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

[Stefan Monnier]

> I've just come across the bug myself, and it is indeed annoying.
> Can you check if this patch, which seems the simplest, serves
> all purposes?

FWIW, I find it rather ugly (makes the two minor modes too tightly
intertwined).

> It also adds a test to prevent future regressions

Another approach might be to do the whitespace erasure in
electric-indent without paying any attention to electric-layout/pair,
and then in electric-pair to explicitly cause (re)indentation
after moving point to between the opened pair?

Yet another option is to tell electric-indent about the final position
of point and have it refrain from deleting whitespace before that final
position, as in the patch below.  WDYT?


        Stefan


diff --git a/lisp/elec-pair.el b/lisp/elec-pair.el
index 3be09d87b4..a14efff241 100644
--- a/lisp/elec-pair.el
+++ b/lisp/elec-pair.el
@@ -551,7 +551,8 @@ electric-pair-post-self-insert-function
                          (goto-char pos)
                          (funcall electric-pair-inhibit-predicate
                                   last-command-event)))))
-         (save-excursion (electric-pair--insert pair)))))
+         (let ((electric-indent--destination (point-marker)))
+           (save-excursion (electric-pair--insert pair))))))
       (_
        (when (and (if (functionp electric-pair-open-newline-between-pairs)
                       (funcall electric-pair-open-newline-between-pairs)
diff --git a/lisp/electric.el b/lisp/electric.el
index 07da2f1d9e..71ebb9cf45 100644
--- a/lisp/electric.el
+++ b/lisp/electric.el
@@ -231,6 +231,14 @@ electric-indent-functions-without-reindent
 not try to reindent lines.  It is normally better to make the major
 mode set `electric-indent-inhibit', but this can be used as a workaround.")
 
+(defun electric-indent--inhibited-p ()
+  (or electric-indent-inhibit
+      (memq indent-line-function
+            electric-indent-functions-without-reindent)))
+
+(defvar electric-indent--destination nil
+  "If non-nil, position to which point will be later restored.")
+
 (defun electric-indent-post-self-insert-function ()
   "Function that `electric-indent-mode' adds to `post-self-insert-hook'.
 This indents if the hook `electric-indent-functions' returns non-nil,
@@ -272,26 +280,26 @@ electric-indent-post-self-insert-function
           (when at-newline
             (let ((before (copy-marker (1- pos) t)))
               (save-excursion
-                (unless
-                    (or (memq indent-line-function
-                              electric-indent-functions-without-reindent)
-                        electric-indent-inhibit)
+                (unless (electric-indent--inhibited-p)
                   ;; Don't reindent the previous line if the
                   ;; indentation function is not a real one.
                   (goto-char before)
                   (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.
-                  (goto-char before)
-                  ;; We were at EOL in marker `before' before the call
-                  ;; to `indent-according-to-mode' but after we may
-                  ;; not be (Bug#15767).
-                  (when (and (eolp))
-                    (delete-horizontal-space t))))))
-          (unless (and electric-indent-inhibit
+                    (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.
+                (goto-char before)
+                ;; We were at EOL in marker `before' before the call
+                ;; to `indent-according-to-mode' but after we may
+                ;; not be (Bug#15767).
+                (when (and (eolp)
+                           ;; Don't delete "trailing space" before point!
+                           (not (and electric-indent--destination
+                                     (= (point) 
electric-indent--destination))))
+                  (delete-horizontal-space t)))))
+          (unless (and (electric-indent--inhibited-p)
                        (not at-newline))
             (condition-case-unless-debug ()
                 (indent-according-to-mode)
diff --git a/test/lisp/electric-tests.el b/test/lisp/electric-tests.el
index 4f1e5729be..0b67fb3f1f 100644
--- a/test/lisp/electric-tests.el
+++ b/test/lisp/electric-tests.el
@@ -876,15 +876,6 @@ electric-layout-for-c-style-du-jour
       (call-interactively (key-binding `[,last-command-event])))
     (should (equal (buffer-string) "int main () {\n  \n}"))))
 
-(define-derived-mode plainer-c-mode c-mode "pC"
-  "A plainer/saner C-mode with no internal electric machinery."
-  (c-toggle-electric-state -1)
-  (setq-local electric-indent-local-mode-hook nil)
-  (setq-local electric-indent-mode-hook nil)
-  (electric-indent-local-mode 1)
-  (dolist (key '(?\" ?\' ?\{ ?\} ?\( ?\) ?\[ ?\]))
-    (local-set-key (vector key) 'self-insert-command)))
-
 (ert-deftest electric-modes-int-main-allman-style ()
   (ert-with-test-buffer ()
     (plainer-c-mode)