Port to ocaml-migrate-parsetree

[let-def]

This is port of the project to ocaml-migrate-parsetree. The library aims at making ppx portable accross compiler versions.

The API is likely to change in the upcoming days, so this is not ready for merging.

[avsm]

Has this interface settled down now @let-def? We're interested in unblocking mirage compilation on OCaml 4.05 to try AFL -- cc @yomimono @talex5

[let-def]

I think so. The library might receive a few update before 4.05, but the API will remain compatible.

[let-def]

Ok, I think the final API made it to ocaml-migrate-parsetree. It has not been merged yet, but it would be nice to rerun tests after (I don't think I can?) and maybe consider merge.

PR: ocaml/opam-repository#8772

[avsm]

restarted Travis

[avsm]

Thanks for this! Could you also add a dependency on ocaml-migrate-parsetree to the opam file, so that CI picks it up?

[let-def]

I am not sure how to express the opam constraints:

The package optionally depends on ocaml-migrate-parsetree and ppx_tools_versioned. If both are available, then the ppx can be built.

Since ocaml-migrate-parsetree is a direct dependency of ppx_tools_versioned, the former can probably be omitted, but that would be slighly underspecifying the constraints. Can you check that the opam file correctly specify the optional deps and the resulting conditional compilation?

[avsm]

Since ocaml-migrate-parsetree is a direct dependency of ppx_tools_versioned

It's fine to just depend on ppx_tools_versioned then, as long as we remember that we need to fix things up if the direct dependencies change in the future. That'll be picked up revdeps testing most likely even if we do forget.

[avsm]

fails on 4.05 but thats expected due to Lwt for now -- thanks for this contribution!