lisp/help.el (describe-mode): Link to the mode's definition.

Fixes: debbugs:8185

lisp/ChangeLog

@@ -1,3 +1,7 @@
+2011-03-13  Juanma Barranquero  <lekktu@gmail.com>
+
+	* help.el (describe-mode): Link to the mode's definition (bug#8185).
+
 2011-03-12  Stefan Monnier  <monnier@iro.umontreal.ca>
 
 	* ebuff-menu.el (electric-buffer-menu-mode-map): Move initialization

lisp/help.el

@@ -871,7 +871,17 @@ whose documentation describes the minor mode."
             (let ((start (point)))
               (insert (format-mode-line mode nil nil buffer))
               (add-text-properties start (point) '(face bold)))))
-	(princ " mode:\n")
+	(princ " mode")
+	(let* ((mode major-mode)
+	       (file-name (find-lisp-object-file-name mode nil)))
+	  (when file-name
+	    (princ (concat " defined in `" (file-name-nondirectory file-name) "'"))
+	    ;; Make a hyperlink to the library.
+	    (with-current-buffer standard-output
+	      (save-excursion
+		(re-search-backward "`\\([^`']+\\)'" nil t)
+		(help-xref-button 1 'help-function-def mode file-name)))))
+	(princ ":\n")
 	(princ (documentation major-mode)))))
   ;; For the sake of IELM and maybe others
   nil)