***Note from Migration:***

Original Title was: Enable Rule-Configuration for the "IN" operator on AttributeValidator to be set from an attribute

I often need to test if an attribute's value is compliant with a comma-separate list of allowable values (enumerated types) held against another attribute. I know I can test for this in a TESTER but it would be nice to have it all in one place, and with a TESTER I still need to build the validation error message.

A workaround is to use Regex operator which does support attribute substitution.