A patch set called headstart makes cargo check on rust-analyzer 54% faster on a 16-core machine, according to benchmark tables the project published on its own repository. It changes both rustc and Cargo, it is not in any Rust release, and its readiness document says in so many words that it is “not yet ready to merge.”
The repository, PowderworksCode/headstart, was created on September 27, 2026. It reached 61 points on Hacker News on October 4 under the title “Emitting metadata early makes building/checking Rust up to twice as fast.”
The patches split the work. A new analysis_interfaces query in rustc finishes the interfaces first and writes an .early-rmeta file. Cargo, behind a -Zheadstart flag, starts downstream crates the moment that file exists, then pauses them and hands back job slots while it waits for the full metadata.
Rust already does a version of this. Cargo’s pipelined compilation starts a downstream crate when the .rmeta file appears, as the compiler team’s April 5, 2019 notes describe. Headstart moves the signal earlier.
| Project | cargo check saving | cargo build saving |
|---|---|---|
| rust-analyzer | 54% (90.1 s to 41.5 s) | 42% (137.9 s to 79.9 s) |
| polars | 49% | 29% |
| wasmtime | 47% | 36% |
| zed | 27% | 25% |
| lemmy | 26% | 30% |
The readiness page says the series is “Ready for the design conversation, and for a draft PR to show with it. Not yet ready to merge.” Running it means building a patched rustc and Cargo with scripts/setup.sh, then calling cargo check -Zheadstart with RUSTC pointed at the patched compiler. With no licence file, it is a benchmark to read, not a tool to adopt.