struct User { active: bool, username: String, email: String, sign_in_count: u64, } fn build_user(email: String, username: String) -> User { User { active: true, username, email, sign_in_count: 1, } }

`build_user` 함수에서 `username`, `email` 은 키값 생략이 되는데 왜 `active` 랑 `sign_in_count` 는 생략하면 안되는거임?