bug#50538: [PATCH] 28.0.50; electric-pair-mode fails to pair double quotes in some cases in CC mode

[Alan Mackenzie <acm@muc.de>]

Hello, Jim.

On Thu, Sep 16, 2021 at 14:36:06 -0700, Jim Porter wrote:
> On 9/16/2021 1:49 PM, Alan Mackenzie wrote:
> > There were two or three minor problems with the patch:

> > 1-/.  CC Mode doesn't use syntax-ppss at all.  This was because way back
> > when, syntax-ppss was buggy, and even now doesn't do the right thing for
> > CC Mode in some edge cases (e.g. with the buffer narrowed and point-min
> > inside a string or comment).  Instead it uses its own internal syntactic
> > cacheing, largely centred around the function c-semi-pp-to-literal.

> > 2/-  Rather than using get-text-property and friends directly, CC Mode
> > uses the macros c-get-char-property, etc.  This is (?was) to maintain
> > compatibility with XEmacs.

> > 3/- (A bit more serious) The patch looks for the last " in the current
> > line without taking account of any escaped new lines.  There is already
> > a CC Mode macro which does all the work here, c-point, which can be given
> > the argument 'eoll for "end of logical line".

> Thanks, I've incorporated all these changes into the attached patch. The 
> only difference between my patch and the version you provided was to 
> keep the `(search-backward "\"")' portion from my patch, since the code 
> needs to find the last double-quote, not the end of line.

Duh!  Sorry about that, I clean forgot about it.  I did say that I
hadn't tested it, though. ;-)

> As an aside, it's probably worth explaining why my patch searches for 
> the last double-quote in the first place. As far as I understand CC 
> Mode, when there's an unterminated double-quote on a line, both the 
> quote and the newline have a string fence property applied to them.

Yes.

> This means we could check the newline for that property, *but* there's
> no guarantee a newline actually exists: `(c-point 'eoll)' could
> actually point to the end of the buffer. To get around that, we search
> backwards for the last double-quote of the (logical) line, since
> that's guaranteed to exist.

Yes.  This is a very important case, the one that occurs when somebody's
typing a new file, or at the end of an existing one.

> If we wanted to, we could avoid searching backwards for the last 
> double-quote when a newline exists, but I'm not sure the gain in 
> performance (likely very small) is worth the extra code complexity.

I'm fairly sure it wouldn't be.

[ Patch snipped but read. ]

-- 
Alan Mackenzie (Nuremberg, Germany).