As a special case, an `enum` is eligible for the "nullable pointer optimization" if it
contains exactly two variants, one of which contains no data and the other contains
-a single field of one of the non-nullable types listed above (or a struct containing such a type).
+a field of one of the non-nullable types listed above (or a struct containing such a type).
This means it is represented as a single pointer, and the non-data variant is represented as a
null pointer. This is called an "optimization", but unlike other optimizations it is guaranteed
to apply to eligible types.