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

> >>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.

I interpret the problem a little differently.
electric-indent-post-self-insert-function, when electric-indent-inhibit
is set, is inhibiting actions which are not really part of electric
indentation, in particular action 2 (above).  This is the heart of the
bug.  The following patch fixes the bug.  It would need tidying up before
being committed:



diff --git a/lisp/electric.el b/lisp/electric.el
index 07da2f1d9e..15a42930c1 100644
--- a/lisp/electric.el
+++ b/lisp/electric.el
@@ -282,9 +282,15 @@ electric-indent-post-self-insert-function
                   (condition-case-unless-debug ()
                       (indent-according-to-mode)
                     (error (throw 'indent-error nil)))
-                  ;; The goal here will be to remove the trailing
-                  ;; whitespace after reindentation of the previous line
-                  ;; because that may have (re)introduced it.
+                  )
+                (unless (memq indent-line-function
+                              electric-indent-functions-without-reindent)
+                  ;; The goal here will be to remove the indentation
+                  ;; whitespace from an otherwise blank line after
+                  ;; typing <CR> twice in succession.  Also to remove
+                  ;; trailing whitespace after reindentation of the
+                  ;; previous line because that may have
+                  ;; (re)introduced it.
                   (goto-char before)
                   ;; We were at EOL in marker `before' before the call
                   ;; to `indent-according-to-mode' but after we may


João and Noam, what're your views on this proposed patch?

-- 
Alan Mackenzie (Nuremberg, Germany).