rustc: Tweak the effect of --jobs on frontend parallelism

[petrochenkov]

We need to be able to express both of these things for the frontend parallelism:

• "Use exactly N threads", --jobs-frontend=N does this.
• "Use some default number of threads, but not larger than N", --jobs=N will do it after this PR.
  The "default number" may be 1 now, or 2 during the upcoming nightly testing, or 4 or 8 after the stabilization if we decide to cap the parallelism.

Part of rust-lang/compiler-team#1005.
r? @bjorn3

[petrochenkov]

Forgot to update this in #160387.

View changes since the review

[rust-log-analyzer]

The job x86_64-gnu-miri failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

tests/pass-dep/libc/libc-time.rs ... ok
tests/pass-dep/libc/libc-time.rs (revision `run`) ... ok

FAILED TEST: tests/pass-dep/libc/libc-epoll-blocking.rs (revision `edge_triggered.run`)
command: MIRI_BE_RUSTC="host" MIRI_ENV_VAR_TEST="0" MIRI_TEMP="/tmp/miri-uitest-skf8TS" RUST_BACKTRACE="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/tmp/miri_ui/0/tests/pass-dep/libc/libc-epoll-blockingedge_triggered"

error: test got exit status: 101, but expected 0
##[error]   --> tests/pass-dep/libc/libc-epoll-blocking.rs:316:58
    |
316 |                 let data = read_exact_array::<4>(fds[0]).unwrap();
    |                                                          ^^^^^^^^^ called `Result::unwrap()` on an `Err` value: Os { code: 9, kind: Uncategorized, message: "Bad file descriptor" }
    |

full stderr:

---
stack backtrace:
   0: __rustc::rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::result::unwrap_failed
   3: libc_epoll_blocking::waiting_threads_unblocked_after_socketpair_close::{closure#0}
   4: std::thread::scoped::scope::<libc_epoll_blocking::waiting_threads_unblocked_after_socketpair_close::{closure#0}, ()>::{closure#0}
   5: <core::panic::unwind_safe::AssertUnwindSafe<std::thread::scoped::scope<libc_epoll_blocking::waiting_threads_unblocked_after_socketpair_close::{closure#0}, ()>::{closure#0}> as core::ops::function::FnOnce<()>>::call_once
   6: std::panicking::catch_unwind::do_call::<core::panic::unwind_safe::AssertUnwindSafe<std::thread::scoped::scope<libc_epoll_blocking::waiting_threads_unblocked_after_socketpair_close::{closure#0}, ()>::{closure#0}>, ()>
   7: __rust_try
   8: std::panicking::catch_unwind::<(), core::panic::unwind_safe::AssertUnwindSafe<std::thread::scoped::scope<libc_epoll_blocking::waiting_threads_unblocked_after_socketpair_close::{closure#0}, ()>::{closure#0}>>
   9: std::thread::scoped::scope::<libc_epoll_blocking::waiting_threads_unblocked_after_socketpair_close::{closure#0}, ()>
  10: libc_epoll_blocking::waiting_threads_unblocked_after_socketpair_close
  11: libc_epoll_blocking::main
  12: <fn() as core::ops::function::FnOnce<()>>::call_once
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

full stdout:
---

Location:
   /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ui_test-0.30.7/src/lib.rs:365

Backtrace omitted. Run with RUST_BACKTRACE=1 environment variable to display it.
Run with RUST_BACKTRACE=full to include source snippets.
error: test failed, to rerun pass `--test ui`

Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools/x86_64-unknown-linux-gnu/release/build/miri/855274d73082e308/out/ui-855274d73082e308` (exit status: 1)
Bootstrap failed while executing `Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo test --target x86_64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color=always --profile=release --manifest-path /checkout/src/tools/miri/Cargo.toml -- [workdir=/checkout]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:197:21
Executed at: src/bootstrap/src/core/build_steps/test.rs:770:19

Command has failed. Rerun with -v to see more details.
test --stage 2 miri cargo-miri`
Currently active steps:
test::Miri { target: x86_64-unknown-linux-gnu } at src/bootstrap/src/core/build_steps/test.rs:696
Build completed unsuccessfully in 0:46:29
  local time: Fri Aug  7 13:23:19 UTC 2026

[petrochenkov]

(Unrelated failure in libc tests.)