ppx is skipped when (modes byte)

[zbaylin]

Expected Behavior

I define a ppx which is expected to compile only as a bytecode library:

(library
 (name my_ppx)
 ...
 (kind ppx_rewriter)
 (modes byte))

I expect this to be applied to my code which depends on my_ppx as a preprocessor

...
  (preprocessor (pps my_ppx))
...

Actual Behavior

The ppx is not applied to the code and extension points remain. This is not the case if I remove the (modes byte).

Reproduction

See above

Specifications

• Version of dune (output of dune --version): 3.12.1
• Version of ocaml (output of ocamlc --version): 4.14.1
• Operating system (distribution and version): macOS Big Sur

[rgrinberg]

Thanks for the report. I reproduced the bug here #9653

What would be the behavior that you would expect?

1. An error telling the user that a native driver could not be built
2. Transparently switch to a bytecode driver. This might create a preprocessor that is slower.

[zbaylin]

Hi @rgrinberg, thanks for helping reproduce the bug.

In my opinion, dune should switch to a bytecode-based driver, or maybe even provide a way of specifying which ppxs to run under bytecode/native mode in the preprocessing atom.

The ppx I'm running depends on being run under the bytecode runtime, and I'm willing to accept the performance trade off to run it.

[rgrinberg]

Okay, switching to bytecode sounds reasonable enough. Do you want to try implementing this?

[zbaylin]

Sure, I can look into it. Do you have any pointers as to where I should look for code regarding ppx building/execution?