Right now we still suffer from extra copies for derived types like ui::KeyEvent. We create a bunch of temporary mojom::*Data types and then copy things over to their file destination. We need to direct copy into the final destination which means we need StructTraits for derived types too. For example:

struct StructTraits<ui::mojom::KeyData, std::unique_ptr<ui::KeyEvent>> {
 .. 
}

would populate the key event specific fields in the KeyEvent.