Make sinks provided to `AbslStringify()` usable with `absl::Format()`.

Such sinks must define ADL-callable `AbslFormatFlush()`. It can just forward to
`Append()`.

PiperOrigin-RevId: 479043790
Change-Id: I5d7d80ca1e17adf03b77726df8a52e2b4e9196ce

absl/strings/BUILD.bazel

@@ -963,8 +963,8 @@ cc_test(
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":str_format",
         ":strings",
-        "//absl/base:core_headers",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -1164,7 +1164,6 @@ cc_test(
         ":cord",
         ":str_format",
         ":strings",
-        "//absl/base:core_headers",
         "@com_google_googletest//:gtest_main",
     ],
 )

absl/strings/CMakeLists.txt

@@ -281,6 +281,7 @@ absl_cc_test(
     ${ABSL_TEST_COPTS}
   DEPS
     absl::strings
+    absl::str_format
     absl::core_headers
     GTest::gmock_main
 )

absl/strings/str_cat.h

@@ -85,6 +85,11 @@ class StringifySink {
 
   bool PutPaddedString(string_view v, int width, int precision, bool left);
 
+  // Support `absl::Format(&sink, format, args...)`.
+  friend void AbslFormatFlush(StringifySink* sink, absl::string_view v) {
+    sink->Append(v);
+  }
+
   template <typename T>
   friend string_view ExtractStringification(StringifySink& sink, const T& v);
 

absl/strings/str_cat_test.cc

@@ -21,6 +21,7 @@
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/strings/str_format.h"
 #include "absl/strings/substitute.h"
 
 #ifdef __ANDROID__
@@ -632,4 +633,21 @@ TEST(StrCat, AbslStringifyExample) {
   EXPECT_EQ(absl::StrCat("a ", p, " z"), "a (10, 20) z");
 }
 
+struct PointStringifyUsingFormat {
+  template <typename FormatSink>
+  friend void AbslStringify(FormatSink& sink,
+                            const PointStringifyUsingFormat& p) {
+    absl::Format(&sink, "(%g, %g)", p.x, p.y);
+  }
+
+  double x = 10.0;
+  double y = 20.0;
+};
+
+TEST(StrCat, AbslStringifyExampleUsingFormat) {
+  PointStringifyUsingFormat p;
+  EXPECT_EQ(absl::StrCat(p), "(10, 20)");
+  EXPECT_EQ(absl::StrCat("a ", p, " z"), "a (10, 20) z");
+}
+
 }  // namespace

absl/strings/str_format.h

@@ -789,6 +789,11 @@ class FormatSink {
     return sink_->PutPaddedString(v, width, precision, left);
   }
 
+  // Support `absl::Format(&sink, format, args...)`.
+  friend void AbslFormatFlush(FormatSink* sink, absl::string_view v) {
+    sink->Append(v);
+  }
+
  private:
   friend str_format_internal::FormatSinkImpl;
   explicit FormatSink(str_format_internal::FormatSinkImpl* s) : sink_(s) {}

absl/strings/str_format_test.cc

@@ -1118,6 +1118,23 @@ TEST_F(FormatExtensionTest, AbslStringifyExample) {
   PointStringify p;
   EXPECT_EQ(absl::StrFormat("a %v z", p), "a (10, 20) z");
 }
+
+struct PointStringifyUsingFormat {
+  template <typename FormatSink>
+  friend void AbslStringify(FormatSink& sink,
+                            const PointStringifyUsingFormat& p) {
+    absl::Format(&sink, "(%g, %g)", p.x, p.y);
+  }
+
+  double x = 10.0;
+  double y = 20.0;
+};
+
+TEST_F(FormatExtensionTest, AbslStringifyExampleUsingFormat) {
+  PointStringifyUsingFormat p;
+  EXPECT_EQ(absl::StrFormat("a %v z", p), "a (10, 20) z");
+}
+
 }  // namespace
 
 // Some codegen thunks that we can use to easily dump the generated assembly for