Constant LOCAL_SPELLING 

pub const LOCAL_SPELLING: &str = "names `http://localhost` in a spelling no client_id is judged in: \
     every local client_id is judged as `http://localhost/?redirect_uri=…&scope=…`, \
     so this part matches no local client; `^http://localhost/` matches every one";

What a predicate is told when it names http://localhost in a spelling no client_id is judged in.