From that experiment all seems to work as expected with the new option except one part:

commit 97b3d69 ("Why does this build?").

I am compiling dune build -p pplumbing at that commit with 5.3.0, and I'd expect the build to fail because err.ml makes use of Pp without having the pp library listed in its dependencies, and the (implicit_transitive_deps false) at that commit.

cc @nojb (this does not relate to the new option, but is a question about (implicit_transitive_deps false) in general).

I am compiling dune build -p pplumbing at that commit with 5.3.0, and I'd expect the build to fail because err.ml makes use of Pp without having the pp library listed in its dependencies, and the (implicit_transitive_deps false) at that commit.

I would expect so as well at first sight. Can you build at that commit and post the build log (_build/log) here?

Ah @nojb I think I understand now, this is I believe thanks to https://github.com/ocaml/dune/blob/82bbef1c62e6f402e22823481c3bde35429a2bad/otherlibs/stdune/src/dune#L11 in stdune which does (re_export pp).

Seems all fine then, I'm happy to consider this testing session satisfactory.

which does (re_export pp).

Yes, this is the explanation. Thanks!

Experimentation is complete - we'll go with #15 when ready.