improve the error message in ->*

pkgs/racket-test/tests/racket/contract/arrow-star.rkt

@@ -748,4 +748,9 @@
                 y)
               'pos
               'neg))
+
+  (contract-syntax-error-test
+   '->*-too-much-stuff
+   #'(->* () #:rest (listof procedure?) () any)
+   #rx"expected the #:rest keyword to be followed only by the range")
   )

racket/collects/racket/contract/private/arrow-val-first.rkt

@@ -775,6 +775,13 @@
                     (values #'() leftover)])]
                 [(rst leftover)
                  (syntax-case leftover ()
+                   [(#:rest rest-expr one-thing another-thing)
+                    (and (not (keyword? #'one-thing))
+                         (not (keyword? #'another-thing)))
+                    (raise-syntax-error
+                     #f
+                     "expected the #:rest keyword to be followed only by the range (possibly with pre- and post-conditions)"
+                     stx #'another-thing)]
                    [(#:rest rest-expr . leftover)
                     (values #'rest-expr #'leftover)]
                    [_ (values #f leftover)])]