Emitting metadata early makes building/checking Rust up to twice as fast
A new optimization technique for the Rust programming language, called 'headstart,' allows dependent crates to begin compiling before their dependencies finish type-checking. This method uses early metadata files to significantly reduce build and check times for large projects.
Why it matters
Improving compilation efficiency is vital for developer productivity in large-scale software engineering projects using the Rust ecosystem.
Start dependent crates before their dependencies finish type-checking.
Every crate waits for the crates it depends on to be fully checked, function bodies included, before it starts. It doesn't need those bodies to type-check itself. It compiles against the dependency's interface, the metadata in its .rmeta file.
Headstart makes rustc write an early metadata file as soon as the interface is checked, and makes cargo start dependents on it. Each crate's bodies are then checked while the crates downstream are already compiling.
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