Add Unsafe factories for string values.

BytesValue::WrapUnsafe and StringValue::WrapUnsafe do not attempt to validate ownership, but avoid copying the underlying string data as much as possible.
PiperOrigin-RevId: 828657464

Author: jnthntatum

common/internal/byte_string.cc

@@ -174,6 +174,19 @@ ByteString::ByteString(const ReferenceCount* absl_nonnull refcount,
                         kMetadataOwnerReferenceCountBit);
 }
 
+ByteString::ByteString(ByteString::ExternalStringTag,
+                       absl::string_view string) {
+  if (string.size() <= kSmallByteStringCapacity) {
+    SetSmall(nullptr, string);
+  } else {
+    SetExternalMedium(string);
+  }
+}
+
+ByteString ByteString::FromExternal(absl::string_view string) {
+  return ByteString(ExternalStringTag{}, string);
+}
+
 google::protobuf::Arena* absl_nullable ByteString::GetArena() const {
   switch (GetKind()) {
     case ByteStringKind::kSmall:
@@ -965,6 +978,14 @@ void ByteString::SetMedium(google::protobuf::Arena* absl_nullable arena,
   }
 }
 
+void ByteString::SetExternalMedium(absl::string_view string) {
+  ABSL_DCHECK_GT(string.size(), kSmallByteStringCapacity);
+  rep_.header.kind = ByteStringKind::kMedium;
+  rep_.medium.size = string.size();
+  rep_.medium.data = string.data();
+  rep_.medium.owner = 0;
+}
+
 void ByteString::SetMedium(google::protobuf::Arena* absl_nullable arena,
                            std::string&& string) {
   ABSL_DCHECK_GT(string.size(), kSmallByteStringCapacity);

common/internal/byte_string.h

@@ -232,6 +232,10 @@ ByteString final {
              const absl::Cord& cord ABSL_ATTRIBUTE_LIFETIME_BOUND)
       : ByteString(Borrowed(borrower, cord)) {}
 
+  // Creates a medium byte string that is backed by an external string. Should
+  // only be called from explicit 'Unsafe' factories.
+  static ByteString FromExternal(absl::string_view string);
+
   ~ByteString() { Destroy(); }
 
   ByteString& operator=(const ByteString& other) noexcept {
@@ -358,6 +362,8 @@ ByteString final {
                                             google::protobuf::Arena* absl_nonnull arena);
   friend struct cel::ArenaTraits<ByteString>;
 
+  struct ExternalStringTag {};
+
   static ByteString Borrowed(Borrower borrower,
                              absl::string_view string
                                  ABSL_ATTRIBUTE_LIFETIME_BOUND);
@@ -368,6 +374,8 @@ ByteString final {
   ByteString(const ReferenceCount* absl_nonnull refcount,
              absl::string_view string);
 
+  ByteString(ExternalStringTag, absl::string_view string);
+
   constexpr ByteStringKind GetKind() const { return rep_.header.kind; }
 
   absl::string_view GetSmall() const {
@@ -451,6 +459,10 @@ ByteString final {
 
   void SetMedium(google::protobuf::Arena* absl_nullable arena, absl::string_view string);
 
+  // This is used to create a medium byte string that is backed by an external
+  // string. Should only be called from explicit 'Unsafe' factories.
+  void SetExternalMedium(absl::string_view string);
+
   void SetMedium(google::protobuf::Arena* absl_nullable arena, std::string&& string);
 
   void SetMedium(google::protobuf::Arena* absl_nonnull arena, const absl::Cord& cord);