Add an experimental nested formatter

include/fmt/format.h

@@ -4198,6 +4198,38 @@ template <typename T> struct formatter<group_digits_view<T>> : formatter<T> {
   }
 };
 
+template <typename T>
+struct nested_view {
+  const formatter<T>* fmt;
+  const T* value;
+};
+
+template <typename T>
+struct formatter<nested_view<T>> {
+  FMT_CONSTEXPR auto parse(format_parse_context& ctx) -> const char* {
+    return ctx.begin();
+  }
+  auto format(nested_view<T> view, format_context& ctx) const
+      -> decltype(ctx.out()) {
+    return view.fmt->format(*view.value, ctx);
+  }
+};
+
+template <typename T>
+struct nested_formatter {
+ private:
+  formatter<T> formatter_;
+
+ public:
+  FMT_CONSTEXPR auto parse(format_parse_context& ctx) -> const char* {
+    return formatter_.parse(ctx);
+  }
+
+  auto nested(const T& value) const -> nested_view<T> {
+    return nested_view<T>{&formatter_, &value};
+  }
+};
+
 // DEPRECATED! join_view will be moved to ranges.h.
 template <typename It, typename Sentinel, typename Char = char>
 struct join_view : detail::view {

test/format-test.cc

@@ -1775,6 +1775,23 @@ TEST(format_test, group_digits_view) {
   EXPECT_EQ(fmt::format("{:8}", fmt::group_digits(1000)), "   1,000");
 }
 
+struct point {
+  double x, y;
+};
+
+FMT_BEGIN_NAMESPACE
+template <>
+struct formatter<point> : nested_formatter<double> {
+  auto format(point p, format_context& ctx) const -> decltype(ctx.out()) {
+    return format_to(ctx.out(), "({}, {})", nested(p.x), nested(p.y));
+  }
+};
+FMT_END_NAMESPACE
+
+TEST(format_test, nested_formatter) {
+  EXPECT_EQ(fmt::format("{:.2f}", point{1, 2}), "(1.00, 2.00)");
+}
+
 enum test_enum { foo, bar };
 auto format_as(test_enum e) -> int { return e; }