Expect the person who tried to do it as well (first telling me OnceCell and RWLock would work) in the end could not get it working either and gave up in half a hour. In the end I moved on, I still program in rust, but it's not a good language when you have to work on the memory level and avoid copying data, even with unsafe, which is sad because unsafe could be more powerful if it actually gave you the power to actually instruct the borrow checker at the end of unsafe block what the state of things should be now, when it can't figure it out itself.
Assigning to RwLock is trivial, so if nobody was able to do that, then shared^mutable limitation was a red herring, and your issue wasn't with thread safety at all. HN-comment-based blind debugging of your program is getting pretty far from the topic of the thread and TFA…
The next most common footgun C-style expectations get you in Rust is self-referential structs, when you create a new object and then take (temporary scope-bound) references to inside of it, and try to store them long-term in unrestricted scope. That is a thing safe Rust doesn't guarantee without Rc/Arc, regardless whether it's thread-safe or not (moves can trivially invalidate such structs in single-threaded code too).
