Proposal

We want to enable and stabilize the next solver and polonius alpha this year. Before that happens, we need to ensure these unstable features are better tested, and that they don't regress.

We want to implement such testing on CI, and this is an MCP because it may incur failures that contributors would see on CI more often than locally, so this is a heads-up and to gather possible concerns (or alternatives).

These features are not fully done, so we cannot turn them on just yet. That, in turn, means that they are currently less tested than we'd like, and regressions can, and do, happen without us noticing. This has happened twice recently, and we want to prevent such breakages from happening both before stabilization, as well as before expanding testing by enabling them on nightly.

We want to add a CI job dedicated to pre-stabilization testing, and that needs to pass on PR CI and on every merge, to:

• bootstrap rustc and build the stdlib at stage 2, with the next solver enabled at stage 1. This may yield new errors when using unstable features in the standard library or rustc. In the future, we will also want to expand testing (by e.g. running the next solver compare-mode on the UI tests) and this may sometimes require blessing new tests, or in rare cases, adding test annotations or revisions.
• run the UI tests under the polonius compare-mode. This may sometimes require blessing new tests, or in rare cases, adding test annotations or revisions.

Mentors or Reviewers

I've discussed this with @jackh726 and @lcnr. I'll do the CI work. t-infra can review.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member who is knowledgeable in the area can second by writing @rustbot second or kickoff a team FCP with @rfcbot fcp $RESOLUTION.
  • Refer to Proposals, Approvals and Stabilization docs for when a second is sufficient, or when a full team FCP is required.
• Once an MCP is seconded, the Final Comment Period begins.
  • Final Comment Period lasts for 10 days after all outstanding concerns are solved.
  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.