Tracking issue for MCP 904: Experiment with replacing specialization in libcore with downcasting

This is a tracking issue for MCP "Experiment with replacing specialization in libcore with downcasting" (rust-lang/compiler-team#904).

[!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 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

Empty so far.

@oli-obk, consider @rustbot claim'ing the work.

Start by reading MCP rust-lang/compiler-team#904, which defines the experiment of replacing specialization in libcore with downcasting. Check the Rust Compiler Development Guide, Rust Forge, and rustc book for documentation affected by the change. Done means the MCP is implemented and applicable documentation updates are completed.