Not only multiple threads, a common mistake is to modify a collection while iterating it, which can happen within a single thread (and it's even worse when it h

Not only multiple threads, a common mistake is to modify a collection while iterating it, which can happen within a single thread (and it's even worse when it happens conditionally, so it'll only fail sometimes). The thing I love the most in Rust is the duality between "&mut" and "&": when you have a "&mut" reference to something, you know nobody else has any "&mut" or a "&" reference to it, so you can modify the data structure without worry; and when you have a "&" reference to something, you know nobody else has any "&mut" reference to it, so you can read the data structure without worry. I wish I had something like that in Java, instead of having to sprinkle "synchronized" everywhere just in case (which not only doesn't protect against a recursive call within the same thread, but also can lead to AB/BA deadlocks... which is made so much more fun by code called from executors which have a ThreadPoolExecutor.CallerRunsPolicy, meaning not only will it happen only in production, it will only happen in production when the system is overloaded. Took us weeks of debugging to reproduce one instance of that...).