Figure out the Dune integration

[aantron]

• Improve argument passing syntax in pps ocaml/dune#2573
• How to support bisect_ppx? ocaml/dune#57

References:

• Environment & build profiles ocaml/dune#419
• Bisect [WIP] ocaml/dune#616
• Some notion of "environment" ocaml/dune#391
• Add support for optional ppx preprocessors ocaml/dune#168

[aantron]

@rgrinberg, I think in order to do #136, Bisect_ppx will have to output an extra file when it preprocesses source, i.e. preprocessing foo.ml would output not only the foo.pp.ml expected today by Dune, but also foo.coverage-metadata, or something like that.

As I understand it, Dune will delete foo.coverage-metadata during a rebuild of the project. Is that right?

[rgrinberg]

Dune would indeed delete it. The list of modules that were linked seems like something should be able to answer however. We could imagine a $ dune build foo.exe.unlinked-modules for example. Would this help at all?

[aantron]

It would help slightly, in that at least we could report that there are some modules for which coverage wasn't generated at all due to not being linked. But the "gold standard" would be to have the coverage metadata available for those modules, so we can simply generate the normal report for them, with all the coverage points unvisited. For that, we need to dump the metadata (location info) somewhere, and the reporter needs to load it.

[aantron]

Actually, that issue might be even more difficult to solve, even with the help of Dune, because we still can't "fix" the case where a module is not even in the dependency cone, so isn't even preprocessed in the first place. I'll have to think about it more. The best thing to do might be nothing.

[aantron]

Closing this. There is now progress in ocaml/dune#57. We will update the Bisect docs afterwards.