item| matches!(item, syn::Item::Use(use_item) if has_glob(&use_item.tree)))
}

pub(crate) fn scope_primitives_known(items: &[syn::Item]) -> bool {
    let mut uncertainty = ScopeUncertainty(false);
    for item in items {
        uncertainty.visit_item(item);
        if uncertainty.0 {
            return false;
        }
    }
    glob_free(items)
}

struct ScopeUncertainty(bool);

impl<'ast> Visit<'ast> for ScopeUncertainty {
    fn visit_attribute(&mut self, attr: &'ast syn::Attribute) {
        self.0 |=
            live(&attr.meta) || attr.path().is_ident("cfg") || attr.path().is_ident("cfg_attr");
    }

    fn visit_item_mod(&mut self, item: &'ast syn::ItemMod) {
        for attr in &item.attrs {
            self.visit_attribute(attr);
        }
    }

    fn visit_item_macro(&mut self, item: &'ast syn::ItemMacro) {
        self.0 |= item.ident.is_none();
        syn::visit::visit_item_macro(self, item);
    }

    fn visit_item_extern_crate(&mut self, _: &'ast syn::ItemExternCrate) {
        self.0 = true;
    }

    fn visit_block(&mut self, _: &'ast syn::Block) {}
}

pub(crate) fn has_glob(tree: &syn::UseTree) -> bool {
    match tree {
        syn::UseTree::Glob(_) => true,
        syn::UseTree::Path(path) => has_glob(&path.tree),
        syn::UseTree::Group(group) => group.items.iter().any(has_glob),
        _ => false,
    }
}

/// A module's items, inline content included.
#[derive(Clone, Copy)]
pub(crate) enum ModuleScope<'a> {
    File(&'a syn::File),
    Inline(&'a syn::ItemMod),
}

impl<'a> ModuleScope<'a> {
    pub(crate) fn items(&self) -> &'a [syn::Item] {
        match self {
            Self::File(file) => &file.items,
            Self::Inline(module) => module.content.as_ref().map_or(&[], |(_, items)| items),
        }
    }

    /// The original declaration's address.
    fn key(&self) -> *const () {
        match self {
            Self::File(file) => core::ptr::from_ref(*file).cast(),
            Self::Inline(module) => core::ptr::from_ref(*module).cast(),
        }
    }

    fn shadows_primitive_crate(&self, name: &str) -> bool {
        let raw = if name == "core" { "r#core" } else { "r#std" };
        let matches = |ident: &syn::Ident| ident == name || ident == raw;
        self.items().iter().any(|item| {
            alpha::type_item_ident(item).is_some_and(matches)
                || matches!(item, syn::Item::Use(import)
                    if alpha::imported_name_matches(&import.tree, matches))
        })
    }
}

/// Every declaration of one module path, in registration order.
#[derive(Clone)]
pub(crate) struct Declarations<'a> {
    first: ModuleScope<'a>,
    rest: Vec<ModuleScope<'a>>,
}

impl<'a> Declarations<'a> {
    pub(crate) fn iter(&self) -> impl Iterator<Item = ModuleScope<'a>> + '_ {
        core::iter::once(self.first).chain(self.rest.iter().copied())
    }

    pub(crate) fn repeated(&self) -> bool {
        !self.rest.is_empty()
    }

    fn primitives_known(&self) -> bool {
        self.iter()
            .all(|scope| scope_primitives_known(scope.items()))
    }

    fn shadows_primitive_crate(&self, name: &str) -> bool {
        self.iter().any(|scope| scope.shadows_primitive_crate(name))
    }
}

/// Module declarations by module path.
pub(crate) type ModuleMap<'a> = BTreeMap<Vec<String>, Declarations<'a>>;

/// Whether the module declaration `scope` at `path` leaves every std name bound to the std
/// item, following each `use super::*` to every declaration of the module it reads.
fn std_names_kept(scopes: &ModuleMap<'_>, path: &[String], scope: &ModuleScope<'_>) -> bool {
    let (kept, supers) = crate::drift::module_std_names(scope.items());
    kept && (1..=supers).all(|levels| {
        let Some(end) = path.len().checked_sub(levels) else {
            return false;
        };
        scopes.get(&path[..end]).is_some_and(|declarations| {
            declarations
                .iter()
                .all(|ancestor| std_names_kept(scopes, &path[..end], &ancestor))
        })
    })
}

/// Module ids by module path and original declaration address.
type ModuleIds = BTreeMap<Vec<String>, Vec<(*const (), ModuleId)>>;

/// Record `scope` as a declaration of `path`, once per original declaration.
pub(crate) fn declare_module<'a>(
    scopes: &mut ModuleMap<'a>,
    path: Vec<String>,
    scope: ModuleScope<'a>,
) {
    match scopes.entry(path) {
        Entry::Vacant(entry) => {
            entry.insert(Declarations {
                first: scope,
                rest: Vec::new(),
            });
        }
        Entry::Occupied(mut entry) => {
            let declarations = entry.get_mut();
            if declarations.iter().all(|known| known.key() != scope.key()) {
                declarations.rest.push(scope);
            }
        }
    }
}

/// Register every inline module body under its module path.
pub(crate) fn register_inline<'a>(
    scopes: &mut ModuleMap<'a>,
    path: &[String],
    items: &'a [syn::Item],
) {
    for item in items {
        if let syn::Item::Mod(mod_item) = item
            && let Some((_, inner)) = &mod_item.content
        {
            let mut child = path.to_vec();
            child.push(unraw(&mod_item.ident.to_string()).to_string());
            declare_module(scopes, child.clone(), ModuleScope::Inline(mod_item));
            register_inline(scopes, &child, inner);
        }
    }
}

/// The attribute facts a module inherits from itself and every ancestor.
#[derive(Clone, Copy, Default)]
struct Ancestry {
    observed: bool,
    blocked: bool,
    /// An attribute on the module or an ancestor may remove the standard prelude.
    bare: bool,
}

impl Ancestry {
    fn absorb(&mut self, attrs: &[syn::Attribute]) {
        self.observed |= live_attr(attrs);
        self.blocked |= blocking_attr(attrs);
        self.bare |= removes_prelude(attrs);
    }
}

fn module_ancestry(scopes: &ModuleMap<'_>) -> BTreeMap<Vec<String>, Ancestry> {
    let mut direct: BTreeMap<Vec<String>, Ancestry> = BTreeMap::new();
    for (path, declarations) in scopes {
        if declarations.repeated() {
            direct.entry(path.clone()).or_default().blocked = true;
        }
        for scope in declarations.iter() {
            if let ModuleScope::File(file) = scope {
                direct.entry(path.clone()).or_default().absorb(&file.attrs);
            }
            for item in scope.items() {
                if let syn::Item::Mod(module) = item {
                    let mut child = path.clone();
                    child.push(ident_name(&module.ident));
                    direct.entry(child).or_default().absorb(&module.attrs);
                }
            }
        }
    }
    scopes
        .keys()
        .map(|path| {
            let mut flags = Ancestry::default();
            for end in 0..=path.len() {
                if let Some(direct) = direct.get(&path[..end]) {
                    flags.observed |= direct.observed;
                    flags.blocked |= direct.blocked;
                    flags.bare |= direct.bare;
                }
            }
            (path.clone(), flags)
        })
        .collect()
}

pub(crate) fn blocked_modules(scopes: &ModuleMap<'_>) -> BTreeSet<Vec<String>> {
    module_ancestry(scopes)
        .into_iter()
        .filter_map(|(path, flags)| flags.blocked.then_some(path))
        .collect()
}

