Start basic docs for pre-stabilization CI job

src/SUMMARY.md

@@ -35,6 +35,7 @@
         - [Cranelift codegen backend](./tests/codegen-backend-tests/cg_clif.md)
         - [GCC codegen backend](./tests/codegen-backend-tests/cg_gcc.md)
     - [Performance testing](./tests/perf.md)
+    - [Pre-stabilization CI job](./tests/pre-stabilization-ci-job.md)
     - [Misc info](./tests/misc.md)
 - [Debugging the compiler](./compiler-debugging.md)
     - [Using the tracing/logging instrumentation](./tracing.md)

src/tests/pre-stabilization-ci-job.md

@@ -0,0 +1,57 @@
+# Pre-stabilization testing of new solver and polonius alpha on CI
+
+We want to improve the continuous test coverage of next solver and polonius
+alpha with a goal to stabilize them in 2026. To achieve this, we need to ensure
+these unstable features are well tested and continuously well tested so they do
+not regress.
+
+This calls for a dedicated CI job [`x86_64-gnu-pre-stabilization`] which is
+blocking in PR CI and thus also Merge CI to catch regressions.
+
+For more context, see:
+
+* [MCP 996: Test new solver and polonius alpha on CI](https://github.com/rust-lang/compiler-team/issues/996)
+* Tracking issue: <https://github.com/rust-lang/rust/issues/157780>
+
+## What is tested
+
+* For next solver:
+    * We want to ensure that we can build the stage 2 compiler and standard
+      library with next solver.
+    * (Later) we also want to ensure that we can bootstrap a full toolchain with
+      the next solver.
+* For polonius alpha:
+    * We wil run the UI test suite under polonius [compare
+      mode](./compiletest.md#compare-modes), against the stage 2 compiler/std
+      built with next solver as aforementioned.
+
+> **Note**
+>
+> Initially, we may test only up to `--stage=1`, but the plan is to expand up to
+> stage 2.
+>
+> To reproduce failures locally:
+>
+> ```console
+> $ RUSTFLAGS_NOT_BOOTSTRAP="-Znext-solver=globally" ../x build library --stage 1
+> ```
+>
+> ```console
+> $ RUSTFLAGS_NOT_BOOTSTRAP="-Znext-solver=globally" ../x test tests/ui --compare-mode polonius --stage 1
+> ```
+
+## What failures and remedies might be specific to this combination?
+
+* This may yield new errors when using unstable features in the standard library
+  or the compiler.
+* This may sometimes require blessing new tests, or in rare cases, adding test
+  annotations or revisions.
+
+## Point of contact
+
+If you have questions or need advice on a test failure in this job, please open
+a new topic in [t-compiler zulip
+channel](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler).
+
+
+[`x86_64-gnu-pre-stabilization`]: https://github.com/rust-lang/rust/pull/157322