Support logging of user-defined types that implement `AbslStringify()`

If a user-defined type has `AbslStringify()` defined, it will always be used for logging over `operator<<`.

`HasAbslStringify` now uses the empty class `UnimplementedSink` for its checks instead of `StringifySink` in order to make it work in cases involving other sinks.

PiperOrigin-RevId: 485710377
Change-Id: Ibdd916151c7abc3269c35fbe79b772867f3d25e1

1 files changed, 55 insertions, 0 deletions

diff --git a/absl/strings/internal/has_absl_stringify.h b/absl/strings/internal/has_absl_stringify.h
new file mode 100644
index 00000000..55a08508
--- /dev/null
+++ b/absl/strings/internal/has_absl_stringify.h
@@ -0,0 +1,55 @@
+// Copyright 2022 The Abseil Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_HAS_ABSL_STRINGIFY_H_
+#define ABSL_STRINGS_INTERNAL_HAS_ABSL_STRINGIFY_H_
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace strings_internal {
+
+// This is an empty class not intended to be used. It exists so that
+// `HasAbslStringify` can reference a universal class rather than needing to be
+// copied for each new sink.
+class UnimplementedSink {
+ public:
+  void Append(size_t count, char ch);
+
+  void Append(string_view v);
+
+  // Support `absl::Format(&sink, format, args...)`.
+  friend void AbslFormatFlush(UnimplementedSink* sink, absl::string_view v);
+};
+
+template <typename T, typename = void>
+struct HasAbslStringify : std::false_type {};
+
+template <typename T>
+struct HasAbslStringify<
+    T, std::enable_if_t<std::is_void<decltype(AbslStringify(
+           std::declval<strings_internal::UnimplementedSink&>(),
+           std::declval<const T&>()))>::value>> : std::true_type {};
+
+}  // namespace strings_internal
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_HAS_ABSL_STRINGIFY_H_