This is an area where Rust ends up feeling different than many languages: xi's rope uses https://doc.rust-lang.org/stable/std/rc/struct.Rc.html#metho... like this: https://github.com/google/xi-editor/blob/422948d62688dcc2ee0... , which gives you both options. This code uses Rc, not Arc, so it's not currently sharable between threads, but Arc has the same API.

Rust cares about sharing more than immutability.