expander: fix lift context for multiple #%module-begins

Also, restore the old syntax-error message for an `if`
without an "else" clause.

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

@@ -2285,6 +2285,39 @@ case of module-leve bindings; it doesn't cover local bindings.
 
 (test 5 dynamic-require ''likely-inlines-across-two-submodules 'result)
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Check that the lift context is different for multiple
+;; `#%module-begin` expansions. That's important, for example, to make
+;; sure that a lift in the first pass record by the contract system
+;; isn't assumed to be from a lexically earlier expression within a
+;; second pass.
+
+(module module-begin-and-unique-context-check racket/base
+  (require (for-syntax racket/base))
+
+  (provide (except-out (all-from-out racket/base)
+                       #%module-begin)
+           (rename-out [module-begin #%module-begin])
+           check-unique-context)
+
+  (define-for-syntax prev-key #f)
+
+  (define-syntax (module-begin stx)
+    (syntax-case stx ()
+      [(_ form ...)
+       (with-syntax ([(pmb new-form ...) (local-expand #'(#%plain-module-begin form ...) 'module-begin null)])
+         #'(#%plain-module-begin new-form ... (check-unique-context)))]))
+
+  (define-syntax (check-unique-context stx)
+    (define key (syntax-local-lift-context))
+    (when (eq? key prev-key)
+      (raise-syntax-error #f "context didn't change"))
+    (set! prev-key key)
+    #'(void)))
+
+(module use-module-begin-and-unique-context-check 'module-begin-and-unique-context-check
+  (#%expression (check-unique-context)))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/expander/expand/expr.rkt

@@ -448,6 +448,8 @@
  (lambda (s ctx)
    (log-expand ctx 'prim-if)
    (define disarmed-s (syntax-disarm s))
+   (define-match bad-m disarmed-s #:try '(_ _ _))
+   (when (bad-m) (raise-syntax-error #f "missing an \"else\" expression" s))
    (define-match m disarmed-s '(if tst thn els))
    (define expr-ctx (as-expression-context ctx))
    (define tail-ctx (as-tail-context expr-ctx #:wrt ctx))

racket/src/expander/expand/module.rkt

@@ -20,6 +20,7 @@
          "require+provide.rkt"
          "../common/module-path.rkt"
          "lift-context.rkt"
+         "lift-key.rkt"
          "../namespace/core.rkt"
          "context.rkt"
          "use-site.rkt"
@@ -309,6 +310,7 @@
                                                 [def-ctx-scopes def-ctx-scopes]
                                                 [need-eventually-defined need-eventually-defined] ; used only at phase 1 and up
                                                 [declared-submodule-names declared-submodule-names]
+                                                [lift-key #:parent root-expand-context (generate-lift-key)]
                                                 [lifts (make-lift-context
                                                         (make-wrap-as-definition self frame-id
                                                                                  inside-scope all-scopes-s
@@ -714,6 +716,7 @@
       (log-expand partial-body-ctx 'next)
       (define exp-body (performance-region
                         ['expand 'form-in-module/1]
+                        ;; --- expand to core form ---
                         (expand (car bodys) partial-body-ctx)))
       (define disarmed-exp-body (syntax-disarm exp-body))
       (define lifted-defns (get-and-clear-lifts! (expand-context-lifts partial-body-ctx)))