The problem with RefCell/Mutex is granularity. If you wrap the entire object in one of them, the mutable borrow you need to write a field blocks anyone else from accessing the object at all, even to read unrelated fields. Note that Swift 5 exclusivity enforcement is imposing exactly this limitation on Swift "structs", but not its heap-allocated "classes".
In Rust, an alternative is to wrap individual fields in RefCell/Mutex, but that results in uglier syntax – you end up writing RefCell/Mutex and .borrow()/.borrow_mut() a lot of times – and adds overhead, especially in the Mutex case (since each Mutex has an associated heap allocation). There are alternatives, like Cell and Atomic*, that avoid the overhead, but have worse usability problems. I've long thought Rust has room for improvement here...
