tl;dr: Zig's incremental compilation, now usable on x86_64-linux via `zig build --watch -fincremental`, achieves 50-70ms rebuilds by tracking a dependency graph of "analysis units" (declaration types, values, function bodies, type layouts) that get selectively re-analyzed when source hashes change. The tightly-integrated linker uses a `MappedFile` abstraction to patch machine code directly into the output binary, resizing regions with exponential growth to amortize costs. Most remaining overhead comes from a reference-graph traversal during flush that could be optimized further; the feature requires Zig master (or 0.17.0) and still has rough edges.