Tracking issue for MCP 861: Make the UI test suite more amenable to multiple editions

This is a tracking issue for MCP 861.

[!NOTE]
Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.

A tracking issue is however not meant for large scale discussion, questions, or bug reports.
Instead, open a dedicated issue for the specific matter, link to this tracking issue and ping the MCP author.
Alternatively, post in the Zulip stream linked from the MCP.

Discussion comments will get marked as off-topic or deleted.
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Steps

• Implement the MCP
• Adjust documentation
  • In the Rust Compiler Development Guide if applicable (e.g., for refactorings)
  • In the Rust Forge if applicable (e.g., for policy changes)
  • In the rustc book if applicable (e.g., for target tier or CLI changes)

Unresolved questions

None so far.

Implementation history

• https://github.com/rust-lang/rust/pull/146166
• https://github.com/rust-lang/rust/pull/147498

Start by reading MCP 861 and the implementation history PRs #146166 and #147498. Check the remaining tracking checklist, including whether documentation changes are needed in the Rust Compiler Development Guide, Rust Forge, or rustc book; done means the MCP and applicable documentation work are complete.