syntax/parse: fix for deterministic compilation

Lift compile-time gensyms to vars bound to run-time gensyms.

racket/collects/syntax/parse/private/parse.rkt

@@ -283,6 +283,20 @@ Some optimizations:
 
 ;; ----
 
+(begin-for-syntax
+  ;; gensym=>var : Hash[(cons Symbol LiftTarget) => Syntax]
+  (define gensym=>var (make-hash))
+
+  ;; lift-gensym : Symbol -> Syntax
+  ;; For deterministic compilation. Maps compile-time gensyms used by
+  ;; action:post to expressions whose values preserve distinctness.
+  (define (lift-gensym g)
+    (define key (cons g (syntax-local-lift-context)))
+    (hash-ref! gensym=>var key
+               (lambda () (syntax-local-lift-expression #'(gensym))))))
+
+;; ----
+
 #|
 Conventions:
   - rhs : RHS
@@ -733,8 +747,10 @@ Conventions:
        [#s(action:do _ (stmt ...))
         #'(let () (no-shadow stmt) ... (#%expression k))]
        [#s(action:post _ pattern group index)
-        #'(let ([pr* (ps-add-post pr 'group 'index)])
+        (with-syntax ([group* (let ([group (syntax->datum #'group)])
-            (parse:A x cx pattern pr* es k))])]))
+                                (if group (lift-gensym group) #'#f))])
+          #'(let ([pr* (ps-add-post pr group* 'index)])
+              (parse:A x cx pattern pr* es k)))])]))
 
 ;; (bind/sides clauses k) : expr[Ans]
 ;; In k: attrs(clauses) are bound.

racket/collects/syntax/parse/private/rep-patterns.rkt

@@ -79,7 +79,7 @@ A ActionPattern is one of
 * (action:and Base (listof ActionPattern))
   (action:parse Base SinglePattern stx)
   (action:do Base (listof stx))
-  (action:post Base ActionPattern Quotable Nat)
+  (action:post Base ActionPattern (U UninternedSymbol #f) Nat)
 
 action:and is desugared below in create-* procedures
 |#