The --script option now enforces lexical binding (bug#79760)

* etc/NEWS: Document the change.
* lisp/startup.el (command-line--load-script): Set 'lexical-binding';
if we delete the first line, insert a new line to preserve line
numbers.
(command-line--eval-script): Set 'lexical-binding'.

etc/NEWS

@@ -3168,6 +3168,16 @@ and blocks Emacs, or does not provide ways to limit how often it runs.
 
 * Incompatible Lisp Changes in Emacs 31.1
 
+** Files loaded from '-x' and '--script' now use lexical binding.
+If you don't have time to adapt your script's code to the lexical
+binding dialect (see (info "(elisp)Converting to Lexical Binding")),
+you can wrap your code in:
+
+    #!/usr/bin/env -S emacs --batch --script
+    (eval
+     '(progn
+        YOUR CODE HERE))
+
 +++
 ** String mutation has been restricted further.
 'aset' on unibyte strings now requires the new character to be a single

lisp/startup.el

@@ -2961,12 +2961,14 @@ nil default-directory" name)
    file file nil t
    (lambda (buffer file)
      (with-current-buffer buffer
+       (setq-local lexical-binding t)
        (goto-char (point-min))
        ;; Removing the #! and then calling `eval-buffer' will make the
        ;; reader not signal an error if it then turns out that the
        ;; buffer is empty.
        (when (looking-at "#!")
-         (delete-line))
+         (delete-line)
+         (insert ";; -*- lexical-binding: t -*-\n"))
        (eval-buffer buffer nil file nil t)))))
 
 (defun command-line--eval-script (file)
@@ -2975,6 +2977,7 @@ nil default-directory" name)
    (lambda (buffer _)
      (with-current-buffer buffer
        (goto-char (point-min))
+       (setq-local lexical-binding t)
        (when (looking-at "#!")
          (forward-line))
        (let (value form)