Fix pcase rx pattern bugs

Two unrelated bugs: A missing type check caused an error in rx
patterns for non-string match targets, and rx patterns did not work at
all in pcase-let or pcase-let*.

Second bug reported by Basil Contovounesios and Ag Ibragimov; fixes
proposed by Stefan Monnier.  Discussion and explanation in thread at
https://lists.gnu.org/archive/html/emacs-devel/2021-02/msg01924.html

* lisp/emacs-lisp/rx.el (rx): Add (pred stringp) to avoid type errors,
and replace the `pred` clause for the actual match with something that
works with pcase-let(*) without being optimised away.
* test/lisp/emacs-lisp/rx-tests.el (rx-pcase): Add test cases.

1 files changed, 11 insertions, 1 deletions

diff --git a/test/lisp/emacs-lisp/rx-tests.el b/test/lisp/emacs-lisp/rx-tests.el
index 12bf4f7978e..fecdcf55aff 100644
--- a/test/lisp/emacs-lisp/rx-tests.el
+++ b/test/lisp/emacs-lisp/rx-tests.el
@@ -171,7 +171,17 @@
   (should (equal (pcase "abc"
                    ((rx (? (let x alpha)) (?? (let y alnum)) ?c)
                     (list x y)))
-                 '("a" "b"))))
+                 '("a" "b")))
+  (should (equal (pcase 'not-a-string
+                   ((rx nonl) 'wrong)
+                   (_ 'correct))
+                 'correct))
+  (should (equal (pcase-let (((rx ?B (let z nonl)) "ABC"))
+                   (list 'ok z))
+                 '(ok "C")))
+  (should (equal (pcase-let* (((rx ?E (let z nonl)) "DEF"))
+                   (list 'ok z))
+                 '(ok "F"))))
 
 (ert-deftest rx-kleene ()
   "Test greedy and non-greedy repetition operators."