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

[Noam Postavsky <npostavs@gmail.com>]

Alan Mackenzie <acm@muc.de> writes:

[rearranged a bit for better flow]

>> 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.

>> Subject: [PATCH] Inhibit electric indent completely in cc mode buffers
>
>> 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).

> I don't quite agree with this.  The problem is confusion between effect
> 2 [above] 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.

So we should have an electric-delete-trailing-whitespace-mode?

>> 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).

>>    (when (boundp 'electric-indent-inhibit) (setq electric-indent-inhibit t))
>> +  (add-hook 'electric-indent-functions 'c-electric-indent-mode-function nil t)

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

> 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.

Hmm, from my point of view, the whole point of the patch is to
*UNcouple* CC mode from electric.el's electric indentation (since CC
mode has its own electric functionality), something that
electric-indent-inhibit does only partially (and based on your response
above I guess the reason for being partial is that there is some
disagreement over which parts exactly constitute "electric
indentation").

>> 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'.

> 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?

Both these commands still exist.  As far as I can tell, `newline' never
removed trailing WS.  It does have some code to delete the "left
margin", but that doesn't seem to be intended for programming modes
where the margin is 0 (disabled).

`newline-and-indent' did, and still does, delete trailing whitespace.
In 24.4, C-j was rebound from `newline-and-indent' to
`electric-newline-and-maybe-indent' which only calls
`newline-and-indent' if `electric-indent-mode' is nil.  Of course c-mode
could rebind it in its mode map (I considered making
`electric-newline-and-maybe-indent' consult `electric-indent-functions'
as well but that won't work because that hook is supposed to run after
the character is inserted).