GoKawiilExperimental Rust patch set 'Headstart' cuts build/check times via early metadata
A new set of patches to rustc and cargo, called 'Headstart' and enabled via -Zearly-metadata and -Zheadstart flags, lets dependent crates begin compiling against a dependency's early interface metadata (.early-rmeta) before that dependency's function bodies are fully type-checked. For cargo check, dependents can run to completion on this early metadata; for cargo build, dependents do their analysis early then wait for full metadata before code generation, releasing their job slot while waiting.