Introduce FieldDescriptor::cpp_string_type() API to replace direct ctype inspection which will be removed in the next breaking change

This should provide the roughly same result as ctype, except that it reflects actual behavior rather than the specification in the proto file.  VIEW will now be visible, and some subtleties around CORD and PIECE will change.

PiperOrigin-RevId: 653677655

src/google/protobuf/descriptor.cc

@@ -3939,6 +3939,29 @@ bool FieldDescriptor::has_optional_keyword() const {
                               is_optional() && !containing_oneof());
 }
 
+FieldDescriptor::CppStringType FieldDescriptor::cpp_string_type() const {
+  ABSL_DCHECK(cpp_type() == FieldDescriptor::CPPTYPE_STRING);
+  switch (features().GetExtension(pb::cpp).string_type()) {
+    case pb::CppFeatures::VIEW:
+      return CppStringType::kView;
+    case pb::CppFeatures::CORD:
+      // In open-source, protobuf CORD is only supported for singular bytes
+      // fields.
+      if (type() != FieldDescriptor::TYPE_BYTES || is_repeated() ||
+          is_extension()) {
+        return CppStringType::kString;
+      }
+      return CppStringType::kCord;
+    case pb::CppFeatures::STRING:
+      return CppStringType::kString;
+    default:
+      // If features haven't been resolved, this is a dynamic build not for C++
+      // codegen.  Just use string type.
+      ABSL_DCHECK(!features().GetExtension(pb::cpp).has_string_type());
+      return CppStringType::kString;
+  }
+}
+
 // Location methods ===============================================
 
 bool FileDescriptor::GetSourceLocation(const std::vector<int>& path,

src/google/protobuf/descriptor.h

@@ -875,6 +875,10 @@ class PROTOBUF_EXPORT FieldDescriptor : private internal::SymbolBase,
   const char* cpp_type_name() const;  // Name of the C++ type.
   Label label() const;                // optional/required/repeated
 
+#ifndef SWIG
+  CppStringType cpp_string_type() const;  // The C++ string type of this field.
+#endif
+
   bool is_required() const;  // shorthand for label() == LABEL_REQUIRED
   bool is_optional() const;  // shorthand for label() == LABEL_OPTIONAL
   bool is_repeated() const;  // shorthand for label() == LABEL_REPEATED
@@ -2932,22 +2936,21 @@ PROTOBUF_EXPORT bool HasPreservingUnknownEnumSemantics(
 
 PROTOBUF_EXPORT bool HasHasbit(const FieldDescriptor* field);
 
+#ifndef SWIG
 // For a string field, returns the effective ctype.  If the actual ctype is
 // not supported, returns the default of STRING.
 template <typename FieldDesc = FieldDescriptor,
           typename FieldOpts = FieldOptions>
 typename FieldOpts::CType EffectiveStringCType(const FieldDesc* field) {
-  ABSL_DCHECK(field->cpp_type() == FieldDescriptor::CPPTYPE_STRING);
-  // Open-source protobuf release only supports STRING ctype and CORD for
-  // sinuglar bytes.
-  if (field->type() == FieldDescriptor::TYPE_BYTES && !field->is_repeated() &&
-      field->options().ctype() == FieldOpts::CORD && !field->is_extension()) {
-    return FieldOpts::CORD;
+  // TODO Replace this function with FieldDescriptor::string_type;
+  switch (field->cpp_string_type()) {
+    case FieldDescriptor::CppStringType::kCord:
+      return FieldOpts::CORD;
+    default:
+      return FieldOpts::STRING;
   }
-  return FieldOpts::STRING;
 }
 
-#ifndef SWIG
 enum class Utf8CheckMode : uint8_t {
   kStrict = 0,  // Parsing will fail if non UTF-8 data is in string fields.
   kVerify = 1,  // Only log an error but parsing will succeed.

src/google/protobuf/descriptor_lite.h

@@ -78,6 +78,17 @@ class FieldDescriptorLite {
     MAX_LABEL = 3,  // Constant useful for defining lookup tables
                     // indexed by Label.
   };
+
+  // Identifies the storage type of a C++ string field.  This corresponds to
+  // pb.CppFeatures.StringType, but is compatible with ctype prior to Edition
+  // 2024.  0 is reserved for errors.
+#ifndef SWIG
+  enum class CppStringType {
+    kView = 1,
+    kCord = 2,
+    kString = 3,
+  };
+#endif
 };
 
 }  // namespace internal

src/google/protobuf/descriptor_unittest.cc

@@ -7594,6 +7594,9 @@ TEST_F(FeaturesTest, Proto2Features) {
   EXPECT_TRUE(message->FindFieldByName("req")->is_required());
   EXPECT_TRUE(file->enum_type(0)->is_closed());
 
+  EXPECT_EQ(message->FindFieldByName("str")->cpp_string_type(),
+            FieldDescriptor::CppStringType::kString);
+
   // Check round-trip consistency.
   FileDescriptorProto proto;
   file->CopyTo(&proto);
@@ -9710,6 +9713,73 @@ TEST_F(FeaturesTest, EnumFeatureHelpers) {
   EXPECT_FALSE(HasPreservingUnknownEnumSemantics(field_legacy_closed));
 }
 
+TEST_F(FeaturesTest, FieldCppStringType) {
+  BuildDescriptorMessagesInTestPool();
+  const std::string file_contents = absl::Substitute(
+      R"pb(
+        name: "foo.proto"
+        syntax: "editions"
+        edition: EDITION_2024
+        message_type {
+          name: "Foo"
+          field {
+            name: "view"
+            number: 1
+            label: LABEL_OPTIONAL
+            type: TYPE_STRING
+          }
+          field {
+            name: "str"
+            number: 2
+            label: LABEL_OPTIONAL
+            type: TYPE_STRING
+            options {
+              features {
+                [pb.cpp] { string_type: STRING }
+              }
+            }
+          }
+          field {
+            name: "cord"
+            number: 3
+            label: LABEL_OPTIONAL
+            type: TYPE_STRING
+            options {
+              features {
+                [pb.cpp] { string_type: CORD }
+              }
+            }
+          }
+          field {
+            name: "cord_bytes"
+            number: 4
+            label: LABEL_OPTIONAL
+            type: TYPE_BYTES
+            options {
+              features {
+                [pb.cpp] { string_type: CORD }
+              }
+            }
+          } $0
+        }
+      )pb",
+      ""
+  );
+  const FileDescriptor* file = BuildFile(file_contents);
+  const Descriptor* message = file->message_type(0);
+  const FieldDescriptor* view = message->field(0);
+  const FieldDescriptor* str = message->field(1);
+  const FieldDescriptor* cord = message->field(2);
+  const FieldDescriptor* cord_bytes = message->field(3);
+
+  EXPECT_EQ(view->cpp_string_type(), FieldDescriptor::CppStringType::kView);
+  EXPECT_EQ(str->cpp_string_type(), FieldDescriptor::CppStringType::kString);
+  EXPECT_EQ(cord_bytes->cpp_string_type(),
+            FieldDescriptor::CppStringType::kCord);
+  EXPECT_EQ(cord->cpp_string_type(), FieldDescriptor::CppStringType::kString);
+
+}
+
 TEST_F(FeaturesTest, MergeFeatureValidationFailed) {
   BuildDescriptorMessagesInTestPool();
   BuildFileInTestPool(pb::TestFeatures::descriptor()->file());