There was a small bug in Cstruct 3.0.0 that initialised the ppx driver too early, which made it fail with some jbuilder-based builds.
I’ve just released Cstruct 3.0.1 that addresses this, and also pulls in ppx_deriving to improve compatibility with the older cstruct.ppx dependency. When enough libraries have moved over to the ppx_cstruct dependency, I will release a new version of Cstruct that removes the older cstruct.ppx entirely.
If you are having any problems with Cstruct 3.0.1 builds, please do get in touch on the issue tracker.