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 <edwin0cheng@gmail.com>

crates/ra_hir_def/src/path.rs

@@ -257,6 +257,7 @@ macro_rules! __known_path {
     (std::ops::Try) => {};
     (std::ops::Neg) => {};
     (std::ops::Not) => {};
+    (std::ops::Index) => {};
     ($path:path) => {
         compile_error!("Please register your known path in the path module")
     };

crates/ra_hir_expand/src/name.rs

@@ -161,6 +161,7 @@ pub mod known {
         Range,
         Neg,
         Not,
+        Index,
         // Builtin macros
         file,
         column,

crates/ra_hir_ty/src/infer.rs

@@ -363,14 +363,26 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
     }
 
     fn resolve_associated_type(&mut self, inner_ty: Ty, assoc_ty: Option<TypeAliasId>) -> Ty {
+        self.resolve_associated_type_with_params(inner_ty, assoc_ty, &[])
+    }
+
+    fn resolve_associated_type_with_params(
+        &mut self,
+        inner_ty: Ty,
+        assoc_ty: Option<TypeAliasId>,
+        params: &[Ty],
+    ) -> Ty {
         match assoc_ty {
             Some(res_assoc_ty) => {
                 let ty = self.table.new_type_var();
+                let builder = Substs::build_for_def(self.db, res_assoc_ty)
+                    .push(inner_ty)
+                    .fill(params.iter().cloned());
                 let projection = ProjectionPredicate {
                     ty: ty.clone(),
                     projection_ty: ProjectionTy {
                         associated_ty: res_assoc_ty,
-                        parameters: Substs::single(inner_ty),
+                        parameters: builder.build(),
                     },
                 };
                 self.obligations.push(Obligation::Projection(projection));
@@ -517,6 +529,12 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
         let struct_ = self.resolver.resolve_known_struct(self.db, &path)?;
         Some(struct_.into())
     }
+
+    fn resolve_ops_index_output(&self) -> Option<TypeAliasId> {
+        let path = path![std::ops::Index];
+        let trait_ = self.resolver.resolve_known_trait(self.db, &path)?;
+        self.db.trait_data(trait_).associated_type_by_name(&name![Output])
+    }
 }
 
 /// The kinds of placeholders we need during type inference. There's separate

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 {

crates/ra_hir_ty/src/tests/traits.rs

@@ -426,6 +426,38 @@ fn indexing_arrays() {
     )
 }
 
+#[test]
+fn infer_ops_index() {
+    let (db, pos) = TestDB::with_position(
+        r#"
+//- /main.rs crate:main deps:std
+
+struct Bar;
+struct Foo;
+
+impl std::ops::Index<u32> for Bar {
+    type Output = Foo;
+}
+
+fn test() {
+    let a = Bar;
+    let b = a[1];
+    b<|>;
+}
+
+//- /std.rs crate:std
+
+#[prelude_import] use ops::*;
+mod ops {
+    pub trait Index<Idx> {
+        type Output;
+    }
+}
+"#,
+    );
+    assert_eq!("Foo", type_at_pos(&db, pos));
+}
+
 #[test]
 fn deref_trait() {
     let t = type_at(