So I just tried¹, and the failure is as follows:

$ ocamlbuild -tag use_ocamldoc test.cmxs
+ /home/gasche/.opam/4.02.3/bin/ocamlopt.opt -shared -I +ocamldoc
ocamldoc.cmxa test.cmx -o test.cmxs
File "_none_", line 1:
Error: Cannot find file ocamldoc.cmxa

It looks like the tag use_ocamldoc expects a file ocamldoc.cmxa to exist,
providing library support for ocamldoc plugins, and it does not seem to
exist -- it seems that the preferred way to use ocamldoc plugins is to use
"ocamldoc.opt -g foo.cmxs", with ocamldoc libraries already linked inside
ocamldoc.opt. So just using

  ocamlbuild -cflags -I,+ocamldoc test.cmxs

works and seems to do what you expect.

It goes without saying that this is not a perfect long-term solution, as it
assumes that ocamldoc libraries are distributed with the stdlib. It would
be cleaner to have an ocamldoc package for them (just like ocamlbuild has a
dummy ocamlfind package indicating the location of its library files), but
it seems that this is not currently the case. It would be nice (but not
super-high priority) to have such a dummy package in ocamlfind in the
future, so that you can just do

  ocamlbuild -use-ocamlfind -package ocamldoc test.cmxs

¹: The most time-consuming part was finding the code of a ocamldoc plugin
to compile, as the documentation examples are incomplete. Next time, feel
free to provide a tarball with a minimal example to reproduce.

On Fri, Dec 18, 2015 at 5:14 PM, Armaël Guéneau <armael.gueneau@ens-lyon.fr>
wrote:

> Hi list,
>
> I'm wondering how one should compile ocamldoc custom generators
> <http://caml.inria.fr/pub/docs/manual-ocaml/ocamldoc.html#sec333> using
> ocamlbuild.
>
> First, the ocamlbuild manual
> (
> <https://github.com/gasche/manual-ocamlbuild/blob/master/manual.md#basic-tags->
> https://github.com/gasche/manual-ocamlbuild/blob/master/manual.md#basic-tags
> -)
> says that the [use_ocamldoc] tag is deprecated; and that a corresponding
> ocamlfind package should be used instead. Is there such a package for
> ocamldoc?
> I did not found one.
>
> Then, I did not manage to build the plugin into .cma/.cmxs files directly
> using
> ocamlbuild. My command line was something along these lines:
>
>    ocamlbuild -use-ocamlfind -tag use_ocamldoc -package compiler-libs
> plugin.cmxs
>
> When trying to build plugin.cma/plugin.cmxs, ocamlbuild complained that
> "Cannot
> find file ocamldoc.cma/ocamldoc.cmxa" (respectively). However, I can build
> plugin.cmo/plugin.cmx using the same command line, then create the
> .cma/.cmxs
> manually using ocamlc/ocamlopt - which I ended up doing.
>
> Is there a better way to do so? If the whole process can be improved
> (how?), I
> would be happy to write the required patches/documentation.
>
> — Armaël
>
>