From a7f81e3cdcdd65077893b170378179ec89eab347 Mon Sep 17 00:00:00 2001
From: Lukas Wirth <lukastw97@gmail.com>
Date: Fri, 27 Jan 2023 11:15:05 +0100
Subject: [PATCH] Record method resolution for index expressions

---
 crates/hir-ty/src/infer/expr.rs   | 12 ++++++++++--
 crates/hir-ty/src/tests/traits.rs |  6 ++----
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/crates/hir-ty/src/infer/expr.rs b/crates/hir-ty/src/infer/expr.rs
index 334434c30ca..1f8fa4a20f9 100644
--- a/crates/hir-ty/src/infer/expr.rs
+++ b/crates/hir-ty/src/infer/expr.rs
@@ -17,7 +17,7 @@
     resolver::resolver_for_expr,
     ConstParamId, FieldId, ItemContainerId, Lookup,
 };
-use hir_expand::name::Name;
+use hir_expand::name::{name, Name};
 use stdx::always;
 use syntax::ast::RangeOp;
 
@@ -741,7 +741,6 @@ fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
                 }
             }
             Expr::Index { base, index } => {
-                // FIXME: note down method resolution for the `index`/`index_mut` function
                 let base_ty = self.infer_expr_inner(*base, &Expectation::none());
                 let index_ty = self.infer_expr(*index, &Expectation::none());
 
@@ -758,6 +757,15 @@ fn infer_expr_inner(&mut self, tgt_expr: ExprId, expected: &Expectation) -> Ty {
                             adj.apply(&mut self.table, base_ty)
                         });
                     self.write_expr_adj(*base, adj);
+                    if let Some(func) =
+                        self.db.trait_data(index_trait).method_by_name(&name!(index))
+                    {
+                        let substs = TyBuilder::subst_for_def(self.db, index_trait, None)
+                            .push(self_ty.clone())
+                            .push(index_ty.clone())
+                            .build();
+                        self.write_method_resolution(tgt_expr, func, substs.clone());
+                    }
                     self.resolve_associated_type_with_params(
                         self_ty,
                         self.resolve_ops_index_output(),
diff --git a/crates/hir-ty/src/tests/traits.rs b/crates/hir-ty/src/tests/traits.rs
index d8c3588acf6..015085bde45 100644
--- a/crates/hir-ty/src/tests/traits.rs
+++ b/crates/hir-ty/src/tests/traits.rs
@@ -2295,10 +2295,8 @@ fn unselected_projection_in_trait_env_no_cycle() {
     // this is not a cycle
     check_types(
         r#"
-//- /main.rs
-trait Index {
-    type Output;
-}
+//- minicore: index
+use core::ops::Index;
 
 type Key<S: UnificationStoreBase> = <S as UnificationStoreBase>::Key;