One can use property testing to automatically generate unit test inputs. The property becomes "does this do something new that I wanted to test for but wasn't?

One can use property testing to automatically generate unit test inputs. The property becomes "does this do something new that I wanted to test for but wasn't?" (or, rather, the negation; inputs "pass" and can be ignored if they do nothing new.) This could be code coverage, or it could be killing code mutants.