Subject: Re: bug#7918: [PATCH] cc-mode: only the first clause of a for-loop should be checked for declarations

From: Daniel Colascione <[email-masked]>
Date: Tue, 1 Mar 2016 10:05:47 -0800

On 03/01/2016 10:02 AM, Alan Mackenzie wrote:
> Hello, Daniel and Lars.
> 
> On Fri, Feb 26, 2016 at 04:48:13PM +1030, Lars Ingebrigtsen wrote:
>> Daniel Colascione <[email-masked]> writes:
> 
>>> // This code has no variable declarations
>>>
>>> void foo() {
>>>     for (; (DWORD) a * b ;)
>>>         ;
>>>
>>>     for (; a * b ;)
>>>         ;
>>> }
>>>
> 
>> I can confirm that the Emacs trunk still highlights the "a" in these
>> examples wrong, and that Daniel's patch seems to fix the issue.
>> However, I'm totally unfamiliar with the cc-mode code, so it would be
>> nice if somebody could look at it before it's applied.
> 
> OK.  I haven't actually tried this patch out, but there are things in it
> I find concerning.
> 
>> === modified file 'lisp/progmodes/cc-fonts.el'
>> --- lisp/progmodes/cc-fonts.el	2010-12-07 12:15:28 +0000
>> +++ lisp/progmodes/cc-fonts.el	2011-01-25 11:10:00 +0000
>> @@ -1080,7 +1080,8 @@
>>  	  ;; o - '<> if the arglist is of angle bracket type;
>>  	  ;; o - 'arglist if it's some other arglist;
>>  	  ;; o - nil, if not in an arglist at all.  This includes the
>> -	  ;;   parenthesised condition which follows "if", "while", etc.
>> +	  ;;   parenthesised condition which follows "if", "while", etc.,
>> +	  ;;   but not "for", which is 'arglist after `;'.
> 
> By what logic is `context' set to 'arglist in a "for" statement?  The
> main function of `context' is to inform `c-forward-decl-or-cast-1' of
> the context in which it is being called.
> 
>>  	  context
>>  	  ;; The position of the next token after the closing paren of
>>  	  ;; the last detected cast.
>> @@ -1109,7 +1110,7 @@
>>  	  ;; `c-forward-decl-or-cast-1' and `c-forward-label' for
>>  	  ;; later fontification.
>>  	  (c-record-type-identifiers t)
>> -	  label-type
>> +	  label-type paren-state most-enclosing-brace
>>  	  c-record-ref-identifiers
>>  	  ;; Make `c-forward-type' calls mark up template arglists if
>>  	  ;; it finds any.  That's necessary so that we later will
>> @@ -1171,7 +1172,6 @@
>>  				 'font-lock-function-name-face))))
>>  	  (c-font-lock-function-postfix limit))
>  
>> -	 
>>  	 (setq start-pos (point))
>>  	 (when
>>  	     ;; The result of the `if' condition below is true when we don't recognize a
> 
> The next hunk attempts to move the detection of a "for" statement here
> from later in the function where it previously was.  Why?
> 
>> @@ -1189,7 +1189,31 @@
>>  	    ;; (e.g. "for (").
>>  	    (let ((type (and (> match-pos (point-min))
>>  			     (c-get-char-property (1- match-pos) 'c-type))))
>> -	      (cond ((not (memq (char-before match-pos) '(?\( ?, ?\[ ?<)))
>> +	      (cond
>> +               (;; Try to not fontify the second and third clauses of
>> +		;; `for' statements as declarations.
>> +		(and (or (eq (char-before match-pos) ?\;)
>> +			 (save-excursion
>> +			   ;; Catch things like for(; (DWORD)(int) x &
>> +			   ;; y; ) without invoking the full might of
>> +			   ;; c-beginning-of-statement-1.
>> +			   (goto-char match-pos)
>> +			   (while (eq (char-before) ?\))
>> +			     (c-go-list-backward)
>> +			     (c-backward-syntactic-ws))
> 
> Here we potentially have an infinite loop when there's an unbalanced ")"
> in the code.  It's critical to check the return from
> `c-go-list-backward' here, too.
> 
>> +			   (eq (char-before) ?\;)))
>> +		     
>> +                     (setq paren-state (c-parse-state))
>> +                     (setq most-enclosing-brace
>> +                           (c-most-enclosing-brace paren-state))
>> +		     (eq (char-after most-enclosing-brace) ?\())
> 
> Rather than using `c-parse-state', this could be done more efficiently
> with `c-up-list-backward'.  There may be the possibility of an error
> here if `c-most-enclosing-brace' returns nil, leading to (char-after
> nil), but maybe that can't happen.  It would certainly be a good idea to
> check for it, though.
> 
>> +		
>> +                ;; After a ";" in a for-block. A declaration can never
>> +                ;; begin after a `;' if the most enclosing paren is a
>> +                ;; `('.
> 
> How do we know we're in a "for" block here?  There is no `looking-at'
> check with the pertinent regexp (c-paren-stmt-key).
> 
>> +		(setq context 'arglist
>> +                      c-restricted-<>-arglists t))
> 
> Again, why is `context' set to 'arglist here?  What effect is this
> intended to have on `c-forward-decl-or-cast-1'?
> 
>> +               ((not (memq (char-before match-pos) '(?\( ?, ?\[ ?<)))
>>  		     (setq context nil
>>  			   c-restricted-<>-arglists nil))
>>  		    ;; A control flow expression
>> @@ -1252,7 +1276,7 @@
>>  		;; Are we at a declarator?  Try to go back to the declaration
>>  		;; to check this.  Note that `c-beginning-of-decl-1' is slow,
>>  		;; so we cache its result between calls.
>> -		(let (paren-state bod-res encl-pos is-typedef)
>> +		(let (bod-res encl-pos is-typedef)
>>  		  (goto-char start-pos)
>>  		  (save-excursion
>>  		    (unless (and decl-search-lim
>> @@ -1318,20 +1342,7 @@
>>  		;; Back up to the type to fontify the declarator(s).
>>  		(goto-char (car decl-or-cast))
>  
>> -		(let ((decl-list
>> -		       (if context
>> -			   ;; Should normally not fontify a list of
>> -			   ;; declarators inside an arglist, but the first
>> -			   ;; argument in the ';' separated list of a "for"
>> -			   ;; statement is an exception.
>> -			   (when (eq (char-before match-pos) ?\()
>> -			     (save-excursion
>> -			       (goto-char (1- match-pos))
>> -			       (c-backward-syntactic-ws)
>> -			       (and (c-simple-skip-symbol-backward)
>> -				    (looking-at c-paren-stmt-key))))
>> -			 t)))
>> -
>> +                (let ((decl-list (not context)))
> 
> Here the setting of decl-list is changed.  Why?  `decl-list''s purpose
> is to instruct `c-font-lock-declarators' whether to fontify just one
> declarator or a whole list of them.  The existing logic is to fontify
> all the declarators in a "for" block, whereas after the patch only the
> first one would be fontified.  Again, why?
> 
>>  		  ;; Fix the `c-decl-id-start' or `c-decl-type-start' property
>>  		  ;; before the first declarator if it's a list.
>>  		  ;; `c-font-lock-declarators' handles the rest.
> 
> Question (for Daniel): has this patch been run through the CC Mode test
> suite, yet?

It has not. It's been years since I even thought about that code. If
you're up for it, I'd rather you supply a separate fix.