Incremental compilation instantly rebuilds the Zig compiler [video]

It reminds me of the speed of the Turbo Pascal IDE 30+ years ago. Compilation was virtually instant, my young self wasn’t even aware it was happening.

The long-term plan is for Zig to expose some form of language server (not LSP) from the compiler itself, so it can use all the information it gets from doing the compilation. Incremental compilation is a key part of this plan, because it means we can get new up-to-date information in milliseconds rather than seconds or minutes when a file changes.

The compiler is set up so that this should be impossible! At the start of an "update", we mark parts of the dependency graph as "outdated" and "potentially outdated", based on which source code changed. From there, we repeatedly use a heuristic to find the "best" thing to analyze; each thing we analyze is guaranteed to remove at least one outdated (/potentially outdated) thing, and compilation terminates once nothing is marked outdated.

The way we do this does unfortunately mean that if our heuristic goes very wrong, we can end up doing much more work than is necessary. There are some real nasty cases with self-referential types right now. However, for simple things like changing function bodies, it works lovely!