[nyanpasu64]

Rust's im[1] and rpds[2] crates are refcounted pointers to immutable data structures, but support mutable operations on &mut instances. When an instance is cloned, it merely creates another pointer. When an instance is modified, it uses Arc::make_mut() to only clone each tree node if it has other users. This approach has runtime overhead, but makes nested updates (foo[0][0].attr = 1) as simple as mutable structures.

This somewhat resembles immer.js (uses a proxy around an immutable structure which records updates). Contrast this approach to Clojure transients (whose children don't magically become transient), and whatever Haskell does (https://news.ycombinator.com/item?id=24740384).

[1]: https://docs.rs/im/

[2]: https://docs.rs/rpds/