Add %{cmt} to "artifacts variables"

[trefis]

%{cmo}, %{cmi}, etc. are supported, but %{cmt} is not, cf. https://dune.readthedocs.io/en/stable/advanced/variables-artifacts.html#variables-for-artifacts

I want to write a rule that works on cmt files, and I can't.
So currently I add a dependency on the cmo (using %{cmo}) and just cross my fingers and hope that the cmt is nearby and not stale.
This feels dirty and fragile.

[Alizter]

Is something like

• feat: %{cmt} and %{cmti} artifact variables #12634

what you are looking for?

[trefis]

Yes!

[rgrinberg]

Don't we plan to switch to cms files anyway? If that's still the plan, this feature would make it harder to get rid of cmt and cmti files.

[Alizter]

@rgrinberg I'm not aware of that plan. Is there an issue about it?

[Alizter]

In the meantime, if you want a workaround, try

(library
 (name mylib))
(rule
 (alias show-cmt-workaround)
 (deps (:cmt .mylib.objs/byte/mylib.cmt))
 (action
  (echo "cmt file (direct path): %{cmt}\n")))

[rgrinberg]

We call these shapes and they are dumped in a new kind of cm-files: .cms(i) The .cms files should be able to replace entirely .cmt files for Merlin usages. As these files are lighter than cmt files we believe that this should speed up the process of building a program in dev mode, thus improving the speed of small iterations when coding.

ocaml/ocaml#10718

[trefis]

Thanks @Alizter ! (Although that doesn't seem very robust either, as it relies on dune internal build dir structure)

[trefis]

@rgrinberg I haven't been following lately, so maybe things have moved but .cms were not merged as part of this PR, so I don't think it's a thing the upstream compiler produces yet.

Regardless, I don't see the compiler losing the ability to produce cmt files even if it can produce cms files, so having a way to refer to them from dune rules (even if they aren't build unless explicitely requested) seems useful?
🙏

[rgrinberg]

Interesting, I thought cms files have been merged upstream a while ago. I think we've completely moved away from them for editor support in oxcaml. In any case, if the upstream compiler can't produce cms files, then we could add more support for cmt's

so having a way to refer to them from dune rules (even if they aren't build unless explicitely requested) seems useful?

It might be hard to do that as these cmt and cmti targets do not have dedicated rules and are produced when compiling bytecode.