allow `begin' wrapper on a submodule to re-expand

Closes PR 12835

collects/scribblings/reference/syntax.scrbl

@@ -269,8 +269,9 @@ form. See also @racket[module-compiled-language-info],
 If a @racket[module] form has a single body @racket[form] and if the
 form is a @racket[#%plain-module-begin] form, then the body
 @racket[form] is traversed to find @racket[module] and
-@racket[module*] forms that are either immediate or under
+@racket[module*] forms that are either immediate, under
-@racket[begin-for-syntax]. (That is, the body is search before adding
+@racket[begin], or under @racket[begin-for-syntax]. (That is, the 
+body is searched before adding
 any lexical context due to the module's initial @racket[module-path]
 import.) Each such module form is given a @indexed-racket['submodule]
 @tech{syntax property} that whose value is the initial module form.

collects/tests/racket/submodule.rktl

@@ -356,6 +356,19 @@
          (require (submod ".." X))
          (define y (add1 x)))))))
 
+;; Check that we can wrap a `begin' around a submodule:
+(parameterize ([current-namespace (make-base-namespace)])
+  (define m (expand
+             '(module m racket/base
+                (module sub racket/base
+                  (require (for-syntax racket/base))
+                  (define-syntax (sym-app stx)
+                    (syntax-case stx ()
+                      [() 10]))))))
+  (eval (syntax-case m ()
+          [(md m r/b (m-b mod))
+           #`(md m r/b (m-b (begin 10 mod)))])))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; `begin-for-syntax' doesn't affect `module' with non-#f language:
 

src/racket/src/module.c

@@ -6402,6 +6402,15 @@ Scheme_Object *do_annotate_submodules(Scheme_Object *fm, int phase)
           changed = 1;
           a = v;
         }
+      } else if (scheme_stx_module_eq3(scheme_begin_stx, v, 
+                                       scheme_make_integer(0), scheme_make_integer(phase), 
+                                       NULL)) {
+        /* found `begin' */
+        v = do_annotate_submodules(a, phase);
+        if (!SAME_OBJ(v, a)) {
+          changed = 1;
+          a = v;
+        }
       }
     }
   }