Atomic missing wait and notify_* while C++20 supports that

[npuichigo]

[joboet]

While a futex-style API exists on all of our tier 1 platforms, it is not available on quite a lot of other targets, including older, but still supported versions of macOS (well, there's more to it, see #122408). We'd basically have to bring in parking_lot for those (and on everything except Windows too, if you want support for e.g. AtomicU8). If you want to make a case for that, please file an ACP.

[workingjubilee]

...I can guarantee that blow-for-blow exacting API compatibility with C++ is certainly not a goal of Rust, so the ACP should focus on the benefits of having the API per se rather than "C++ has it".

[workingjubilee]

We can reopen this as a tracking issue after the ACP is accepted. Or that can be a new issue. Whatever floats your boat.