/// Whether attributes may remove the standard prelude: `no_std`, `no_implicit_prelude`, or a
/// `cfg_attr` that may apply either.
pub(crate) fn removes_prelude(attrs: &[syn::Attribute]) -> bool {
    attrs.iter().any(|attr| {
        let path = attr.path();
        path.is_ident("no_std")
            || path.is_ident("no_implicit_prelude")
            || (path.is_ident("cfg_attr")
                && attr
                    .meta
                    .to_token_stream()
                    .into_iter()
                    .any(|token| prelude_attribute_token(&token)))
    })
}

fn prelude_attribute_token(token: &proc_macro2::TokenTree) -> bool {
    match token {
        proc_macro2::TokenTree::Ident(ident) => ident == "no_std" || ident == "no_implicit_prelude",
        proc_macro2::TokenTree::Group(group) => group
            .stream()
            .into_iter()
            .any(|token| prelude_attribute_token(&token)),
        _ => false,
    }
}

/// Whether a trait among `items`, at any depth, declares a method with one of `names`, which
/// the readings of standard method calls leave to the compiler.
pub(crate) fn declares_trait_method(items: &[syn::Item], names: &[&str]) -> bool {
    struct Search<'a>(&'a [&'a str], bool);
    impl syn::visit::Visit<'_> for Search<'_> {
        fn visit_trait_item_fn(&mut self, method: &syn::TraitItemFn) {
            let name = ident_name(&method.sig.ident);
            self.1 |= self.0.contains(&name.as_str());
        }
    }
    let mut search = Search(names, false);
    for item in items {
        syn::visit::Visit::visit_item(&mut search, item);
    }
    search.1
}

/// Whether every `use` among `items`, at any depth, imports from `std`, `core` or `alloc`, so
/// it brings no trait method beyond the standard library's into view.
pub(crate) fn standard_imports(items: &[syn::Item]) -> bool {
    struct Search(bool);
    impl syn::visit::Visit<'_> for Search {
        fn visit_item_use(&mut self, item: &syn::ItemUse) {
            self.0 &= standard_tree(&item.tree);
        }
    }
    fn standard_tree(tree: &syn::UseTree) -> bool {
        match tree {
            syn::UseTree::Path(path) => {
                matches!(ident_name(&path.ident).as_str(), "std" | "core" | "alloc")
            }
            syn::UseTree::Group(group) => group.items.iter().all(standard_tree),
            _ => false,
        }
    }
    let mut search = Search(true);
    for item in items {
        syn::visit::Visit::visit_item(&mut search, item);
    }
    search.0
}

pub(crate) fn qualified_name(module: &[String], name: &str) -> String {
    let mut out = String::from("__dejadoc_inherited");
    for part in module
        .iter()
        .map(|part| unraw(part))
        .chain(core::iter::once(unraw(name)))
    {
        qualified_part(&mut out, part);
    }
    out
}

pub(crate) fn qualified_part(out: &mut String, part: &str) {
    const HEX: &[u8; 16] = b"0123456789abcdef";
    out.push('_');
    for byte in part.bytes() {
        out.push(char::from(HEX[usize::from(byte >> 4)]));
        out.push(char::from(HEX[usize::from(byte & 15)]));
    }
}

#[derive(Clone)]
pub(crate) struct ImportedTarget {
    pub(crate) canon: String,
    pub(crate) prim: Option<PrimTy>,
    module: Option<Vec<String>>,
    macro_resolution: Option<crate::macros::MacroResolution>,
}

impl ImportedTarget {
    /// The facts `self` shares with `other`, `None` when they name different items.
    fn agree(self, other: &Self) -> Option<Self> {
        (self.canon == other.canon).then(|| Self {
            prim: self.prim.filter(|prim| other.prim == Some(*prim)),
            macro_resolution: self
                .macro_resolution
                .filter(|standard| other.macro_resolution == Some(*standard)),
            module: self
                .module
                .filter(|module| other.module.as_ref() == Some(module)),
            canon: self.canon,
        })
    }
}

/// One resolved `use` target.
pub(crate) enum UseTarget {
    /// An external path.
    External,
    /// An indexed item's name and occupied namespaces.
    Item(ImportedTarget, [bool; 5]),
    /// A name the context could not resolve.
    Unresolved,
}

const IMPORT_NAMESPACES: [Ns; 3] = [Ns::Value, Ns::Type, Ns::Macro];

#[derive(Default)]
enum UnknownImport {
    #[default]
    None,
    Unique(ImportedTarget),
    Ambiguous,
}

#[derive(Default)]
struct ImportedName {
    resolved: [Option<ImportedTarget>; 5],
    unknown: UnknownImport,
    /// Whether a `cfg` or `cfg_attr` gates one of the `use` items binding the alias.
    gated: bool,
}

impl ImportedName {
    /// Make every namespace the opaque module-scoped alias when gated imports
    /// bind the alias to both a resolved item and an unknown target.
    fn settle(&mut self, from: &[String], alias: &str) {
        if self.gated {
            for target in self.resolved.iter_mut().flatten() {
                target.macro_resolution = Some(crate::macros::MacroResolution::Opaque);
            }
            if let UnknownImport::Unique(target) = &mut self.unknown {
                target.macro_resolution = Some(crate::macros::MacroResolution::Opaque);
            }
        }
        if !self.gated
            || matches!(self.unknown, UnknownImport::None)
            || self.resolved.iter().all(Option::is_none)
        {
            return;
        }
        for slot in self.resolved.iter_mut().filter(|slot| slot.is_some()) {
            *slot = Some(ImportedTarget {
                canon: qualified_name(from, alias),
                prim: None,
                module: None,
                macro_resolution: None,
            });
        }
        self.unknown = UnknownImport::Ambiguous;
    }
}

/// Whether a `cfg` or `cfg_attr` attribute gates the item carrying `attrs`.
fn cfg_gated(attrs: &[syn::Attribute]) -> bool {
    attrs
        .iter()
        .any(|attr| attr.path().is_ident("cfg") || attr.path().is_ident("cfg_attr"))
}

struct ImportPath<'a> {
    module: &'a [String],
    name: &'a str,
    parent: Option<&'a Self>,
}

impl ImportPath<'_> {
    fn contains(&self, module: &[String], name: &str) -> bool {
        let mut active = Some(self);
        while let Some(query) = active {
            if query.module == module && query.name == name {
                return true;
            }
            active = query.parent;
        }
        false
    }
}

struct UseBinder<'a> {
    scopes: &'a ModuleMap<'a>,
    from: &'a [String],
    import: &'a syn::ItemUse,
    imports: &'a mut BTreeMap<String, ImportedName>,
    only_name: Option<&'a str>,
    depth: usize,
    active: Option<&'a ImportPath<'a>>,
    steps: &'a Cell<usize>,
}

fn import_slots(
    slots: &mut [Option<ImportedTarget>; 5],
    namespaces: [bool; 5],
    target: ImportedTarget,
) {
    let mut active = namespaces
        .into_iter()
        .enumerate()
        .filter_map(|(index, present)| present.then_some(index))
        .peekable();
    while let Some(index) = active.next() {
        if active.peek().is_some() {
            slots[index] = Some(target.clone());
        } else {
            slots[index] = Some(target);
            break;
        }
    }
}

