Tracking Issue for async drop codegen

This is a tracking issue for the async drop codegen from MCP "727" (rust-lang/compiler-team#727).
The feature gate for the issue is #![feature(async_drop))].

About tracking issues

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 about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
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 async drop codegen
  • AsyncDrop trait (#121801)
  • async_drop_in_place_raw/async drop glue generation support for
    • Trivially destructible types (integers, bools, floats, string slices, pointers, references, etc.), arrays and slices (array pointer is unsized into slice pointer), ADTs (enums, structs, unions), tuple-like types (tuples, closures) (#121801)
    • Dynamic types (dyn Trait)
      • One proposed design: https://zetanumbers.github.io/book/async-drop-design.html#dyn_trait
      • https://github.com/rust-lang/rust/pull/145316
    • Coroutines (#123948)
  • Type level support (possibly outside of the scope of this tracking issue)
    • Union rejects non-trivially async destructible fields
    • AsyncDrop implementation requires same bounds as type definition
    • Synchronously undroppable types
  • Optimizations
    • Skip trivially destructible fields (optimization) (#124662)
    • New TyKind::AdtAsyncDestructor and get rid of combinators (obsoleted by https://github.com/rust-lang/rust/pull/123948)
  • Automatic async drop at the end of the scope in async context (#123948)
• Adjust documentation (see instructions on rustc-dev-guide)
• Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Async drop for dynamic types (dyn Trait)
  • One proposed design: https://zetanumbers.github.io/book/async-drop-design.html#dyn_trait
• Drop of async destructors
• AsyncDrop and Drop should somehow be combined to not duplicate implementations (possibly outside of the scope of this tracking issue)

Implementation history

• #121801
• #124662
• #123948
• Many fixes after #123948 - https://github.com/rust-lang/rust/issues?q=label%3AF-async_drop%20is%3Apr
• https://github.com/rust-lang/rust/pull/156649

Start with the async drop codegen tracking checklist and the linked MCP 727 discussion, then review the open dynamic-type, type-level-support, documentation, and stabilization items. The linked issues and pull requests provide the implementation history; completion requires resolving the remaining checklist items and preparing documentation and stabilization work.