Hold the saved values weakly.

collects/tests/xrepl/xrepl.rkt

@@ -87,6 +87,9 @@
   4
   -> «(list ^ ^^ ^^^ ^^^^)»
   '(4 3 2 1)
+  -> «(collect-garbage)»
+  -> «^»
+  ; ^: saved value #1 was garbage-collected [,bt for context]
   -> «(module foo racket (define x 123))»
   -> «,en foo»
   'foo> «x»

collects/xrepl/saved-values.rkt

@@ -49,7 +49,12 @@
             [else (err "unknown name pattern for a saved-value reference")]))
     (unless (pair? saved)          (err "no saved values, yet"))
     (when (n . > . (length saved)) (err "no ~a saved values, yet" n))
-    #`'#,(list-ref saved (sub1 n)))
+    ;; the values are either `#f', or a weak box holding the value
+    (define r
+      (let ([b (list-ref saved (sub1 n))])
+        (and b (or (weak-box-value b)
+                   (err "saved value #~a was garbage-collected" n)))))
+    #`'#,r)
   (syntax-case stx (set!)
     [(set! id . xs) (raise-syntax-error 'set! "cannot set history reference")]
     [(id . xs) (datum->syntax stx (cons (ref #'id) #'xs) stx)]

collects/xrepl/xrepl.rkt

@@ -1198,7 +1198,8 @@
 ;; saved interaction values (can be #f to disable saving)
 (define saved-values (make-parameter '()))
 (define (save-values! xs)
-  (let ([xs (filter (λ (x) (not (void? x))) xs)]) ; don't save void values
+  (let* ([xs (filter (λ (x) (not (void? x))) xs)] ; don't save void values
+         [xs (map (λ (x) (and x (make-weak-box x))) xs)]) ; save weakly
     (unless (null? xs)
       ;; the order is last, 2nd-to-last, ..., same from prev interactions
       ;; the idea is that `^', `^^', etc refer to the values as displayed

collects/xrepl/xrepl.scrbl

@@ -451,9 +451,19 @@ The rationale for this is that @racketidfont{^} always refers to the
 last @emph{printed} result, @racketidfont{^^} to the one before that,
 etc.
 
-These bindings are made available only if they are not already defined,
+The bindings are made available only if they are not already defined.
-and if they are not modified.  This means that if you have code that
+This means that if you have code that uses these names, it will continue
-uses these names, it will continue to work as usual.
+to work as usual.
+
+The bindings are identifier macros that expand to the literal saved
+values; so referring to a saved value that is missing (because not
+enough values were shown) raises a syntax error.  In addition, the
+values are held in a @tech{weak reference}, so they can disappear after
+a garbage-collection.
+
+Note that this facility can be used to ``transfer'' values from one
+namespace to another---but beware of struct values that might come from
+a different instantiation of a module.
 
 @; ---------------------------------------------------------------------
 @section{Hacking XREPL}