fix validation of known structure mutators

Specifically, fix the case where the structure type for the mutator
includes "auto" fields (with no corresponding constructor argument).

pkgs/racket-test-core/tests/racket/optimize.rktl

@@ -6210,6 +6210,26 @@
       ((proc)))
     (void proc proc)))
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Make sure validation doesn't fail for importing a setter of a
+;; structure type that has auto fields:
+
+(module provides-a-mutator-for-a-struct-with-an-auto-field racket/base
+  (provide foo set-foo-y!)
+  (struct foo (x [y #:mutable] [z #:auto])))
+
+(let ([e `(module uses-mutator-with-an-auto-field racket/base
+           (require 'provides-a-mutator-for-a-struct-with-an-auto-field)
+           (provide f)
+           (define (f x)
+             (and x
+                  (set-foo-y! x 1))))]
+      [o (open-output-bytes)])
+  (write (compile e) o)
+  (parameterize ([read-accept-compiled #t])
+    (eval (read (open-input-bytes (get-output-bytes o)))))
+  ((dynamic-require ''uses-mutator-with-an-auto-field 'f) #f)) 
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/racket/src/struct.c

@@ -3608,7 +3608,7 @@ int scheme_check_structure_shape(Scheme_Object *e, Scheme_Object *expected)
     return (v == STRUCT_PROC_SHAPE_PRED);
   } else if (i == SCHEME_PRIM_STRUCT_TYPE_INDEXED_SETTER) {
     st = (Scheme_Struct_Type *)SCHEME_PRIM_CLOSURE_ELS(e)[0];
-    return (v == ((st->num_islots << STRUCT_PROC_SHAPE_SHIFT)
+    return (v == ((st->num_slots << STRUCT_PROC_SHAPE_SHIFT)
                   | STRUCT_PROC_SHAPE_SETTER));
   } else if (i == SCHEME_PRIM_STRUCT_TYPE_INDEXED_GETTER) {
     int pos = SCHEME_INT_VAL(SCHEME_PRIM_CLOSURE_ELS(e)[1]);