praxis_runtime/

descriptor.rs

//! Type descriptors: the vtable-equivalent for runtime objects (§11.4).
//!
//! Every GC object carries a pointer to a [`TypeDescriptor`] that centralizes
//! all payload-aware operations — tracing, dropping, formatting, equality, and
//! hashing. The compiler generates one descriptor per type and emits code that
//! reaches these function pointers through the object header. The point of the
//! design (§11.4) is that there are no scattered type switches in generated or
//! runtime code: every operation routes through a descriptor.

use std::collections::hash_map::DefaultHasher;
use std::fmt;
use std::hash::{Hash, Hasher};

/// The closed set of built-in runtime types (§11.4).
///
/// This enum *is* the type-id registry: a descriptor's [`TypeId`] is derived
/// from its variant, so two built-ins cannot be labelled with the same id.
/// Uniqueness reduces to enum-discriminant uniqueness, which rustc already
/// enforces.
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
#[repr(u32)]
pub enum BuiltinTypeId {
    Unit = 0,
    Bool,
    Int,
    Byte,
    Char,
    Float,
    Text,
    Vec,
    Deque,
    Grid,
    Map,
    Set,
    Counter,
    MinHeap,
    MaxHeap,
    BitSet,
    Tuple,
    Record,
    Enum,
    Closure,
    VarCell,
    /// `Range` (§4.11, ADR-059). New variants are appended, so no existing id
    /// ever moves.
    Range,
}

impl BuiltinTypeId {
    /// Number of built-in types. Kept honest by [`BUILTINS`]'s array length and
    /// by `builtins_are_indexed_by_their_id`.
    pub const COUNT: usize = 22;

    /// Total inverse of the discriminant. A `match` rather than a `transmute`,
    /// so an out-of-range word yields `None` instead of an invalid enum value.
    pub const fn from_u32(v: u32) -> Option<BuiltinTypeId> {
        use BuiltinTypeId::*;
        Some(match v {
            0 => Unit,
            1 => Bool,
            2 => Int,
            3 => Byte,
            4 => Char,
            5 => Float,
            6 => Text,
            7 => Vec,
            8 => Deque,
            9 => Grid,
            10 => Map,
            11 => Set,
            12 => Counter,
            13 => MinHeap,
            14 => MaxHeap,
            15 => BitSet,
            16 => Tuple,
            17 => Record,
            18 => Enum,
            19 => Closure,
            20 => VarCell,
            21 => Range,
            _ => return None,
        })
    }

    /// This built-in's descriptor. The inverse of
    /// [`TypeDescriptor::as_builtin`].
    pub fn descriptor(self) -> &'static TypeDescriptor {
        BUILTINS[self as usize]
    }
}

/// An opaque, interned identifier for a type. Equality on `TypeId` *is* type
/// identity for descriptor-table lookups.
///
/// The inner word is **private**: the only producers are
/// [`TypeDescriptor::builtin`] (which derives it from a [`BuiltinTypeId`]) and
/// the test-only escape hatch, so a hand-written integer literal cannot
/// impersonate a built-in.
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]
pub struct TypeId(u32);

impl TypeId {
    #[inline]
    pub const fn to_u32(self) -> u32 {
        self.0
    }

    /// The built-in this id names, or `None` for a non-built-in (today: only
    /// the test descriptors, which live at the top of the `u32` range).
    #[inline]
    pub const fn as_builtin(self) -> Option<BuiltinTypeId> {
        BuiltinTypeId::from_u32(self.0)
    }
}

/// The tracer a descriptor's `trace` function receives during GC. The collector
/// supplies a concrete implementation whose own `trace` method enqueues child
/// references onto the mark worklist (ADR-011).
pub trait Tracer {
    /// Mark a `GcRef` as reachable and arrange for it to be traced.
    fn trace(&mut self, reference: crate::GcRef);
}

/// A hashing sink used by structural hash descriptors (§5.5). Concrete
/// implementations feed bytes into a hash state; [`StructHasher`] is the
/// built-in implementation used by the scalar and collection descriptors.
pub trait DynamicHasher {
    fn write_bytes(&mut self, bytes: &[u8]);
    fn finish(&self) -> u64;
}

