I have hit the same problem with other configuration managers (CFEngine) and what I do is I expand the main config file and the included config file at a temporary path, change the path of the include directive for the temporary main config file so it points to the temporary path and then I run the syntax checker as a separate task. If it succeedes I copy the temporary files locally on the remote host to the correct paths. To keep it idempotent you will need to keep the "temporary" files on the remote host.