cs: avoid abuse of vector-ref for struct-ref

Use `$object-ref`, instead. Otherwise, cptypes can detect the abuse of
`#3%vector-ref` and make unwanted transformations.

racket/src/cs/Makefile

@@ -242,6 +242,7 @@ future2-demo: $(BUILDDIR)rumble.$(CSO)
 
 RUMBLE_SRCS = rumble/define.ss \
               rumble/virtual-register.ss \
+              rumble/layout.ss \
               rumble/check.ss \
 	      rumble/syntax-rule.ss \
 	      rumble/letrec.ss \

racket/src/cs/rumble.sls

@@ -706,6 +706,7 @@
 
   (include "rumble/define.ss")
   (include "rumble/virtual-register.ss")
+  (include "rumble/layout.ss")
   (include "rumble/begin0.ss")
   (include "rumble/letrec.ss")
   (include "rumble/syntax-rule.ss")

racket/src/cs/rumble/foreign.ss

@@ -128,11 +128,6 @@
 
 ;; ----------------------------------------
 
-;; HACK: hardwired numbers that depend on the tagging regime
-;; and other representation details
-(define bytevector-content-offset 9)
-(define vector-content-offset (if (> (fixnum-width) 32) 9 5))
-
 (define (object->addr v) ; call with GC disabled
   (#%$object-address v 0))
 

racket/src/cs/rumble/layout.ss

@@ -0,0 +1,5 @@
+;; HACK: hardwired numbers that depend on the tagging regime
+;; and other representation details
+(define bytevector-content-offset 9)
+(define vector-content-offset (if (> (fixnum-width) 32) 9 5))
+(define record-content-offset vector-content-offset)

racket/src/cs/rumble/struct.ss

@@ -974,9 +974,9 @@
           (putprop (record-type-uid rtd) 'guards new-guards))))))
 
 (define (unsafe-struct*-ref s i)
-  (#3%vector-ref s i))
+  (#%$object-ref 'scheme-object s (fx+ record-content-offset (fx* i (foreign-sizeof 'void*)))))
 (define (unsafe-struct*-set! s i v)
-  (#3%vector-set! s i v))
+  (#%$object-set! 'scheme-object s (fx+ record-content-offset (fx* i (foreign-sizeof 'void*))) v))
 (define (unsafe-struct? v r)
   (#3%record? v r))