/// Merge one more `use` of `alias` into its slots, a namespace bound to two
/// different items becoming the opaque module-scoped alias.
fn merge_import_slots(
    slots: &mut [Option<ImportedTarget>; 5],
    namespaces: [bool; 5],
    target: &ImportedTarget,
    from: &[String],
    alias: &str,
) {
    for (slot, present) in slots.iter_mut().zip(namespaces) {
        if !present {
            continue;
        }
        *slot = Some(match slot.take() {
            None => target.clone(),
            Some(current) => current.agree(target).unwrap_or_else(|| ImportedTarget {
                canon: qualified_name(from, alias),
                prim: None,
                module: None,
                macro_resolution: None,
            }),
        });
    }
}

fn bind_imports(
    frame: &mut Frame,
    from: &[String],
    imports: BTreeMap<String, ImportedName>,
    next_id: &mut usize,
) {
    for (alias, mut imported) in imports {
        imported.settle(from, &alias);
        let unknown = match imported.unknown {
            UnknownImport::None => None,
            UnknownImport::Unique(target) => Some(target),
            UnknownImport::Ambiguous => Some(ImportedTarget {
                canon: qualified_name(from, &alias),
                prim: None,
                module: None,
                macro_resolution: None,
            }),
        };
        if let Some(target) = unknown {
            let mut unclaimed = [false; 5];
            for ns in IMPORT_NAMESPACES {
                unclaimed[ns as usize] =
                    imported.resolved[ns as usize].is_none() && frame.lookup(ns, &alias).is_none();
            }
            import_slots(&mut imported.resolved, unclaimed, target);
        }
        for ns in IMPORT_NAMESPACES {
            if let Some(target) = imported.resolved[ns as usize].take() {
                frame.bind(
                    ns,
                    &alias,
                    Binding {
                        id: BindingId(*next_id),
                        canon: target.canon,
                        origin: Origin::Use,
                        prim: if ns == Ns::Type { target.prim } else { None },
                        target: None,
                        raw: false,
                        port: 0,
                        value_port: None,
                        helper: None,
                        macro_resolution: target.macro_resolution,
                    },
                );
                *next_id += 1;
            }
        }
    }
}

fn use_tree_bindings(binder: &mut UseBinder<'_>, prefix: &[String], tree: &syn::UseTree) {
    if binder.depth >= 64 {
        return;
    }
    binder.depth += 1;
    match tree {
        syn::UseTree::Path(path) => {
            let mut next = prefix.to_vec();
            next.push(ident_name(&path.ident));
            use_tree_bindings(binder, &next, &path.tree);
        }
        syn::UseTree::Name(name) if name.ident == "self" && !prefix.is_empty() => {
            bind_use_name(
                binder,
                prefix,
                prefix.last().expect("nonempty import prefix"),
            );
        }
        syn::UseTree::Name(name) => {
            let mut path = prefix.to_vec();
            path.push(ident_name(&name.ident));
            bind_use_name(binder, &path, &name.ident.to_string());
        }
        syn::UseTree::Rename(rename) => {
            let mut path = prefix.to_vec();
            if rename.ident != "self" || path.is_empty() {
                path.push(ident_name(&rename.ident));
            }
            bind_use_name(binder, &path, &rename.rename.to_string());
        }
        syn::UseTree::Group(group) => {
            for inner in &group.items {
                use_tree_bindings(binder, prefix, inner);
            }
        }
        syn::UseTree::Glob(_) => {}
    }
    binder.depth -= 1;
}

fn bind_use_name(binder: &mut UseBinder<'_>, path: &[String], alias: &str) {
    if binder.only_name.is_some_and(|name| unraw(alias) != name) || path.is_empty() {
        return;
    }
    let target = resolve_use_at(
        binder.scopes,
        binder.from,
        path,
        binder.import.leading_colon.is_some(),
        binder.depth,
        binder.active,
        binder.steps,
    );
    let imported = if let Some(imported) = binder.imports.get_mut(alias) {
        imported
    } else {
        binder.imports.entry(alias.to_string()).or_default()
    };
    imported.gated |= cfg_gated(&binder.import.attrs);
    let target = match target {
        UseTarget::Item(target, namespaces) => {
            if imported.resolved.iter().all(Option::is_none) {
                import_slots(&mut imported.resolved, namespaces, target);
            } else {
                merge_import_slots(
                    &mut imported.resolved,
                    namespaces,
                    &target,
                    binder.from,
                    alias,
                );
            }
            return;
        }
        UseTarget::External => ImportedTarget {
            canon: qualified_name(&path[..path.len() - 1], &path[path.len() - 1]),
            prim: external_prim(path).filter(|_| {
                binder
                    .scopes
                    .get(binder.from)
                    .expect("indexed importing module")
                    .primitives_known()
            }),
            module: None,
            macro_resolution: crate::macros::MacroResolution::external_path(path),
        },
        UseTarget::Unresolved => ImportedTarget {
            canon: qualified_name(binder.from, alias),
            prim: None,
            module: None,
            macro_resolution: None,
        },
    };
    imported.unknown = match imported.unknown {
        UnknownImport::None => UnknownImport::Unique(target),
        UnknownImport::Unique(_) | UnknownImport::Ambiguous => UnknownImport::Ambiguous,
    };
}

/// Resolve a use path against the module map, `from` the importing module,
/// spending `steps` of the repeated-module budget.
pub(crate) fn resolve_use(
    scopes: &ModuleMap<'_>,
    from: &[String],
    path: &[String],
    leading_colon: bool,
    steps: &Cell<usize>,
) -> UseTarget {
    resolve_use_at(scopes, from, path, leading_colon, 0, None, steps)
}

