Only include AtomicWaker with std build

[Nemo157]

Workaround for #842

This is not my preferred solution, but it's the best solution I can think of that works on stable rust

[cramertj]

I'd like to keep AtomicWaker usable on platforms that support it, even if they aren't using no_std. Do you know of specific platforms that don't support AtomicUsize for which you could cfg out AtomicWaker?

[Nemo157]

The only platform I know of/care about right now is thumbv6m-none-eabi. I assume there are other embedded platforms that don’t, but I can start with just excluding that.

[Nemo157]

So, #[cfg(not(target = "thumbv6m-none-eabi"))] doesn't seem to be supported (not listed in the reference and using it doesn't exclude the code from compiling).

#[cfg(not(target_arch = "arm"))] works, but is far too broad since (for example) that includes armv7-apple-ios which supports atomic instructions.

As mentioned in the linked ticket #[cfg(target_has_atomic = "ptr")] is the correct solution, but that requires the unstable cfg_target_has_atomic feature.

None of the other cfg options seem relevant.

[Nemo157]

Since #854 introduced nightly feature flags, I've pushed an alternative branch that uses #[cfg_attr(feature = "nightly", cfg(target_has_atomic = "ptr"))] to disable AtomicWaker when building with the nightly feature for a target that doesn't support it. If you'd prefer that I can replace this PR with it.

[cramertj]

@Nemo157 That sounds great to me, so long as you're fine sticking to nightly when using thumbv6m-none-eabi. (it's better than not being able to use it at all, certainly!)

[Nemo157]

I have many other things tying me to nightly at the moment, so 🤷‍♂. I’ll wait for #871 to be merged then open a PR to avoid conflicts.