I see that the current ExUnit source code has support for rich failure messages on a small whitelist of “recognized” assertion patterns, namely:

• assert match?(left, right), assert left = right
• refute match?(left, right), refute left = right
• assert left === right, refute left !== right,
• assert left !== right, refute left === right,
• assert left == right, refute left != right,
• assert left != right, refute left == right,
• assert left >= right, refute left < right,
• assert left > right, refute left <= right
• assert left <= right, refute left > right
• assert left < right, refute left >= right
• assert left ~= right
• refute left ~= right

Would there be any interest in a pull request adding support for an additional pattern:

• refute left and not right (implication, right must be true whenever left is)

This pattern is coming up repeatedly as the most robust, developer-intent-communicating form for some unittests I’m writing:

  property "delete returns a well-formed multiset whenever input is well-formed" do
    check all ms <- t(term(), strict: false),
          value <- term(),
          count <- one_of([non_negative_integer(), constant(:all)]) do
      result = One9.Ms.delete(ms, value, count)

      refute One9.Ms.well_formed?(ms) and not One9.Ms.well_formed?(result)
    end
  end

  property "put 0 copies doesn't corrupt struct" do
    check all multiset <- t(term()), value <- term() do
      result = One9.Multiset.put(multiset, value, 0)

      assert One9.Ms.well_formed?(result.counts)
      assert One9.Multiset.equals?(result, multiset)
      refute One9.Multiset.member?(result, value) and not One9.Multiset.member?(multiset, value)
    end
  end

And having these tests augmented with a little bit of extra verbiage and awesome debug formatting, like the comparison checks currently have, would be a minor boon.