Use `annotate-snippets` as the default emitter on nightly

[Muscraft]

Proposal

There has been an ongoing effort to use annotate-snippets to render compiler diagnostics. This effort started in rust#59346 and was a project goal in 2024H2 and 2025H1. With rust#148007 being merged, AnnotateSnippetEmitter now matches rust's output with minimal test differences. As a step towards using annotate-snippets as the default emitter, and to get feedback, I propose that annotate-snippets be enabled as the default emitter on nightly. This change should have minimal end-user impact and is very easy to undo if there are widespread issues.

Mentors or Reviewers

I (@Muscraft) am currently maintaining annotate-snippets and will happily handle any changes/fixes that need to be made.

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.

[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that.
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cc @rust-lang/compiler

[davidtwco]

@rustbot second