/// The repeated-module declarations one budget may visit.
const UNION_STEPS: usize = 1024;

fn resolve_use_at(
    scopes: &ModuleMap<'_>,
    from: &[String],
    path: &[String],
    leading_colon: bool,
    depth: usize,
    active: Option<&ImportPath<'_>>,
    steps: &Cell<usize>,
) -> UseTarget {
    if depth >= 64 || path.len() > 64 {
        return UseTarget::Unresolved;
    }
    let Some(first) = path.first().map(|name| unraw(name)) else {
        return UseTarget::Unresolved;
    };
    if leading_colon {
        return UseTarget::External;
    }
    let local = scopes.get(from).expect("indexed importing module");
    if matches!(first, "std" | "core") && !local.shadows_primitive_crate(first) {
        return UseTarget::External;
    }
    let (mut base, mut rest) = match first {
        "crate" => (Vec::new(), &path[1..]),
        "self" => (from.to_vec(), &path[1..]),
        _ => (from.to_vec(), path),
    };
    while rest.first().is_some_and(|name| unraw(name) == "super") {
        if base.pop().is_none() {
            return UseTarget::Unresolved;
        }
        rest = &rest[1..];
    }
    let Some(mut scope) = scopes.get(&base) else {
        return UseTarget::Unresolved;
    };
    if rest.is_empty() {
        let Some(name) = base.last() else {
            return UseTarget::Unresolved;
        };
        return UseTarget::Item(
            ImportedTarget {
                canon: qualified_name(&base[..base.len() - 1], name),
                prim: None,
                macro_resolution: None,
                module: Some(base.clone()),
            },
            [false, true, false, false, false],
        );
    }
    for (index, name) in rest.iter().enumerate() {
        let name = unraw(name);
        if index + 1 == rest.len() {
            return resolve_item(scopes, scope, &base, name, depth + 1, active, steps);
        }
        let mut next_path = base.clone();
        next_path.push(name.to_string());
        if !scopes.contains_key(&next_path) {
            let UseTarget::Item(target, namespaces) =
                resolve_item(scopes, scope, &base, name, depth + 1, active, steps)
            else {
                return UseTarget::Unresolved;
            };
            if !namespaces[1] {
                return UseTarget::Unresolved;
            }
            let Some(module) = target.module else {
                return UseTarget::Unresolved;
            };
            next_path = module;
        }
        scope = scopes
            .get(&next_path)
            .expect("indexed import target module");
        base = next_path;
    }
    UseTarget::Unresolved
}

/// `name` in the module at `module`, every declaration of a repeated module
/// naming one item and only the facts they all prove kept.
fn resolve_item(
    scopes: &ModuleMap<'_>,
    declarations: &Declarations<'_>,
    module: &[String],
    name: &str,
    depth: usize,
    active: Option<&ImportPath<'_>>,
    steps: &Cell<usize>,
) -> UseTarget {
    if !declarations.repeated() {
        let items = declarations.first.items();
        return resolve_declared(scopes, items, module, name, depth, active, steps);
    }
    let mut merged: Option<(ImportedTarget, [bool; 5])> = None;
    for scope in declarations.iter() {
        steps.set(steps.get() + 1);
        if steps.get() > UNION_STEPS {
            return UseTarget::Unresolved;
        }
        let items = scope.items();
        match resolve_declared(scopes, items, module, name, depth, active, steps) {
            UseTarget::Item(target, namespaces) => {
                merged = Some(match merged {
                    None => (target, namespaces),
                    Some((current, seen)) => {
                        let Some(target) = current.agree(&target) else {
                            return UseTarget::Unresolved;
                        };
                        (
                            target,
                            core::array::from_fn(|ns| seen[ns] || namespaces[ns]),
                        )
                    }
                });
            }
            UseTarget::Unresolved if !may_name(items, name) => {}
            UseTarget::External | UseTarget::Unresolved => return UseTarget::Unresolved,
        }
    }
    merged.map_or(UseTarget::Unresolved, |(target, namespaces)| {
        UseTarget::Item(target, namespaces)
    })
}

/// Whether `items` may bind `name` beyond the items they declare.
fn may_name(items: &[syn::Item], name: &str) -> bool {
    items.iter().any(|item| match item {
        syn::Item::Use(import) => {
            has_glob(&import.tree)
                || alpha::imported_name_matches(&import.tree, |ident| ident_name(ident) == name)
        }
        syn::Item::ExternCrate(krate) => {
            let bound = krate
                .rename
                .as_ref()
                .map_or(&krate.ident, |(_, rename)| rename);
            ident_name(bound) == name
        }
        syn::Item::Macro(item) => item.ident.is_none(),
        _ => false,
    })
}

fn resolve_declared(
    scopes: &ModuleMap<'_>,
    items: &[syn::Item],
    module: &[String],
    name: &str,
    depth: usize,
    active: Option<&ImportPath<'_>>,
    steps: &Cell<usize>,
) -> UseTarget {
    if depth >= 64 {
        return UseTarget::Unresolved;
    }
    let mut namespaces = [false; 5];
    for item in items {
        let value = match item {
            syn::Item::Fn(item) => Some(&item.sig.ident),
            syn::Item::Const(item) => Some(&item.ident),
            syn::Item::Static(item) => Some(&item.ident),
            _ => None,
        };
        if value.is_some_and(|ident| ident_name(ident) == name) {
            namespaces[Ns::Value as usize] = true;
        }
        if alpha::type_item_ident(item).is_some_and(|ident| ident_name(ident) == name) {
            namespaces[Ns::Type as usize] = true;
            if let syn::Item::Struct(item) = item
                && !matches!(item.fields, syn::Fields::Named(_))
            {
                namespaces[Ns::Value as usize] = true;
            }
        }
        if let syn::Item::Macro(item) = item
            && item
                .ident
                .as_ref()
                .is_some_and(|ident| ident_name(ident) == name)
        {
            namespaces[Ns::Macro as usize] = true;
        }
    }
    if namespaces.iter().any(|present| *present) {
        let prim = items.iter().find_map(|item| match item {
            syn::Item::Type(alias) if ident_name(&alias.ident) == name => {
                alias_chain_prim(alias, &items.iter(), scope_primitives_known(items), 0).or_else(
                    || resolved_alias_prim(scopes, module, alias, depth + 1, active, steps),
                )
            }
            _ => None,
        });
        UseTarget::Item(
            ImportedTarget {
                canon: qualified_name(module, name),
                prim,
                macro_resolution: None,
                module: items.iter().find_map(|item| {
                    if let syn::Item::Mod(item) = item
                        && ident_name(&item.ident) == name
                    {
                        let mut path = module.to_vec();
                        path.push(name.to_string());
                        scopes.contains_key(&path).then_some(path)
                    } else {
                        None
                    }
                }),
            },
            namespaces,
        )
    } else {
        resolve_imported_item(scopes, items, module, name, depth, active, steps)
    }
}

fn resolve_imported_item(
    scopes: &ModuleMap<'_>,
    items: &[syn::Item],
    module: &[String],
    name: &str,
    depth: usize,
    active: Option<&ImportPath<'_>>,
    steps: &Cell<usize>,
) -> UseTarget {
    if active.is_some_and(|query| query.contains(module, name)) {
        return UseTarget::Unresolved;
    }
    let query = ImportPath {
        module,
        name,
        parent: active,
    };
    let mut namespaces = [false; 5];
    let mut imports = BTreeMap::new();
    for item in items {
        if let syn::Item::Use(import) = item {
            use_tree_bindings(
                &mut UseBinder {
                    scopes,
                    from: module,
                    import,
                    imports: &mut imports,
                    only_name: Some(name),
                    depth: depth + 1,
                    active: Some(&query),
                    steps,
                },
                &[],
                &import.tree,
            );
        }
    }
    let Some((alias, mut imported)) = imports.into_iter().find(|(alias, _)| unraw(alias) == name)
    else {
        return UseTarget::Unresolved;
    };
    imported.settle(module, &alias);
    let mut target: Option<ImportedTarget> = None;
    for (index, slot) in imported.resolved.into_iter().enumerate() {
        if let Some(resolved) = slot {
            if target
                .as_ref()
                .is_some_and(|current| current.canon != resolved.canon)
            {
                return UseTarget::Unresolved;
            }
            namespaces[index] = true;
            target = Some(resolved);
        }
    }
    if target.is_none()
        && let UnknownImport::Unique(external) = imported.unknown
        && external.prim.is_some()
    {
        namespaces[1] = true;
        target = Some(external);
    }
    target.map_or(UseTarget::Unresolved, |target| {
        UseTarget::Item(target, namespaces)
    })
}

