Headstart is a Rust project designed to kick off dependent crates before their dependencies finish type-checking by emitting an early metadata file (.early-rmeta) as soon as a crate’s interface is checked, enabling downstream crates to compile against interfaces rather than full bodies.

Cargo changes introduce a -Zearly-metadata flag across compilations, enabling dependents to start on early metadata in both cargo check and cargo build, and the system pauses and resumes work to optimize resource use while dependencies complete.

Rustc updates add a new analysis_interfaces query to separate item interfaces from function bodies, generate early metadata, and later swap in full metadata for code generation when ready.

If a body contains an error, the overall build still fails with the same diagnostics and exit status; progress messages and cross-crate order may vary, and Cargo reports a crate’s output only after all dependencies succeed, dropping the crate if any dependency fails.

Reported performance gains include up to 54% faster cargo check and up to 42% faster cargo build on the default Rust frontend for real projects, with up to 25% improvements using the parallel frontend (-Zthreads=8); gains scale with machine cores.

The repository provides benchmarks, sanity checks, and real-project simulations to compare headstart-enabled builds against baselines, ensuring diagnostics, exit statuses, and outputs remain consistent while evaluating performance.

Extensive documentation and tests accompany the project, including a design document on how early metadata works and its risks, results measurements, and readiness assessments for upstream adoption.

In practice, performance measurements cover diverse real projects to illustrate the impact of early metadata on build throughput and reliability.

Usage guidance includes a setup script and example commands for running headstart with cargo check or cargo build, configuration via environment variables and Cargo.toml, and additional scripts for error verification, incremental changes, and benchmarks.