Ok let's say there's existing code that requires that a type is not Clone. Then that type definitely would not have #[derive(Clone)] applied to it. So it would

Ok let's say there's existing code that requires that a type is not Clone. Then that type definitely would not have #[derive(Clone)] applied to it. So it would not be affected by the change. So it would not be broken. It's only a breaking change if code that previously worked stops working without changing the code.