Reborrow traits

[nikomatsakis]

Metadata
Point of contact	@aapoalas
Team(s)	compiler, lang
Goal document	2025h2/autoreborrow-traits

Summary

Bring up a language RFC for autoreborrow traits and land nightly support for the traits.

Tasks and status

• Discussion and moral support (lang )
• Standard reviews (compiler )
• Lang-team experiment (lang )
• Do the work (@aapoalas)

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[aapoalas]

Initial implementation of a Reborrow trait for types with only lifetimes with exclusive reference semantics is working but not yet upstreamed not in review. CoerceShared implementation is not yet started.

Proper composable implementation will likely require a different tactic than the current one. Safety and validity checks are currently absent as well and will require more work.

[aapoalas]

We've worked towards coherence checking of the CoerceShared trait, and have come to a conclusion that (at least as a first step) only one lifetime, the first one, shall participate in reborrowing. Problems abound with how to store the field mappings for CoerceShared.