I agree that it's rarely a good idea to do this; it's a red flag at minimum when you see it. I'm just trying to reject the notion that it's categorically unsoun

I agree that it's rarely a good idea to do this; it's a red flag at minimum when you see it. I'm just trying to reject the notion that it's categorically unsound to do this. I've personally seen the safe-trait example while reviewing a crate, and there were indeed plenty of comments documenting the assumptions. As long as there's absolutely no way for anyone to cause these assumptions to be broken from the outside, there's no way to cause UB, and it's not really useful to call the interface unsound.