Allow regexp-quote to return its argument

* src/search.c (Fregexp_quote): Only allocate a new string if needed.
* doc/lispref/searching.texi (Regexp Functions):
* etc/NEWS (Incompatible Lisp Changes): Document.

doc/lispref/searching.texi

@@ -1562,6 +1562,9 @@ whitespace:
  (concat "\\s-" (regexp-quote string) "\\s-"))
 @end group
 @end example
+
+The returned string may be @var{string} itself if it does not contain
+any special characters.
 @end defun
 
 @cindex optimize regexp

etc/NEWS

@@ -2157,6 +2157,10 @@ valid event type.
 be specified in image specs representing the entire bitmap as a single
 bool vector.
 
++++
+** 'regexp-quote' may return its argument string.
+If the argument needs no quoting, it can be returned instead of a copy.
+
 
 * Lisp Changes in Emacs 27.1
 

src/search.c

@@ -3138,10 +3138,12 @@ DEFUN ("regexp-quote", Fregexp_quote, Sregexp_quote, 1, 1, 0,
     }
 
   Lisp_Object result
-    = make_specified_string (temp,
-			     SCHARS (string) + backslashes_added,
-			     out - temp,
-			     STRING_MULTIBYTE (string));
+    = (backslashes_added > 0
+       ? make_specified_string (temp,
+                                SCHARS (string) + backslashes_added,
+                                out - temp,
+                                STRING_MULTIBYTE (string))
+       : string);
   SAFE_FREE ();
   return result;
 }