Emit Retags in Codegen

[nikomatsakis]

Metadata
Point of contact	@icmccorm
Team(s)	compiler, opsem
Goal document	2025h2/codegen_retags

Summary

Allow codegen backends to implement the MIR Retag intrinsic, and add a similar intrinsic to the LLVM backend.

Tasks and status

• Implementation (@icmccorm)
• Author RFC (@icmccorm)
• Design meeting (opsem, compiler )
• Standard reviews (opsem, compiler )
• Dedicated reviewer (opsem, compiler )
• RFC decision (opsem, compiler )
• Discussion and moral support (opsem, compiler )

[nikomatsakis]

This issue is intended for status updates only.

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

[icmccorm]

Here's our first status update!

• We've been experimenting with a few different ways of emitting retags in codegen, as well as a few different forms that retags should take at this level. We think we've settled on a set of changes that's worth sending out to the community for feedback, likely as a pre-RFC. You can expect more engagement from us on this level in the next couple of weeks.

• We've used these changes to create an initial working prototype for BorrowSanitizer that supports finding Tree Borrows violations in tiny, single-threaded Rust programs. We're working on getting Miri's test suite ported over to confirm that everything is working correctly and that we've quashed any false positives or false negatives.

• This coming Monday, I'll be presenting on BorrowSanitizer and this project goal at the Workshop on Supporting Memory Safety in LLVM. Please reach out if you're attending and would like to chat more in person!

[icmccorm]

We've posted a pre-RFC for feedback, and we'll continue updating and expanding the draft here. This reflects most of the current state of the implementation, aside from tracking interior mutability precisely, which is still TBD but is described in the RFC.