Use a pointer instead of a reference for specs in arg_formatter

Per PR suggestion.

include/fmt/format.h

@@ -3580,19 +3580,19 @@ template <typename Char> struct arg_formatter {
   using context = buffer_context<Char>;
 
   iterator out;
-  const format_specs<Char>& specs;
+  const format_specs<Char>* specs;
   locale_ref locale;
 
-  arg_formatter(buffer_appender<Char> it, const format_specs<Char>& s)
+  arg_formatter(buffer_appender<Char> it, const format_specs<Char>* s)
       : out(it), specs(s) {}
 
-  arg_formatter(buffer_appender<Char> it, const format_specs<Char>& s,
+  arg_formatter(buffer_appender<Char> it, const format_specs<Char>* s,
                 locale_ref l)
       : out(it), specs(s), locale(l) {}
 
   template <typename T>
   FMT_CONSTEXPR FMT_INLINE auto operator()(T value) -> iterator {
-    return detail::write(out, value, specs, locale);
+    return detail::write(out, value, *specs, locale);
   }
   auto operator()(typename basic_format_arg<context>::handle) -> iterator {
     // User-defined types are handled separately because they require access
@@ -4220,7 +4220,7 @@ void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
           specs.precision, specs.precision_ref, context);
       if (begin == end || *begin != '}')
         on_error("missing '}' in format string");
-      auto f = arg_formatter<Char>{context.out(), specs, context.locale()};
+      auto f = arg_formatter<Char>{context.out(), &specs, context.locale()};
       context.advance_to(visit_format_arg(f, arg));
       return begin;
     }

include/fmt/printf.h

@@ -228,13 +228,13 @@ class printf_arg_formatter : public arg_formatter<Char> {
   context_type& context_;
 
   OutputIt write_null_pointer(bool is_string = false) {
-    auto s = this->specs;
+    auto s = *this->specs;
     s.type = presentation_type::none;
     return write_bytes(this->out, is_string ? "(null)" : "(nil)", s);
   }
 
  public:
-  printf_arg_formatter(OutputIt iter, format_specs<Char>& s, context_type& ctx)
+  printf_arg_formatter(OutputIt iter, format_specs<Char>* s, context_type& ctx)
       : base{iter, s}, context_(ctx) {}
 
   OutputIt operator()(monostate value) { return base::operator()(value); }
@@ -244,7 +244,7 @@ class printf_arg_formatter : public arg_formatter<Char> {
     // MSVC2013 fails to compile separate overloads for bool and Char so use
     // std::is_same instead.
     if (std::is_same<T, Char>::value) {
-      format_specs<Char> fmt_specs = this->specs;
+      format_specs<Char> fmt_specs = *this->specs;
       if (fmt_specs.type != presentation_type::none &&
           fmt_specs.type != presentation_type::chr) {
         return (*this)(static_cast<int>(value));
@@ -269,13 +269,13 @@ class printf_arg_formatter : public arg_formatter<Char> {
   /** Formats a null-terminated C string. */
   OutputIt operator()(const char* value) {
     if (value) return base::operator()(value);
-    return write_null_pointer(this->specs.type != presentation_type::pointer);
+    return write_null_pointer(this->specs->type != presentation_type::pointer);
   }
 
   /** Formats a null-terminated wide C string. */
   OutputIt operator()(const wchar_t* value) {
     if (value) return base::operator()(value);
-    return write_null_pointer(this->specs.type != presentation_type::pointer);
+    return write_null_pointer(this->specs->type != presentation_type::pointer);
   }
 
   OutputIt operator()(basic_string_view<Char> value) {
@@ -545,7 +545,7 @@ void vprintf(buffer<Char>& buf, basic_string_view<Char> format,
 
     // Format argument.
     out = visit_format_arg(
-        printf_arg_formatter<iterator, Char>(out, specs, context), arg);
+        printf_arg_formatter<iterator, Char>(out, &specs, context), arg);
   }
   write(out, basic_string_view<Char>(start, to_unsigned(it - start)));
 }