Don't keep warning about unescaped literals (Bug#36068)

* lisp/emacs-lisp/bytecomp.el (byte-compile-from-buffer): Restore lost
let-binding of lread--unescaped-character-literals, so that unescaped
literals warning will only apply to the form just read.
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-tests--unescaped-char-literals): Expand test to check that
we don't keep warning about old unescaped literals.

Author: npostavs

lisp/emacs-lisp/bytecomp.el

@@ -2093,8 +2093,9 @@ With argument ARG, insert value in current buffer after the form."
 		 (not (eobp)))
 	  (setq byte-compile-read-position (point)
 		byte-compile-last-position byte-compile-read-position)
-	  (let ((form (read inbuffer))
-                (warning (byte-run--unescaped-character-literals-warning)))
+          (let* ((lread--unescaped-character-literals nil)
+                 (form (read inbuffer))
+                 (warning (byte-run--unescaped-character-literals-warning)))
             (when warning (byte-compile-warn "%s" warning))
 	    (byte-compile-toplevel-file-form form)))
 	;; Compile pending forms at end of file.

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

@@ -559,19 +559,25 @@ bytecompiled code, and their results compared.")
   "Check that byte compiling warns about unescaped character
 literals (Bug#20852)."
   (should (boundp 'lread--unescaped-character-literals))
-  (bytecomp-tests--with-temp-file source
-    (write-region "(list ?) ?( ?; ?\" ?[ ?])" nil source)
-    (bytecomp-tests--with-temp-file destination
-      (let* ((byte-compile-dest-file-function (lambda (_) destination))
-            (byte-compile-error-on-warn t)
-            (byte-compile-debug t)
-            (err (should-error (byte-compile-file source))))
-        (should (equal (cdr err)
-                       (list (concat "unescaped character literals "
-                                     "`?\"', `?(', `?)', `?;', `?[', `?]' "
-                                     "detected, "
-                                     "`?\\\"', `?\\(', `?\\)', `?\\;', `?\\[', "
-                                     "`?\\]' expected!"))))))))
+  (let ((byte-compile-error-on-warn t)
+        (byte-compile-debug t))
+    (bytecomp-tests--with-temp-file source
+      (write-region "(list ?) ?( ?; ?\" ?[ ?])" nil source)
+      (bytecomp-tests--with-temp-file destination
+        (let* ((byte-compile-dest-file-function (lambda (_) destination))
+               (err (should-error (byte-compile-file source))))
+          (should (equal (cdr err)
+                         `(,(concat "unescaped character literals "
+                                    "`?\"', `?(', `?)', `?;', `?[', `?]' "
+                                    "detected, "
+                                    "`?\\\"', `?\\(', `?\\)', `?\\;', `?\\[', "
+                                    "`?\\]' expected!")))))))
+    ;; But don't warn in subsequent compilations (Bug#36068).
+    (bytecomp-tests--with-temp-file source
+      (write-region "(list 1 2 3)" nil source)
+      (bytecomp-tests--with-temp-file destination
+        (let ((byte-compile-dest-file-function (lambda (_) destination)))
+          (should (byte-compile-file source)))))))
 
 (ert-deftest bytecomp-tests--old-style-backquotes ()
   "Check that byte compiling warns about old-style backquotes."