Function compile_rule 

pub fn compile_rule<F>(str: &str, build_fn: F) -> Result<Vec<PathBuf>>
where
    F: FnMut(&Yaml) -> Result<Vec<PathBuf>>,

Given a string that should be Yaml, it calls build_fn with that string. The build function/closure should process the Yaml as appropriate and capture any errors and write them to std_err. The returned value should be a Vector containing the paths of all the files that were included.