fix `variable-reference->module-source' for submodules

Unlike the `variable-reference->resolved-module-path', the module
source one doesn't include a submodule path.

collects/scribblings/reference/namespaces.scrbl

@@ -414,6 +414,8 @@ result is @racket[#f].}
 If @racket[varref] refers to a @tech{module-level variable}, the
 result is a path or symbol naming the module's source (which is
 typically, but not always, the same as in the resolved module path).
+If the module is a submodule, the result corresponds to the enclosing
+top-level module's source.
 
 If @racket[varref] refers to a @tech{top-level variable}, then the
 result is @racket[#f].}

collects/tests/racket/submodule.rktl

@@ -496,6 +496,22 @@
 
 (test 3 dynamic-require '(submod 'module+-example-2 a b) 'x)
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Check module-source for submodule:
+
+(let ()
+  (define (go set-name get-name)
+    (parameterize ([current-namespace (make-base-namespace)])
+      (parameterize ([current-module-declare-source set-name])
+        (eval '(module m racket/base
+                 (module+ sub
+                   (provide v)
+                   (define v (variable-reference->module-source
+                              (#%variable-reference))))))
+        (test get-name dynamic-require '(submod 'm sub) 'v))))
+  (go #f 'm)
+  (go 'other 'other))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Check that various shaodwings are allowed:
 

src/racket/src/module.c

@@ -6064,8 +6064,16 @@ static Scheme_Object *do_module_execute(Scheme_Object *data, Scheme_Env *genv,
   if (!SCHEME_FALSEP(src)) {
     src = scheme_intern_resolved_module_path(src);
     m->modsrc = src;
-  } else
-    m->modsrc = m->modname;
+  } else {
+    src = m->modname;
+    if (m->submodule_path && !SCHEME_NULLP(m->submodule_path)) {
+      src = scheme_resolved_module_path_value(src);
+      if (SCHEME_PAIRP(src))
+        src = SCHEME_CAR(src);
+      src = scheme_intern_resolved_module_path(src);
+    }
+    m->modsrc = src;
+  }
 
   if (genv)
     env = genv;