Fix scan-sexps error for C-defined variables

helpful.el

@@ -1259,7 +1259,10 @@ If the source code cannot be found, return the sexp used."
                   ;; function.
                   (progn
                     (setq pos (line-beginning-position))
-                    (forward-list)
+                    ;; HACK Use the elisp syntax table even though the file is a
+                    ;; C file. This is a temporary workaround for issue #329.
+                    (with-syntax-table emacs-lisp-mode-syntax-table
+                      (forward-list))
                     (forward-char)
                     (narrow-to-region pos (point)))
                 ;; Narrow to the top-level definition.

test/helpful-unit-test.el

@@ -782,6 +782,12 @@ in."
     (should
      (s-starts-with-p "(defun " source))))
 
+(ert-deftest helpful--source-c-fn ()
+  (-let* (((buf pos _opened) (helpful--definition 'mode-line-format nil))
+          (source (helpful--source 'mode-line-format nil buf pos)))
+    (should
+     (s-starts-with-p "DEFVAR" (s-trim-left source)))))
+
 (ert-deftest helpful--source-autoloaded ()
   "We should include the autoload cookie."
   (-let* (((buf pos _opened) (helpful--definition #'helpful-at-point t))