[GrumpySloth]

Mutation in Rust is much more controlled than in SML or OCaml. Any record or variable (sic!) can contain a ref cell, or a special data type like an array, which can be mutated regardless of the type of the thing it’s contained within. That’s the ML approach to mutation.

In Rust on the other hand, even though ref cells are a thing as well, the main approach to mutation is through mutable bindings and mutable references, which preserve const-correctness (inspired by C++), i.e. there is a certain degree of transitivity in the guarantees around immutability. When I see an immutable reference in Rust and pass it to a function (or a method), then, outside of few special cases, I can reasonably expect it to stay unmodified. In ML I can’t, unless the code is written in a very unoptimal way, i.e. with persistent data structures anywhere and everywhere, and still the type system won’t give me any hints about that.