/// The primitive an external `std::primitive` or `core::primitive` path
/// names, when it names one.
pub(crate) fn external_prim(path: &[String]) -> Option<PrimTy> {
    if let [first, middle, third] = path
        && middle == "primitive"
        && (first == "std" || first == "core")
    {
        return crate::scope::primitive_suffix(third);
    }
    None
}

/// The proven primitive a type alias names, its target resolved through
/// the sibling items, the chain bounded to sixteen hops.
fn alias_chain_prim<'a>(
    alias: &syn::ItemType,
    siblings: &(impl Iterator<Item = &'a syn::Item> + Clone),
    prim_fallback: bool,
    hops: usize,
) -> Option<PrimTy> {
    if hops > 16 || !prim_fallback || !alias.generics.params.is_empty() {
        return None;
    }
    let target = alpha::single_segment_type_name(&alias.ty)?;
    let target = unraw(&target);
    for item in siblings.clone() {
        match item {
            syn::Item::Type(other) if ident_name(&other.ident) == target => {
                return alias_chain_prim(other, siblings, prim_fallback, hops + 1);
            }
            syn::Item::Use(use_item) => {
                if alpha::imported_name_matches(&use_item.tree, |ident| ident_name(ident) == target)
                {
                    return None;
                }
            }
            other => {
                if let Some(ident) = alpha::type_item_ident(other)
                    && ident_name(ident) == target
                {
                    return None;
                }
            }
        }
    }
    crate::scope::primitive_suffix(target)
}

fn alias_type_path(mut ty: &syn::Type) -> Option<&syn::Path> {
    for _ in 0..64 {
        match ty {
            syn::Type::Paren(paren) => ty = &paren.elem,
            syn::Type::Group(group) => ty = &group.elem,
            syn::Type::Path(path)
                if path.qself.is_none()
                    && path
                        .path
                        .segments
                        .iter()
                        .all(|segment| segment.arguments.is_empty()) =>
            {
                return Some(&path.path);
            }
            _ => return None,
        }
    }
    None
}

fn resolved_alias_prim(
    scopes: &ModuleMap<'_>,
    module: &[String],
    alias: &syn::ItemType,
    depth: usize,
    active: Option<&ImportPath<'_>>,
    steps: &Cell<usize>,
) -> Option<PrimTy> {
    if depth >= 64 || !alias.generics.params.is_empty() || blocking_attr(&alias.attrs) {
        return None;
    }
    let scope = scopes.get(module)?;
    if !scope.primitives_known() {
        return None;
    }
    let path = alias_type_path(&alias.ty)?;
    let names: Vec<String> = path
        .segments
        .iter()
        .map(|segment| ident_name(&segment.ident))
        .collect();
    match resolve_use_at(
        scopes,
        module,
        &names,
        path.leading_colon.is_some(),
        depth + 1,
        active,
        steps,
    ) {
        UseTarget::Item(target, namespaces) if namespaces[1] => target.prim,
        UseTarget::External => external_prim(&names),
        UseTarget::Unresolved if names.len() == 1 && !scope.shadows_primitive_crate(&names[0]) => {
            crate::scope::primitive_suffix(&names[0])
        }
        _ => None,
    }
}

fn seed_alias_prim(
    scopes: &ModuleMap<'_>,
    module: &[String],
    alias: &syn::ItemType,
    frame: &Frame,
    prim_fallback: bool,
    steps: &Cell<usize>,
) -> Option<PrimTy> {
    if !alias.generics.params.is_empty() || blocking_attr(&alias.attrs) {
        return None;
    }
    let path = alias_type_path(&alias.ty)?;
    if path.leading_colon.is_none() && path.segments.len() == 1 {
        let name = ident_name(&path.segments[0].ident);
        return match frame.lookup(Ns::Type, &name) {
            Some(binding) => binding.prim,
            None if prim_fallback => crate::scope::primitive_suffix(&name),
            None => None,
        };
    }
    resolved_alias_prim(scopes, module, alias, 0, None, steps)
}

fn bind_item_ident(
    frame: &mut Frame,
    module: &[String],
    ns: Ns,
    ident: &syn::Ident,
    origin: Origin,
    prim: Option<PrimTy>,
    next_id: &mut usize,
) {
    let name = ident.to_string();
    let binding = Binding {
        id: BindingId(*next_id),
        canon: qualified_name(module, &name),
        origin,
        target: None,
        raw: false,
        prim,
        port: 0,
        value_port: None,
        helper: None,
        macro_resolution: None,
    };
    frame.bind(ns, &name, binding);
    *next_id += 1;
}

fn bind_extern_crate(
    frame: &mut Frame,
    module: &[String],
    item: &syn::ItemExternCrate,
    next_id: &mut usize,
) {
    let ident = item.rename.as_ref().map_or(&item.ident, |(_, alias)| alias);
    let name = ident.to_string();
    frame.bind(
        Ns::Type,
        &name,
        Binding {
            id: BindingId(*next_id),
            canon: qualified_name(module, &name),
            origin: Origin::Use,
            target: None,
            raw: false,
            prim: None,
            port: 0,
            value_port: None,
            helper: None,
            macro_resolution: crate::macros::MacroResolution::external_crate(item),
        },
    );
    *next_id += 1;
}

fn bind_macro_rule(
    frame: &mut Frame,
    module: &[String],
    item: &syn::ItemMacro,
    next_id: &mut usize,
) {
    if let Some(ident) = &item.ident {
        bind_item_ident(
            frame,
            module,
            Ns::Macro,
            ident,
            Origin::MacroRule,
            None,
            next_id,
        );
    }
}

/// The identity a module item seeds at its own spelling: the namespace it
/// occupies, its identifier, and the prim its alias target proves.
fn item_seed<'a>(
    item: &'a syn::Item,
    siblings: &(impl Iterator<Item = &'a syn::Item> + Clone),
    prim_fallback: bool,
) -> Option<(Ns, &'a syn::Ident, Origin, Option<PrimTy>)> {
    match item {
        syn::Item::Fn(i) => Some((Ns::Value, &i.sig.ident, Origin::Item, None)),
        syn::Item::Const(i) => Some((Ns::Value, &i.ident, Origin::Item, None)),
        syn::Item::Static(i) => Some((Ns::Value, &i.ident, Origin::Item, None)),
        syn::Item::Struct(i) => Some((Ns::Type, &i.ident, Origin::Item, None)),
        syn::Item::Enum(i) => Some((Ns::Type, &i.ident, Origin::Item, None)),
        syn::Item::Union(i) => Some((Ns::Type, &i.ident, Origin::Item, None)),
        syn::Item::Trait(i) => Some((Ns::Type, &i.ident, Origin::Item, None)),
        syn::Item::TraitAlias(i) => Some((Ns::Type, &i.ident, Origin::Item, None)),
        syn::Item::Mod(i) => Some((Ns::Type, &i.ident, Origin::Item, None)),
        syn::Item::Type(i) => Some((
            Ns::Type,
            &i.ident,
            Origin::TypeAlias,
            alias_chain_prim(i, siblings, prim_fallback, 0),
        )),
        _ => None,
    }
}

