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

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).