fix: comletion detail shows {unknown} for impl Trait in return position

crates/hir/src/lib.rs

@@ -1358,9 +1358,9 @@ pub fn name(self, db: &dyn HirDatabase) -> Name {
     /// Get this function's return type
     pub fn ret_type(self, db: &dyn HirDatabase) -> Type {
         let resolver = self.id.resolver(db.upcast());
-        let ret_type = &db.function_data(self.id).ret_type;
-        let ctx = hir_ty::TyLoweringContext::new(db, &resolver);
-        let ty = ctx.lower_ty(ret_type);
+        let substs = TyBuilder::placeholder_subst(db, self.id);
+        let callable_sig = db.callable_item_signature(self.id.into()).substitute(Interner, &substs);
+        let ty = callable_sig.ret().clone();
         Type::new_with_resolver_inner(db, &resolver, ty)
     }
 

crates/ide_completion/src/tests/expression.rs

@@ -602,3 +602,42 @@ fn variant    fn() -> Enum
         "#]],
     );
 }
+
+#[test]
+fn detail_impl_trait_in_return_position() {
+    check_empty(
+        r"
+//- minicore: sized
+trait Trait<T> {}
+fn foo<U>() -> impl Trait<U> {}
+fn main() {
+    self::$0
+}
+",
+        expect![[r"
+            tt Trait
+            fn main() fn()
+            fn foo()  fn() -> impl Trait<U>
+        "]],
+    );
+}
+
+#[test]
+fn detail_async_fn() {
+    // FIXME: #11438
+    check_empty(
+        r#"
+//- minicore: future, sized
+trait Trait<T> {}
+async fn foo() -> u8 {}
+fn main() {
+    self::$0
+}
+"#,
+        expect![[r"
+            tt Trait
+            fn main() fn()
+            fn foo()  async fn() -> impl Future<Output = u8>
+        "]],
+    );
+}