Create new error code E0740 for visibility restrictions to ancestor module issues

src/librustc_resolve/build_reduced_graph.rs

@@ -256,9 +256,9 @@ impl<'a, 'b> BuildReducedGraphVisitor<'a, 'b> {
                                 if self.r.is_accessible_from(vis, parent_scope.module) {
                                     vis
                                 } else {
-                                    let msg =
-                                        "visibilities can only be restricted to ancestor modules";
-                                    self.r.session.span_err(path.span, msg);
+                                    struct_span_err!(self.r.session, path.span, E0741,
+                                        "visibilities can only be restricted to ancestor modules")
+                                        .emit();
                                     ty::Visibility::Public
                                 }
                             }

src/librustc_resolve/error_codes.rs

@@ -1953,4 +1953,5 @@ struct Foo<X = Box<Self>> {
 //  E0470, removed
     E0577,
     E0578,
+    E0740,
 }