zapcc VS ClangBuildAnalyzer

C++ modules helps with the parsing problem similar to precompiled headers, but it doesn't help with the code execution at compile time problem. All your overload matching, free function lookup, SFINAE, concept matching, and consteval code needs executing and that can take very considerable time. Other than JITing all that stuff, and maybe running an in-memory server like https://github.com/yrnkrn/zapcc, I don't know what more can be done here.

If you use clang, that might be of interest: https://github.com/yrnkrn/zapcc

> On another note: C++ compiler should by default keep statistics about the chain of #include's / parsing during compilation and dump it to a file at the end and also summarize how badly you're re-parsing the same .h files during build.

Not exactly that, but do you know clang's -ftime-trace and tools like https://github.com/aras-p/ClangBuildAnalyzer which help analyzing where time is actually spent? (In small repeated headers I don't see much of a problem, but they of course may contain not so small things ...)

You can also use the following when you want to inspect multiple files: https://github.com/aras-p/ClangBuildAnalyzer

ClangBuildAnalyzer reports on parsing, build, and link time, whereas IncludeGuardian only reports on parsing time.

https://github.com/aras-p/ClangBuildAnalyzer is a very useful tool to quantify the cost of different headers (and other costly parts of the compile such as template instantiations). It doesn’t help with actually fixing such problems, but it’s a pretty good ruler to measure where the time is spent.

If you can compile with Clang, I suggest you to try ClangBuildAnalyzer