That's why Cell<T> and RefCell<T> are a thing. Both allow you to mutate shared references, but disable shared access across threads. The qcell crate even includes a version of Cell/RefCell that's "branded" by a region/lifetime, just like in this proposal.
As I remember, Cell only allows moving/copying data from/to cell so if you have a 128-byte object inside do you have to copy it to modify? Or this can be optimized?
