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

[Noam Postavsky]

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.

* 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)
+
 
 ;; Support for C
 
-- 
2.11.0