Disable subparts of a project

Desired Behavior

I try to replace a configure file with dune-configurator. One important feature of our configure file is the ability to disable parts of the complete system:

$ ./configure --disable-foo

It prevents feature foo to be compiled, tested, and installed. Which is useful for both users and (mostly) developers.

We have the following architecture:

- dune project
- dune
- src/plugins/
    - p1
        - dune-project
        - dune
    - p2
        ...
    ...

Currently, when at least one plugin is disabled, the configure generates a dune file in src/plugins, that contains:

(include_subdirs no)
(data_only_dirs <disabled-plugins-directories>)

But, as far as i understand, it is not possible to do that with dune-configurator since we cannot generate a dune file that would be immediately used by the current build process.

Example

My initial idea was to generate a .enabled file in each plugin directory (each containing true or false), and to modify plugins dune file like this:

(library
 (name p1)
  (optional)
  (public_name proj-p1.core)
  (enabled_if %{read:.enabled}) ; << added line
  (libraries proj.kernel))

However, it happens that dune does not like it, and asks me to file this issue:

File "src/plugins/p1/dune", line 38, characters 14-30:
38 |   (enabled_if %{read:.enabled})
                   ^^^^^^^^^^^^^^^^
Error: Only architecture, system, model, os_type, ccomp_type, profile,
ocaml_version and context_name variables are allowed in this 'enabled_if'
field. If you think that read should also be allowed, please file an issue
about it.

Is there something we can do about it? Did I miss a simpler solution?

(FYI @bobot)

Start with the plugin layout under src/plugins/, the plugin dune files such as src/plugins/p1/dune, and the generated src/plugins/dune described in the issue. Investigate how dune-configurator output is consumed during the current build and the restrictions on enabled_if; done should provide a supported way to disable a plugin from compilation, testing, and installation.