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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:

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, 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:

$ RUSTFLAGS_NOT_BOOTSTRAP="-Znext-solver=globally" ../x build library --stage 1

$ 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.