> There are other approaches to shared memory, like ML-style mutable pointers to immutable data (perfected in Clojure) and actors. Rust has nothing to do with them, and as far as I understand the core choices made by the language make implementing them very problematic.

Would you mind elaborating on this? At least off the top of my head a mut Arc<T> seems like it should suffice for a mutable pointer to immutable data, and it's not obvious to me what about actors makes implementing them in Rust very problematic.