> I’ve never understood why Rust doesn’t require the `Unpin` trait in `mem::swap` and derivatives instead of having `Pin` Pin and Unpin came years after Rust h

> I’ve never understood why Rust doesn’t require the `Unpin` trait in `mem::swap` and derivatives instead of having `Pin` Pin and Unpin came years after Rust had reached 1.0; wouldn't adding a new bound to an existing function be a breaking change?