Rust can use its own bovine superpowers here https://doc.rust-lang.org/std/borrow/enum.Cow.html to enable developers to write code that's invariant over whether it's dealing with a fresh copy of something, or just a borrowed reference. So, it's not quite correct to say that Swift's use of copy-on-write gives it better performance. Rust can express the exact same pattern quite easily, and do it with far more generality.