Make `old-style-backquotes' variable internal

* src/lread.c (load_warn_old_style_backquotes, Fload, read1)
(syms_of_lread): Rename `old-style-backquotes' to
`lread--old-style-backquotes', and clarify that it's for internal
use only.
* lisp/emacs-lisp/bytecomp.el (byte-compile-from-buffer): Rename
variable.
* test/src/lread-tests.el (lread-tests--old-style-backquotes): Add
unit test.
* emacs-lisp/bytecomp-tests.el
(bytecomp-tests--old-style-backquotes): Add unit test.

2 files changed, 24 insertions, 0 deletions

diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index 84004a9264a..d15bd8b6e65 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -530,6 +530,21 @@ literals (Bug#20852)."
                                      "`?\"', `?(', `?)', `?;', `?[', `?]' "
                                      "detected!"))))))))
 
+(ert-deftest bytecomp-tests--old-style-backquotes ()
+  "Check that byte compiling warns about old-style backquotes."
+  (should (boundp 'lread--old-style-backquotes))
+  (bytecomp-tests--with-temp-file source
+    (write-region "(` (a b))" 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 "!! The file uses old-style backquotes !!
+This functionality has been obsolete for more than 10 years already
+and will be removed soon.  See (elisp)Backquote in the manual.")))))))
+
 ;; Local Variables:
 ;; no-byte-compile: t
 ;; End:
diff --git a/test/src/lread-tests.el b/test/src/lread-tests.el
index 685ea682e29..98cbb6a301d 100644
--- a/test/src/lread-tests.el
+++ b/test/src/lread-tests.el
@@ -155,4 +155,13 @@ literals (Bug#20852)."
     (load "somelib" nil t)
     (should (string-suffix-p "/somelib.el" (caar load-history)))))
 
+(ert-deftest lread-tests--old-style-backquotes ()
+  "Check that loading warns about old-style backquotes."
+  (lread-tests--with-temp-file file-name
+    (write-region "(` (a b))" nil file-name)
+    (should (equal (load file-name nil :nomessage :nosuffix) t))
+    (should (equal (lread-tests--last-message)
+                   (concat (format-message "Loading `%s': " file-name)
+                           "old-style backquotes detected!")))))
+
 ;;; lread-tests.el ends here