Currently, all packages that generate messages have a dependency on https://github.com/ros2/rosidl_defaults/blob/master/rosidl_default_runtime/package.xml . In turn, the rosidl_default_runtime package depends on rosidl_generator_cpp, which it really shouldn't. What we should do instead is make another package (let's call it rosidl_cpp_runtime for now), and have the headers that the generated message code depends on live there (basically all of the headers in https://github.com/ros2/rosidl/blob/master/rosidl_generator_cpp/include/rosidl_generator_cpp). Once we have that, then rosidl_default_runtime can depend on rosidl_cpp_runtime, breaking the dependency on the generator.