Unbind 'C-M-i' in Text mode

Remove the binding of 'C-M-i' to 'ispell-complete-word' in Text mode.
Define a new 'ispell-completion-at-point' function and add that to
'completion-at-point-functions' in Text mode, such that
'completion-at-point' provides the same word completions as
'ispell-complete-word' does OOTB.

* lisp/textmodes/ispell.el (ispell-completion-at-point): New function.
* lisp/textmodes/text-mode.el (text-mode): Add it to 'c-a-p-functions'.
(text-mode-map): Remove 'C-M-i' binding.
(text-mode-meta-tab-ispell-complete-word): New user option.
* etc/NEWS: Announce it.
* doc/emacs/fixit.texi (Spelling)
* doc/emacs/text.texi (Text Mode)
* doc/lispref/modes.texi (Basic Major Modes)
(Example Major Modes): Update.
* lisp/mail/sendmail.el (mail-abbrevs-loaded)
* lisp/nxml/nxml-mode.el (nxml-mode-map): Remove superfluous binding.

doc/emacs/fixit.texi

@@ -295,8 +295,8 @@ Kill the spell-checker subprocess.
 @item M-@key{TAB}
 @itemx @key{ESC} @key{TAB}
 @itemx C-M-i
-Complete the word before point based on the spelling dictionary
-(@code{ispell-complete-word}).
+Complete the word before point based on the spelling dictionary and
+other completion sources (@code{completion-at-point}).
 @item M-x flyspell-mode
 Enable Flyspell mode, which highlights all misspelled words.
 @item M-x flyspell-prog-mode
@@ -398,14 +398,11 @@ Quit interactive spell-checking and kill the spell-checker subprocess.
 Show the list of options.
 @end table
 
-@findex ispell-complete-word
-  In Text mode and related modes, @kbd{M-@key{TAB}}
-(@code{ispell-complete-word}) performs in-buffer completion based on
-spelling correction.  Insert the beginning of a word, and then type
-@kbd{M-@key{TAB}}; this shows a list of completions.  (If your
+  Use the command @kbd{M-@key{TAB}} (@code{completion-at-point}) to
+complete the word at point.  Insert the beginning of a word, and then
+type @kbd{M-@key{TAB}} to select from a list of completions.  (If your
 window manager intercepts @kbd{M-@key{TAB}}, type @w{@kbd{@key{ESC}
-@key{TAB}}} or @kbd{C-M-i}.)  Each completion is listed with a digit or
-character; type that digit or character to choose it.
+@key{TAB}}} or @kbd{C-M-i}.)
 
 @cindex @code{ispell} program
 @findex ispell-kill-ispell

doc/emacs/text.texi

@@ -943,12 +943,12 @@ situations where you shouldn't change the major mode---in mail
 composition, for instance.
 
 @kindex M-TAB @r{(Text mode)}
-  Text mode binds @kbd{M-@key{TAB}} to @code{ispell-complete-word}.
-This command performs completion of the partial word in the buffer
-before point, using the spelling dictionary as the space of possible
-words.  @xref{Spelling}.  If your window manager defines
-@kbd{M-@key{TAB}} to switch windows, you can type @kbd{@key{ESC}
-@key{TAB}} or @kbd{C-M-i} instead.
+  The command @kbd{M-@key{TAB}} (@code{completion-at-point}) performs
+completion of the partial word in the buffer before point, using the
+spelling dictionary as the space of possible words by default.
+@xref{Spelling}.  If your window manager defines @kbd{M-@key{TAB}} to
+switch windows, you can type @kbd{@key{ESC} @key{TAB}} or @kbd{C-M-i}
+instead.
 
 @vindex text-mode-hook
   Entering Text mode runs the mode hook @code{text-mode-hook}

doc/lispref/modes.texi

@@ -986,9 +986,9 @@ allows users to customize a single mode hook
 @deffn Command text-mode
 Text mode is a major mode for editing human languages.  It defines the
 @samp{"} and @samp{\} characters as having punctuation syntax
-(@pxref{Syntax Class Table}), and binds @kbd{M-@key{TAB}} to
-@code{ispell-complete-word} (@pxref{Spelling,,, emacs, The GNU Emacs
-Manual}).
+(@pxref{Syntax Class Table}), and arranges for
+@code{completion-at-point} to complete words based on the spelling
+dictionary (@pxref{Completion in Buffers}).
 
 An example of a major mode derived from Text mode is HTML mode.
 @xref{HTML Mode,,SGML and HTML Modes, emacs, The GNU Emacs Manual}.
@@ -1382,15 +1382,6 @@ the conventions listed above:
     st)
   "Syntax table used while in `text-mode'.")
 @end group
-
-;; @r{Create the keymap for this mode.}
-@group
-(defvar-keymap text-mode-map
-  :doc "Keymap for `text-mode'.
-Many other modes, such as `mail-mode' and `outline-mode', inherit all
-the commands defined in this map."
-  "C-M-i" #'ispell-complete-word)
-@end group
 @end smallexample
 
   Here is how the actual mode command is defined now:

etc/NEWS

@@ -1132,6 +1132,15 @@ showcases all their customization options.
 
