Function extract_lib_features 

fn extract_lib_features<'c, 'p>(
    contents: &'c str,
    file: &'p Path,
    mf: &mut (dyn Send + Sync + FnMut(Result<(&'c str, Feature), &'static str>, &'p Path, usize)),
)

mf gets the feature or an error if it is invalid, the file path passed as file, and the attribute’s line number.