Function check_raw_common

fn check_raw_common<F>(src: &str, mode: Mode, callback: &mut F)
where
    F: FnMut(Range<usize>, Result<char, EscapeError>),

Takes a contents of a string literal (without quotes) and produces a sequence of characters or errors. NOTE: Raw strings do not perform any explicit character escaping, here we only produce errors on bare CR.