>People should stop freaking out about this. The "more inefficient code" you might get by adding a few .clone() or Rc<RefCell<…>> when the borrow checker complains about something is still way more efficient than anything written in alternative "safe" languages.
But you don't have to use a "safe language". C++ can avoid a lot of copies and be just as safe from an absolute point of view. The borrow checker isn't god and Rust isn't perfect. In an absolute sense the C++ code you can write could be more correct and efficient(it just doesn't tend to be).
You can always just go down the UnsafeCell/pointers way and will still be safer than C++ as the borrow checker still works for inside unsafe, and that the few places where a memory error can lurk will be highlighted by unsafe blocks.
But you don't have to use a "safe language". C++ can avoid a lot of copies and be just as safe from an absolute point of view. The borrow checker isn't god and Rust isn't perfect. In an absolute sense the C++ code you can write could be more correct and efficient(it just doesn't tend to be).