Co-authored-by: Josh Triplett <[email protected]>

Follow up to rust-lang#92746. Address rust-lang#94510.

…aahc

Add #[track_caller] to track callers when initializing poisoned Once

This PR is for this Issue.
rust-lang#87707

With this fix, we expect to be able to track the caller when poisoned Once is initialized.

…lacrum

Remove num_cpus dependency from bootstrap, build-manifest and rustc_s…

…ession

`std::threads::available_parallelism` was stabilized in rust 1.59.

r? ```````````````````````````@Mark-Simulacrum```````````````````````````

Rename JoinHandle::is_running to is_finished.

This is renaming `is_running` to `is_finished` as discussed on the tracking issue here: rust-lang#90470 (comment)

Taking some of the docs suggestions from rust-lang#94033

…r=Mark-Simulacrum

Remove some dead code from rustbuild

This should reduce build time a tiny bit.

Do not recover from `Ty?` in macro parsing

Follow up to rust-lang#92746. Address rust-lang#94510.

Use if let instead of manual match

Factored out of rust-lang#94139 . `if let` is better here than both `let ... else` and `let ... = match`.