[glhaynes]

Swift arrays have pass-by-value semantics (under the cover, they use copy-on-write to keep performance good) and are thus safe to pass to functions without concern that they'll be modified. (Unless it's passed as an `inout` parameter, which requires a prepended "&" at the call site.)

[zozbot123]

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.

[glhaynes]

I don't mean to imply that Swift's array performance is better than anything other than what its own performance would be if it didn't implement COW. (My intention was to convey that the normal way of passing arrays comes with confidence that your copy won't be modified by the callee.) Rust sounds nice, I'll have to try it sometime.

[millstone]

Are you sure that code expresses the same pattern? It looks like Rust's COW can only transition from Borrowed to Owned by making a copy. Swift's COW works by dynamically inspecting the reference count.

[giornogiovanna]

Right, unless I'm mistaken, what you want is `Rc` with `Rc::make_mut`[0], not `Cow`.

[0]: https://doc.rust-lang.org/stable/std/rc/struct.Rc.html#metho...