/// The built-in [`DynamicHasher`] backed by [`DefaultHasher`]. Used by every
/// descriptor's `hash` callback.
pub struct StructHasher(DefaultHasher);

impl StructHasher {
    pub fn new() -> Self {
        StructHasher(DefaultHasher::new())
    }
}

impl Default for StructHasher {
    fn default() -> Self {
        Self::new()
    }
}

impl DynamicHasher for StructHasher {
    fn write_bytes(&mut self, bytes: &[u8]) {
        // `Hasher::write` consumes the bytes into the hash state.
        self.0.write(bytes);
    }

    fn finish(&self) -> u64 {
        self.0.finish()
    }
}

/// Convenience: feed any `Hash` value into a [`DynamicHasher`] byte-wise.
pub(crate) fn hash_value<H: DynamicHasher + ?Sized, T: Hash + ?Sized>(hasher: &mut H, value: &T) {
    // Route through a shim Hasher so we don't re-implement Hash for each scalar.
    struct HasherShim<'a, H: ?Sized>(&'a mut H);
    impl<H: DynamicHasher + ?Sized> Hasher for HasherShim<'_, H> {
        #[inline]
        fn write(&mut self, bytes: &[u8]) {
            self.0.write_bytes(bytes);
        }
        #[inline]
        fn finish(&self) -> u64 {
            self.0.finish()
        }
    }
    value.hash(&mut HasherShim(hasher));
}

/// `trace` callback shape: receive a pointer to the object payload (the bytes
/// after the header) plus a tracer, and report any `GcRef`s stored inside.
///
/// # Safety
/// The `payload` pointer must point at a value of the descriptor's type for the
/// duration of the call.
pub type TraceFn = unsafe fn(payload: *mut u8, tracer: &mut dyn Tracer);

/// `drop_value` callback shape: release Rust-owned resources held in the
/// payload (e.g. the backing `Vec<GcRef>` of a `Vec[T]`). Invoked during sweep
/// (§12.5).
///
/// # Safety
/// `payload` must point at a value of the descriptor's type, and afterwards the
/// memory is no longer valid.
pub type DropFn = unsafe fn(payload: *mut u8);

/// Which of the two renderings a `format` callback is producing.
///
/// Exactly one type reads this — `Text` — and it is one bit rather than a second
/// callback per descriptor because of *nesting*: a `Vec[Text]` renders its
/// elements through the element descriptor's `format`, so whatever distinguishes
/// the two renderings has to travel down that recursion. A `format_debug` field
/// beside `format` would not: `vec_format` would have to know which of the two
/// it was itself running as in order to pick the right one for its elements, and
/// that knowledge is exactly this enum.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum FormatStyle {
    /// The program's own rendering: `out(v)`, `"{v}"` interpolation, `to_text()`
    /// and `praxis run`'s result line. A `Text` is its characters, because that
    /// is what a program printing a string means (§16.1, and §8.1 for the
    /// interpolation that shares the callback).
    Display,
    /// The **debugger's** rendering: a locals row, a TUI pane cell, `p EXPR`. A
    /// `Text` is a quoted literal here, because a display that gives each value
    /// one line and no other context cannot afford a value that renders as zero
    /// characters, as a newline, or as something an adjacent `"` could have
    /// ended.
    Debug,
}

/// The writer a `format` callback appends to, carrying the [`FormatStyle`] it is
/// rendering under.
///
/// A wrapper rather than an extra parameter, so that a container passing its
/// writer to an element descriptor passes the style with it and cannot forget
/// to.
pub struct FormatSink<'a> {
    out: &'a mut dyn fmt::Write,
    style: FormatStyle,
}

impl<'a> FormatSink<'a> {
    /// A sink in the program's own rendering.
    pub fn display(out: &'a mut dyn fmt::Write) -> FormatSink<'a> {
        FormatSink {
            out,
            style: FormatStyle::Display,
        }
    }

    /// A sink in the debugger's rendering.
    pub fn debug(out: &'a mut dyn fmt::Write) -> FormatSink<'a> {
        FormatSink {
            out,
            style: FormatStyle::Debug,
        }
    }

