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);