Required trait for constants used in pattern matches.

Constants are only allowed as patterns if (a) their type implementsPartialEq, and (b) interpreting the value of the constant as a patternis equivalent to callingPartialEq. This ensures that constants used aspatterns cannot expose implementation details in an unexpected way orcause semver hazards.

This trait ensures point (b).Any type that derivesPartialEq automatically implements this trait.

Implementing this trait (which is unstable) is a way for type authors to explicitly allowcomparing const values of this type; that operation will recursively compare all fields(including private fields), even if that behavior differs fromPartialEq. This can make itsemver-breaking to add further private fields to a type.