Inside Zig's Incremental Compilation: Millisecond Rebuilds

The Zig core team has matured incremental compilation from a proof-of-concept into a feature used daily on real projects. The compiler detects which functions and declarations changed since the last build, recompiles only that code, and patches the resulting bytes directly into the output binary. In a demo on Fizzy, a pixel editor app, the initial build takes about 5 seconds while subsequent rebuilds after code changes complete in just 50-70 milliseconds.

The pipeline splits into two main stages: file processing, where source files are parsed into an AST and lowered into ZIR (an untyped SSA-form intermediate representation), and semantic analysis, which handles type checking and comptime evaluation. Because file processing is a pure function of file contents, it's trivially parallelizable and easy to cache incrementally by storing ZIR on disk — an optimization enabled by default for years.

The real challenge lies in semantic analysis. The team models fine-grained dependencies between 'analysis units' — struct/union layout, declaration types, constant values, and function bodies — so that only affected units get re-analyzed when something changes. This reflects deliberate language design choices made over the years specifically to enable fast incremental compilation. The feature currently lives on Zig's master branch, since 0.16.0 lacks linker support that will ship with 0.17.0.