Make the rx eval form use lexical binding when active

Previously, it always used dynamic binding.

* lisp/emacs-lisp/rx.el (rx--expand-eval): Heed `lexical-binding`.
* test/lisp/emacs-lisp/rx-tests.el (rx-tests--x, rx-tests--get-x)
(rx-eval): Add test case.
* etc/NEWS: Announce.

etc/NEWS

@@ -1280,6 +1280,10 @@ name.  Previously, Eshell only did this for MS-Windows systems. To
 restore the old behavior, you can set 'eshell-pwd-convert-function' to
 'identity'.
 
+---
+** The rx 'eval' form now uses the current elisp dialect for evaluation.
+Previously, its argument was always evaluated using dynamic binding.
+
 
 * Lisp Changes in Emacs 31.1
 

lisp/emacs-lisp/rx.el

@@ -1072,7 +1072,7 @@ Return (REGEXP . PRECEDENCE)."
   "Expand `eval' arguments.  Return a new rx form."
   (unless (and body (null (cdr body)))
     (error "rx `eval' form takes exactly one argument"))
-  (eval (car body)))
+  (eval (car body) lexical-binding))
 
 (defun rx--translate-eval (body)
   "Translate the `eval' form.  Return (REGEXP . PRECEDENCE)."

test/lisp/emacs-lisp/rx-tests.el

@@ -485,11 +485,18 @@
     (should (equal (rx "" (regexp x) (eval ""))
                    "a*"))))
 
+(eval-when-compile
+  (defvar rx-tests--x "LEX")
+  (defun rx-tests--get-x () rx-tests--x))
+
 (ert-deftest rx-eval ()
   (should (equal (rx (eval (list 'syntax 'symbol)))
                  "\\s_"))
   (should (equal (rx "a" (eval (concat)) "b")
-                 "ab")))
+                 "ab"))
+  (should (equal (rx (eval (funcall (lambda (rx-tests--x) (rx-tests--get-x))
+                                    "DYN")))
+                 "LEX")))
 
 (ert-deftest rx-literal ()
   (should (equal (rx (literal "$a"))