Function parse_format_string

extlibs/fmt/include/fmt/format.h:2575–2624

template <bool IS_CONSTEXPR, typename Char, typename Handler>
FMT_CONSTEXPR void parse_format_string(basic_string_view<Char> format_str,
                                       Handler&& handler) {
  struct pfs_writer {
    FMT_CONSTEXPR void operator()(const Char* begin, const Char* end) {
      if (begin == end) return;
      for (;;) {
        const Char* p = nullptr;
        if (!find<IS_CONSTEXPR>(begin, end, '}', p))
          return handler_.on_text(begin, end);
        ++p;
        if (p == end || *p != '}')
          return handler_.on_error("unmatched '}' in format string");
        handler_.on_text(begin, p);
        begin = p + 1;
      }
    }
    Handler& handler_;
  } write{handler};
  auto begin = format_str.data();
  auto end = begin + format_str.size();
  while (begin != end) {
    // Doing two passes with memchr (one for '{' and another for '}') is up to
    // 2.5x faster than the naive one-pass implementation on big format strings.
    const Char* p = begin;
    if (*begin != '{' && !find<IS_CONSTEXPR>(begin + 1, end, '{', p))
      return write(begin, end);
    write(begin, p);
    ++p;
    if (p == end) return handler.on_error("invalid format string");
    if (static_cast<char>(*p) == '}') {
      handler.on_arg_id();
      handler.on_replacement_field(p);
    } else if (*p == '{') {
      handler.on_text(p, p + 1);
    } else {
      p = parse_arg_id(p, end, id_adapter<Handler, Char>{handler});
      Char c = p != end ? *p : Char();
      if (c == '}') {
        handler.on_replacement_field(p);
      } else if (c == ':') {
        p = handler.on_format_specs(p + 1, end);
        if (p == end || *p != '}')
          return handler.on_error("unknown format specifier");
      } else {
        return handler.on_error("missing '}' in format string");
      }
    }
    begin = p + 1;
  }
}

template <typename T, typename ParseContext>
FMT_CONSTEXPR const typename ParseContext::char_type* parse_format_specs(