Headstart prototype overlaps Rust checks to shorten builds

The Headstart project reports that its prototype patches for Rust's compiler and Cargo build tool make clean checks up to 54% faster and clean builds up to 42% faster across 13 real projects on 16 cores. The patches let dependent components begin checking against interface information before their dependencies finish checking function bodies. For builds, dependent components wait for complete dependency information before generating code. Dependency errors still fail the build, but speculative work can be discarded, errors can arrive later, and memory use increases. The patches are intended for upstream pull requests; gains shrink on smaller machines, with wide builds on four cores coming out even.