Fall back to Rc on targets without atomics

[smaeul]

alloc::sync::Arc is unavailable on targets without atomics, such as ESP32-C3, which is riscv32imc. In this case, use Rc instead, and make the user responsible for synchronization.

I'm not sure if it is okay to do the fallback silently, or if you'd prefer to gate this behind a feature. This is the same condition as used by alloc, so this change should only affect targets which previously would fail to build.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.89%. Comparing base (3db1dd3) to head (8f20e9b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
- Coverage   90.90%   90.89%   -0.02%     
==========================================
  Files          55       55              
  Lines        7456     7456              
==========================================
- Hits         6778     6777       -1     
- Misses        678      679       +1     

Files with missing lines	Coverage Δ
src/mqtt/common/arc_payload.rs	88.76% <ø> (ø)
src/mqtt/connection/packet_builder.rs	97.75% <ø> (ø)
src/mqtt/packet/v3_1_1/publish.rs	95.74% <ø> (ø)
src/mqtt/packet/v5_0/publish.rs	94.48% <ø> (ø)

... and 1 file with indirect coverage changes

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• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[redboltz]

Thank you for the PR.

I’ve been thinking about whether automatic detection and switching is really the right choice.

#[cfg(target_has_atomic = "ptr")]

Arc uses A as Atomic, but it silently falls back to a non-atomic reference counter. That’s my first concern.
In practice, this is fine because most multi-threaded environments provide atomic operations for ptr.
In other words, if #[cfg(not(target_has_atomic = "ptr"))] is true, we can assume the environment is single-threaded.

[smaeul]

In other words, if #[cfg(not(target_has_atomic = "ptr"))] is true, we can assume the environment is single-threaded.

Yes, that is my thought as well.

[redboltz]

It seems that github actions' environmental issue is happened. I just cleared cache and re-run the CI process.

[redboltz]

Thank you for updating! It looks good to me.

[redboltz]

@smaeul Do you have any other (preparing) pull request? If it doesn't, I will release the merged version as the minior updated version. Please let me know.

[smaeul]

@smaeul Do you have any other (preparing) pull request? If it doesn't, I will release the merged version as the minior updated version. Please let me know.

Thanks for the quick review! I have opened issues or PRs for everything I have encountered so far.