use std::mem;

struct Diamond {}

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

fn advance(state: &mut State) {
    unsafe {
        let new_state = match state {
            State::Early(diamond) => {
                State::Late(mem::replace(diamond, mem::uninitialized()))
            }
            _ => unreachable!(),
        };
        mem::forget(mem::replace(state, new_state));
    }
}

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