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Shallowly bail from coerce_unsized more #142941

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3dc5b4a
Shallowly bail from coerce_unsized more
compiler-errors Jun 24, 2025
3d115ac
Add a note and one more fast path
compiler-errors Jun 24, 2025
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46 changes: 41 additions & 5 deletions compiler/rustc_hir_typeck/src/coercion.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -515,11 +515,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
///
/// [unsized coercion](https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions)
#[instrument(skip(self), level = "debug")]
fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
source = self.shallow_resolve(source);
target = self.shallow_resolve(target);
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why remove these, are we guaranteed that they are shallow resolved? if so, please convert them into debug asserts

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yes, we shallowly resolve at the top of the coerce entrypoint

debug!(?source, ?target);

fn coerce_unsized(&self, source: Ty<'tcx>, target: Ty<'tcx>) -> CoerceResult<'tcx> {
// We don't apply any coercions incase either the source or target
// aren't sufficiently well known but tend to instead just equate
// them both.
Expand All @@ -532,6 +528,46 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
return Err(TypeError::Mismatch);
}

// These targets are known to never be RHS in `LHS: CoerceUnsized<RHS>`.
// That's because these are built-in types for which a core-provided impl
// doesn't exist, and for which a user-written impl is invalid.
//
// This is technically incomplete when users write impossible bounds like
// `where T: CoerceUnsized<usize>`, for example, but that trait is unstable
// and coercion is allowed to be incomplete. The only case where this matters
// is impossible bounds.
match target.kind() {
ty::Bool
| ty::Char
| ty::Int(_)
| ty::Uint(_)
| ty::Float(_)
| ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
| ty::Str
| ty::Array(_, _)
| ty::Slice(_)
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array to slice is always Deref an not CoerceUnsized? 🤔

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@compiler-errors compiler-errors Jun 25, 2025 •
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Slices are the RHS for Unsize, not CoerceUnsized. The latter is for pointers not data.

| ty::FnDef(_, _)
| ty::FnPtr(_, _)
| ty::Dynamic(_, _, _)
| ty::Closure(_, _)
| ty::CoroutineClosure(_, _)
| ty::Coroutine(_, _)
| ty::CoroutineWitness(_, _)
| ty::Never
| ty::Tuple(_) => return Err(TypeError::Mismatch),
_ => {}
}
// Additionally, we ignore `&str -> &str` coercions, which happen very
// commonly since strings are one of the most used argument types in Rust,
// we do coercions when type checking call expressions.
if let ty::Ref(_, source_pointee, ty::Mutability::Not) = *source.kind()
&& source_pointee.is_str()
&& let ty::Ref(_, target_pointee, ty::Mutability::Not) = *target.kind()
&& target_pointee.is_str()
{
return Err(TypeError::Mismatch);
}

let traits =
(self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
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