Rust Project Goals

Update this cycle is the same as last time - rust-lang/rfcs#3874 and rust-lang/rfcs#3875 are progressing, with feedback being addressed and checkboxes checked, and we're still working out what the implementation would look like.

No major updates this cycle - we're still working through feedback on rust-lang/rfcs#3874 and rust-lang/rfcs#3875 and prototyping the implementation to be prepared.

rust-lang/rfcs#3873 has been merged and an FCP has been started on rust-lang/rfcs#3874 and rust-lang/rfcs#3875 - those both have some feedback for me to respond to that I'll get to as soon as I can

I posted a "year in review" for cargo-semver-checks here: https://predr.ag/blog/cargo-semver-checks-2025-year-in-review/

It has a section on how I think we should move forward in 2026 and beyond: https://predr.ag/blog/cargo-semver-checks-2025-year-in-review/#the-path-forward-for-2026-and-beyond

Status update March 9, 2026

After the initial PR from the last update was merged, we shifted our focus to resolving some of the known issues. Notably, locking blocks the Cargo job queue slowly causing thread starvation if many build units are held by another Cargo instance.

We investigated adding the ability for Cargo to "suspend" a job internally while waiting for a lock, but we felt this change was a bit invasive and did not fit well with how the job queue was designed. Instead we plan to change our design to acquire all build unit locks prior to running the job queue. (see https://github.com/rust-lang/cargo/pull/16657)

At the same time, we have continued to refine the new build-dir to prepare it for a call for testing and eventual stabilization. (https://github.com/rust-lang/cargo/pull/16542, https://github.com/rust-lang/cargo/pull/16502, https://github.com/rust-lang/cargo/pull/16515, https://github.com/rust-lang/cargo/pull/16514)

Finally we decided to split .cargo-lock into 2 locks to allow cargo check and cargo build to run in parallel when artifact-dir == build-dir (and -Zfine-grain-locking is enabled)

I suspect this may be the last update on this goal, as the 2026 slate of goals is coming up. While I did not renew this goal for 2026, I do plan to continue work on this and eventually stabilize this within this year.

Status update January 15, 2026

Fine grain locking for build-dir was merged in https://github.com/rust-lang/cargo/pull/16155 and now available on nightly via -Zfine-grain-locking unstable flag. 🎉

There are some known issues we'd like to address before doing a formal call for testing. Notably, improving blocking messages, fixing potential thread starvation in Cargo's job queue when locks block, and investigate increasing rlimits to reduce risk of hitting max file descriptors for large projects.

I am hopeful that these issues will be resolved over the coming month and we can do a call for testing to start gathering feedback from the community on whether the new locking strategy improves workflows.

Key developments

• Cargo's FCP has ended

Blockers

• https://github.com/rust-lang/rust/issues/152254

Key developments

• FCP has ended on frontmatter support, just awaiting merge (https://github.com/rust-lang/rust/pull/148051)
• Cargo script has entered FCP (https://github.com/rust-lang/cargo/pull/16569)

Blockers

• Potential issues around edition, see https://github.com/rust-lang/rust/issues/152254

Key developments

• #146377 has been decided and merged

Blockers

• T-lang discussing CR / text direction feedback: https://github.com/rust-lang/rust/pull/148051#issuecomment-3638326490
• T-rustdoc deciding on and implementing how they want frontmatter handled in doctests