[throwaway613834]

Yeah that makes sense! It strikes me as pretty similar to volatile-correctness in C++ (it's a technique -- look it up if you haven't heard of it) with the additional constraint that there is only ever one mutable reference, for better or for worse.

[Rusky]

For cases where you need more than one mutable reference, you can use `Cell` (single-threaded) or atomics, which allow mutation through shared references.