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

[Stefan Monnier <monnier@IRO.UMontreal.CA>]

>> 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).

Note that `newline` itself already does some subset of 2 (before running
electric-indent's post-self-insert hook).

> 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:

I haven't followed enough of the discussion to understand why that might
cause a bug.

> 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

I don't understand why you distinguish

    electric-indent-inhibit

from

    (memq indent-line-function
          electric-indent-functions-without-reindent)

When I introduced these, electric-indent-functions-without-reindent was
only meant to paper over those pre-existing cases that don't set
electric-indent-inhibit.

So, I'd suggest an even simpler patch which just closes the `unless`
earlier.  Would that work?


        Stefan


diff --git a/lisp/electric.el b/lisp/electric.el
index 07da2f1d9e..beb3348755 100644
--- a/lisp/electric.el
+++ b/lisp/electric.el
@@ -281,7 +281,7 @@ electric-indent-post-self-insert-function
                   (goto-char before)
                   (condition-case-unless-debug ()
                       (indent-according-to-mode)
-                    (error (throw 'indent-error nil)))
+                    (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.
@@ -290,7 +290,7 @@ electric-indent-post-self-insert-function
                   ;; to `indent-according-to-mode' but after we may
                   ;; not be (Bug#15767).
                   (when (and (eolp))
-                    (delete-horizontal-space t))))))
+                    (delete-horizontal-space t)))))
           (unless (and electric-indent-inhibit
                        (not at-newline))
             (condition-case-unless-debug ()