irement is also trivially satisfied if all accesses (atomic or not) are
    done from the same thread.
* If this atomic type is *not* lock-free:
  * Any accesses to the value behind `ptr` must have a happens-before relationship
    with accesses via the returned value (or vice-versa).
  * Any concurrent accesses to the value behind `ptr` for the duration of lifetime `'a` must
    be compatible with operations performed by this atomic type.
* This method must not be used to create overlapping or mixed-size atomic
  accesses, as these are not supported by the memory model.

[valid]: core::ptr#safety