Zig's Incremental Compilation Internals

Zig core team member Andrew Kelley detailed the internals of the Zig compiler's incremental compilation feature in a blog post published Monday, explaining how the system achieves rebuild speeds of 50 to 70 milliseconds for complex applications. Kelley said the feature has progressed from a proof-of-concept to a viable tool used daily by the core team over recent release cycles. He noted that while Zig 0.16.0 supports incremental compilation, it lacks necessary linker features implemented on the master branch, meaning users on tagged releases must wait for version 0.17.0. The compiler pipeline splits work into two main phases. The first phase processes source files into Zig Intermediate Representation (ZIR) using parsing and AstGen passes. Kelley said this phase is embarrassingly parallel and has utilized on-disk caching for years, making it near-instantaneous for incremental builds. The second phase, semantic analysis, handles type checking and comptime evaluation. Kelley said this is the most difficult part to incrementalize. The compiler divides this work into "analysis units" covering struct layouts, declaration types, constant values, and function bodies. Dependencies between units are tracked in a graph to determine what requires recompilation when source files change.