Support portable atomic

[ElliottjPierce]

This lets users use this crate on targets that don't support atomics.

cargo check --no-default-features --target thumbv6m-none-eabi Fails on main.

But cargo check --no-default-features --target thumbv6m-none-eabi --features portable-atomic,portable-atomic/fallback,portable-atomic/critical-section passes on this PR.

This is blocking work in dependent PRs, for example, see here.

[taiki-e]

Thanks for the PR! The implementation looks good.

Could you update CI config to check the case where this feature enabled?

It would be sufficient to replace the following two with the commands used in async-task (this and this),

async-channel/.github/workflows/ci.yml

Lines 63 to 64 in 3ce7937

	run: cargo hack build --all --no-dev-deps
	- run: cargo hack build --all --target thumbv7m-none-eabi --no-default-features --no-dev-deps

async-channel/.github/workflows/ci.yml

Lines 77 to 78 in 3ce7937

	- run: cargo hack build --rust-version
	- run: cargo hack build --no-default-features --rust-version

and add two additional cases (cargo test --feature portable-atomic and cargo test --no-default-features --feature portable-atomic) after the following.

async-channel/.github/workflows/ci.yml

Lines 55 to 56 in 3ce7937

	- run: cargo test
	- run: cargo test --no-default-features

[ElliottjPierce]

@taiki-e Thanks! I'm no expert with CI, so I'm not certain this does what you're asking, but I think it does. Does it look right to you now?

[taiki-e]

non atomic platforms

thumbv7m has atomics...
You have to use thumbv6m if you want to check target without atomics.

[ElliottjPierce]

I'll be honest, I don't know that much about these platforms. I copied this check from bevy ci. If you wan't to change, the 7 to a 6, go for it. (Or I can if you prefer.) But I don't have the context to know which is what y'all want to use here.

[taiki-e]

I see. I actually prefer to check both, so I added the change for that.

[ElliottjPierce]

Sounds good. Thanks!

[taiki-e]

Published in 2.4.0.