chore: replace num_cpus with available_parallelism - #2946

https://github.com/rust-lang/futures-rs/actions/runs/14322140475/job/40140977417

Failed to compile iovec 0.1.0 - doesn't seem to be related. It should be resolved to 0.1.4 and previously runs all passed. This patch doesn't touch it.

iovec crate is used by old dependencies that are enabled via the compat/compat-io feature, but given that our traits are planned to remain basically unchanged between futures 0.3 and 0.4, it is not a problem to remove compat and compat-io features in 0.4. (From a maintenance standpoint, their removal is also welcome because they were not easy to maintain (e.g., #2514).)

Thanks for your information! Seems I can take a look at dropping dependency to iovec in another PR. For this one, we'd on hold until then or I can rebase on master and ping you once CI is happy.

Seems libc's version causes this issue.

Previously, we have:

  Adding libc v0.2.26 (available: v0.2.172)

Now, it becomes:

  Adding libc v0.2.0 (available: v0.2.172)

Maybe num_cpus add a requirement to help indicates the version. I'll try to add a specific libc version and see.

For iovec itself, it's depends on tokio's code path (also mio and bytes), it should deserve a new patch to investigate.

@taiki-e This patch should be ready for merge now.

The clippy related changes are attached because we have a clippy CI check that runs on latest compiler. I suppose it's intended.