dune pkg outdated should solve and compare

[Alizter]

Currently dune pkg outdated simply compares the packages in the lock file to the newer versions found in the opam repository. This is wrong since it means we are advertising versions that may not be relevant to the solution.

One way to fix this would be to do a solve and compare the new solution to the old. Then we would get a much more accurate picture of which packages are considered outdated.

[rgrinberg]

I think you misunderstood the point of dune pkg outdated. The point of it is not to show you what new solution you would get if you simply solved again. You can already do that if you just run dune pkg lock followed by git diff. The point of it is to show how out of date is your build plans vs what is "optimal".

[Alizter]

@rgrinberg Yes, but I think its useless to tell the user that their packages are out-of-date if they can't exactly re-lock to use them.

[rgrinberg]

How is it useless? It tells them what are the potential benefits from fiddling with their constraints and re-locking. Without this feature, users have no way of knowing if there's a point to even start this possibly time consuming process.

[rgrinberg]

I mentioned that the functionality is redundant as it can be achieved through a dune pkg lock followed by a revert, but I still wouldn't mind if this was first class. I don't particularly mind having a --dry-run argument dune pkg lock to make it do what you described.