/// The seed frame of one module scope: its items at their identity
/// spelling, the prims its type aliases and use imports prove.
pub(crate) fn seed_frame(
    scopes: &ModuleMap<'_>,
    path: &[String],
    scope: &ModuleScope<'_>,
    prim_fallback: bool,
) -> Frame {
    seed_scope(scopes, path, &scope.items().iter(), None, prim_fallback)
}

pub(crate) fn seed_items(
    scopes: &ModuleMap<'_>,
    module: &[String],
    items: &[&syn::Item],
    outer: &Frame,
    prim_fallback: bool,
) -> Frame {
    seed_scope(
        scopes,
        module,
        &items.iter().copied(),
        Some(outer),
        prim_fallback,
    )
}

fn seed_scope<'a>(
    scopes: &ModuleMap<'_>,
    path: &[String],
    items: &(impl Iterator<Item = &'a syn::Item> + Clone),
    outer: Option<&Frame>,
    prim_fallback: bool,
) -> Frame {
    let mut frame = outer.cloned().unwrap_or_default();
    let mut next_id = frame.next_id();
    let mut imports = BTreeMap::new();
    let steps = Cell::new(0);
    for item in items.clone() {
        if let Some((ns, ident, origin, prim)) =
            item_seed(item, items, prim_fallback && outer.is_none())
        {
            bind_item_ident(&mut frame, path, ns, ident, origin, prim, &mut next_id);
            if let syn::Item::Enum(item) = item {
                frame.set_variants(&ident_name(&item.ident), item);
            }
            if let syn::Item::Struct(item) = item
                && !matches!(item.fields, syn::Fields::Named(_))
            {
                bind_item_ident(
                    &mut frame,
                    path,
                    Ns::Value,
                    &item.ident,
                    Origin::Item,
                    None,
                    &mut next_id,
                );
            }
            continue;
        }
        match item {
            syn::Item::ExternCrate(item) => {
                bind_extern_crate(&mut frame, path, item, &mut next_id);
            }
            syn::Item::Use(use_item) => {
                frame.glob |= has_glob(&use_item.tree);
                use_tree_bindings(
                    &mut UseBinder {
                        scopes,
                        from: path,
                        import: use_item,
                        imports: &mut imports,
                        only_name: None,
                        depth: 0,
                        active: None,
                        steps: &steps,
                    },
                    &[],
                    &use_item.tree,
                );
            }
            syn::Item::Macro(item) => bind_macro_rule(&mut frame, path, item, &mut next_id),
            _ => {}
        }
    }
    bind_imports(&mut frame, path, imports, &mut next_id);
    for _ in 0..16 {
        let mut changed = false;
        for item in items.clone() {
            if let syn::Item::Type(alias) = item {
                if outer.is_none()
                    && alias_type_path(&alias.ty).is_some_and(|path| path.segments.len() == 1)
                {
                    continue;
                }
                let name = ident_name(&alias.ident);
                let lexical_shadow = outer.is_some()
                    && alias_type_path(&alias.ty).is_some_and(|target| {
                        target.leading_colon.is_none()
                            && target.segments.len() > 1
                            && target.segments.first().is_some_and(|first| {
                                let first_name = ident_name(&first.ident);
                                !matches!(first_name.as_str(), "crate" | "self" | "super")
                                    && items.clone().any(|item| {
                                        alpha::type_item_ident(item)
                                            .is_some_and(|ident| ident_name(ident) == first_name)
                                    })
                            })
                    });
                let prim = if lexical_shadow {
                    None
                } else {
                    seed_alias_prim(scopes, path, alias, &frame, prim_fallback, &steps)
                };
                if let Some(binding) = frame.bindings_mut(Ns::Type).get_mut(&name)
                    && binding.origin == Origin::TypeAlias
                    && binding.prim != prim
                {
                    binding.prim = prim;
                    changed = true;
                }
            }
        }
        if !changed {
            break;
        }
    }
    frame
}

/// Bind one generic parameter in `namespaces` under one id and its owner role `label`.
fn bind_generic_param(
    frame: &mut Frame,
    next_id: &mut usize,
    namespaces: &[Ns],
    ident: &syn::Ident,
    origin: Origin,
    label: String,
) {
    let name = ident.to_string();
    let binding = Binding {
        id: BindingId(*next_id),
        canon: label,
        origin,
        target: None,
        raw: false,
        prim: None,
        port: 0,
        value_port: None,
        helper: None,
        macro_resolution: None,
    };
    for ns in namespaces {
        frame.bind(*ns, &name, binding.clone());
    }
    *next_id += 1;
}

/// The owner's generic parameters, each bound under its role label, `labels` in parameter order.
fn generics_frame(
    module: ModuleId,
    modules: &[Module],
    generics: &syn::Generics,
    labels: Vec<String>,
) -> Frame {
    let mut frame = Frame::default();
    let mut next_id = modules[module.0].seed.next_id();
    for (param, label) in generics.params.iter().zip(labels) {
        let (namespaces, ident, origin): (&[Ns], _, _) = match param {
            syn::GenericParam::Type(ty_param) => (&[Ns::Type], &ty_param.ident, Origin::Generic),
            syn::GenericParam::Lifetime(lifetime) => {
                (&[Ns::Lifetime], &lifetime.lifetime.ident, Origin::Lifetime)
            }
            syn::GenericParam::Const(r#const) => {
                (&[Ns::Value, Ns::Type], &r#const.ident, Origin::Generic)
            }
        };
        bind_generic_param(&mut frame, &mut next_id, namespaces, ident, origin, label);
    }
    frame
}

/// The role labels of a trait's own parameters, their argument positions.
fn trait_roles(generics: &syn::Generics) -> Vec<String> {
    (0..generics.params.len())
        .map(|position| alloc::format!("{OWNER_PARAMETER}t_{position}"))
        .collect()
}

/// The role labels of an impl's parameters, in parameter order. A role is the path of argument
/// positions to the first appearance in the self type, then in the trait arguments. Parameters
/// appearing in neither take their declaration order among those parameters.
fn impl_roles(item: &syn::ItemImpl) -> Vec<String> {
    let mut roles = OwnerRoles {
        params: &item.generics,
        labels: BTreeMap::new(),
    };
    let mut path = String::from("s");
    roles.ty(&item.self_ty, &mut path);
    if let Some((trait_path, _)) = &item.trait_ {
        let mut path = String::from("t");
        roles.path_args(trait_path, &mut path, &mut 0);
    }
    let mut unplaced = 0;
    item.generics
        .params
        .iter()
        .map(|param| {
            let role = roles
                .labels
                .get(&param_key(param))
                .cloned()
                .unwrap_or_else(|| {
                    unplaced += 1;
                    alloc::format!("b_{}", unplaced - 1)
                });
            alloc::format!("{OWNER_PARAMETER}{role}")
        })
        .collect()
}

