Correctly fill default type parameters

crates/ra_syntax/src/ast/make.rs

@@ -13,6 +13,10 @@ pub fn name_ref(text: &str) -> ast::NameRef {
     ast_from_text(&format!("fn f() {{ {}; }}", text))
 }
 
+pub fn type_arg(text: &str) -> ast::TypeArg {
+    ast_from_text(&format!("impl F<{}> for G {{}}", text))
+}
+
 pub fn path_segment(name_ref: ast::NameRef) -> ast::PathSegment {
     ast_from_text(&format!("use {};", name_ref))
 }