 * Incompatible Lisp Changes in Emacs 30.1
 
++++
+** 'M-TAB' now invokes 'completion-at-point' also in Text mode.
+Text mode no longer binds 'M-TAB' to 'ispell-complete-word', and
+instead this mode arranges for 'completion-at-point', globally bound
+to 'M-TAB', to perform word completion as well.  If you want 'M-TAB'
+to invoke 'ispell-complete-word', as it did in previous Emacs
+versions, customize the new option
+'text-mode-meta-tab-ispell-complete-word' to non-nil.
+
 ** 'pp' and 'pp-to-string' now always include a terminating newline.
 In the past they included a terminating newline in most cases but not all.
 

lisp/mail/sendmail.el

@@ -269,7 +269,6 @@ The default value matches citations like `foo-bar>' plus whitespace."
 (defvar mail-abbrevs-loaded nil)
 (defvar mail-mode-map
   (let ((map (make-sparse-keymap)))
-    (define-key map "\M-\t" 'completion-at-point)
     (define-key map "\C-c?" 'describe-mode)
     (define-key map "\C-c\C-f\C-t" 'mail-to)
     (define-key map "\C-c\C-f\C-b" 'mail-bcc)

lisp/nxml/nxml-mode.el

@@ -390,7 +390,6 @@ reference.")
   "C-c C-u" #'nxml-insert-named-char
   "C-c C-o" nxml-outline-prefix-map
   "/"       #'nxml-electric-slash
-  "M-TAB"   #'completion-at-point
   "S-<mouse-2>" #'nxml-mouse-hide-direct-text-content)
 
 (defvar nxml-font-lock-keywords

lisp/textmodes/ispell.el

@@ -3680,6 +3680,28 @@ If APPEND is non-nil, don't erase previous debugging output."
 	  (if (>= column (- (window-width) 2))
 	      (scroll-left (max (- column (window-width) -3) 10)))))))
 
+;;;###autoload
+(defun ispell-completion-at-point ()
+  "Word completion function for use in `completion-at-point-functions'."
+  (pcase (bounds-of-thing-at-point 'word)
+    (`(,beg . ,end)
+     (when (and (< beg (point)) (<= (point) end))
+       (let* ((word (buffer-substring-no-properties beg end))
+              (len (length word))
+              (inhibit-message t)
+              (all (cons word (ispell-lookup-words word)))
+              (cur all))
+         (while cur
+           (unless (string-prefix-p word (car cur))
+             (setcar cur (concat word (substring (car cur) len))))
+           (while (when-let ((next (cadr cur)))
+                    (not (string-prefix-p word next t)))
+             (setcdr cur (cddr cur)))
+           (setq cur (cdr cur)))
+         (list beg end (cdr all)
+               :annotation-function (lambda (_) " Dictionary word")
+               :exclusive 'no))))))
+
 
 ;;; Interactive word completion.
 ;; Forces "previous-word" processing.  Do we want to make this selectable?

lisp/textmodes/text-mode.el

@@ -73,8 +73,20 @@
 (defvar-keymap text-mode-map
   :doc "Keymap for `text-mode'.
 Many other modes, such as `mail-mode' and `outline-mode', inherit
-all the commands defined in this map."
-  "C-M-i" #'ispell-complete-word)
+all the commands defined in this map.")
+
+(defcustom text-mode-meta-tab-ispell-complete-word nil
+  "Whether M-TAB invokes `ispell-complete-word' in Text mode.
+
+This user option only takes effect when you customize it in
+Custom or with `setopt', not with `setq'."
+  :group 'text
+  :type 'boolean
+  :version "30.1"
+  :set (lambda (sym val)
+         (if (set sym val)
+	     (keymap-set text-mode-map "C-M-i" #'ispell-complete-word)
+           (keymap-unset text-mode-map "C-M-i" t))))
 
 (easy-menu-define text-mode-menu text-mode-map
   "Menu for `text-mode'."
@@ -131,7 +143,8 @@ Turning on Text mode runs the normal hook `text-mode-hook'."
 
   ;; Enable text conversion in this buffer.
   (setq-local text-conversion-style t)
-  (add-hook 'context-menu-functions 'text-mode-context-menu 10 t))
+  (add-hook 'context-menu-functions 'text-mode-context-menu 10 t)
+  (add-hook 'completion-at-point-functions #'ispell-completion-at-point 10 t))
 
 (define-derived-mode paragraph-indent-text-mode text-mode "Parindent"
   "Major mode for editing text, with leading spaces starting a paragraph.