implement initial type inference for index expressions

crates/ra_hir/src/expr.rs

@@ -245,6 +245,10 @@ pub enum Expr {
         rhs: ExprId,
         op: Option<BinaryOp>,
     },
+    Index {
+        base: ExprId,
+        index: ExprId,
+    },
     Lambda {
         args: Vec<PatId>,
         arg_types: Vec<Option<TypeRef>>,
@@ -399,6 +403,10 @@ impl Expr {
                 f(*lhs);
                 f(*rhs);
             }
+            Expr::Index { base, index } => {
+                f(*base);
+                f(*index);
+            }
             Expr::Field { expr, .. }
             | Expr::Await { expr }
             | Expr::Try { expr }
@@ -887,10 +895,14 @@ where
                 };
                 self.alloc_expr(Expr::Literal(lit), syntax_ptr)
             }
+            ast::ExprKind::IndexExpr(e) => {
+                let base = self.collect_expr_opt(e.base());
+                let index = self.collect_expr_opt(e.index());
+                self.alloc_expr(Expr::Index { base, index }, syntax_ptr)
+            }
 
             // FIXME implement HIR for these:
             ast::ExprKind::Label(_e) => self.alloc_expr(Expr::Missing, syntax_ptr),
-            ast::ExprKind::IndexExpr(_e) => self.alloc_expr(Expr::Missing, syntax_ptr),
             ast::ExprKind::RangeExpr(_e) => self.alloc_expr(Expr::Missing, syntax_ptr),
             ast::ExprKind::MacroCall(e) => {
                 let ast_id = self

crates/ra_hir/src/ty/infer.rs

@@ -1279,6 +1279,12 @@ impl<'a, D: HirDatabase> InferenceContext<'a, D> {
                 }
                 _ => Ty::Unknown,
             },
+            Expr::Index { base, index } => {
+                let _base_ty = self.infer_expr(*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
+            }
             Expr::Tuple { exprs } => {
                 let mut ty_vec = Vec::with_capacity(exprs.len());
                 for arg in exprs.iter() {

crates/ra_hir/src/ty/tests.rs

@@ -2655,6 +2655,20 @@ fn test() -> u64 {
     );
 }
 
+#[test]
+fn indexing_arrays() {
+    assert_snapshot_matches!(
+        infer("fn main() { &mut [9][2]; }"),
+        @r###"
+[10; 26) '{ &mut...[2]; }': ()
+[12; 23) '&mut [9][2]': &mut {unknown}
+[17; 20) '[9]': [i32;_]
+[17; 23) '[9][2]': {unknown}
+[18; 19) '9': i32
+[21; 22) '2': i32"###
+    )
+}
+
 #[test]
 fn infer_macros_expanded() {
     assert_snapshot_matches!(