feature: notation for expected failures

[dustingetz]

(tests
 (p/run (! (p/for [x ~(m/watch input)] x)))
 % := [1]
 (reset! input [1 2])
 % := [1 2] ;; FAIL
 )

Syntax idea: % := [1 2] :! [1], which indicates desired expectation simultaneously with known current expected error.

Perhaps % :! [1 2] ; [1] is sufficient?

[dustingetz]

This works in master, obviously could be a lot tighter

(defmethod t/assert-expr :hyperfiddle.rcf/fails? [msg form]
  (let [[_= & args] form
        form        (cons ::fails? (map impl/original-form args))]
    `(let [lhs#           (identity ~(first args)) ; (identity …) guards against :clojure.spec.alpha/invalid being passed to `let` macroexpansion
           rhs#           (identity ~(second args))
           [result# env#] (u/unifier* lhs# rhs#)]
       (if (u/failed? env#)
         (do (do-report {:type     :hyperfiddle.rcf/pass
                         :message  ~msg,
                         :expected '~form
                         :actual   result#})
             result#)
         (do (do-report {:type     :hyperfiddle.rcf/fail
                         :message  ~msg,
                         :expected '~form
                         :actual   (u/explain env#)})
             lhs#)))))