[galangalalgol]

Moving stuff into a closure is a perfect example of that. Try to capture an arcmutex to a bunch of threads in a closure? The compiler will note that arc can't be copied and to try moving it. It will then say you are trying to move the same arc to multiple threads, so you clone the arc. And if you try to just use an Rc it will tell you that can't be sent between threads (not thread safe) use arc instead. Those messages can absolutely guide you a long way before you realize your whole concept won't work, and that is frustrating, but those steps usually make you understand the whole issue.