cs: fix chaperone-struct on accessor/mutator taking a position

Closes #2570

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

@@ -3420,6 +3420,39 @@
   (test #t has-impersonator-prop:contracted? group-rows*)
   (test 1 'apply (group-rows* #:group 10)))
 
+;; ----------------------------------------
+;; Check that position-consuming accessor and mutators work with
+;; `impersonate-struct`.
+
+(let ()
+  (define-values (struct:s make-s s? s-ref s-set!)
+    (make-struct-type 's #f 1 0 #f))
+
+  (define a-s (make-s 0))
+  
+  (test '(0)
+        s-ref
+        (impersonate-struct
+         a-s
+         s-ref
+         (lambda (k v) (list v))
+         s-set!
+         (lambda (k v) (list v)))
+        0)
+  
+  (test (void)
+        s-set!
+        (impersonate-struct
+         a-s
+         s-ref
+         (lambda (k v) (list v))
+         s-set!
+         (lambda (k v) (list v)))
+        0
+        7)
+
+  (test '(7) s-ref a-s 0))
+  
 ;; ----------------------------------------
 
 (report-errs)

racket/src/cs/rumble/struct.ss

@@ -398,10 +398,16 @@
        #t))
 
 (define (struct-accessor-procedure-rtd+pos v)
-  (with-global-lock* (hashtable-ref struct-field-accessors v #f)))
+  (if (position-based-accessor? v)
+      (cons (position-based-accessor-rtd v)
+            (position-based-accessor-offset v))
+      (with-global-lock* (hashtable-ref struct-field-accessors v #f))))
 
 (define (struct-mutator-procedure-rtd+pos v)
-  (with-global-lock* (hashtable-ref struct-field-mutators v #f)))
+  (if (position-based-mutator? v)
+      (cons (position-based-mutator-rtd v)
+            (position-based-mutator-offset v))
+      (with-global-lock* (hashtable-ref struct-field-mutators v #f))))
 
 ;; This indirection prevents the whole-program optimizer from inlining
 ;; the `with-glocal-lock*` expansion --- which, at the time of