Merge #3263

3263: Implement unsizing coercions using Chalk r=matklad a=flodiebold

These are coercions like `&[T; n] -> &[T]`, which are handled by the `Unsize` and `CoerceUnsized` traits. The impls for `Unsize` are all built in to the compiler and require special handling, so we need to provide them to Chalk.

This adds the following `Unsize` impls:
 - `Unsize<[T]> for [T; _]`
 - `Unsize<dyn Trait> for T where T: Trait`
 - `Unsize<dyn SuperTrait> for dyn SubTrait`

Hence we are still missing the 'unsizing the last field of a generic struct' case.

Co-authored-by: Florian Diebold <[email protected]>
Co-authored-by: Florian Diebold <[email protected]>

1 files changed, 4 insertions, 1 deletions

diff --git a/crates/ra_hir_ty/src/lower.rs b/crates/ra_hir_ty/src/lower.rs
index 52da34574..092977e93 100644
--- a/crates/ra_hir_ty/src/lower.rs
+++ b/crates/ra_hir_ty/src/lower.rs
@@ -239,7 +239,10 @@ impl Ty {
     ) -> Ty {
         let ty = match resolution {
             TypeNs::TraitId(trait_) => {
-                let trait_ref = TraitRef::from_resolved_path(ctx, trait_, resolved_segment, None);
+                // if this is a bare dyn Trait, we'll directly put the required ^0 for the self type in there
+                let self_ty = if remaining_segments.len() == 0 { Some(Ty::Bound(0)) } else { None };
+                let trait_ref =
+                    TraitRef::from_resolved_path(ctx, trait_, resolved_segment, self_ty);
                 return if remaining_segments.len() == 1 {
                     let segment = remaining_segments.first().unwrap();
                     let associated_ty = associated_type_by_name_including_super_traits(