Function rename_marker_from 

pub fn rename_marker_from<'a>(raw: &'a str, marker: &str) -> Option<&'a str>

The old name in a whole rename marker raw, as cratestack-migrate reads it: marker ("@rename" or "@@rename"), directly its (, arguments parse_rename_from takes, and a ) that ends the text. None for anything else — which the migrator treats as no marker.