`convert_latin1_to_utf8` doesn't accept length field for `utf8_output` pointer about simdutf

[lemire]

The simdutf library has always worked in a two pass model: first compute how much memory is needed, and then we transcode.

      size_t expected_utf8words =
            simdutf::utf8_length_from_latin1(latin1_output.get(), latin1words);
      std::unique_ptr<char[]> utf8_output{ new char[expected_utf8words] };
        // convert to UTF-8
      size_t utf8words = simdutf::convert_latin1_to_utf8(
            latin1_output.get(), latin1words, utf8_output.get());

      size_t expected_latin1words = simdutf::latin1_length_from_utf8(source.c_str(), source.size());
      std::unique_ptr<char[]> latin1_output{
        new char[expected_latin1words]
      };
      // convert to latin1
      size_t latin1words = simdutf:: convert_utf8_to_latin1(
          source.c_str(), source.size(), latin1_output.get());

wouldn't that would that usually be slower than checking if there's enough length after each non-ascii character is written?

Computing the output size is much faster than transcoding.

If my answer is not satisfactory, can you elaborate?