/// A parameter's lookup key, lifetimes apart from type and const names.
fn param_key(param: &syn::GenericParam) -> (bool, String) {
    match param {
        syn::GenericParam::Lifetime(lifetime) => (true, lifetime.lifetime.ident.to_string()),
        syn::GenericParam::Type(ty_param) => (false, ty_param.ident.to_string()),
        syn::GenericParam::Const(r#const) => (false, r#const.ident.to_string()),
    }
}

/// The first appearance of each owner parameter in its owner type, as a path of positions.
struct OwnerRoles<'a> {
    params: &'a syn::Generics,
    labels: BTreeMap<(bool, String), String>,
}

impl OwnerRoles<'_> {
    /// Record `ident` at `path` when it names a parameter not yet placed.
    fn place(&mut self, lifetime: bool, ident: &syn::Ident, path: &str) {
        let key = (lifetime, ident.to_string());
        if !self.labels.contains_key(&key)
            && self
                .params
                .params
                .iter()
                .any(|param| param_key(param) == key)
        {
            self.labels.insert(key, path.to_string());
        }
    }

    /// Place the parameters of `ty`, numbering its direct children by syntactic position.
    fn ty(&mut self, ty: &syn::Type, path: &mut String) {
        match ty {
            syn::Type::Path(type_path) if type_path.qself.is_none() => {
                if let Some(ident) = type_path.path.get_ident() {
                    self.place(false, ident, path);
                } else {
                    self.path_args(&type_path.path, path, &mut 0);
                }
            }
            syn::Type::Reference(reference) => {
                if let Some(lifetime) = &reference.lifetime {
                    let len = enter(path, &mut 0);
                    self.place(true, &lifetime.ident, path);
                    path.truncate(len);
                }
                let len = enter(path, &mut 1);
                self.ty(&reference.elem, path);
                path.truncate(len);
            }
            syn::Type::Array(array) => {
                let len = enter(path, &mut 0);
                self.ty(&array.elem, path);
                path.truncate(len);
                let len = enter(path, &mut 1);
                self.expr(&array.len, path);
                path.truncate(len);
            }
            syn::Type::Tuple(tuple) => {
                let mut index = 0;
                for elem in &tuple.elems {
                    let len = enter(path, &mut index);
                    self.ty(elem, path);
                    path.truncate(len);
                }
            }
            syn::Type::Slice(syn::TypeSlice { elem, .. })
            | syn::Type::Ptr(syn::TypePtr { elem, .. })
            | syn::Type::Paren(syn::TypeParen { elem, .. })
            | syn::Type::Group(syn::TypeGroup { elem, .. }) => {
                let len = enter(path, &mut 0);
                self.ty(elem, path);
                path.truncate(len);
            }
            other => self.occurrences(|visitor| visitor.visit_type(other), path),
        }
    }

    /// Place the parameters of a path's angle-bracketed arguments, numbered across all segments.
    /// Other parameters stay unplaced and take their declaration order among the unplaced.
    fn path_args(&mut self, path_syntax: &syn::Path, path: &mut String, index: &mut usize) {
        for segment in &path_syntax.segments {
            if let syn::PathArguments::AngleBracketed(arguments) = &segment.arguments {
                for argument in &arguments.args {
                    let len = enter(path, index);
                    self.argument(argument, path);
                    path.truncate(len);
                }
            }
        }
    }

    fn argument(&mut self, argument: &syn::GenericArgument, path: &mut String) {
        match argument {
            syn::GenericArgument::Type(ty) => self.ty(ty, path),
            syn::GenericArgument::Lifetime(lifetime) => self.place(true, &lifetime.ident, path),
            syn::GenericArgument::Const(expr) => self.expr(expr, path),
            other => self.occurrences(|visitor| visitor.visit_generic_argument(other), path),
        }
    }

    fn expr(&mut self, expr: &syn::Expr, path: &str) {
        if let syn::Expr::Path(expr_path) = expr
            && expr_path.qself.is_none()
            && let Some(ident) = expr_path.path.get_ident()
        {
            self.place(false, ident, path);
        } else {
            self.occurrences(|visitor| visitor.visit_expr(expr), path);
        }
    }

    /// Place the parameters a syntax node `visit` reaches, each occurrence numbered in order.
    fn occurrences(&mut self, visit: impl FnOnce(&mut Occurrences<'_, '_>), path: &str) {
        let mut visitor = Occurrences {
            roles: self,
            path,
            count: 0,
        };
        visit(&mut visitor);
    }
}

/// Append child `index` to `path`, advancing it, and return the length to truncate back to.
fn enter(path: &mut String, index: &mut usize) -> usize {
    let len = path.len();
    path.push('_');
    path.push_str(&index.to_string());
    *index += 1;
    len
}

/// Parameter occurrences inside syntax with no positional structure, numbered in visit order.
struct Occurrences<'r, 'g> {
    roles: &'r mut OwnerRoles<'g>,
    path: &'r str,
    count: usize,
}

impl Occurrences<'_, '_> {
    fn place(&mut self, lifetime: bool, ident: &syn::Ident) {
        let label = alloc::format!("{}_v{}", self.path, self.count);
        self.count += 1;
        self.roles.place(lifetime, ident, &label);
    }
}

impl<'ast> Visit<'ast> for Occurrences<'_, '_> {
    fn visit_path(&mut self, path: &'ast syn::Path) {
        if let Some(ident) = path.get_ident() {
            self.place(false, ident);
        }
        syn::visit::visit_path(self, path);
    }

    fn visit_lifetime(&mut self, lifetime: &'ast syn::Lifetime) {
        self.place(true, &lifetime.ident);
    }
}

