expander: fix problem related to (local-expand 'module-begin ...)

When `local-expand` is used for a 'module-begin context, use a fresh
binding -> definition-unreadable-symbol table for the nested
expansion. That way, the table used for the main expansion is
unchanged, and re-expanding or evaluating the expanded module will
arrive at the same unreadable symbols as the initial expansion.

The report and example are from Alexis.

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

@@ -2535,6 +2535,53 @@ case of module-leve bindings; it doesn't cover local bindings.
       (compile/eval u-code)
       (test 'ns-val dynamic-require ''u 'v))))
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Another example to check that re-expansion generates definition
+;; names consistent with the previoud expansion.
+
+(parameterize ([current-namespace (make-base-namespace)])
+  (define modbeg-trampoline
+    '(module modbeg-trampoline racket/base
+       (require (for-syntax racket/base
+                            syntax/strip-context))
+
+       (provide (rename-out [module-begin #%module-begin]))
+
+       (define-syntax (module-begin stx)
+         (syntax-case stx ()
+           [(_ lang . body)
+            #'(#%plain-module-begin (module-begin-trampoline lang . body))]))
+
+       (define-syntax (module-begin-trampoline stx)
+         (syntax-case stx ()
+           [(_ lang . body)
+            (with-syntax ([[modbeg . [body* ...]] (syntax-local-introduce
+                                                   (syntax-local-lift-require
+                                                    (strip-context #'lang)
+                                                    (datum->syntax #f (cons '#%module-begin
+                                                                            (strip-context #'body)))))])
+              (with-syntax ([body** (if (= (length (syntax->list #'(body* ...))) 1)
+                                        (error "oops")
+                                        #'(modbeg . [body* ...]))])
+                (with-syntax ([(modbeg* form ...) (local-expand #'body** 'module-begin #f)])
+                  (with-syntax ([body*** (local-expand #'(modbeg* form ...) 'module-begin (list #'module*))])
+                    (with-syntax ([(modbeg**:#%plain-module-begin form* ...) #'body***])
+                      (syntax-track-origin #`(begin form* ...) #'body*** #'modbeg**))))))]))))
+
+  (define m-use
+    '(module m-use 'modbeg-trampoline racket/base
+       (require racket/contract/base)
+       (provide (contract-out [f (-> number? number?)]))
+       (define (f x) (+ x 42))
+       (module* main racket
+         (require (submod ".."))
+         (f 10))))
+
+  (eval modbeg-trampoline)
+  (eval (expand m-use))
+
+  (dynamic-require '(submod 'm-use main) #f))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/expander/expand/module.rkt

@@ -3,6 +3,7 @@
          "../common/struct-star.rkt"
          "../common/performance.rkt"
          "../syntax/syntax.rkt"
+         "../syntax/debug.rkt"
          "../syntax/property.rkt"
          "../syntax/scope.rkt"
          "../syntax/taint.rkt"
@@ -233,7 +234,7 @@
        (hash-clear! compiled-submodules)
        (set-box! compiled-module-box #f))
      (set! again? #t)
-     
+
      ;; In case a nested `#%module-begin` expansion is forced, save
      ;; and restore the module-expansion state:
      (define ctx (struct*-copy expand-context mb-init-ctx
@@ -249,7 +250,8 @@
                                                             #:copy-requires requires+provides))
                                   (with-save-and-restore ([requires+provides new-requires+provides]
                                                           [compiled-submodules (make-hasheq)]
-                                                          [compiled-module-box (box #f)])
+                                                          [compiled-module-box (box #f)]
+                                                          [defined-syms (make-hasheq)])
                                     (module-begin-k s ctx)))]))
 
      ;; In case `#%module-begin` expansion is forced on syntax that