Rust Project Goals

We've now opened our first batch of RFCs: rust-lang/rfcs#3873, rust-lang/rfcs#3874 and rust-lang/rfcs#3875

Recently we've been working on feedback on the multi-staged format of the RFC. We've also shared the RFC outside of our sync call group to people from a variety of project teams and potential users too.

We're now receiving feedback that is much more detail-oriented, as opposed to being about the direction and scope of the RFC, which is a good indication that the overall strategy for shipping this RFC seems promising. We're continuing to address feedback to ensure the RFC is clear, consistent and technically feasible. David's feeling is that we've probably got another couple rounds of feedback from currently involved people and then we'll invite more people from various groups before publishing parts of the RFC formally.

Key developments

• rust-lang/rust#148051

Blockers:

• rustdoc deciding on and implementing how they want frontmatter handled in doctests

Key developments:

• Overall polish
  • https://github.com/rust-lang/rust/pull/145751
  • https://github.com/rust-lang/rust/pull/145754
  • https://github.com/rust-lang/rust/pull/146106
  • https://github.com/rust-lang/rust/pull/146137
  • https://github.com/rust-lang/rust/pull/146211
  • https://github.com/rust-lang/rust/pull/146340
  • https://github.com/rust-lang/rust/pull/145568
  • https://github.com/rust-lang/cargo/pull/15878
  • https://github.com/rust-lang/cargo/pull/15886
  • https://github.com/rust-lang/cargo/pull/15899
  • https://github.com/rust-lang/cargo/pull/15914
  • https://github.com/rust-lang/cargo/pull/15927
  • https://github.com/rust-lang/cargo/pull/15939
  • https://github.com/rust-lang/cargo/pull/15952
  • https://github.com/rust-lang/cargo/pull/15972
  • https://github.com/rust-lang/cargo/pull/15975
• rustfmt work
  • https://github.com/rust-lang/rust/pull/145617
  • https://github.com/rust-lang/rust/pull/145766
• Reference work
  • https://github.com/rust-lang/reference/pull/1974

Status update as of November 1

Key developments:

• Draft PR for exposing implied bounds in rustdoc JSON: https://github.com/rust-lang/rust/pull/148379
• A concrete plan for how that new info turns into dozens of new lints covering many kinds of bounds

Linting ?Sized and 'static bounds turned out to be quite a bit more complex than I anticipated. The key issue is that seeing T: Foo + ?Sized does not guarantee that T can be unsized, since we might have Foo: Sized which renders the ?Sized relaxation ineffective. Similarly, seeing T: Foo might also non-obviously imply T: 'static via a similar implied bound.

Failure to correctly account for implied bounds would lead to catastrophic false-positives and false-negatives. For example, changing T: Foo to T: Foo + 'static could be a major breaking change or a no-op, depending on whether we have Foo: 'static (either directly or implicitly via other trait bounds).

We cannot determine implied bounds using information present in rustdoc JSON today, so the rustdoc team and I have been iterating on the best way to compute and include that information in rustdoc JSON. Assuming something similar to the aforementioned PR becomes part of rustdoc JSON, cargo-semver-checks stands to gain several dozen new lints covering these tricky cases over trait associated types, generic type parameters, and APIT/RPIT/RPITIT.

Just removed the duplicate posts, guessing from a script that had a bad day.

Key developments:

• libtest2:
  • #[test] macro added
  • Support for should_panic
  • Support for ignore
  • Support for custom error types
  • compile-fail tests for macros

Blockers

• None

Help wanted:

• Round out libtest compatibility

Key developments:

• libtest2
  • libtest env variables were deprecated, reducing the API surface for custom test harnesses, https://github.com/rust-lang/rust/pull/145269
  • libtest2 was updated to reflect deprecations
  • https://github.com/assert-rs/libtest2/pull/105
  • libtest2 is now mostly in shape for use
• json schema
  • https://github.com/assert-rs/libtest2/pull/107
  • https://github.com/assert-rs/libtest2/pull/108
  • https://github.com/assert-rs/libtest2/pull/111
  • https://github.com/assert-rs/libtest2/pull/120
  • starting exploration of extension through custom messages, see https://github.com/assert-rs/libtest2/pull/122

New areas found for further exploration

• Failable discovery
• Nested discovery

Instead of using a full-fledged database like SQLite, we switched to a basic JSONL-based logging system to collect build metrics. A simple design doc can be found here: https://hackmd.io/K5-sGEJeR5mLGsJLXqsHrw.

Here are the recent pull requests:

• https://github.com/rust-lang/cargo/pull/16150
• https://github.com/rust-lang/cargo/pull/16179

To enable it, set CARGO_BUILD_ANALYSIS_ENABLED=true or set the Cargo config file like this:

[build.analysis]
enabled = true

As of today (nightly-2025-11-03), it currently emits build-started and timing-info two log events to $CARGO_HOME/log/ (~/.cargo/log/ by default). The shape of timing-info JSON is basically the shape of the unstable --timing=json. I anticipate when this is stabilized we don't need --timing=json.

The build.analysis.enable is a non-blocking unstable feature. Unless bugs, should be able to set unconditionally even on stable toolchain. When not supported, it would just warn the unknown config merely.

Cargo tracking issue: https://github.com/rust-lang/cargo/issues/15844. The first implementation was https://github.com/rust-lang/cargo/pull/15845 in August that added build.analysis.enabled = true to unconditionally generate timing HTML. Further implementations tasks is listed in https://github.com/rust-lang/cargo/issues/15844#issuecomment-3192779748.

Haven't yet got any progress in September.

Status update November 21, 2025

October was largely spent working out design details of the build cache and locking design.

https://github.com/rust-lang/cargo/pull/16155 was opened with an initial implementation for fine grain locking for Cargo's build-dir however it needs to be reworked after the design clarifications mentioned above.

In November I had a change of employer so I my focus was largely on that. However, we did make some progress towards locking in https://github.com/rust-lang/cargo/pull/16230 which no longer lock the artifact-dir for cargo check. This is expected to land in 1.93.0.

I'm hoping to push fine grain locking forward later this month and in December.

Status update October 6, 2025

The build-dir was split out of target-dir as part of https://github.com/rust-lang/cargo/issues/14125 and scheduled for stabilization in Rust 1.91.0. 🎉

Before re-organizing the build-dir layout we wanted to improve the existing layout tests to make sure we do not make any unexpected changes. This testing harness improvement was merged in https://github.com/rust-lang/cargo/pull/15874.

The initial build-dir layout reorganization PR has been posted https://github.com/rust-lang/cargo/pull/15947 and discussion/reviews are under way.