/// Index every function `scope` declares, nested modules included.
#[expect(
    clippy::too_many_arguments,
    reason = "the index carries its original module and observer evidence"
)]
#[expect(
    clippy::too_many_lines,
    reason = "each item kind carries its original owner and ancestry"
)]
fn index_scope<'a>(
    decls: &mut BTreeMap<(*const syn::Signature, *const syn::Block), Decl<'a>>,
    module_ids: &ModuleIds,
    modules: &[Module],
    path: &[String],
    scope: ModuleScope<'a>,
    observed: bool,
    macro_input: bool,
    helper_blocked: bool,
    gen_frames: &mut BTreeMap<*const syn::Generics, Frame>,
) {
    let module = module_ids
        .get(path)
        .and_then(|ids| ids.iter().find(|(key, _)| *key == scope.key()))
        .expect("a module id")
        .1;
    let prim_fallback = modules[module.0].prim_fallback;
    let observed = observed || modules[module.0].observed;
    let helper_blocked = helper_blocked || modules[module.0].helper_blocked;
    for item in scope.items() {
        match item {
            syn::Item::Fn(f) => {
                index_fn(
                    decls,
                    FnDecl {
                        module,
                        prim_fallback,
                        owner: Owner::Free,
                        observed,
                        macro_input,
                        helper_module: path,
                        helper_blocked,
                        attrs: &f.attrs,
                        generics: None,
                    },
                    &f.sig,
                    &f.block,
                );
            }
            syn::Item::Mod(m) => {
                if m.content.is_some() {
                    let mut child = path.to_vec();
                    child.push(unraw(&m.ident.to_string()).to_string());
                    index_scope(
                        decls,
                        module_ids,
                        modules,
                        &child,
                        ModuleScope::Inline(m),
                        observed || live_attr(&m.attrs),
                        macro_input || crate::drift::has_macro_attribute(&m.attrs),
                        helper_blocked || blocking_attr(&m.attrs),
                        gen_frames,
                    );
                }
            }
            syn::Item::Impl(i) => {
                let text = i.self_ty.to_token_stream().to_string();
                let owner = if i.trait_.is_some() {
                    Owner::Trait(text)
                } else {
                    Owner::Inherent(text)
                };
                let observed = observed || live_attr(&i.attrs);
                let macro_input = macro_input || crate::drift::has_macro_attribute(&i.attrs);
                let key = &raw const i.generics;
                gen_frames
                    .entry(key)
                    .or_insert_with(|| generics_frame(module, modules, &i.generics, impl_roles(i)));
                for member in &i.items {
                    if let syn::ImplItem::Fn(f) = member {
                        index_fn(
                            decls,
                            FnDecl {
                                module,
                                prim_fallback,
                                owner: owner.clone(),
                                observed,
                                macro_input,
                                helper_module: path,
                                helper_blocked: helper_blocked || blocking_attr(&i.attrs),
                                attrs: &f.attrs,
                                generics: Some(key),
                            },
                            &f.sig,
                            &f.block,
                        );
                    }
                }
            }
            syn::Item::Trait(t) => {
                let text = t.ident.to_string();
                let observed = observed || live_attr(&t.attrs);
                let macro_input = macro_input || crate::drift::has_macro_attribute(&t.attrs);
                let key = &raw const t.generics;
                gen_frames.entry(key).or_insert_with(|| {
                    generics_frame(module, modules, &t.generics, trait_roles(&t.generics))
                });
                for member in &t.items {
                    if let syn::TraitItem::Fn(f) = member
                        && let Some(block) = &f.default
                    {
                        index_fn(
                            decls,
                            FnDecl {
                                module,
                                prim_fallback,
                                owner: Owner::Trait(text.clone()),
                                observed,
                                macro_input,
                                attrs: &f.attrs,
                                helper_module: path,
                                helper_blocked: helper_blocked || blocking_attr(&t.attrs),
                                generics: Some(key),
                            },
                            &f.sig,
                            block,
                        );
                    }
                }
            }
            _ => {}
        }
    }
}

/// The module facts one indexed function is recorded with.
#[expect(
    clippy::struct_excessive_bools,
    reason = "primitive, observer, macro input and helper facts of a function are independent"
)]
struct FnDecl<'a, 'path> {
    module: ModuleId,
    prim_fallback: bool,
    owner: Owner,
    observed: bool,
    /// An enclosing inline module, impl or trait carries an attribute macro reading the function.
    macro_input: bool,
    helper_module: &'path [String],
    helper_blocked: bool,
    attrs: &'a [syn::Attribute],
    generics: Option<*const syn::Generics>,
}

fn index_fn<'a>(
    decls: &mut BTreeMap<(*const syn::Signature, *const syn::Block), Decl<'a>>,
    facts: FnDecl<'a, '_>,
    sig: &syn::Signature,
    block: &syn::Block,
) {
    let key = (
        core::ptr::from_ref::<syn::Signature>(sig),
        core::ptr::from_ref::<syn::Block>(block),
    );
    decls.insert(
        key,
        Decl {
            module: facts.module,
            attrs: facts.attrs,
            owner: facts.owner,
            prim_fallback: facts.prim_fallback,
            observed: facts.observed || live_attr(facts.attrs),
            enclosing_observed: facts.macro_input,
            helper_module: facts.helper_module.to_vec(),
            helper_blocked: facts.helper_blocked || blocking_attr(facts.attrs),
            generics: facts.generics,
        },
    );
}

/// Whether `meta` takes part in the comparison. `cfg`, the hints that
/// leave the body's meaning alone, the attributes that export a symbol,
/// and a `cfg_attr` wrapping only those do not.
pub(crate) fn live(meta: &syn::Meta) -> bool {
    let path = meta.path();
    if path.is_ident("cfg_attr") {
        return wrapped(meta, 1).is_none_or(|metas| metas.iter().any(live));
    }
    if path.is_ident("unsafe") {
        return wrapped(meta, 0).is_none_or(|metas| metas.iter().any(live));
    }
    // A `doc` computed by a macro runs at compile time and can fail.
    if path.is_ident("doc")
        && matches!(meta, syn::Meta::NameValue(nv) if matches!(nv.value, syn::Expr::Macro(_)))
    {
        return true;
    }
    let inert = [
        "cfg",
        "inline",
        "cold",
        "must_use",
        "track_caller",
        "doc",
        "allow",
        "warn",
        "deny",
        "forbid",
        "expect",
        "no_mangle",
        "export_name",
    ];
    !inert.iter().any(|name| path.is_ident(name))
        && path.segments.first().is_none_or(|s| s.ident != "rustfmt")
}

/// Whether any attribute in `attrs` takes part in the comparison.
pub(crate) fn live_attr(attrs: &[syn::Attribute]) -> bool {
    attrs.iter().any(|attr| live(&attr.meta))
}

/// Whether `meta` can change a helper's body or observe its caller.
pub(crate) fn blocking_meta(meta: &syn::Meta) -> bool {
    let path = meta.path();
    path.is_ident("cfg")
        || path.is_ident("cfg_attr")
        || path.is_ident("track_caller")
        || (path.is_ident("unsafe")
            && wrapped(meta, 0).is_none_or(|metas| metas.iter().any(blocking_meta)))
        || live(meta)
}

/// Whether any attribute in `attrs` can change a helper's denotation.
pub(crate) fn blocking_attr(attrs: &[syn::Attribute]) -> bool {
    attrs.iter().any(|attr| blocking_meta(&attr.meta))
}

/// The metas `meta` wraps after its first `skip` arguments, `None` when
/// they don't parse.
///
/// ```
/// let attr: syn::Attribute = syn::parse_quote!(#[cfg_attr(unix, inline, cold)]);
/// let metas = syn_canon::wrapped(&attr.meta, 1).unwrap();
/// assert!(metas[0].path().is_ident("inline") && metas[1].path().is_ident("cold"));
/// ```
#[must_use]
pub fn wrapped(meta: &syn::Meta, skip: usize) -> Option<Vec<syn::Meta>> {
    let syn::Meta::List(list) = meta else {
        return None;
    };
    let parser = syn::punctuated::Punctuated::<syn::Meta, syn::Token![,]>::parse_terminated;
    list.parse_args_with(parser)
        .ok()
        .map(|metas| metas.into_iter().skip(skip).collect())
}