Merge #3387

3387: Type inference for slice patterns r=flodiebold a=JoshMcguigan

Fixes #3043 

Notes to reviewer:

1. This only works if `expected` is `Ty::Apply`. I'm not sure of the implications of this.
1. This only works if the slice pattern only has a prefix. I think this means it doesn't work for subslice patterns, which are currently only available behind a feature flag.

Co-authored-by: Josh Mcguigan <[email protected]>

1 files changed, 17 insertions, 0 deletions

diff --git a/crates/ra_hir_ty/src/infer/pat.rs b/crates/ra_hir_ty/src/infer/pat.rs
index bf8ea192b..623e52599 100644
--- a/crates/ra_hir_ty/src/infer/pat.rs
+++ b/crates/ra_hir_ty/src/infer/pat.rs
@@ -185,6 +185,23 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 self.write_pat_ty(pat, bound_ty);
                 return inner_ty;
             }
+            Pat::Slice { prefix, slice: _slice, suffix } => {
+                let (container_ty, elem_ty) = match &expected {
+                    ty_app!(TypeCtor::Array, st) => {
+                        (TypeCtor::Array, st.as_single().clone())
+                    },
+                    ty_app!(TypeCtor::Slice, st) => {
+                        (TypeCtor::Slice, st.as_single().clone())
+                    },
+                    _ => (TypeCtor::Slice, Ty::Unknown),
+                };
+
+                for pat_id in prefix.iter().chain(suffix) {
+                    self.infer_pat(*pat_id, &elem_ty, default_bm);
+                }
+
+                Ty::apply_one(container_ty, elem_ty)
+            }
             _ => Ty::Unknown,
         };
         // use a new type variable if we got Ty::Unknown here