bug#35454: 26.2.50; CC-Mode fontification fails inside macro

[Alan Mackenzie <acm@muc.de>]

Hello, Mauro.

On Wed, May 01, 2019 at 19:31:48 -0300, Mauro Aranda wrote:
> Alan Mackenzie <acm@muc.de> writes:

> Hello Alan.  Thanks for looking into this bug!

> > Please try out the patch below.  On my system, it corrects the
> > fontification in both your test file and editfns.c.

> I've applied the patch and tried the recipe I provided, and it works fine.

> However, when I visit editfns.c and search for EXTRA_CONTEXT_FIELDS,
> like I said in my report, I see the following problem with this variables:
> struct buffer *buffer_a;
> struct buffer *buffer_b;
> unsigned char *deletions;
> unsigned char *insertions;

> All but deletions have face font-lock-variable-name-face.

> I can't seem to come up with a simple recipe to reproduce the problem,
> so I refer you to that part of editfns.c.

Thanks, I didn't notice these last night, but I can see them now.

> All the following steps, separately with emacs -Q (or you could kill the
> buffer if you want)
> 1) C-x C-f editfns.c
> C-s extra RET
> I observe deletions without its correspondent face and if I type:
> SPC DEL
> deletions gets font-lock-variable-name-face face.  However, if I
> revert the buffer with M-x revert-buffer RET yes RET buffer_a, deletions
> and the first 'buffer' lose their faces.

The problem with "deletions" seems to be triggered by the 2-line comment
in the macro not having a backslash escaping the \n.  In nearly 30 years
hacking C, I've never seen this before, and didn't even know it was
valid syntax.  However, this means at least four very commonly used
functions (c-beginning-of-macro, c-end-of-macro, c-forward-sws, and
c-backward-sws) are going to have to be amended to deal with it, and
this is inevitably going to make CC Mode slower.  :-(

I can't see at the moment any pattern with the fontification on
buffer_a.  Sometimes just scrolling the buffer a few lines makes
buffer_a lose its fontification.  But if an empty pair of parentheses is
put after EXTRA_CONTEXT_FIELDS, all these problems go away.  So they
seem related to what I half-fixed last night.

> 2) C-x C-f editfns.c
> C-s deletions RET
> I see that deletions has the right face.  But
> M-x revert-buffer
> makes it lose it (but *buffer_a keeps its face).

> 3) C-x C-f editfns.c
> C-s extra RET
> deletions without font-lock-variable-name-face.
> C-l C-l
> M-x revert-buffer
> deletions now has font-lock-variable-name-face.

> That is all the testing I could do, sorry for not being able to come up
> with a better recipe.  Let me know if you see the same behavior, or what
> else I could try.

Well, that seems like quite a lot of testing to me.  :-)  Thanks for
doing it and reporting back to me so quickly.

I'll be working on the outstanding bugs here in the next few days.

> Best regards,
> Mauro.

-- 
Alan Mackenzie (Nuremberg, Germany).