rx: fix `or' ordering by adding argument to regexp-opt

The rx `or' form may reorder its arguments in an unpredictable way,
contrary to user expectation, since it sometimes uses `regexp-opt'.
Add a NOREORDER option to `regexp-opt' for preventing it from
producing a reordered regexp (Bug#34641).

* doc/lispref/searching.texi (Regular Expression Functions):
* etc/NEWS (Lisp Changes in Emacs 27.1):
Describe the new regexp-opt NOREORDER argument.
* lisp/emacs-lisp/regexp-opt.el (regexp-opt): Add NOREORDER.
Make no attempt at regexp improvement if the set of strings contains
a prefix of another string.
(regexp-opt--contains-prefix): New.
* lisp/emacs-lisp/rx.el (rx-or): Call regexp-opt with NOREORDER.
* test/lisp/emacs-lisp/rx-tests.el: Test rx `or' form match order.

2 files changed, 35 insertions, 5 deletions

diff --git a/lisp/emacs-lisp/regexp-opt.el b/lisp/emacs-lisp/regexp-opt.el
index 63786c1508c..d0c5f2d3fc4 100644
--- a/lisp/emacs-lisp/regexp-opt.el
+++ b/lisp/emacs-lisp/regexp-opt.el
@@ -84,7 +84,7 @@
 ;;; Code:
 
 ;;;###autoload
-(defun regexp-opt (strings &optional paren)
+(defun regexp-opt (strings &optional paren noreorder)
   "Return a regexp to match a string in the list STRINGS.
 Each string should be unique in STRINGS and should not contain
 any regexps, quoted or not.  Optional PAREN specifies how the
@@ -111,8 +111,14 @@ nil
     necessary to ensure that a postfix operator appended to it will
     apply to the whole expression.
 
-The resulting regexp is equivalent to but usually more efficient
-than that of a simplified version:
+The optional argument NOREORDER, if nil or omitted, allows the
+returned regexp to match the strings in any order.  If non-nil,
+the match is guaranteed to be performed in the order given, as if
+the strings were made into a regexp by joining them with the
+`\\|' operator.
+
+Up to reordering, the resulting regexp is equivalent to but
+usually more efficient than that of a simplified version:
 
  (defun simplified-regexp-opt (strings &optional paren)
    (let ((parens
@@ -133,7 +139,15 @@ than that of a simplified version:
 	   (open (cond ((stringp paren) paren) (paren "\\(")))
 	   (sorted-strings (delete-dups
 			    (sort (copy-sequence strings) 'string-lessp)))
-	   (re (regexp-opt-group sorted-strings (or open t) (not open))))
+	   (re
+            ;; If NOREORDER is non-nil and the list contains a prefix
+            ;; of another string, we give up all attempts at optimisation.
+            ;; There is plenty of room for improvement (Bug#34641).
+            (if (and noreorder (regexp-opt--contains-prefix sorted-strings))
+                (concat (or open "\\(?:")
+                        (mapconcat #'regexp-quote strings "\\|")
+                        "\\)")
+              (regexp-opt-group sorted-strings (or open t) (not open)))))
       (cond ((eq paren 'words)
 	     (concat "\\<" re "\\>"))
 	    ((eq paren 'symbols)
@@ -313,6 +327,22 @@ CHARS should be a list of characters."
           (concat "[" dash caret "]"))
       (concat "[" bracket charset caret dash "]"))))
 
+
+(defun regexp-opt--contains-prefix (strings)
+  "Whether STRINGS contains a proper prefix of one of its other elements.
+STRINGS must be a list of sorted strings without duplicates."
+  (let ((s strings))
+    ;; In a lexicographically sorted list, a string always immediately
+    ;; succeeds one of its prefixes.
+    (while (and (cdr s)
+                (not (string-equal
+                      (car s)
+                      (substring (cadr s) 0 (min (length (car s))
+                                                 (length (cadr s)))))))
+      (setq s (cdr s)))
+    (cdr s)))
+
+
 (provide 'regexp-opt)
 
 ;;; regexp-opt.el ends here
diff --git a/lisp/emacs-lisp/rx.el b/lisp/emacs-lisp/rx.el
index 715cd608c46..ca756efb493 100644
--- a/lisp/emacs-lisp/rx.el
+++ b/lisp/emacs-lisp/rx.el
@@ -393,7 +393,7 @@ FORM is of the form `(and FORM1 ...)'."
   (rx-group-if
    (if (memq nil (mapcar 'stringp (cdr form)))
        (mapconcat (lambda (x) (rx-form x '|)) (cdr form) "\\|")
-     (regexp-opt (cdr form)))
+     (regexp-opt (cdr form) nil t))
    (and (memq rx-parent '(: * t)) rx-parent)))