Improve locale retrieval code

include/fmt/chrono.h

@@ -383,6 +383,25 @@ auto write(OutputIt out, const std::tm& time, const std::locale& loc,
   return std::copy(str.begin(), str.end(), out);
 }
 
+template <typename F>
+auto with_locale(locale_ref loc_ref, F&& f) -> decltype(f(std::locale{})) {
+  if (loc_ref) return f(loc_ref.template get<std::locale>());
+  // in contrast to the case above, classic locale is returned by reference
+  // without introducing a temporary object
+  // (locale copy ctor & dtor are relatively expensive)
+  return f(get_classic_locale());
+}
+
+template <typename F>
+auto with_locale(bool localized, locale_ref loc_ref, F&& f)
+    -> decltype(f(std::locale{})) {
+  if (localized) return f(loc_ref.template get<std::locale>());
+  // in contrast to the case above, classic locale is returned by reference
+  // without introducing a temporary object
+  // (locale copy ctor & dtor are relatively expensive)
+  return f(get_classic_locale());
+}
+
 }  // namespace detail
 
 FMT_MODULE_EXPORT_BEGIN
@@ -1474,21 +1493,15 @@ struct chrono_formatter {
   void write_pinf() { std::copy_n("inf", 3, out); }
   void write_ninf() { std::copy_n("-inf", 4, out); }
 
-  template <class Callback, class... Args>
-  void do_format_tm(const std::locale& loc, const tm& time, Callback cb,
-                    Args... args) {
-    auto w = tm_writer_type(loc, out, time);
-    (w.*cb)(std::forward<Args>(args)...);
-    out = w.out();
-  }
   template <class Callback, class... Args>
   void format_tm(const tm& time, Callback cb, Args... args) {
     if (isnan(val)) return write_nan();
-    if (localized)
-      do_format_tm(context.locale().template get<std::locale>(), time, cb,
-                   std::forward<Args>(args)...);
-    else
-      do_format_tm(get_classic_locale(), time, cb, std::forward<Args>(args)...);
+    with_locale(localized, context.locale(),
+                [this, &time, &cb, &args...](const std::locale& loc) {
+                  auto w = tm_writer_type(loc, out, time);
+                  (w.*cb)(std::forward<Args>(args)...);
+                  out = w.out();
+                });
   }
 
   void on_text(const char_type* begin, const char_type* end) {
@@ -1640,14 +1653,6 @@ template <typename Char> struct formatter<weekday, Char> {
  private:
   bool localized = false;
 
-  template <typename It>
-  auto do_format(It out, const std::tm& time, const std::locale& loc) const
-      -> It {
-    auto w = detail::tm_writer<It, Char>(loc, out, time);
-    w.on_abbr_weekday();
-    return w.out();
-  }
-
  public:
   FMT_CONSTEXPR auto parse(basic_format_parse_context<Char>& ctx)
       -> decltype(ctx.begin()) {
@@ -1661,12 +1666,15 @@ template <typename Char> struct formatter<weekday, Char> {
 
   template <typename FormatContext>
   auto format(weekday wd, FormatContext& ctx) const -> decltype(ctx.out()) {
-    auto time = std::tm();
-    time.tm_wday = static_cast<int>(wd.c_encoding());
-    if (localized)
-      return this->do_format(ctx.out(), time,
-                             ctx.locale().template get<std::locale>());
-    return this->do_format(ctx.out(), time, detail::get_classic_locale());
+    auto out = ctx.out();
+    return with_locale(
+        localized, ctx.locale(), [&out, &wd](const std::locale& loc) {
+          auto time = std::tm();
+          time.tm_wday = static_cast<int>(wd.c_encoding());
+          auto w = detail::tm_writer<decltype(out), Char>(loc, out, time);
+          w.on_abbr_weekday();
+          return w.out();
+        });
   }
 };
 
@@ -1843,18 +1851,6 @@ template <typename Char> struct formatter<std::tm, Char> {
     return end;
   }
 
-  template <typename It>
-  It do_format(It out, const std::tm& tm, const std::locale& loc) const {
-    auto w = detail::tm_writer<It, Char>(loc, out, tm);
-    if (spec_ == spec::year_month_day)
-      w.on_iso_date();
-    else if (spec_ == spec::hh_mm_ss)
-      w.on_iso_time();
-    else
-      detail::parse_chrono_format(specs.begin(), specs.end(), w);
-    return w.out();
-  }
-
  public:
   template <typename ParseContext>
   FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
@@ -1864,10 +1860,17 @@ template <typename Char> struct formatter<std::tm, Char> {
   template <typename FormatContext>
   auto format(const std::tm& tm, FormatContext& ctx) const
       -> decltype(ctx.out()) {
-    if (const auto& loc_ref = ctx.locale())
-      return this->do_format(ctx.out(), tm,
-                             loc_ref.template get<std::locale>());
-    return this->do_format(ctx.out(), tm, detail::get_classic_locale());
+    auto out = ctx.out();
+    return with_locale(ctx.locale(), [this, &tm, &out](const std::locale& loc) {
+      auto w = detail::tm_writer<decltype(out), Char>(loc, out, tm);
+      if (spec_ == spec::year_month_day)
+        w.on_iso_date();
+      else if (spec_ == spec::hh_mm_ss)
+        w.on_iso_time();
+      else
+        detail::parse_chrono_format(specs.begin(), specs.end(), w);
+      return w.out();
+    });
   }
 };