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remove confusing comment

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`?0: CoerceUnsized<SomeTy>` can definitely apply because
`?0` matches any type, same for `SomeTy: CoerceUnsized<?0>`
This commit is contained in:
lcnr 2022-11-25 16:35:10 +01:00
parent a215b7b4df
commit 4f07008419
1 changed files with 3 additions and 21 deletions
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24
compiler/rustc_hir_typeck/src/coercion.rs
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@ -503,27 +503,9 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
target = self.shallow_resolve(target);
debug!(?source, ?target);
// These 'if' statements require some explanation.
// The `CoerceUnsized` trait is special - it is only
// possible to write `impl CoerceUnsized<B> for A` where
// A and B have 'matching' fields. This rules out the following
// two types of blanket impls:
//
// `impl<T> CoerceUnsized<T> for SomeType`
// `impl<T> CoerceUnsized<SomeType> for T`
//
// Both of these trigger a special `CoerceUnsized`-related error (E0376)
//
// We can take advantage of this fact to avoid performing unnecessary work.
// If either `source` or `target` is a type variable, then any applicable impl
// would need to be generic over the self-type (`impl<T> CoerceUnsized<SomeType> for T`)
// or generic over the `CoerceUnsized` type parameter (`impl<T> CoerceUnsized<T> for
// SomeType`).
//
// However, these are exactly the kinds of impls which are forbidden by
// the compiler! Therefore, we can be sure that coercion will always fail
// when either the source or target type is a type variable. This allows us
// to skip performing any trait selection, and immediately bail out.
// 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.
if source.is_ty_var() {
debug!("coerce_unsized: source is a TyVar, bailing out");
return Err(TypeError::Mismatch);