staged_pps doesn't cope well with bisect_ppx -conditional

[NathanReb]

I reported this issue on bisect_ppx here first.

Basically what happened is that I needed to use both ppx_import and bisect_ppx on the same library resulting in a preprocess stanza looking like:

(preprocess
  (staged_pps
    ppx_import
    ...
    bisect_ppx -conditional
  )
)

This resulted in the library code always being instrumented. I assume the -conditional isn't properly passed somehow but I don't know much about the behaviour of staged_pps.

I suppose the fix for #57 will probably solve this issue but I can imagine a deeper preprocessor composition problem being involved here.

I'm as always happy to provide an example project and further detail as well as working on fix if needed!

The syntax for passing flags to ppx rewriters is the following:

(staged_pps ppx_import ... bisect_ppx -- -conditional)

i.e. the flags are separated from the ppxs by a --. No using the -- should result in an error as dune would end up looking for a -conditional ppx rewriter. This looks like a bug in dune.

Just to be sure, if you add the missing --, does the issue go away?

Actually never mind, I read the code again and -conditional should be recognised as a flag.

Could you grep for -ppx in _build/log to see what -ppx command are being passed to the compiler?

[NathanReb]

I tried the adding the -- just to check and the issue does persist even then.

It appears the command passed to -ppx is always just ppx.exe --as-ppx:

$ grep "\-ppx" _build/log 
$ (cd _build/default && /home/nathan/.opam/4.07.1/bin/ocamldep.opt -modules -ppx '.ppx/fd8acca92aaadae9d8099f94851fc84c/ppx.exe --as-ppx' -impl main.ml) > _build/default/.main.eobjs/main.ml.d
$ (cd _build/default && /home/nathan/.opam/4.07.1/bin/ocamlc.opt -w @a-4-29-40-41-42-44-45-48-58-59-60-40 -strict-sequence -strict-formats -short-paths -keep-locs -g -ppx '.ppx/fd8acca92aaadae9d8099f94851fc84c/ppx.exe --as-ppx' -bin-annot -I .main.eobjs -I /home/nathan/.opam/4.07.1/lib/bisect_ppx/runtime -no-alias-deps -opaque -o .main.eobjs/main.cmo -c -impl main.ml)
$ (cd _build/default && /home/nathan/.opam/4.07.1/bin/ocamlopt.opt -w @a-4-29-40-41-42-44-45-48-58-59-60-40 -strict-sequence -strict-formats -short-paths -keep-locs -g -ppx '.ppx/fd8acca92aaadae9d8099f94851fc84c/ppx.exe --as-ppx' -I .main.eobjs -I /home/nathan/.opam/4.07.1/lib/bisect_ppx/runtime -intf-suffix .ml -no-alias-deps -opaque -o .main.eobjs/main.cmx -c -impl main.ml)

This is from a small project with a hello world and the following dune file:

(executable
  (public_name main)
  (preprocess
    (staged_pps bisect_ppx -conditional)
  )
)

Indeed, it was just an omission. Fixed in #1779