Add `AbslStringify()` support to `absl::Cord`

PiperOrigin-RevId: 576149980
Change-Id: I0a7e3df7a01edc78ee5d15d8d8e82e7bbc5fc0f3

absl/strings/cord.h

@@ -765,6 +765,14 @@ class Cord {
     cord->Append(part);
   }
 
+  // Support automatic stringification with absl::StrCat and absl::StrFormat.
+  template <typename Sink>
+  friend void AbslStringify(Sink& sink, const absl::Cord& cord) {
+    for (absl::string_view chunk : cord.Chunks()) {
+      sink.Append(chunk);
+    }
+  }
+
   // Cord::SetExpectedChecksum()
   //
   // Stores a checksum value with this non-empty cord instance, for later

absl/strings/cord_test.cc

@@ -2656,6 +2656,13 @@ TEST_P(CordTest, Format) {
   EXPECT_EQ(c, "There were 0003 little pigs.And 1   bad wolf!");
 }
 
+TEST_P(CordTest, Stringify) {
+  absl::Cord c =
+      absl::MakeFragmentedCord({"A ", "small ", "fragmented ", "Cord", "."});
+  MaybeHarden(c);
+  EXPECT_EQ(absl::StrCat(c), "A small fragmented Cord.");
+}
+
 TEST_P(CordTest, Hardening) {
   absl::Cord cord("hello");
   MaybeHarden(cord);

absl/strings/internal/str_format/arg.h

@@ -158,6 +158,7 @@ template <typename T>
 auto FormatConvertImpl(const T& v, FormatConversionSpecImpl,
                        FormatSinkImpl* sink)
     -> std::enable_if_t<!std::is_enum<T>::value &&
+                            !std::is_same<T, absl::Cord>::value &&
                             std::is_void<decltype(AbslStringify(
                                 std::declval<FormatSink&>(), v))>::value,
                         ArgConvertResult<FormatConversionCharSetInternal::v>> {