[windows] `staged_pps` passes incorrect path separator to `cmd.exe`

[ejgallego]

I am trying to build some of my packages with native Windows opam switches as created by opam 2.2.

Things work mostly fine, except for my projects using staged_pps:

$ (cd _build/default && C:\Users\User\AppData\Local\opam\default\bin\ocamldep.opt.exe -modules -ppx ".ppx/1f4626a10100bf9c3a282e83fe079d93/ppx.exe --as-ppx --cookie \"library-name=\\\"serlib_funind\\\"\"" -impl serlib/plugins/funind/ser_g_indfun.ml) > _build/default/serlib/plugins/funind/.serlib_funind.objs/serlib_funind__Ser_g_indfun.impl.d
> '.ppx' is not recognized as an internal or external command,
> operable program or batch file.
> File "serlib/plugins/funind/ser_g_indfun.ml", line 1:
> Error: Error while running external preprocessor
> Command line: .ppx/1f4626a10100bf9c3a282e83fe079d93/ppx.exe --as-ppx --cookie "library-name=\"serlib_funind\"" "C:\Users\User\AppData\Local\Temp\build_17b787_dune\camlppx7ef3dd" "C:\Users\User\AppData\Local\Temp\build_17b787_dune\camlppx361290"

As identified by @nojb , the problem seems to stem from the use of .ppx/1f4626a10100bf9c3a282e83fe079d93/ppx.exe as the executable name, cmd.exe will handle this as .ppx /option1 /option2 leading to the error message.

Using .ppx\1f4626a10100bf9c3a282e83fe079d93\ppx.exe as the path seems to work (famous last words).

(*) I couldn't try a dune patch to confirm changing that would solve the issue due to other opam 2.2 + windows problems, in particular with pinning of git repos.

[nojb]

@ejgallego can you give nojb@150354b a try? (It is a quick hack, not sure it will suffice.)

[ejgallego]

Thanks so much @nojb , I will, once I sort some pinning problems out.