use std::mem;

struct Diamond {}

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

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

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