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, 45 insertions, 3 deletions

diff --git a/absl/log/internal/log_message.h b/absl/log/internal/log_message.h
index 37a267c0..992bb630 100644
--- a/absl/log/internal/log_message.h
+++ b/absl/log/internal/log_message.h
@@ -41,6 +41,7 @@
 #include "absl/log/internal/nullguard.h"
 #include "absl/log/log_entry.h"
 #include "absl/log/log_sink.h"
+#include "absl/strings/internal/has_absl_stringify.h"
 #include "absl/strings/string_view.h"
 #include "absl/time/time.h"
 
@@ -153,8 +154,17 @@ class LogMessage {
   template <int SIZE>
   LogMessage& operator<<(char (&buf)[SIZE]) ABSL_ATTRIBUTE_NOINLINE;
 
-  // Default: uses `ostream` logging to convert `v` to a string.
-  template <typename T>
+  // Types that support `AbslStringify()` are serialized that way.
+  template <typename T,
+            typename std::enable_if<
+                strings_internal::HasAbslStringify<T>::value, int>::type = 0>
+  LogMessage& operator<<(const T& v) ABSL_ATTRIBUTE_NOINLINE;
+
+  // Types that don't support `AbslStringify()` but do support streaming into a
+  // `std::ostream&` are serialized that way.
+  template <typename T,
+            typename std::enable_if<
+                !strings_internal::HasAbslStringify<T>::value, int>::type = 0>
   LogMessage& operator<<(const T& v) ABSL_ATTRIBUTE_NOINLINE;
 
   // Note: We explicitly do not support `operator<<` for non-const references
@@ -205,12 +215,44 @@ class LogMessage {
   std::ostream stream_;
 };
 
+// Helper class so that `AbslStringify()` can modify the LogMessage.
+class StringifySink final {
+ public:
+  explicit StringifySink(LogMessage& message) : message_(message) {}
+
+  void Append(size_t count, char ch) { message_ << std::string(count, ch); }
+
+  void Append(absl::string_view v) { message_ << v; }
+
+  // For types that implement `AbslStringify` using `absl::Format()`.
+  friend void AbslFormatFlush(StringifySink* sink, absl::string_view v) {
+    sink->Append(v);
+  }
+
+ private:
+  LogMessage& message_;
+};
+
+// Note: the following is declared `ABSL_ATTRIBUTE_NOINLINE`
+template <typename T,
+          typename std::enable_if<strings_internal::HasAbslStringify<T>::value,
+                                  int>::type>
+LogMessage& LogMessage::operator<<(const T& v) {
+  StringifySink sink(*this);
+  // Replace with public API.
+  AbslStringify(sink, v);
+  return *this;
+}
+
 // Note: the following is declared `ABSL_ATTRIBUTE_NOINLINE`
-template <typename T>
+template <typename T,
+          typename std::enable_if<!strings_internal::HasAbslStringify<T>::value,
+                                  int>::type>
 LogMessage& LogMessage::operator<<(const T& v) {
   stream_ << log_internal::NullGuard<T>().Guard(v);
   return *this;
 }
+
 inline LogMessage& LogMessage::operator<<(
     std::ostream& (*m)(std::ostream& os)) {
   stream_ << m;