Assertf with no arguments does not make sense.

src/boot/boot.janet

@@ -154,11 +154,6 @@
        ,v
        (,error ,(if err err (string/format "assert failure in %j" x))))))
 
-(defmacro assertf
-  "Convenience macro that combines `assert` and `string/format`."
-  [x & args]
-  ~(as-macro ,assert ,x (,string/format ,;args)))
-
 (defmacro defdyn
   ``Define an alias for a keyword that is used as a dynamic binding. The
   alias is a normal, lexically scoped binding that can be used instead of
@@ -209,6 +204,16 @@
   [fmt & args]
   (error (string/format fmt ;args)))
 
+(defmacro assertf
+  "Convenience macro that combines `assert` and `string/format`."
+  [x fmt & args]
+  (def v (gensym))
+  ~(do
+     (def ,v ,x)
+     (if ,v
+       ,v
+       (,errorf ,fmt ,;args))))
+
 (defmacro default
   ``Define a default value for an optional argument.
   Expands to `(def sym (if (= nil sym) val sym))`.``

test/suite-boot.janet

@@ -987,11 +987,10 @@
 (setdyn *debug* nil)
 
 # issue #1516
-(assert (assertf true) "assertf 1 argument")
+(assert-error "assertf 1 argument" (macex '(assertf true)))
 (assert (assertf true "fun message") "assertf 2 arguments")
 (assert (assertf true "%s message" "mystery") "assertf 3 arguments")
 (assert (assertf (not nil) "%s message" "ordinary") "assertf not nil")
-(assert-error "assertf error 1" (assertf false))
 (assert-error "assertf error 2" (assertf false "fun message"))
 (assert-error "assertf error 3" (assertf false "%s message" "mystery"))
 (assert-error "assertf error 4" (assertf nil "%s %s" "alice" "bob"))