Merge #2592

2592: Add std::ops::Index support for infering r=edwin0cheng a=edwin0cheng

see also #2534

Seem like this can't fix #2534 for this case:

```rust
fn foo3(bar: [usize; 2]) {
    let baz = bar[1];   // <--- baz is still unknown ?
    println!("{}", baz);
}
```

Co-authored-by: Edwin Cheng <[email protected]>

1 files changed, 8 insertions, 4 deletions

diff --git a/crates/ra_hir_ty/src/infer/expr.rs b/crates/ra_hir_ty/src/infer/expr.rs
index 6917c183b..8be567917 100644
--- a/crates/ra_hir_ty/src/infer/expr.rs
+++ b/crates/ra_hir_ty/src/infer/expr.rs
@@ -422,10 +422,14 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 }
             }
             Expr::Index { base, index } => {
-                let _base_ty = self.infer_expr_inner(*base, &Expectation::none());
-                let _index_ty = self.infer_expr(*index, &Expectation::none());
-                // FIXME: use `std::ops::Index::Output` to figure out the real return type
-                Ty::Unknown
+                let base_ty = self.infer_expr_inner(*base, &Expectation::none());
+                let index_ty = self.infer_expr(*index, &Expectation::none());
+
+                self.resolve_associated_type_with_params(
+                    base_ty,
+                    self.resolve_ops_index_output(),
+                    &[index_ty],
+                )
             }
             Expr::Tuple { exprs } => {
                 let mut tys = match &expected.ty {