We need to seriously think about what to do with C++ modules

Summary

Note: Everything that follows is purely my personal opinion as an individual. It should not be seen as any sort of policy of the Meson build system or any other person or organization. It is also not my intention to throw anyone involved in this work under a bus. Many people have worked to the best of their abilities on C++ modules, but that does not mean we can't analyze the current situation with a critical eye.The lead on this post is a bit pessimistic, so let's just get it out of the way.If C++ modules can not show a 5× compilation time speedup (preferably 10×) on multiple existing open source code base, modules should be killed and taken out of the standard. Without this speedup pouring any more resources into modules is just feeding the sunk cost fallacy. That seems like a harsh thing to say for such a massive undertaking that promises to make things so much better. It is not something that you can just belt out and then mic drop yourself out. So let's examine the whole thing in unnecessarily deep detail. You might want to grab a cup of $beverage before continuing, this is going to take a while.What do we want?For the average developer the main visible advantages would be the following, ordered from the most important to the least.Much faster compilation times.If you look at old presentations and posts from back in the day when modules were voted in (approximately 2018-2019), this is the big talking point. This makes perfect sense, as the "header inclusion" way is an O(N²) algorithm and parsing C++ source code is slow. Splitting the code between source and header files is busywork one could do without. The core idea behind modules is that if you can store the "headery" bit in a preprocessed binary format that can be loaded from disk, things become massively faster.Then, little by little, build speed seems to fall by the wayside and the focus starts shifting towards "build isolation". This means avoiding bugs caused by things like macro leakage, weird namespace...