[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...