Add operator overloading to atomic types.

[Aatch]

The C++ atomic types overload specified operators, defaulting to a sequentially consistent ordering, we should do the same.

Due to the semantics of atomic operations, however, this is blocked on assignment operator overloading (since a += b is a different operation to c = a + b) (#5992)

[brson]

This worries me a bit. Using overloaded operators for atomics seems prone to abuse or misuse. Using methods makes it clear something special's going on, order of evaluation doesn't depend on operator precedence. I believe that evaluating complex math with atomics is not common.

[thestinger]

Visiting for triage, I agree that these are unlikely to be used very often and could cause confusion. I think almost every use case for atomics in Rust will be to implement concurrent data structures, with a rare case of a static mut atomic like a global counter for debugging or performance statistics.

They aren't really part of the user-level concurrency idioms in Rust (message passing), so it's okay if they aren't pretty.

[sanxiyn]

Triage. Do we actually want this? I personally don't. While "C++ does it" is usually a good argument, it is not an absolute argument.

[alexcrichton]

Closing this due to the general opinion being that atomics should force you to think about memory ordering and related semantics.