`buildtool_export_depend` and `build_export_depend`

I was looking at the dependencies of some packages and noticed that they sometimes depend on amend_cmake (take for instance ros-kilted-rmw-connextdds). These build dependencies sometimes include a large number of unwanted dependencies. And I was trying to see if we can slim that down.

I tracked this down to the buildtool_export_depends and build_export_depends being added as run-dependencies.

To clarify this is what REP 149 says:

buildtool_export_depend

Declares a rosdep key or ROS package name that this package exports which must be compiled and run on the build system, not the target system. > For cross-compilation, one must distinguish these from target build dependencies, which may be linked with your package and must be compiled for the target architecture.

<build_export_depend>

Declares a rosdep key or ROS package name that this package needs as part of some build interface it exports. For system packages, the rosdep key will normally specify the "development" package, which frequently ends in "-dev".

The <build_export_depend> declares a transitive build dependency. A common example is when one of your dependencies provides a header file included in some header exported by your package. Even if your package does not use that header when building itself, other packages depending on your header will require those transitive dependencies when they are built.

I translate this to; if we are building a package baz that has a dependency on a package foobar:

• If foobar is a dependency, we should also include its build_export_depends in the host environment.
• If foobar is a dependency, we should also include its buildtool_export_depends in the build environment.

I don't think that at the moment there is good mechanism to do this with the conda build infrastructure.

I'm wondering what could potentially be a good approach to properly support this mechanism.

Start by tracing where buildtool_export_depends and build_export_depends become run-dependencies in the conda recipe generation flow. Compare that behavior with the REP 149 semantics quoted in the issue and the conda build host/build environments; done means correctly separating these dependencies without adding unwanted runtime dependencies.