Move `InitializationErrorString` into the secondary vtable, retrieved from `ClassData`.

This removes one virtual function from MessageLite with zero per-type increase.

PiperOrigin-RevId: 574994142

src/google/protobuf/message.cc

@@ -195,8 +195,13 @@ static std::string GetTypeNameImpl(const MessageLite& msg) {
   return DownCast<const Message&>(msg).GetDescriptor()->full_name();
 }
 
+static std::string InitializationErrorStringImpl(const MessageLite& msg) {
+  return DownCast<const Message&>(msg).InitializationErrorString();
+}
+
 constexpr MessageLite::DescriptorMethods Message::kDescriptorMethods = {
     GetTypeNameImpl,
+    InitializationErrorStringImpl,
 };
 
 namespace internal {

src/google/protobuf/message.h

@@ -271,7 +271,7 @@ class PROTOBUF_EXPORT Message : public MessageLite {
 
   // Like FindInitializationErrors, but joins all the strings, delimited by
   // commas, and returns them.
-  std::string InitializationErrorString() const override;
+  std::string InitializationErrorString() const;
 
   // Clears all unknown fields from this message and all embedded messages.
   // Normally, if unknown tag numbers are encountered when parsing a message,

src/google/protobuf/message_lite.cc

@@ -68,6 +68,14 @@ void MessageLite::OnDemandRegisterArenaDtor(Arena* arena) {
 }
 
 std::string MessageLite::InitializationErrorString() const {
+  auto* data = GetClassData();
+  ABSL_DCHECK(data != nullptr);
+
+  if (data->descriptor_methods != nullptr) {
+    // For !LITE messages, we use the descriptor method function.
+    return data->descriptor_methods->initialization_error_string(*this);
+  }
+
   return "(cannot determine missing fields for lite message)";
 }
 

src/google/protobuf/message_lite.h

@@ -235,7 +235,7 @@ class PROTOBUF_EXPORT MessageLite {
   // This is not implemented for Lite messages -- it just returns "(cannot
   // determine missing fields for lite message)".  However, it is implemented
   // for full messages.  See message.h.
-  virtual std::string InitializationErrorString() const;
+  std::string InitializationErrorString() const;
 
   // If |other| is the exact same class as this, calls MergeFrom(). Otherwise,
   // results are undefined (probably crash).
@@ -523,6 +523,7 @@ class PROTOBUF_EXPORT MessageLite {
   // costs in MessageLite.
   struct DescriptorMethods {
     std::string (*get_type_name)(const MessageLite&);
+    std::string (*initialization_error_string)(const MessageLite&);
   };
   struct ClassData {
     // Note: The order of arguments in the functions is chosen so that it has