Should opam file generation use promotion

[NathanReb]

I'm opening this following a recent discussion with @rgrinberg.

It's quite easy to forget about opam generation given that the opam file is silently updated whenever one runs dune build. As a result it's also quite easy not to commit the updated opam file when upating the dune-project file since dune build/runtest will just generate the new one and succeed thus not triggering a CI failure. One can always add an explicit check but that's a bit tedious.

If opam generation was done through the promotion mechanism, an out-of-date opam file would trigger a build failure in CI, preventing such mistakes from slipping in!

I'd be happy to work on this change if you think it's a good idea!

I guess there's an issue here though since opam generation isn't a versioned feature and that can be considered a breaking change.

[ghost]

The proposed behaviour seems good to me. That said I heard that opam devs started working on a feature allowing opam files to be generated on the fly when pinning a package, which would allow to not commit the opam files in the repository at all. We could also wait for that and then we wouldn't have to decide whether this is a breaking change or not.

[NathanReb]

Yeah, not committing .opam files would definitely be the preferred option. Let's wait and see then!

[NathanReb]

Do you have any news from the opam team on that feature? I'm starting to wonder if it wouldn't be worth having a solution to this in dune in the meantime. It could use the dune lang version to determine which behaviour it should have so that it doesn't break anything.

[rgrinberg]

Let's CC @dra27, @AltGr, @rjbou. They should have some up to date information.

[rjbou]

We are focusing on the 2.1.0 beta release expected on february. Once done, I'll focus on the opam file generation.
Related issues ocaml/opam/issues/3793 ocaml/opam/issues/3797 ocaml/opam/issues/2497