Function rustc_lexer::unescape::unescape_str_or_byte_str

fn unescape_str_or_byte_str<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 escaped characters or errors.