fix printing of prefabs with print-unreadable is #f

Closes #3827

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

@@ -107,6 +107,9 @@
   (ptest "#<a>" (a 1 2))
   (ptest "(b 1 2)" (b 1 2))
   (ptest "'#s(c 1 2)" (c 1 2))
+  (test "#s(c 1 2)" 'prefab
+        (parameterize ([print-unreadable #f])
+          (format "~s" (c 1 2))))
 
   (let ([s (b 1 2)])
     (ptest "(list (cons (b 1 2) 0) (cons (b 1 2) 0))" (list (cons s 0) (cons s 0))))

racket/src/bc/src/print.c

@@ -2276,7 +2276,7 @@ print(Scheme_Object *obj, int notdisplay, int compact, Scheme_Hash_Table *ht,
         vec = scheme_struct_to_vector(obj, (notdisplay >= 3) ? qq_ellipses : NULL, pp->inspector);
         SCHEME_VEC_ELS(vec)[0] = prefab;
         print_vector(vec, notdisplay, compact, ht, mt, pp, 1);
-      } else if (compact || !pp->print_unreadable) {
+      } else if (compact || (!pp->print_unreadable && !SCHEME_PREFABP(obj))) {
         cannot_print(pp, notdisplay, obj, ht, compact);
       } else if (scheme_is_writable_struct(obj)) {
         if (notdisplay == 3) {

racket/src/cs/schemified/io.scm

@@ -22081,8 +22081,10 @@
                                                (config-get
                                                 config_0
                                                 1/print-unreadable))
-                                            (not
-                                             (|#%app| printable-regexp? v_0))
+                                            (if (not (prefab-struct-key v_0))
+                                              (not
+                                               (|#%app| printable-regexp? v_0))
+                                              #f)
                                             #f)
                                           #f)
                                       (fail-unreadable who_0 v_0)

racket/src/io/print/main.rkt

@@ -299,6 +299,7 @@
         (write-string/max "#<hash>" o max-length)])]
     [(and (eq? mode WRITE-MODE)
           (not (config-get config print-unreadable))
+          (not (prefab-struct-key v))
           ;; Regexps are a special case: custom writers that produce readable input
           (not (printable-regexp? v)))
      (fail-unreadable who v)]