btw Cargo has this cool thing where it’ll first look for a build.rs and run that (potentially generating files) before anything else. This is used quite a lot in all sorts of ways, and would be a nice way to avoid writing rules in dune.
In general most of my gripes with dune is that it’s like writing Makefiles: it uses very few conventions, and doesn’t make much assumptions about the directory structure or the name of the files. It’s too flexible and so everybody uses it differently and every OCaml project is structured differently. Ideally, the build system should be as invisible as possible and just work™.