Tracking issue for MCP 997: Desugar async blocks in HIR instead of MIR

This is a tracking issue for MCP "Desugar async blocks in HIR instead of MIR" (rust-lang/compiler-team#997).

[!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.
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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.

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

Start by reading MCP rust-lang/compiler-team#997, then review the tracking checklist and its linked Rust Compiler Development Guide, Rust Forge, rustc book, and Unstable Book references. Done means the MCP is implemented and any applicable documentation updates are completed; this issue does not identify implementation files or tests.