bug#21871: Emacs Lisp Mode (at least): spurious parens in column 0 don't get bold red highlighting.

[Dmitry Gutov <dgutov@yandex.ru>]

On 11/12/2015 08:54 PM, Alan Mackenzie wrote:

> The fix to bug #16247 meant no longer setting syntax-begin-function to a
> non-nil value.  This is the condition which used to cause the appropriate
> font-lock-keywords form to get added to lisp-font-lock-keywords-1/2.  It
> no longer is.

Looking into this, I'm not sure we still want to highlight them. The 
aforementioned bug, now fixed, mirrored the justifications that we give 
in the manual and the comments for the highlighting of parens in the 0th 
column:

"The convention speeds up many Emacs operations, which would otherwise 
have to scan back to the beginning of the buffer to analyze the syntax 
of the code."

and

;; Try to detect when a string or comment contains something that
;; looks like a defun and would thus confuse font-lock.

We don't have to scan back to the beginning of the buffer, we can use 
syntax-ppss (and it's more reliable with bug#16247 fixed).

font-lock doesn't get confused by something looking like a defun inside 
a docstring (try it; I wasn't able to get it highlight something wrong).

M-x beginning-of-defun does get confused, though. If *that* is problem 
what we want to detect, I think the patch should look like this:

diff --git a/lisp/font-lock.el b/lisp/font-lock.el
index 8ee9f69..eed2766 100644
--- a/lisp/font-lock.el
+++ b/lisp/font-lock.el
@@ -1786,13 +1786,10 @@ font-lock-compile-keywords
  	  (cons t (cons keywords
  			(mapcar #'font-lock-compile-keyword keywords))))
      (if (and (not syntactic-keywords)
-	     (let ((beg-function syntax-begin-function))
-	       (or (eq beg-function 'beginning-of-defun)
-                   (if (symbolp beg-function)
-                       (get beg-function 'font-lock-syntax-paren-check))))
-	     (not beginning-of-defun-function))
+             (not beginning-of-defun-function)
+             open-paren-in-column-0-is-defun-start)
  	;; Try to detect when a string or comment contains something that
-	;; looks like a defun and would thus confuse font-lock.
+	;; looks like a defun and would thus confuse beginning-of-defun.
  	(nconc keywords
  	       `((,(if defun-prompt-regexp
  		       (concat "^\\(?:" defun-prompt-regexp "\\)?\\s(")