Rust Project Goals

Here's our December status update!

• We have revised our prototype of the pre-RFC based on Ralf Jung's feedback. Now, instead of having two different retag functions for operands and places, we emit a single __rust_retag intrinsic in every situation. We also track interior mutability precisely. At this point, the implementation is mostly stable and seems to be ready for an MCP.

• There's been some discussion here and in the pre-RFC about whether or not Rust will still have explicit MIR retag statements. We plan on revising our implementation so that we no longer rely on MIR retags to determine where to insert our lower-level retag calls. This should be a relatively straightforward change to the current prototype. If anything, it should make these changes easier to merge upstream, since they will no longer affect Miri.

• BorrowSanitizer continues to gain new features, and we've started testing it on our first real crate (lru) (which has uncovered a few new bugs in our implementation). The two core Tree Borrows features that we have left to support are error reporting and garbage collection. Once these are finished, we will be able to expand our testing to more real-world libraries and confirm that we are passing each of Miri's test cases (and likely find more bugs lurking in our implementation). Our instrumentation pass ignores global and thread-local state for now, and it does not support atomic memory accesses outside of atomic load and store instructions. These operations should be relatively straightforward to add once we've finished higher-priority items.

• Performance is slow. We do not know exactly how slow yet, since we've been focusing on feature support over benchmarking and optimization. This is at least partially due to the lack of garbage collection, based on what we're seeing from profiling. We will have a better sense of what our performance is like once we can compare against Miri on more real-world test cases.

As for what's next, we plan on posting an MCP soon, now that it's clear that we will be able to do without MIR retags. You can expect a more detailed status update on BorrowSanitizer by the end of January. This will discuss our implementation and plans for 2026. We will post that here and on our project website.

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.

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!