Comment by nananana9

  template <typename T>
  struct Slice {
      T* data = nullptr;
      size_t size = nullptr;

      T& operator[](size_t index) {
        if (index >= size) crash_the_program();
        return data[index];
      }
  };

If you're considering this extension, just use C++ and 5 lines of standard, portable, no-weird-annotations code instead.