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.

1 files changed, 7 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 29ed7ab4819..7c95988ff52 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1642,6 +1642,13 @@ MS-Windows.
 ** New module environment function 'process_input' to process user
 input while module code is running.
 
++++
+** The function 'regexp-opt' accepts an additional optional argument.
+By default, the regexp returned by 'regexp-opt' may match the strings
+in any order.  If the new third argument is 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 '\|'.
+
 
 * Changes in Emacs 27.1 on Non-Free Operating Systems