Required trait for constants used in pattern matches.
Any type that derives
PartialEq automatically implements this trait,
regardless of whether its type-parameters implement
const item contains some type that does not implement this trait,
then that type either (1.) does not implement
PartialEq (which means the
constant will not provide that comparison method, which code generation
assumes is available), or (2.) it implements its own version of
PartialEq (which we assume does not conform to a structural-equality
In either of the two scenarios above, we reject usage of such a constant in a pattern match.