Optimize buffered padding with chunked fills

Fill available appender storage in bulk, continuing across buffer growth and flushes
Preserve the constexpr-compatible scalar fallback
Add testing of large padded sizes

include/fmt/format.h

@@ -544,6 +544,20 @@ FMT_CONSTEXPR20 auto fill_n(T* out, Size count, char value) -> T* {
   memset(out, value, to_unsigned(count));
   return out + count;
 }
+template <typename T>
+FMT_CONSTEXPR auto fill_n(basic_appender<T> out, size_t n, T value)
+    -> basic_appender<T> {
+  auto& buf = get_container(out);
+  while (n != 0 && !is_constant_evaluated(true)) {
+    buf.try_reserve(buf.size() + n);
+    auto count = min_of(n, buf.capacity() - buf.size());
+    auto ptr = buf.data() + buf.size();
+    buf.try_resize(buf.size() + count);
+    detail::fill_n(ptr, count, value);
+    n -= count;
+  }
+  return fill_n<basic_appender<T>, size_t, T>(out, n, value);
+}
 
 template <typename T, typename V, typename OutputIt>
 FMT_CONSTEXPR auto copy(basic_string_view<V> s, OutputIt out) -> OutputIt {

test/format-test.cc

@@ -2254,6 +2254,7 @@ TEST(format_test, formatted_size) {
 #if FMT_USE_LOCALE
   EXPECT_EQ(2u, fmt::formatted_size(std::locale(), "{}", 42));
 #endif
+  EXPECT_EQ(602u, fmt::formatted_size("[{:*>600}]", 42));
 }
 
 TEST(format_test, format_to_no_args) {
@@ -2327,6 +2328,11 @@ TEST(format_test, format_to_n) {
   result = fmt::format_to_n(buffer, 3, "{}", std::string(1000, '*'));
   EXPECT_EQ(1000u, result.size);
   EXPECT_EQ("***x", fmt::string_view(buffer, 4));
+
+  result = fmt::format_to_n(buffer, 0, "{:!>600}", 42);
+  EXPECT_EQ(600u, result.size);
+  EXPECT_EQ(buffer, result.out);
+  EXPECT_EQ("***x", fmt::string_view(buffer, 4));
 }
 
 struct test_output_iterator {
@@ -2354,6 +2360,10 @@ TEST(format_test, format_to_n_output_iterator) {
   char buf[10] = {};
   fmt::format_to_n(test_output_iterator{buf}, 10, "{}", 42);
   EXPECT_STREQ(buf, "42");
+  auto result = fmt::format_to_n(test_output_iterator{buf}, 3, "{:*>600}", 42);
+  EXPECT_EQ(600u, result.size);
+  EXPECT_EQ(buf + 3, result.out.data);
+  EXPECT_STREQ(buf, "***");
 }
 
 TEST(format_test, vformat_to) {

test/xchar-test.cc

@@ -82,6 +82,7 @@ TEST(xchar_test, format) {
   if (sizeof(wchar_t) == 4)
     EXPECT_EQ(fmt::format(fmt::runtime(L"{:𓀨>3}"), 42), L"𓀨42");
   EXPECT_EQ(fmt::format(L"{}c{}", L"ab", 1), L"abc1");
+  EXPECT_EQ(std::wstring(600, L'\u03a9'), fmt::format(L"{:\u03a9>600}", L""));
 }
 
 TEST(xchar_test, is_formattable) {