Dune error - Ppx dependency on a non-ppx library

I’m getting a strange dune error from trying to compile a project with pre-4.07 OCaml, Dune 2.6 and ppxes that are built with jbuilder:

File "bin/dune", line 6, characters 7-14:
6 |   (pps ppx_let)))
Error: Ppx dependency on a non-ppx library "ppx_let". If "ppx_let" is in fact
a ppx rewriter library, it should have (kind ppx_rewriter) in its dune file.

I can’t reproduce it with a minimal example (i.e. where ppx_let is the only library). It’s on CI here but I also get it locally. Does anyone have any ideas what might be causing it?

I think it’s just an issue with you using an extremely old version of ppx_let. The version of ppx_let that you’re using is still on jbuild files, which don’t generate the correct metadata. In particular, the (installed in dune-package files) metadata doesn’t declare that ppx_let is a rewriter.

Thanks! I was a bit confused because the jbuild file did seem to specify it as a ppx rewriter.