Fix pcase 'rx' patterns with a single named submatch (bug#48477)

pcase 'rx' patterns with a single named submatch, like

  (rx (let x "a"))

would always succeed because of an over-optimistic transformation.
Patterns with 0 or more than 1 named submatches were not affected.

Reported by Philipp Stephani.

* lisp/emacs-lisp/rx.el (rx--pcase-macroexpander):
Special case for a single named submatch.
* test/lisp/emacs-lisp/rx-tests.el (rx-pcase): Add tests.

1 files changed, 14 insertions, 0 deletions

diff --git a/test/lisp/emacs-lisp/rx-tests.el b/test/lisp/emacs-lisp/rx-tests.el
index 2dd1bca22d1..4828df0de92 100644
--- a/test/lisp/emacs-lisp/rx-tests.el
+++ b/test/lisp/emacs-lisp/rx-tests.el
@@ -166,6 +166,20 @@
                         (backref 1))
                     (list u v)))
                  '("1" "3")))
+  (should (equal (pcase "bz"
+                   ((rx "a" (let x nonl)) (list 1 x))
+                   (_ 'no))
+                 'no))
+  (should (equal (pcase "az"
+                   ((rx "a" (let x nonl)) (list 1 x))
+                   ((rx "b" (let x nonl)) (list 2 x))
+                   (_ 'no))
+                 '(1 "z")))
+  (should (equal (pcase "bz"
+                   ((rx "a" (let x nonl)) (list 1 x))
+                   ((rx "b" (let x nonl)) (list 2 x))
+                   (_ 'no))
+                 '(2 "z")))
   (let ((k "blue"))
     (should (equal (pcase "<blue>"
                      ((rx "<" (literal k) ">") 'ok))