Deprecate >> in the PPX

[aantron]

See discussion.

We should probably have the PPX emit a warning each time >> is used.

cc @Richard-Degenne @dkim @Drup

[raphael-proust]

I'm giving that a shot. First draft is raphael-proust@4048ec5

Is there a standard way to emit warnings from ppxs? Currently it just uses prerr_string inline. I didn't find examples online.

[aantron]

Off the top of my head, I think you can insert [@..@ocaml.ppwarning] annotations: see https://caml.inria.fr/pub/docs/manual-ocaml/extn.html#sec247. I've never tried this.

There are also some warning functions in one of the modules in the compiler's parsing directory. However, the PPX's version of that is coming from compiler-libs, and linked into the PPX, so I think it will end up printing to stderr – just with the same formatting as the compiler would use. ppwarning sounds like the better option.

[Drup]

Yes, warning should be done by adding some ocaml.ppwarning annotations in the emited OCaml code, using Ast_mapper.attribute_of_warning. Don't ever print yourself, it will break formatting.