那是 Emacs 自带的功能，定义indicate-buffer-boundaries变量就可以显示，这样设置就可以显示在左边了：
(defun indicate-buffer-boundaries-left ()
(setq indicate-buffer-boundaries 'left))
(add-hook 'prog-mode-hook 'indicate-buffer-boundaries-left)
(add-hook 'text-mode-hook 'indicate-buffer-boundaries-left)
如果要全局开启，可以这样：
(setq-default indicate-buffer-boundaries 'left)