chore: Avoid allocation if PollSemaphore is unused

[NeoLegends]

Motivation

Cloning PollSemaphore allocates, even if it ends up unused (e. g. if .try_acquire_owned is used as a fast-path). This is due to the underlying ReusableBoxFuture which is always initialized and holding a future.

Solution

This turns the ReusableBoxFuture optional, and only initializes it on the first call to .poll_acquire, when it's actually needed.

The size of the semaphore doesn't change, rustc is able to exploit the niche in NonNull inside of ReusableBoxFuture, but this does introduce an additional branch, so not sure this optimization is worth it in the end.

If desired I can file a similar PR for PollSender.

[Darksonn]

Thank you for the PR.

[Darksonn]

Another alternative that sometimes works is to initialize the ReusableBoxFuture with a zero-sized future, which also would not allocate. But it probably doesn't help much in this specific case.

[NeoLegends]

Yup! But that wouldn't let us fast-path .try_acquire anymore without an explicit flag, would it?

[Darksonn]

Yeah, I don't think it would help here, but it could be done for PollSender, and maybe also the watch/broadcast wrappers in tokio-stream.

[NeoLegends]

Thank you for the feedback ❤️

I added another commit adding a fast-path for when a permit is immediately available.

[Darksonn]

Thank you for the PR. 😃

[Darksonn]

If desired I can file a similar PR for PollSender.

Such a PR would be accepted. The same is also the case for the broadcast and watch wrappers in tokio-stream.

[NeoLegends]

🎉