There's compiler-level traits like `Iterator` and `Future` which enforce references. If wanting to do intrusive pointers into them, one risks creating overlapping references: https://github.com/tokio-rs/tokio/issues/3399
References to UnsafeCell<> should still be safe, because the only thing you can do with an &UnsafeCell<T> is extract a possibly-mutating raw pointer to T. (UnsafeCell<> is also special in that, like Cell<>, you can mutate through it without incurring UB.)