    /// The style this sink renders under.
    #[must_use]
    pub fn style(&self) -> FormatStyle {
        self.style
    }

    /// A sink over `out` in a style read off another one.
    ///
    /// For the callbacks that render a part into a scratch `String` before
    /// placing it — `map_format` orders its entries by key and has to have them
    /// rendered to place them (ADR-138 decision 4). Such a callback holds its
    /// [`style`](Self::style) across the buffer and rebuilds a sink around it,
    /// which is what keeps a `Map[Text, Text]` quoting its keys and values in
    /// the debugger, exactly as a `Vec[Text]` does without needing a buffer at
    /// all.
    ///
    /// Every scratch buffer is a place the style could be dropped, and dropping
    /// it is silent — the value still renders, just in the other rendering. The
    /// style is `Copy` so that carrying it across is the easy thing to write.
    pub fn styled(out: &'a mut dyn fmt::Write, style: FormatStyle) -> FormatSink<'a> {
        FormatSink { out, style }
    }
}

impl fmt::Write for FormatSink<'_> {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        self.out.write_str(s)
    }
}

/// `format` callback shape: append the user-visible representation of the value
/// to the given writer, in that writer's [`FormatStyle`].
///
/// # Safety
/// `payload` must point at a value of the descriptor's type.
pub type FormatFn = unsafe fn(payload: *const u8, out: &mut FormatSink<'_>);

/// `equals` callback shape: structural equality between two values of the same
/// descriptor. `None` on the descriptor means the type is not equatable.
///
/// # Safety
/// Both pointers must point at values of the descriptor's type.
pub type EqualsFn = unsafe fn(a: *const u8, b: *const u8) -> bool;

/// `hash` callback shape: feed the value's structural identity into a hasher.
/// `None` on the descriptor means the type is not hashable.
///
/// # Safety
/// `payload` must point at a value of the descriptor's type.
pub type HashFn = unsafe fn(payload: *const u8, hasher: &mut dyn DynamicHasher);

/// `owned_bytes` callback shape: how many bytes *outside* the object's
/// `[header|payload]` block this value owns — the `Box<str>` behind a `Text`,
/// the `Vec`'s buffer behind a `Vec[T]`, the `HashMap`'s table behind a
/// `Map[K,V]`.
///
/// `None` on the descriptor means "nothing beyond the payload", which is the
/// truth for every scalar and the reason this is opt-in rather than a required
/// constructor argument.
///
/// The collector's pacing counter reads it at allocation. Without it a 1 MiB
/// `Text` would charge the same 40 bytes as an `Int`, and a text-heavy program
/// would under-report its own pressure by essentially its whole footprint.
///
/// # Safety
/// `payload` must point at a value of the descriptor's type.
pub type OwnedBytesFn = unsafe fn(payload: *const u8) -> usize;

/// `compare` callback shape: total ordering between two values of the same
/// descriptor. `None` on the descriptor means the type has no container order.
///
/// This is the ordering a **container** imposes — a heap's `Ord`, a sort, the
/// sequence a `Map` or `Set` prints and iterates in — and it is total, including
/// over `Float` NaN (which sorts last and equals itself). The source-level `<`
/// on a `Float` keeps IEEE semantics and is a different operation; see ADR-045.
///
/// Populated on every type a `Map` key or `Set` member can be: `Int`, `Byte`,
/// `Char`, `Float`, `Text`, `Bool`, `Unit`, `Range`, and tuples, records and
/// enums recursing through their element types. `None` on the eleven that can
/// never be one — the nine collections, `Closure` and `VarCell` (ADR-138
/// decision 1). That is deliberately a *different* set from
/// `praxis_hir::capability::supports_ord`, which is the source language's `<`
/// and `sorted()`: a tuple has a container order and no `<`, and
/// `(1, 2) < (1, 3)` is still `Y006` (ADR-138 decision 3).
///
/// # Safety
/// Both pointers must point at values of the descriptor's type.
pub type CompareFn = unsafe fn(a: *const u8, b: *const u8) -> std::cmp::Ordering;

