Update parallel compilation status for August 2026

[eeminionn]

What changed

I went through the four date markers in src/parallel-rustc.md and checked the surrounding text against the current compiler source instead of only changing the month.

This updates the markers to August 2026 and also:

• documents the current --jobs* options and the one-job front-end default;
• replaces the removed parallel-compiler split with the current runtime synchronization behavior;
• updates WorkerLocal, the parallel helper names, and representative call sites;
• points from the completed query-parallelization tracker to the active parallel front-end tracker; and
• records that rustdoc HTML rendering is still serial.

Why

Several claims around the dates had fallen behind the implementation. In particular, -Z threads now directs users to --jobs-frontend, par_iter is no longer exported, the HIR helper names changed, and the old query-parallelization tracker was completed in June 2026. Updating only the dates would have made the page look freshly verified while leaving those details wrong.

The result should give compiler contributors current flags and source names while keeping the existing warning that this area is still evolving.

Verification

• Checked the documented helpers and call sites against rust-lang/rust main at 73dc9167f1cd099e525c9ade2e068d1907b78564.
• Confirmed that Tracking Issue for Parallel Rustc Front-end rust#113349 and Parallelize rustdoc rendering rust#82741 are open, while Query Parallelization Tracking Issue rust#48685 is closed as completed.
• Confirmed that all new documentation links return HTTP 200.
• Ran git diff --check.

This addresses the four src/parallel-rustc.md entries in #2948.

@jyn514, I rewrote the description in my own words and included the checks I used for each dated claim. Thanks again for pointing out that the verification needed to be explicit.

[jyn514]

@eeminionn thank you for writing the description in your own words, this is a lot better now! i do want to note that we try to discourage LLM-generated documentation — i've read through it and in practice i think it's fine, but please write documentation in your own words going forwards.

View changes since this review

[jyn514]

could you note that these flags (and the ones below) are unstable?

[jyn514]

please write this in terms of how things work now, not compared to how they used to be.

[jyn514]

could you change this to reuse the new [tracking] link you added?

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.