Omit default types in HirDisplay SourceCode mode

crates/ra_hir_ty/src/tests/display_source_code.rs

@@ -21,3 +21,30 @@ fn bar() {
     );
     assert_eq!("foo::Foo", displayed_source_at_pos(&db, pos));
 }
+
+#[test]
+fn omit_default_type_parameters() {
+    let (db, pos) = TestDB::with_position(
+        r"
+        //- /main.rs
+        struct Foo<T = u8> { t: T }
+        fn main() {
+            let foo = Foo { t: 5 };
+            foo<|>;
+        }
+        ",
+    );
+    assert_eq!("Foo", displayed_source_at_pos(&db, pos));
+
+    let (db, pos) = TestDB::with_position(
+        r"
+        //- /main.rs
+        struct Foo<K, T = u8> { k: K, t: T }
+        fn main() {
+            let foo = Foo { k: 400, t: 5 };
+            foo<|>;
+        }
+        ",
+    );
+    assert_eq!("Foo<i32>", displayed_source_at_pos(&db, pos));
+}