struct Diamond {}

enum State {
    Early(Diamond),
    Late(Diamond),
    Transitioning,
}

fn advance(state: &mut State) {
    match std::mem::replace(state, State::Transitioning) {
        State::Early(diamond) => {
            *state = State::Late(diamond);
        }
        _ => unreachable!(),
    }
}

fn main() {
    let mut state = State::Early(Diamond {});
    advance(&mut state);
}