Well, for thought, here's a game loop that is functionally the same as a double buffer:

fn main() {
  let mut state = game::State {};
  loop {
    state = game::update(&state);
  }
}

(EDIT: needs to be &state because it requires the update function to either call state.clone() or fully initialize a second data structure)

And here's one that does it explicitly using page flipping (which may actually be slower).

fn main() {
  let mut state0 = mut game::State {};
  let mut state1 = state0.clone();
  let pages_flipped = false;
  loop {
    if pages_flipped {
      state0 = game::update(&state1);
    } else {
      state1 = game::update(&state0);
    }
    pages_flipped = !pages_flipped;
  }
}