bug#35254: 27.0.50; cc-mode/electric-pair-mode/electric-layout-mode: bad trailing whitespace behavior in cc-mode

[Alan Mackenzie <acm@muc.de>]

Hello, Noam.

On Fri, May 10, 2019 at 23:12:06 -0400, Noam Postavsky wrote:
> Dima Kogan <dima@secretsauce.net> writes:

> > Hi.

> > I'm seeing a regression introduced in 2019/01 by an update meant to make
> > electric-pair-mode and electric-layout-mode play nicely:

> >   http://git.savannah.gnu.org/cgit/emacs.git/commit/?id=fd943124439b7644392919bca8bc2a77e6316d92

> > Recipe:

> > 1. 'emacs -Q' with any emacs more recent than that patch

> > 2. Open up tst.c that contains this:

> > int main(int argc, char* argv[])
> > {
> >     return 0;
> > }


> > 3. Move the point to the beginning of the line containing "return 0"

> > 4. RET RET RET RET RET

> > Now there're a bunch of new lines between the { and the "return 0", as
> > expected. But these lines aren't empty: they contain the initial
> > indentation whitespace. This whitespace shouldn't be there; and it
> > wasn't there prior to this patch.

> Right, the problem is that electric-indent-inhibit only partially
> disables electric indent, and the commit you reference changes which
> parts.  The patch below disables it more completely.  Note however, that
> this makes RET not do any electric indentation at all, just like in the
> good old days of Emacs 24.3.

I don't quite agree with this.  The problem is confusion between effect
2 (below) and electric indentation.  This effect 2 is fundamental editor
functionality, and should be independent, MUST be independent of anything
called "electric indentation".  The two things are conceptually
unrelated.

> Possibly c-mode should rebind RET to a c-electric-return command or
> something.

CC Mode should be able to rely on the proper working of basic editor
functionality.  It shouldn't have to implement its own version of
`newline'.

> >>From 1103fdfbf2be55d68d44151498c2598fd622ac08 Mon Sep 17 00:00:00 2001
> From: Noam Postavsky <npostavs@gmail.com>
> Date: Fri, 10 May 2019 16:40:27 -0400
> Subject: [PATCH] Inhibit electric indent completely in cc mode buffers
>  (Bug#35254)

> Electric indent mode's post-self-insert hook entry has 3 effects:

> 1. Indent the previous line.
> 2. Remove trailing whitespace from the previous line.
> 3. Indent the current line (when at beginning of line).

> The change from 2019-01-22 "electric-layout-mode kicks in before
> electric-pair-mode", makes 'electric-indent-inhibit' inhibit 1 and 2,
> whereas before then it inhibited only 1.  While cc mode provides its
> own electric commands and therefore sets 'electric-indent-inhibit', it
> doesn't implement an electric newline command.  So if only one of
> effects 2 and 3 from Electric indent mode occur, then hitting RET will
> leave trailing whitespace.

> * lisp/progmodes/cc-mode.el (c-electric-indent-mode-function): New
> function.
> (c-basic-common-init): Add it to electric-indent-functions to disable
> electric indent completely (while still letting the
> electric-indent-mode command/setting to control c-electric-flag as
> before).
> ---
>  lisp/progmodes/cc-mode.el | 5 +++++
>  1 file changed, 5 insertions(+)

> diff --git a/lisp/progmodes/cc-mode.el b/lisp/progmodes/cc-mode.el
> index bd62fc754a..e41f1101d0 100644
> --- a/lisp/progmodes/cc-mode.el
> +++ b/lisp/progmodes/cc-mode.el
> @@ -634,6 +634,7 @@ (defun c-basic-common-init (mode default-style)
>    ;; messing up CC Mode's, and set `c-electric-flag' if `electric-indent-mode'
>    ;; has been called by the user.
>    (when (boundp 'electric-indent-inhibit) (setq electric-indent-inhibit t))
> +  (add-hook 'electric-indent-functions 'c-electric-indent-mode-function nil t)
>    ;; CC-mode should obey Emacs's generic preferences, tho only do it if
>    ;; Emacs's generic preferences can be set per-buffer (Emacs>=24.4).
>    (when (fboundp 'electric-indent-local-mode)
> @@ -2143,6 +2144,10 @@ (defun c-electric-indent-local-mode-hook ()
>      (setq c-electric-flag electric-indent-mode)
>      (c-update-modeline)))

> +(defun c-electric-indent-mode-function (char)
> +  ;; We never want `electric-indent-mode' to do anything.
> +  'no-indent)
> +
>  \f
>  ;; Support for C

I'm against this patch.  It is an unpleasant workaround in CC Mode for
problems in the Emacs core.  CC Mode has set electric-indent-inhibit to
t, and the Emacs core should respect that setting.  Using
electric-indent-functions in the way suggested couples CC Mode
undesirably with electric indentation, possibly leading to future
problems when electric indentation gets changed in the future.

In previous Emacs versions (?23.x, ?24.x) there were two simple commands
`newline' and `newline-and-indent'.  As far as I remember, they both
removed trailing WS from the "old" line, possibly as part of the filling
which was done.  They worked, and worked well.  Why can't we get this
functionality back again?

> -- 
> 2.11.0

-- 
Alan Mackenzie (Nuremberg, Germany).