Extend list of supported methods in ValueToString

Also support |const char* ValueType::data const| in
|base::trace_event::ValueToString|. Thus support turning more types into
std::string (for instance |IPC::MessageView|). This will provide more
detailed information in captured traces.

Bug: 1103623
Change-Id: I897325212fe2202f782e6b8c445a404aba71582a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2356304Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Karel Král <karelkral@google.com>
Cr-Commit-Position: refs/heads/master@{#798158}

base/trace_event/trace_conversion_helper.h

@@ -37,7 +37,7 @@ class ValueToStringPriority : public ValueToStringPriority<N + 1> {};
 template <>
 // The number must be the same as in the fallback version of
 // |ValueToStringHelper|.
-class ValueToStringPriority<4> {};
+class ValueToStringPriority<5> {};
 
 // Use SFINAE to decide how to extract a string from the given parameter.
 
@@ -87,10 +87,20 @@ ValueToStringHelper(ValueToStringPriority<3>,
   return OstreamValueToString(value);
 }
 
+// If there is |ValueType::data| whose return value can be used to construct
+// |std::string|, use it.
+template <typename ValueType>
+decltype(std::string(std::declval<const ValueType>().data()))
+ValueToStringHelper(ValueToStringPriority<4>,
+                    const ValueType& value,
+                    std::string /* unused */) {
+  return value.data();
+}
+
 // Fallback returns the |fallback_value|. Needs to have |ValueToStringPriority|
 // with the highest number (to be called last).
 template <typename ValueType>
-std::string ValueToStringHelper(ValueToStringPriority<4>,
+std::string ValueToStringHelper(ValueToStringPriority<5>,
                                 const ValueType& /* unused */,
                                 std::string fallback_value) {
   return fallback_value;

base/trace_event/trace_conversion_helper_unittest.cc

@@ -109,5 +109,35 @@ TEST(TraceEventConversionHelperTest, HasBoth) {
   EXPECT_EQ("HasBoth::ToString", answer);
 }
 
+class HasData {
+ public:
+  const char* data() const { return "HasData"; }
+};
+
+TEST(TraceEventConversionHelperTest, HasData) {
+  std::string answer = ValueToString(HasData());
+  EXPECT_EQ("HasData", answer);
+}
+
+class HasNonConstData {
+ public:
+  const char* data() { return "HasNonConstData"; }
+};
+
+TEST(TraceEventConversionHelperTest, HasNonConstData) {
+  std::string answer = ValueToString(HasNonConstData(), "fallback");
+  EXPECT_EQ("fallback", answer);
+}
+
+class HasDataOfWrongType {
+ public:
+  void data() {}
+};
+
+TEST(TraceEventConversionHelperTest, HasDataOfWrongType) {
+  std::string answer = ValueToString(HasDataOfWrongType(), "fallback");
+  EXPECT_EQ("fallback", answer);
+}
+
 }  // namespace trace_event
 }  // namespace base