Use portable-atomic to allow use on targets without atomic instructions

[9names]

Some targets do not natively support atomics (eg rp2040, esp32-c3).
By adding portable-atomic, these targets can select a method to emulate atomics.

It might be nice to make this a feature instead to avoid forcing it on for everyone.
Happy to get some feedback on this.

[j-g00da]

Thanks! This is great, but it should definitely be behind a feature flag. critical-section and unsafe-assume-single-core features should also be exposed.

[9names]

critical-section and unsafe-assume-single-core features should also be exposed.

This would not be idiomatic. For portable-atomic it's usually the binary crate that specifies these features.
See https://github.com/taiki-e/portable-atomic#usage

But re-reading the usage example above has reminded me that for the features this needs I think I should have written it like this:

portable-atomic = { version = "1.3", default-features = false, features = ["require-cas"] }

[orhun]

Would be nice to have this under a feature flag and then it should be good to go :)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.24%. Comparing base (c557892) to head (0db79a3).

Additional details and impacted files

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##             main      #30   +/-   ##
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  Coverage   76.24%   76.24%           
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  Files           8        8           
  Lines        1229     1229           
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  Hits          937      937           
  Misses        292      292           

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[j-g00da]

Run fmt and I think this is good to go, thanks!

Suggested commit message: feat: Add portable-atomic feature for targets without atomic instructions.

On a side note, it would be nice to make the ci run with std / no std / no std with portable-atomic separately. (In a separate PR.)

After releasing new version we can integrate it into Ratatui.

cc: @joshka

[j-g00da]

LGTM

[joshka]

I think it might be useful to add some more docs to this (just the same things that you've mentioned in the justification of the PR). Basically something that will help answer "Why should I enable this feature and what does it change?".

Right now the doc that will show up on the features lists is "use portable-atomic to polyfill CAS atomics on targets that do not have them", which for someone unfamiliar with those things might be a little too bare. This also applies to the same idea on the Ratatui side. Think about the docs.rs output needs of this.