Make parallel frontend CI job compile tests in parallel

[zetanumbers]

It is time to have a CI job to catch parallel frontend issues for tests. This PR sets RUST_TEST_THREADS to 1 to sequentially execute each test to achieve max thread utilization by a single rustc process. Then iteration-count option is set to 2 to repeat each test since parallel frontend issues usually don't reproduce reliably.

try-job: optional-x86_64-gnu-parallel-frontend

[rustbot]

r? @jieyouxu

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The reviewer was selected based on:

• Owners of files modified in this PR: infra-ci
• infra-ci expanded to Kobzol, Mark-Simulacrum, jdno, jieyouxu, marcoieni
• Random selection from Mark-Simulacrum, jdno, jieyouxu, marcoieni

[jieyouxu]

@bors try jobs=optional-x86_64-gnu-parallel-frontend

[rust-bors]

💔 Test for 93be885 failed: CI. Failed job:

• try - optional-x86_64-gnu-parallel-frontend (web logs, enhanced plaintext logs)

A workflow was considered to be a failure because it took only 526s. The minimum duration for CI workflows is configured to be 600s.

[jieyouxu]

@bors try

[rust-bors]

💔 Test for 1810706 failed: CI. Failed job:

• try - optional-x86_64-gnu-parallel-frontend (web logs, enhanced plaintext logs)

A workflow was considered to be a failure because it took only 502s. The minimum duration for CI workflows is configured to be 600s.

[rust-log-analyzer]

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

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

# Compile each test sequentially to achieve max thread utilization by a single rustc process
ENV RUST_TEST_THREADS 1

# Build the toolchain with multiple parallel frontend threads and then run tests
ENV SCRIPT python3 ../x.py --stage 2 test --set rust.parallel-frontend-threads=4 "--" --parallel-frontend-threads=4 --iteration-count=2
#!/bin/sh

# ignore-tidy-linelength

set -ex
---
x.py completions check
x.py help check
[TIMING:end] test::Tidy {  } -- 31.941
[TIMING:start] test::BootstrapPy {  }
usage: python3 -m unittest [-h] [-v] [-q] [--locals] [-f] [-c] [-b]
                           [-k TESTNAMEPATTERNS]
                           [tests ...]
python3 -m unittest: error: unrecognized arguments: --parallel-frontend-threads=4 --iteration-count=2
Command `/usr/bin/python3 -m unittest bootstrap_test.py --parallel-frontend-threads=4 --iteration-count=2 [workdir=/checkout/src/bootstrap/]` failed with exit code 2
Created at: src/bootstrap/src/core/build_steps/test.rs:3715:35
Executed at: src/bootstrap/src/core/build_steps/test.rs:3727:41

--- BACKTRACE vvv
   0: <bootstrap::utils::exec::DeferredCommand>::finish_process
             at /checkout/src/bootstrap/src/utils/exec.rs:939:17
   1: <bootstrap::utils::exec::DeferredCommand>::wait_for_output::<&bootstrap::utils::exec::ExecutionContext>
             at /checkout/src/bootstrap/src/utils/exec.rs:831:21
   2: <bootstrap::utils::exec::ExecutionContext>::run
             at /checkout/src/bootstrap/src/utils/exec.rs:741:45
   3: <bootstrap::utils::exec::BootstrapCommand>::run::<&bootstrap::core::builder::Builder>
             at /checkout/src/bootstrap/src/utils/exec.rs:339:27
   4: <bootstrap::core::build_steps::test::BootstrapPy as bootstrap::core::builder::Step>::run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:3727:41
   5: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::test::BootstrapPy>
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1596:36
   6: <bootstrap::core::build_steps::test::BootstrapPy as bootstrap::core::builder::Step>::make_run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:3711:21
   7: <bootstrap::core::builder::StepDescription>::maybe_run
             at /checkout/src/bootstrap/src/core/builder/mod.rs:476:13
   8: bootstrap::core::builder::cli_paths::match_paths_to_steps_and_run
             at /checkout/src/bootstrap/src/core/builder/cli_paths.rs:141:22
   9: <bootstrap::core::builder::Builder>::run_step_descriptions
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1139:9
  10: <bootstrap::core::builder::Builder>::execute_cli
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1118:14
  11: <bootstrap::Build>::build
             at /checkout/src/bootstrap/src/lib.rs:803:25
  12: bootstrap::main
             at /checkout/src/bootstrap/src/bin/main.rs:130:11
  13: <fn() as core::ops::function::FnOnce<()>>::call_once
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/core/src/ops/function.rs:250:5
  14: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/sys/backtrace.rs:166:18
  15: std::rt::lang_start::<()>::{closure#0}
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/rt.rs:206:18
  16: <&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe as core::ops::function::FnOnce<()>>::call_once
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/core/src/ops/function.rs:287:21
  17: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:581:40
  18: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:544:19
  19: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::marker::Sync + core::panic::unwind_safe::RefUnwindSafe, i32>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panic.rs:359:14
  20: std::rt::lang_start_internal::{closure#0}
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/rt.rs:175:24
  21: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/0417c25868d6dfbd1c291dfeae950504faa6f790/library/std/src/panicking.rs:581:40
---
  28: __libc_start_main
  29: _start


Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --set rust.parallel-frontend-threads=4 -- --parallel-frontend-threads=4 --iteration-count=2`
Build completed unsuccessfully in 0:01:14
  local time: Fri Jun 12 15:04:34 UTC 2026
  network time: Fri, 12 Jun 2026 15:04:34 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[petrochenkov]

@zetanumbers is on vacation until July 15, but we want these changes sooner, so #158307 resubmits and tries to fix them.