/// Centralized table of operations on a value's payload (§11.4).
///
/// Exact Rust types may evolve, but all payload-aware operations must live here
/// rather than in scattered type switches.
///
/// `id`, `size` and `align` are private and *derived*: a built-in descriptor is
/// constructible only through [`TypeDescriptor::builtin`], which takes the
/// [`BuiltinTypeId`] the id comes from and the payload type the layout comes
/// from. "A descriptor whose id names a different type" and "a descriptor whose
/// size disagrees with its payload" are therefore unrepresentable.
#[derive(Clone, Copy)]
pub struct TypeDescriptor {
    id: TypeId,
    pub name: &'static str,
    size: usize,
    align: usize,
    pub trace: TraceFn,
    pub drop_value: DropFn,
    pub format: FormatFn,
    pub equals: Option<EqualsFn>,
    pub hash: Option<HashFn>,
    pub compare: Option<CompareFn>,
    /// Bytes this value owns outside its allocation block, for GC pacing.
    /// `None` means none — the scalar case, and the default. Set with
    /// [`TypeDescriptor::with_owned_bytes`].
    pub owned_bytes: Option<OwnedBytesFn>,
}

impl TypeDescriptor {
    /// The only constructor for a built-in descriptor. `id` is derived from
    /// `builtin`; `size`/`align` are derived from the payload type `P`.
    ///
    /// Built-in descriptors must be declared as `static`, never `const`: a
    /// `const` reference is a promoted rvalue with no guaranteed unique
    /// address, and descriptor *pointer* identity is what the runtime compares.
    #[allow(clippy::too_many_arguments)]
    pub const fn builtin<P>(
        builtin: BuiltinTypeId,
        name: &'static str,
        trace: TraceFn,
        drop_value: DropFn,
        format: FormatFn,
        equals: Option<EqualsFn>,
        hash: Option<HashFn>,
        compare: Option<CompareFn>,
    ) -> TypeDescriptor {
        TypeDescriptor {
            id: TypeId(builtin as u32),
            name,
            size: std::mem::size_of::<P>(),
            align: std::mem::align_of::<P>(),
            trace,
            drop_value,
            format,
            equals,
            hash,
            compare,
            owned_bytes: None,
        }
    }

    /// Test-only descriptor whose id is outside the built-in range by
    /// construction, so a fixture can never collide with a real type.
    #[cfg(test)]
    #[allow(clippy::too_many_arguments)]
    pub const fn for_test<P>(
        n: u32,
        name: &'static str,
        trace: TraceFn,
        drop_value: DropFn,
        format: FormatFn,
        equals: Option<EqualsFn>,
        hash: Option<HashFn>,
        compare: Option<CompareFn>,
    ) -> TypeDescriptor {
        TypeDescriptor {
            id: TypeId(u32::MAX - n),
            name,
            size: std::mem::size_of::<P>(),
            align: std::mem::align_of::<P>(),
            trace,
            drop_value,
            format,
            equals,
            hash,
            compare,
            owned_bytes: None,
        }
    }

    /// Declare that this type owns memory outside its allocation block, and how
    /// to measure it.
    ///
    /// A builder rather than a constructor argument because the default —
    /// "nothing beyond the payload" — is right for every scalar and for
    /// `VarCell` and `Range`, and a required argument would make each of those
    /// declarations spell out the same `None`.
    #[must_use]
    pub const fn with_owned_bytes(self, owned_bytes: OwnedBytesFn) -> TypeDescriptor {
        TypeDescriptor {
            id: self.id,
            name: self.name,
            size: self.size,
            align: self.align,
            trace: self.trace,
            drop_value: self.drop_value,
            format: self.format,
            equals: self.equals,
            hash: self.hash,
            compare: self.compare,
            owned_bytes: Some(owned_bytes),
        }
    }

    /// Bytes `payload` owns outside its allocation block, or 0 if this type
    /// owns nothing beyond its payload.
    ///
    /// # Safety
    /// `payload` must point at a value of this descriptor's type.
    #[inline]
    pub unsafe fn owned_bytes_of(&self, payload: *const u8) -> usize {
        match self.owned_bytes {
            // SAFETY: forwarded from this function's contract.
            Some(f) => unsafe { f(payload) },
            None => 0,
        }
    }

    /// This descriptor's type identity.
    #[inline]
    pub const fn id(&self) -> TypeId {
        self.id
    }

