ci: Make the x86_64-gnu-parallel-frontend job non-optional

src/ci/docker/host-x86_64/optional-x86_64-gnu-parallel-frontend/Dockerfile → src/ci/docker/host-x86_64/x86_64-gnu-parallel-frontend/Dockerfile

@@ -39,5 +39,5 @@ ENV RUST_CONFIGURE_ARGS \
 # Tests are still compiled serially at the moment (intended to be changed in follow-ups).
 # Running compiletest only tests for parallel frontend, then the rest without compiletest-only
 # options (i.e. `--parallel-frontend-threads=4 --iteration-count=2`)
-COPY ./scripts/optional-x86_64-gnu-parallel-frontend.sh /scripts/
+COPY ./scripts/x86_64-gnu-parallel-frontend.sh /scripts/
 ENV SCRIPT="Must specify DOCKER_SCRIPT for this image"

src/ci/github-actions/jobs.yml

@@ -382,11 +382,10 @@ auto:
   - name: x86_64-gnu
     <<: *job-linux-4c
 
-  - name: optional-x86_64-gnu-parallel-frontend
-    # This test can be flaky, so do not cancel CI if it fails, for now.
-    continue_on_error: true
+  - name: x86_64-gnu-parallel-frontend
+    doc_url: https://rustc-dev-guide.rust-lang.org/tests/x86_64-gnu-parallel-frontend.html
     env:
-      DOCKER_SCRIPT: optional-x86_64-gnu-parallel-frontend.sh
+      DOCKER_SCRIPT: x86_64-gnu-parallel-frontend.sh
     <<: *job-linux-4c
 
   # This job ensures commits landing on nightly still pass the full

src/doc/rustc-dev-guide/src/tests/x86_64-gnu-parallel-frontend.md

@@ -0,0 +1,12 @@
+# Parallel frontend testing on CI
+
+If you see any test failures in `tests/ui` from the CI job `x86_64-gnu-parallel-frontend`, please
+add `//@ ignore-parallel-frontend triage` to the failing test, even if your PR is otherwise
+entirely unrelated to parallel compiler or its testing.
+In some time people from the parallel rustc working group will triage the failing test, make a
+tracking issue for it, and try to debug the problem.
+
+For more context, see:
+
+* [MCP: Stabilization strategy for rustc parallel frontend](https://github.com/rust-lang/compiler-team/issues/1005)
+* Tracking issue: <https://github.com/rust-lang/rust/issues/118698>

tests/ui/specialization/min_specialization/next-solver-region-resolution.rs

@@ -1,4 +1,5 @@
 //@ compile-flags: -Znext-solver=globally
+//@ ignore-parallel-frontend query cycle
 // ICE regression test for https://github.com/rust-lang/rust/issues/151327
 
 #![feature(min_specialization)]

tests/ui/specialization/min_specialization/next-solver-region-resolution.stderr

@@ -1,27 +1,27 @@
 error[E0391]: cycle detected when coherence checking all impls of trait `Foo`
-  --> $DIR/next-solver-region-resolution.rs:6:1
+  --> $DIR/next-solver-region-resolution.rs:7:1
    |
 LL | trait Foo {
    | ^^^^^^^^^
    |
    = note: ...which requires building specialization graph of trait `Foo`...
 note: ...which requires computing whether impls specialize one another...
-  --> $DIR/next-solver-region-resolution.rs:12:1
+  --> $DIR/next-solver-region-resolution.rs:13:1
    |
 LL | / impl<'a, T> Foo for &'a T
 LL | | where
 LL | |     Self::Item: 'a,
    | |___________________^
-note: ...which requires computing normalized predicates of `<impl at $DIR/next-solver-region-resolution.rs:18:1: 20:21>`...
-  --> $DIR/next-solver-region-resolution.rs:18:1
+note: ...which requires computing normalized predicates of `<impl at $DIR/next-solver-region-resolution.rs:19:1: 21:21>`...
+  --> $DIR/next-solver-region-resolution.rs:19:1
    |
 LL | / impl<'a, T> Foo for &T
 LL | | where
 LL | |     Self::Item: Baz,
    | |____________________^
    = note: ...which again requires coherence checking all impls of trait `Foo`, completing the cycle
-note: cycle used when checking that `<impl at $DIR/next-solver-region-resolution.rs:12:1: 14:20>` is well-formed
-  --> $DIR/next-solver-region-resolution.rs:12:1
+note: cycle used when checking that `<impl at $DIR/next-solver-region-resolution.rs:13:1: 15:20>` is well-formed
+  --> $DIR/next-solver-region-resolution.rs:13:1
    |
 LL | / impl<'a, T> Foo for &'a T
 LL | | where