Allow zero-argument rx `or' and `seq' forms

Make the rx `or' and `seq' forms accept zero arguments to produce a
never-matching regexp and an empty string, respectively.

* lisp/emacs-lisp/rx.el: Require cl-extra.
(rx-constituents, rx-or): Permit zero args.
(rx): Amend doc string for `or' and `seq'.
* test/lisp/emacs-lisp/rx-tests.el (rx-or, rx-seq): Test the change.
* etc/NEWS (Changes in Specialized Modes and Packages): Mention the change.

1 files changed, 6 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 9ca98c370e6..72702a9aaac 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1321,6 +1321,12 @@ when given in a string.  Previously, '(any "\x80-\xff")' would match
 characters U+0080...U+00FF.  Now the expression matches raw bytes in
 the 128...255 range, as expected.
 
+*** The rx 'or' and 'seq' forms no longer require any arguments.
+(or) produces a regexp that never matches anything, while (seq)
+matches the empty string, each being an identity for the operation.
+This also works for their aliases: '|' for 'or'; ':', 'and' and
+'sequence' for 'seq'.
+
 ** Frames
 
 +++