    /// Which built-in this descriptor is, if any.
    #[inline]
    pub const fn as_builtin(&self) -> Option<BuiltinTypeId> {
        self.id.as_builtin()
    }

    /// Size in bytes of this type's payload.
    #[inline]
    pub const fn size(&self) -> usize {
        self.size
    }

    /// Alignment in bytes of this type's payload.
    #[inline]
    pub const fn align(&self) -> usize {
        self.align
    }

    /// True iff values of this type participate in structural equality (§5.5).
    #[inline]
    pub fn is_equatable(&self) -> bool {
        self.equals.is_some()
    }

    /// True iff values of this type have a structural hash (§5.5).
    ///
    /// Not the same as "may be a `Map` key": a `Vec` hashes and can never be a
    /// key (ADR-057 D4). That question is
    /// `praxis_hir::capability::supports_hash_stable`.
    #[inline]
    pub fn is_hashable(&self) -> bool {
        self.hash.is_some()
    }

    /// True iff values of this type have a **container** order — the sequence a
    /// `Map`, `Set`, `Counter` or heap puts them in (ADR-138).
    ///
    /// Not the source language's `<`: that is
    /// `praxis_hir::capability::supports_ord`, and it is a strictly smaller set
    /// on purpose. A tuple answers `true` here and is still refused by `<`.
    #[inline]
    pub fn is_orderable(&self) -> bool {
        self.compare.is_some()
    }
}

/// A descriptor together with the Rust type of the payload it describes.
///
/// [`TypeDescriptor::builtin`] takes the payload type `P`, derives `size`/`align`
/// from it, and then **erases it**. An allocator that took the payload as a bare
/// generic could therefore only compare widths at *runtime* —
/// `gc_alloc(ctx, &scalars::INT, 0)` passes an `i32`, because Rust's default
/// integer type is not `i64`, and aborts the process with "payload size mismatch
/// for descriptor Int" from inside `extern "C"`. That is the non-unwinding panic
/// across the ABI §10.4 forbids, and it cannot fire until the wrong call runs.
///
/// `Payload<T>` re-attaches the type. The pairing is checked once, where the
/// handle is declared — [`Payload::new`] is a `const fn` whose assertions run
/// during const evaluation, so a `static`/`const` handle whose `T` is not its
/// descriptor's payload **fails to compile**. And because the allocators take
/// the handle and the value together, the value's type is checked at every call
/// site by ordinary type inference. Neither mistake reaches a runtime assert.
pub struct Payload<T: Copy> {
    descriptor: &'static TypeDescriptor,
    /// `fn() -> T` rather than `T`: invariance is not wanted here, and this
    /// marker leaves `Payload<T>` `Copy`/`Send`/`Sync` whatever `T` is.
    _payload: std::marker::PhantomData<fn() -> T>,
}

// Derived impls would demand `T: Clone`/`T: Copy` bounds that the marker makes
// unnecessary — a handle is two words of shared metadata, not a value.
impl<T: Copy> Clone for Payload<T> {
    fn clone(&self) -> Self {
        *self
    }
}
impl<T: Copy> Copy for Payload<T> {}

impl<T: Copy> Payload<T> {
    /// Pair `descriptor` with the payload type `T`.
    ///
    /// Declare the result as a `const` or `static` — that is what makes the
    /// check a compile-time one. Called in a runtime expression the assertions
    /// are ordinary ones, which is the situation this type exists to remove.
    ///
    /// # Panics
    /// During const evaluation, if `T`'s layout is not the one `descriptor`
    /// declares.
    #[must_use]
    pub const fn new(descriptor: &'static TypeDescriptor) -> Payload<T> {
        assert!(
            std::mem::size_of::<T>() == descriptor.size(),
            "payload type is not this descriptor's width"
        );
        assert!(
            std::mem::align_of::<T>() == descriptor.align(),
            "payload type is not this descriptor's alignment"
        );
        Payload {
            descriptor,
            _payload: std::marker::PhantomData,
        }
    }

    /// The descriptor this handle carries. Its *address* is the type's identity,
    /// and the handle holds the one `static`, so that identity survives.
    #[must_use]
    pub const fn descriptor(self) -> &'static TypeDescriptor {
        self.descriptor
    }

