Imagine you're building a skyscraper. Each floor's structural steel must be certified before the next floor begins — but the certification process checks two things: the load-bearing columns (which the next floor actually stands on) and the interior plumbing (which it doesn't). Today, Rust's build system makes every floor wait for the full inspection. Headstart splits the inspection in two, letting the next floor start as soon as the columns pass. The committed claim: by separating rustc's analysis into interface checking and function-body checking, and emitting an early .rmeta file between the two phases, downstream crates can begin type-checking against dependency interfaces while bodies are still being verified. This is not a new compiler — it's a 9-patch series (6 rustc, 3 cargo) that modifies the existing toolchain to exploit parallelism that was always structurally available but never surfaced. The results are substantial and well-measured. On 16 cores, clean cargo check builds of 13 real-world projects (rust-analyzer, Zed, Bevy, Lemmy, Polars, and others) see speedups of up to 54%. Cargo build gains up to 42%. Even with the parallel front end (-Zthreads=8), which already covers some of this ground, headstart adds up to 25% on top. The gains come from cores that would otherwise sit idle — on 4-core machines, the numbers drop to 13-24% for check and 13-15% for build, with wide dependency graphs breaking even. Architecturally, this is a scheduling optimization, not an algorithmic one. It belongs to the family of build-system parallelism techniques — pipelined compilation, as explored in other ecosystems — applied to Rust's specific metadata/codegen split. The key structural insight is that .rmeta files already contain everything a downstream crate needs for type-checking; the only missing piece was emitting them earlier and teaching cargo to act on the notification. For cargo build, dependents do all analysis on early metadata, then pause and yield their job slot until full metadata arrives for codegen. The integrity story is unusually strong for a toolchain contribution. The project includes exhaustive correctness tests: error-propagation scripts comparing diagnostics, exit status, and JSON output with and without headstart; incremental-edit sequences covering interface breaks and impl additions; metadata-swap tests at every optimization level; and a full sweep of 53 rustc-perf compile benchmarks with -Zearly-metadata-verify. The claim that error behavior is preserved — same diagnostics, same exit status, only cross-crate message ordering and progress lines can differ — is tested, not asserted. Cargo suppresses a crate's output until all its dependencies succeed, dropping it if one fails. The costs are explicit and honestly stated: wasted downstream work when a dependency body has an error, slightly delayed error reporting, and higher peak memory from overlapping compilations. These are real tradeoffs, not hand-waved limitations. The design document (docs/design.md) details what early metadata leaves out and where risks live. The obvious next step is upstreaming. The patches are structured as a commit series intended to become pull requests to rustc and cargo. The gap between 'working proof-of-concept with benchmarks' and 'merged into nightly' is political and engineering review, not fundamental research. The author has not run the patches through the full rust-lang CI matrix or tested on exotic targets — likely a scoping decision to demonstrate value before investing in the long tail of platform support.
