It's not just Arc<T> vs. Rc<T> that's relevant for thread safety, though. Pretty much any kind of shared mutability requires extra protection (locks or atomicity) to work safely across threads, so there has to be some way to indicate whether or not that extra protection is present. Not to mention objects that interact with FFI, such as mutex locks, which must be unlocked from the same thread. It would be a huge performance drain to demand that "every value everywhere must be usable from every thread".