That's a fair point, but even mandating "choose one of Ninja/CMake/etc." would be helpful in terms of at least getting a little bit of metadata out. This is obv

That's a fair point, but even mandating "choose one of Ninja/CMake/etc." would be helpful in terms of at least getting a little bit of metadata out. This is obviously non-trivial, and I'm not sure how much metadata is even retrievable from my examples, but it's better than nothing. (Ninja and CMake operate at sort of different levels, but that's OK for this purpose, I think. At least you can ask things like "if X changed, what else would change?" without actually having to run a compiler.) (AFAIUI Cabal:Custom had exactly the same reasoning, btw. "It's too difficult! Let's just give up and allow anything.") It might be a practical reality that this is just unavoidable if you want to achieve success, but at a meta-level I think deserves thinking about whether builds should actually be allowed the full Turing Complete treatment. At least, theoretically: As long as we continue 'allowing' Custom for practical reasons and without serious thought, there can be no progress. (Btw, I'm quite mad which it comes to this type of thing. I spend way too much time thinking about build systems, etc.) EDIT: This problem manifests EVERYWHERE: Docker builds randomly guess wrong about whether inputs have changed, cabal-install gets it wrong, SBT/Zinc doesn't quite track dependencies correctly, etc. etc. and it's infuriating.