Fix #428

Add binding check for generate verb in loops. The check is present
in other loop verbs.

src/boot/boot.janet

@@ -471,10 +471,12 @@
       :in (each-template binding object [rest])
       :iterate (iterate-template binding object rest)
       :generate (with-syms [f s]
+                   (def ds (if (idempotent? binding) binding (gensym)))
                   ~(let [,f ,object]
                      (while true
-                       (def ,binding (,resume ,f))
+                       (def ,ds (,resume ,f))
                        (if (= :dead (,fiber/status ,f)) (break))
+                       ,;(if (= ds binding) [] [~(def ,binding ,ds)])
                        ,rest)))
       (error (string "unexpected loop verb " verb)))))
 

test/suite8.janet

@@ -319,4 +319,10 @@ neldb\0\0\0\xD8\x05printG\x01\0\xDE\xDE\xDE'\x03\0marshal_tes/\x02
 (check-jdn "a string")
 (check-jdn @"a buffer")
 
+# Issue 428
+(var result nil)
+(defn f [] (yield {:a :ok}))
+(assert-no-error "issue 428 1" (loop [{:a x} :generate (fiber/new f)] (set result x)))
+(assert (= result :ok) "issue 428 2")
+
 (end-suite)