Initial part of adding nullability annotations to C++ generated code

Add protobuf local macro similar to the very recently added absl version abseil/abseil-cpp@cd9dd42 The macro form allows Wnullability-completeness to check (vs absl's template form of annotation)

Introduce a flag+option that is default off. We will flip it on when ready and flip it on for one example file to see how it works as we add annotations.

The annotations will also be off in open source until we are ready.

PiperOrigin-RevId: 708322571

src/google/protobuf/compiler/cpp/generator.cc

@@ -51,6 +51,7 @@ std::string NumberedCcFileName(absl::string_view basename, int number) {
 absl::flat_hash_map<absl::string_view, std::string> CommonVars(
     const Options& options) {
   bool is_oss = options.opensource_runtime;
+  bool annotate_nullability = options.annotate_nullability && !is_oss;
   return {
       {"proto_ns", std::string(ProtobufNamespace(options))},
       {"pb", absl::StrCat("::", ProtobufNamespace(options))},
@@ -67,6 +68,9 @@ absl::flat_hash_map<absl::string_view, std::string> CommonVars(
       {"hrule_thick", kThickSeparator},
       {"hrule_thin", kThinSeparator},
 
+      {"nullable", annotate_nullability ? "PROTOBUF_NULLABLE" : ""},
+      {"nonnull", annotate_nullability ? "PROTOBUF_NONNULL" : ""},
+
       // Warning: there is some clever naming/splitting here to avoid extract
       // script rewrites.  The names of these variables must not be things that
       // the extract script will rewrite.  That's why we use "CHK" (for example)
@@ -186,6 +190,16 @@ bool CppGenerator::Generate(const FileDescriptor* file,
       file_options.force_eagerly_verified_lazy = true;
     } else if (key == "experimental_strip_nonfunctional_codegen") {
       file_options.strip_nonfunctional_codegen = true;
+    } else if (key == "annotate_nullability") {
+      int enable_annotate_nullability = 0;
+      if (absl::SimpleAtoi(value, &enable_annotate_nullability) &&
+          (enable_annotate_nullability == 0 ||
+           enable_annotate_nullability == 1)) {
+        file_options.annotate_nullability = enable_annotate_nullability != 0;
+      } else {
+        *error = absl::StrCat("Invalid value for ", key, ": ", value);
+        return false;
+      }
     } else {
       *error = absl::StrCat("Unknown generator option: ", key);
       return false;

src/google/protobuf/compiler/cpp/options.h

@@ -72,6 +72,9 @@ struct Options {
   bool force_inline_string = false;
 #endif  // !PROTOBUF_STABLE_EXPERIMENTS
   bool strip_nonfunctional_codegen = false;
+  // TODO: clean this up once the change is complete
+  // and rolled out.
+  bool annotate_nullability = false;
 };
 
 }  // namespace cpp

src/google/protobuf/port_def.inc

@@ -902,6 +902,18 @@ namespace internal {
 #define PROTOBUF_DEPRECATE_AND_INLINE() [[deprecated]]
 #endif
 
+#ifdef PROTOBUF_NONNULL
+#error PROTOBUF_NONNULL was previously defined
+#endif
+#if defined(__clang__) && !defined(__OBJC__) && \
+    ABSL_HAVE_FEATURE(nullability_on_classes)
+#define PROTOBUF_NONNULL _Nonnull
+#define PROTOBUF_NULLABLE _Nullable
+#else
+#define PROTOBUF_NONNULL
+#define PROTOBUF_NULLABLE
+#endif
+
 namespace google {
 namespace protobuf {
 namespace internal {

src/google/protobuf/port_undef.inc

@@ -63,6 +63,8 @@
 #undef PROTOBUF_TC_PARAM_DECL
 #undef PROTOBUF_DEBUG
 #undef PROTOBUF_NO_THREADLOCAL
+#undef PROTOBUF_NONNULL
+#undef PROTOBUF_NULLABLE
 
 #ifdef PROTOBUF_FUTURE_BREAKING_CHANGES
 #undef PROTOBUF_FUTURE_BREAKING_CHANGES