Make whitespace.el's cleanup add missing final newline

* lisp/whitespace.el (whitespace-cleanup-region): If cleaning
up at end of file, add missing newline if indicated by
'whitespace-style'.  (Bug#71499)

* etc/NEWS: Announce the change in behavior.

etc/NEWS

@@ -36,6 +36,12 @@ applies, and please also update docstrings as needed.
 
 * Changes in Specialized Modes and Packages in Emacs 31.1
 
+---
+** Whitespace
+'whitespace-cleanup' now adds missing newline at end of file If
+'whitespace-style' includes 'missing-newline-at-eof (which is the
+default), the 'whitespace-cleanup' function will fix this when run.
+
 
 * New Modes and Packages in Emacs 31.1
 

lisp/whitespace.el

@@ -1465,6 +1465,11 @@ The problems cleaned up are:
    If `whitespace-style' includes the value
    `space-after-tab::space', replace TABs by SPACEs.
 
+5. missing newline at end of file.
+   If `whitespace-style' includes the value `missing-newline-at-eof',
+   and the cleanup region includes the end of file, add a final newline
+   if it is not there already.
+
 See `whitespace-style', `indent-tabs-mode' and `tab-width' for
 documentation."
   (interactive "@r")
@@ -1545,7 +1550,16 @@ documentation."
          ((memq 'space-before-tab::space whitespace-style)
           (whitespace-replace-action
            'untabify rstart rend
-           whitespace-space-before-tab-regexp 2))))
+           whitespace-space-before-tab-regexp 2)))
+        ;; PROBLEM 5: missing newline at end of file
+        (and (memq 'missing-newline-at-eof whitespace-style)
+             (> (point-max) (point-min))
+             (= (point-max) (without-restriction (point-max)))
+             (/= (char-before (point-max)) ?\n)
+             (not (and (eq selective-display t)
+                       (= (char-before (point-max)) ?\r)))
+             (goto-char (point-max))
+             (ignore-errors (insert "\n"))))
       (set-marker rend nil))))		; point marker to nowhere
 
 

test/lisp/whitespace-tests.el

@@ -8,7 +8,6 @@
 ;; it under the terms of the GNU General Public License as published by
 ;; the Free Software Foundation, either version 3 of the License, or
 ;; (at your option) any later version.
-
 ;; GNU Emacs is distributed in the hope that it will be useful,
 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
@@ -94,6 +93,20 @@ buffer's content."
     (should (equal (whitespace-tests--cleanup-string "a  \n\t \n\n")
                    "a  \n"))))
 
+(ert-deftest whitespace-cleanup-missing-newline-at-eof ()
+  (let ((whitespace-style '(empty missing-newline-at-eof)))
+    (should (equal (whitespace-tests--cleanup-string "")
+                   ""))
+    (should (equal (whitespace-tests--cleanup-string "a")
+                   "a\n"))
+    (should (equal (whitespace-tests--cleanup-string "a\n\t")
+                   "a\n"))
+    (should (equal (whitespace-tests--cleanup-string "a\n\t ")
+                   "a\n"))
+    (should (equal (whitespace-tests--cleanup-string "a\n\t ")
+                   "a\n"))
+    (should (equal (whitespace-tests--cleanup-string "\n\t")
+                   ""))))
 
 ;; We cannot call whitespace-mode because it will do nothing in batch
 ;; mode.  So we call its innards instead.