Merge #4445

4445: Correctly fill default type parameters r=flodiebold a=montekki

Fixes #3877 

So, basically even if the parameters are omitted from the `impl` block, check the parameters in `trait` if they have a default type, and if they do go from `hir` to `ast::TypeArg`. I've added a helper for that but I am not sure that it's a proper way to go from `hir` to `ast` here.

Co-authored-by: Fedor Sakharov <fedor.sakharov@gmail.com>

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_ref(text: &str) -> ast::TypeRef {
+    ast_from_text(&format!("impl {} for D {{}};", text))
+}
+
 pub fn path_segment(name_ref: ast::NameRef) -> ast::PathSegment {
     ast_from_text(&format!("use {};", name_ref))
 }