I'd just like to throw in a note of concern for the way closures quietly confer reference semantics on the values they capture. If not for this slippery hole in the language guarantees, value semantics and the law of exclusivity would be enough to support the provable thread-safety of most code. In related threads I have seen lots of discussion of actors and queues and other concurrency mediators, but I haven't seen any attention given to this issue with ordinary code they may execute. I think move-only closures may be an important part of the answer and I wonder about attacking concurrency without an ownership model that supports non-copyable types.
/cc @saeta