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, 12 insertions, 0 deletions

diff --git a/crates/ra_hir_ty/src/traits.rs b/crates/ra_hir_ty/src/traits.rs
index e83449957..2317fcac3 100644
--- a/crates/ra_hir_ty/src/traits.rs
+++ b/crates/ra_hir_ty/src/traits.rs
@@ -335,6 +335,12 @@ pub struct ClosureFnTraitImplData {
     fn_trait: FnTrait,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct UnsizeToSuperTraitObjectData {
+    trait_: TraitId,
+    super_trait: TraitId,
+}
+
 /// An impl. Usually this comes from an impl block, but some built-in types get
 /// synthetic impls.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
@@ -343,6 +349,12 @@ pub enum Impl {
     ImplBlock(ImplId),
     /// Closure types implement the Fn traits synthetically.
     ClosureFnTraitImpl(ClosureFnTraitImplData),
+    /// [T; n]: Unsize<[T]>
+    UnsizeArray,
+    /// T: Unsize<dyn Trait> where T: Trait
+    UnsizeToTraitObject(TraitId),
+    /// dyn Trait: Unsize<dyn SuperTrait> if Trait: SuperTrait
+    UnsizeToSuperTraitObject(UnsizeToSuperTraitObjectData),
 }
 /// This exists just for Chalk, because our ImplIds are only unique per module.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]