make-text-button no longer modifies its string arg

* etc/NEWS: Mention this.
* lisp/apropos.el (apropos-library-button):
* lisp/ibuf-ext.el (ibuffer-old-saved-filters-warning):
There’s no longer a need copy make-text-button’s string arg.
* lisp/button.el (make-text-button): Return a copy of a string arg.
Delay making the copy until after error-checking.

etc/NEWS

@@ -476,6 +476,11 @@ are 'eq'.  To compare contents, use 'compare-window-configurations'
 instead.  This change helps fix a bug in 'sxhash-equal', which returned
 incorrect hashes for window configurations and some other objects.
 
+** When its first argument is a string, 'make-text-button' no longer
+modifies the string's text properties; instead, it uses and returns
+a copy of the string.  This helps avoid trouble when strings are
+shared or constants.
+
 ---
 ** The obsolete function 'thread-alive-p' has been removed.
 

lisp/apropos.el

@@ -661,7 +661,7 @@ Return list of symbols and documentation found."
 (defun apropos-library-button (sym)
   (if (null sym)
       "<nothing>"
-    (let ((name (copy-sequence (symbol-name sym))))
+    (let ((name (symbol-name sym)))
       (make-text-button name nil
                         'type 'apropos-library
                         'face 'apropos-symbol

lisp/button.el

@@ -341,15 +341,14 @@ If the property `button-data' is present, it will later be used
 as the argument for the `action' callback function instead of the
 default argument, which is the button itself.
 
-BEG can also be a string, in which case it is made into a button.
+BEG can also be a string, in which case a copy of it is made into
+a button and returned.
 
 Also see `insert-text-button'."
   (let ((object nil)
         (type-entry
 	 (or (plist-member properties 'type)
 	     (plist-member properties :type))))
-    (when (stringp beg)
-      (setq object beg beg 0 end (length object)))
     ;; Disallow setting the `category' property directly.
     (when (plist-get properties 'category)
       (error "Button `category' property may not be set directly"))
@@ -362,6 +361,10 @@ Also see `insert-text-button'."
       (setcar type-entry 'category)
       (setcar (cdr type-entry)
               (button-category-symbol (cadr type-entry))))
+    (when (stringp beg)
+      (setq object (copy-sequence beg))
+      (setq beg 0)
+      (setq end (length object)))
     ;; Now add all the text properties at once.
     (add-text-properties beg end
                          ;; Each button should have a non-eq `button'

lisp/ibuf-ext.el

@@ -202,7 +202,7 @@ The format has been repaired and the variable modified accordingly.
 You can save the current value through the customize system by
 either clicking or hitting return "
           (make-text-button
-           (copy-sequence "here") nil
+           "here" nil
            'face '(:weight bold :inherit button)
            'mouse-face '(:weight normal :background "gray50" :inherit button)
            'follow-link t