Function rustc_lexer::unescape::unescape_str_or_byte_str

fn unescape_str_or_byte_str<F>(src: &str, is_byte: bool, 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.