    /// Read the payload at `payload` as this handle's `T`.
    ///
    /// The **width is the compiler's**: it is `size_of::<T>()`, and
    /// [`Payload::new`] proved during const evaluation that that is exactly the
    /// descriptor's declared width. A caller therefore cannot pick a width, and
    /// cannot pick the wrong one — a hand-written read of a one-byte `Bool`
    /// through an `i64` consumes seven bytes of arena padding the allocator
    /// never initialized.
    ///
    /// This is the read half of what `Payload<T>` already does for allocation.
    /// It does **not** check the object's descriptor — a handle names a type but
    /// a raw payload pointer carries no header — so callers that hold a `GcRef`
    /// should reach for the wrapper that checks identity first.
    ///
    /// # Safety
    /// `payload` must point at an initialized payload of this handle's type,
    /// aligned for `T`.
    #[must_use]
    #[inline]
    pub unsafe fn read(self, payload: *const u8) -> T {
        // SAFETY: the caller guarantees `payload` is an initialized, aligned
        // payload of this descriptor's type, and `Payload::new` already proved
        // `T`'s layout is that type's layout.
        unsafe { payload.cast::<T>().read() }
    }
}

impl<T: Copy> fmt::Debug for Payload<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Payload")
            .field("descriptor", &self.descriptor.name)
            .field("size", &std::mem::size_of::<T>())
            .finish()
    }
}

impl fmt::Debug for TypeDescriptor {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TypeDescriptor")
            .field("id", &self.id)
            .field("name", &self.name)
            .field("size", &self.size)
            .field("align", &self.align)
            .field("equatable", &self.is_equatable())
            .field("hashable", &self.is_hashable())
            .finish()
    }
}

/// Every built-in descriptor, indexed by its [`BuiltinTypeId`] discriminant.
///
/// This is the registry `BuiltinTypeId::descriptor` reads and the array
/// `builtins_are_indexed_by_their_id` walks; adding a variant without adding an
/// entry here is a compile error on the array length.
pub static BUILTINS: [&TypeDescriptor; BuiltinTypeId::COUNT] = [
    &crate::scalars::UNIT,
    &crate::scalars::BOOL,
    &crate::scalars::INT,
    &crate::scalars::BYTE,
    &crate::scalars::CHAR,
    &crate::scalars::FLOAT,
    &crate::text::TEXT,
    &crate::collections::VEC,
    &crate::collections::DEQUE,
    &crate::collections::GRID,
    &crate::maps::MAP,
    &crate::maps::SET,
    &crate::maps::COUNTER,
    &crate::heaps::MIN_HEAP,
    &crate::heaps::MAX_HEAP,
    &crate::bitset::BITSET,
    &crate::tuples::TUPLE,
    &crate::records::RECORD,
    &crate::enums::ENUM,
    &crate::closures::CLOSURE,
    &crate::var_cell::VAR_CELL,
    &crate::range::RANGE,
];

/// [`BUILTINS`] as raw addresses, for
/// [`RuntimeContext::descriptors`](crate::RuntimeContext::descriptors) to hold
/// by value (ADR-116).
///
/// **Derived rather than written out, which is the point.** Generated code
/// proves a value's type by loading slot `id` of that array and comparing it
/// against the header's descriptor word (ADR-102), so a slot holding a
/// neighbour's descriptor would be a proof of the wrong type. Mapping
/// `BUILTINS` here leaves the registry as the one place the index-to-descriptor
/// correspondence is stated, and `builtins_are_indexed_by_their_id` as the one
/// gate on it.
///
/// A `fn` and not a `const fn`: const evaluation may not read a `static`, and
/// `BUILTINS` is one deliberately — the addresses *are* the identities
/// (`builtin_descriptors_have_a_stable_address`). It is called once per
/// [`Runtime::context`](crate::Runtime::context), which is once per program
/// run, not per call into generated code.
#[must_use]
pub fn builtin_descriptor_addresses() -> [*const TypeDescriptor; BuiltinTypeId::COUNT] {
    BUILTINS.map(|d| d as *const TypeDescriptor)
}

