I haven't filed an RFC yet, but I have a plan that I'm optimistic can land sometime that would let you write:

  struct User {
    id: Id,
    email: EMail,
    name: String = String::new(),
    age: u16 = 0,
  }
  
  let user = User { id, email, .. }; // valid
  let user = User { id, .. }; // complain about email missing

It would leverage `const` expressions, so it would desugar effectively to the same as if you had written:

  const DEFAULT_USER_NAME: String = String::new();
  const DEFAULT_USER_AGE: u16 = 0;

  struct User {
    id: Id,
    email: EMail,
    name: String,
    age: u16,
  }
  
  let user = User { id, email, name: DEFAULT_USER_NAME, age: DEFAULT_USER_AGE };

The above assumes that String::new will be const at some point (it can be).

If this ever lands, then there will be less need to write Builder traits by hand.

Also, at some point the free-standing `default()` function that calls `Default::default()` will land, making it that much shorter to write (without any new features like I'm proposing).