relationship with atomic accesses via
  the returned value (or vice-versa).
  * In other words, time periods where the value is accessed atomically may not
    overlap with periods where the value is accessed non-atomically.
  * This requirement is trivially satisfied if `ptr` is never used non-atomically
    for the duration of lifetime `'a`. Most use cases should be able to follow
    this guideline.
  * This requirement 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