Function parse_username 

pub fn parse_username<P>(
    ext: &mut Extensions,
    parser: P,
    username_ref: impl AsRef<str>,
) -> Result<String, Box<dyn Error + Send + Sync>>
where
    P: UsernameLabelParser,
    <P as UsernameLabelParser>::Error: Into<Box<dyn Error + Send + Sync>>,

Parse a username, extracting the username (first part) and passing everything else to the UsernameLabelParser.