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

    /// Smoke test: a descriptor can be constructed and copied, and the
    /// `is_equatable` / `is_hashable` flags reflect the optional callbacks.
    /// The function pointers here are dummies that must never be called — the
    /// point is that the *type* is well-formed.
    unsafe fn dummy_trace(_: *mut u8, _: &mut dyn Tracer) {}
    unsafe fn dummy_drop(_: *mut u8) {}
    unsafe fn dummy_format(_: *const u8, _: &mut FormatSink<'_>) {}
    unsafe fn dummy_eq(a: *const u8, b: *const u8) -> bool {
        a == b
    }
    unsafe fn dummy_hash(_: *const u8, _: &mut dyn DynamicHasher) {}

    #[test]
    fn descriptor_constructs_and_reports_capabilities() {
        static EQUATABLE_ONLY: TypeDescriptor = TypeDescriptor::for_test::<i64>(
            0,
            "EquatableOnly",
            dummy_trace,
            dummy_drop,
            dummy_format,
            Some(dummy_eq),
            None,
            None,
        );
        assert!(EQUATABLE_ONLY.is_equatable());
        assert!(!EQUATABLE_ONLY.is_hashable());
        assert!(!EQUATABLE_ONLY.is_orderable());

        static HASHABLE: TypeDescriptor = TypeDescriptor::for_test::<[u64; 2]>(
            1,
            "Key",
            dummy_trace,
            dummy_drop,
            dummy_format,
            Some(dummy_eq),
            Some(dummy_hash),
            None,
        );
        assert!(HASHABLE.is_equatable());
        assert!(HASHABLE.is_hashable());
        assert_eq!(HASHABLE.size(), 16);
        assert_eq!(HASHABLE.align(), 8);
    }

    /// A test descriptor's id is outside the built-in range by construction, so
    /// a fixture can never be mistaken for a real type.
    #[test]
    fn test_descriptor_ids_are_not_builtins() {
        static PROBE: TypeDescriptor = TypeDescriptor::for_test::<u8>(
            0,
            "Probe",
            dummy_trace,
            dummy_drop,
            dummy_format,
            None,
            None,
            None,
        );
        assert_eq!(PROBE.as_builtin(), None);
    }

    #[test]
    fn builtin_type_ids_are_globally_unique() {
        let mut by_id = std::collections::BTreeMap::new();

        for descriptor in BUILTINS {
            if let Some(previous) = by_id.insert(descriptor.id(), descriptor.name) {
                panic!(
                    "built-in descriptors {previous} and {} share {:?}; descriptor IDs are runtime type identity",
                    descriptor.name,
                    descriptor.id()
                );
            }
        }
        assert_eq!(by_id.len(), BuiltinTypeId::COUNT);
    }

    /// The registry is a lookup table: `BUILTINS[b as usize]` must be the
    /// descriptor whose id *is* `b`. Without this, `BuiltinTypeId::descriptor`
    /// would silently return a neighbour.
    #[test]
    fn builtins_are_indexed_by_their_id() {
        for (index, descriptor) in BUILTINS.iter().enumerate() {
            assert_eq!(
                descriptor.id().to_u32(),
                index as u32,
                "BUILTINS[{index}] is {} whose id is {:?}",
                descriptor.name,
                descriptor.id()
            );
            let builtin = BuiltinTypeId::from_u32(index as u32).expect("index is in range");
            assert!(std::ptr::eq(builtin.descriptor(), *descriptor));
        }
        assert!(BuiltinTypeId::from_u32(BuiltinTypeId::COUNT as u32).is_none());
    }

    /// Built-in descriptors are `static`, so their address is their identity.
    /// Two reads of the same descriptor must produce the same pointer — this is
    /// what lets the runtime compare descriptors by pointer rather than by id.
    #[test]
    fn builtin_descriptors_have_a_stable_address() {
        assert!(std::ptr::eq(&crate::scalars::INT, &crate::scalars::INT));
        assert!(std::ptr::eq(
            BuiltinTypeId::Int.descriptor(),
            &crate::scalars::INT
        ));
        assert!(!std::ptr::eq(
            &crate::scalars::FLOAT,
            &crate::text::TEXT as &TypeDescriptor
        ));
    }
}