Tracking issue for MCP 999: Deny `todo!()` in tidy

This is a tracking issue for MCP "Deny todo!() in tidy" (rust-lang/compiler-team#999).

[!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.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the MCP
• Adjust the 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)
  • In the Unstable Book if applicable (e.g., for unstable CLI changes)

Unresolved questions

None so far.

Implementation history

Empty so far.

@jdonszelmann, consider @rustbot claim'ing the work.

The issue names no source files or tests; begin by reading compiler-team#999 and the tracking checklist. Review the linked Rust Compiler Development Guide, Rust Forge, rustc book, and Unstable Book to determine which documentation applies. Done means the MCP is implemented and the applicable documentation is updated.