Hi,

I was trying to use (preprocessor_deps) as a way to pull files (required at runtime) into _build/, and noticed the following behaviour.

When there is no (preprocess) field, (preprocessor_deps) is completely ignored.

For example, the following dune file does not trigger any error or warning (but uncommenting the (preprocess) field triggers an error if the file does not exist of course).

(executable
 (name e)
 ;(preprocess (pps ppx_deriving.std))
 (preprocessor_deps does-not-exist.txt))

I think that we could detect the problem a bit more eagerly, for example by issuing an error or a warning when (preprocess_deps) is set but (preprocess) is not.

What do you think?