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Re: font-locking and open parens in column zero

From: martin rudalics
Subject: Re: font-locking and open parens in column zero
Date: Thu, 14 Sep 2006 10:41:46 +0200
User-agent: Mozilla Thunderbird 1.0 (Windows/20041206)

Richard Stallman schrieb:
>     When `c-beginning-of-syntax' doesn't find a cached position it calls
>     `c-beginning-of-defun-1' which, on Emacs, calls `beginning-of-defun':
> Do you think that is a bug?
> Is it doing the wrong thing here?

No.  But with a paren-in-column-zero within a comment, fontification may
fail just as it fails for Elisp.

>     (1) With emacs -Q open ~/src/syntax.c
>     (2) Execute
>     (defun foo ()
>        (interactive)
>        (re-search-forward "string-to-syntax")
>        (forward-line 6)
>        (recenter 0))
>     This will fontify the entire body of `string_to_syntax' as a C string
>     due to the left paren in column zero of the doc-string.
> That seems like a bug to me, more or less.  But it doesn't seem to
> happen often; I tried a little exploring in a C file and it usually
> seems not to be fooled by open-parens in column 0.
> Can you figure out what it is about this case that causes
> it to fail?

It's the standard paren-in-column-zero problem which is decribed in the
manual and not considered a bug in Elisp fontification either.  I think,
it should happen as frequently for C as it happens for Elisp.  Hence a
warning face for such parens could be useful just as it is for Elisp.
Since I have stealth fontification turned off, this problem might bite
me more frequently.

Note also the strange dichotomy between this problem amd the
`open-paren-in-column-0-is-defun-start' option.  C mode sets this to nil
with the explanation

  ;; In Emacs 21 and later it's possible to turn off the ad-hoc
  ;; heuristic that open parens in column 0 are defun starters.  Since
  ;; we have c-state-cache, that heuristic isn't useful and only causes
  ;; trouble, so turn it off.

That motivation is strange since, if the c-state-cache does not contain
anything useful, C mode has to use `beginning-of-defun'.  Moreover, if
`defun-prompt-regexp' is nil, `open-paren-in-column-0-is-defun-start'
does _not_ affect the behavior of `beginning-of-defun' and the later
does search for an open paren in column zero.

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