Fill some auto-generated docstrings

* lisp/emacs-lisp/easy-mmode.el (define-minor-mode)
(define-globalized-minor-mode): Fill auto-generated documentation
strings.  (Bug#44858)
* lisp/subr.el (internal--fill-string-single-line)
(internal--format-docstring-line): New functions.

lisp/emacs-lisp/derived.el

@@ -306,11 +306,13 @@ No problems result if this variable is not bound.
       ;; Use a default docstring.
       (setq docstring
 	    (if (null parent)
-		;; FIXME filling.
-		(format "Major-mode.\nUses keymap `%s'%s%s." map
+                (concat
+                 "Major-mode.\n"
+                 (internal--format-docstring-line
+                  "Uses keymap `%s'%s%s." map
                   (if abbrev (format "%s abbrev table `%s'"
                                      (if syntax "," " and") abbrev) "")
-			(if syntax (format " and syntax-table `%s'" syntax) ""))
+                  (if syntax (format " and syntax-table `%s'" syntax) "")))
 	      (format "Major mode derived from `%s' by `define-derived-mode'.
 It inherits all of the parent's attributes, but has its own keymap%s:
 
@@ -336,20 +338,22 @@ which more-or-less shadow%s %s's corresponding table%s."
     (unless (string-match (regexp-quote (symbol-name hook)) docstring)
       ;; Make sure the docstring mentions the mode's hook.
       (setq docstring
-	    (concat docstring
+            (concat docstring "\n\n"
+                    (internal--format-docstring-line
+                     "%s%s%s"
                      (if (null parent)
-			"\n\nThis mode "
+                         "This mode "
                        (concat
-		       "\n\nIn addition to any hooks its parent mode "
+                        "In addition to any hooks its parent mode "
                         (if (string-match (format "[`‘]%s['’]"
                                                   (regexp-quote
                                                    (symbol-name parent)))
                                           docstring)
                             nil
                           (format "`%s' " parent))
-		       "might have run,\nthis mode "))
+                        "might have run, this mode "))
                      (format "runs the hook `%s'" hook)
-		    ", as the final or penultimate step\nduring initialization.")))
+                     ", as the final or penultimate step during initialization."))))
 
     (unless (string-match "\\\\[{[]" docstring)
       ;; And don't forget to put the mode's keymap.

lisp/emacs-lisp/easy-mmode.el

@@ -278,8 +278,10 @@ For example, you could write
          ((not globalp)
           `(progn
              :autoload-end
-             (defvar ,mode ,init-value ,(format "Non-nil if %s is enabled.
-Use the command `%s' to change this variable." pretty-name mode))
+             (defvar ,mode ,init-value
+               ,(concat (format "Non-nil if %s is enabled.\n" pretty-name)
+                        (internal--format-docstring-line
+                         "Use the command `%s' to change this variable." mode)))
              (make-variable-buffer-local ',mode)))
          (t
 	  (let ((base-doc-string
@@ -455,24 +457,23 @@ on if the hook has explicitly disabled it.
          (make-variable-buffer-local ',MODE-major-mode))
        ;; The actual global minor-mode
        (define-minor-mode ,global-mode
-	 ;; Very short lines to avoid too long lines in the generated
-	 ;; doc string.
-	 ,(format "Toggle %s in all buffers.
-With prefix ARG, enable %s if ARG is positive;
-otherwise, disable it.  If called from Lisp, enable the mode if
-ARG is omitted or nil.
-
-%s is enabled in all buffers where
-`%s' would do it.
-
-See `%s' for more information on
-%s.%s"
-		  pretty-name pretty-global-name
-		  pretty-name turn-on mode pretty-name
+         ,(concat (format "Toggle %s in all buffers.\n" pretty-name)
+                  (internal--format-docstring-line
+                   "With prefix ARG, enable %s if ARG is positive; otherwise, \
+disable it.  If called from Lisp, enable the mode if ARG is omitted or nil.\n\n"
+                   pretty-global-name)
+                  (internal--format-docstring-line
+                   "%s is enabled in all buffers where `%s' would do it.\n\n"
+                   pretty-name turn-on)
+                  (internal--format-docstring-line
+                   "See `%s' for more information on %s."
+                   mode pretty-name)
                   (if predicate
-                      (format "\n\n`%s' is used to control which modes
-this minor mode is used in."
-                              MODE-predicate)
+                      (concat
+                       "\n\n"
+                       (internal--format-docstring-line
+                        "`%s' is used to control which modes this minor mode is used in."
+                        MODE-predicate))
                     ""))
          :global t ,@group ,@(nreverse extra-keywords)
 

lisp/subr.el

@@ -5973,4 +5973,22 @@ seconds."
      ;; Continue running.
      nil)))
 
+(defun internal--fill-string-single-line (str)
+  "Fill string STR to `fill-column'.
+This is intended for very simple filling while bootstrapping
+Emacs itself, and does not support all the customization options
+of fill.el (for example `fill-region')."
+  (if (< (string-width str) fill-column)
+      str
+    (let ((fst (substring str 0 fill-column))
+          (lst (substring str fill-column)))
+      (if (string-match ".*\\( \\(.+\\)\\)$" fst)
+          (setq fst (replace-match "\n\\2" nil nil fst 1)))
+      (concat fst (internal--fill-string-single-line lst)))))
+
+(defun internal--format-docstring-line (string &rest objects)
+  "Format a documentation string out of STRING and OBJECTS.
+This is intended for internal use only."
+  (internal--fill-string-single-line (apply #'format string objects)))
+
 ;;; subr.el ends here