rx: Improve 'or' compositionality (bug#37659)

Perform 'regexp-opt' on nested 'or' forms, and after expansion of
user-defined and 'eval' forms.  Characters are now turned into strings
for wider 'regexp-opt' scope.  This preserves the longest-match
semantics for string in 'or' forms over composition.

* doc/lispref/searching.texi (Rx Constructs): Document.
* lisp/emacs-lisp/rx.el (rx--normalise-or-arg)
(rx--all-string-or-args): New.
(rx--translate-or): Normalise arguments first, and check for strings
in subforms.
(rx--expand-eval): Extracted from rx--translate-eval.
(rx--translate-eval): Call rx--expand-eval.
* test/lisp/emacs-lisp/rx-tests.el (rx-or, rx-def-in-or): Add tests.
* etc/NEWS: Announce.

1 files changed, 6 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index e9dfd266b46..6e2b1fe00e2 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2325,6 +2325,12 @@ expressions from simpler parts.
 +++
 *** 'not' argument can now be a character or single-char string.
 
++++
+*** Nested 'or' forms of strings guarantee a longest match.
+For example, (or (or "IN" "OUT") (or "INPUT" "OUTPUT")) now matches
+the whole string "INPUT" if present, not just "IN".  Previously, this
+was only guaranteed inside a single 'or' form of string literals.
+
 ** Frames
 
 +++