Function absl_nullable arena

common/memory.h:199–203

  explicit operator bool() const noexcept { return !IsNone(ptr_); }

  google::protobuf::Arena* absl_nullable arena() const noexcept {
    return (ptr_ & Owner::kBits) == Owner::kArenaBit
               ? reinterpret_cast<google::protobuf::Arena*>(ptr_ & Owner::kPointerMask)
               : nullptr;
  }

  void reset() noexcept {
    Destroy(ptr_);