Atomic compare exchange shouldn't require the failure ordering to be as strong as the success ordering

[m-ou-se]

Right now, Rust does not support compare_exchange(a, b, Acquire, Release): we require the success ordering to be at least as strong as the failure ordering, which means we must upgrade the success ordering to AcqRel in this example.

C++ proposal P0418r2 argues this is unnecessary.

The restriction has been removed in C++17. We should consider doing the same in Rust.

[tmiasko]

Duplicate of #68464.

[m-ou-se]

Oh, thanks!