Revert "Raise an error when detecting old-style backquotes."

This reverts commit 9613690f6e51e2f2aa2bcbbede3e209d08cfaaad.

5 files changed, 46 insertions, 18 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 0f4c6ae40f3..2332ba4d1f8 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -87,9 +87,6 @@ them through 'format' first.  Even that is discouraged: for ElDoc
 support, you should set 'eldoc-documentation-function' instead of
 calling 'eldoc-message' directly.
 
-** Old-style backquotes now generate an error.  They have been
-generating warnings for a decade.
-
 
 * Lisp Changes in Emacs 27.1
 
diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index 45fa188d6c8..590db570c56 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -2048,8 +2048,14 @@ 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* ((lread--unescaped-character-literals nil)
+          (let* ((lread--old-style-backquotes nil)
+                 (lread--unescaped-character-literals nil)
                  (form (read inbuffer)))
+            ;; Warn about the use of old-style backquotes.
+            (when lread--old-style-backquotes
+              (byte-compile-warn "!! 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."))
             (when lread--unescaped-character-literals
               (byte-compile-warn
                "unescaped character literals %s detected!"
diff --git a/src/lread.c b/src/lread.c
index c073fc4ce6d..6bc93b14817 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -1003,11 +1003,14 @@ load_error_handler (Lisp_Object data)
   return Qnil;
 }
 
-static _Noreturn void
-load_error_old_style_backquotes (void)
+static void
+load_warn_old_style_backquotes (Lisp_Object file)
 {
-  AUTO_STRING (format, "Loading `%s': old-style backquotes detected!");
-  xsignal1 (Qerror, CALLN (Fformat_message, format, Vload_file_name));
+  if (!NILP (Vlread_old_style_backquotes))
+    {
+      AUTO_STRING (format, "Loading `%s': old-style backquotes detected!");
+      CALLN (Fmessage, format, file);
+    }
 }
 
 static void
@@ -1279,6 +1282,10 @@ Return t if the file exists and loads successfully.  */)
 
   version = -1;
 
+  /* Check for the presence of old-style quotes and warn about them.  */
+  specbind (Qlread_old_style_backquotes, Qnil);
+  record_unwind_protect (load_warn_old_style_backquotes, file);
+
   /* Check for the presence of unescaped character literals and warn
      about them. */
   specbind (Qlread_unescaped_character_literals, Qnil);
@@ -3171,7 +3178,10 @@ read1 (Lisp_Object readcharfun, int *pch, bool first_in_list)
            first_in_list exception (old-style can still be obtained via
            "(\`" anyway).  */
         if (!new_backquote_flag && first_in_list && next_char == ' ')
-          load_error_old_style_backquotes ();
+          {
+            Vlread_old_style_backquotes = Qt;
+            goto default_label;
+          }
         else
           {
             Lisp_Object value;
@@ -3222,7 +3232,10 @@ read1 (Lisp_Object readcharfun, int *pch, bool first_in_list)
             return list2 (comma_type, value);
           }
         else
-          load_error_old_style_backquotes ();
+          {
+            Vlread_old_style_backquotes = Qt;
+            goto default_label;
+          }
       }
     case '?':
       {
@@ -3410,6 +3423,7 @@ read1 (Lisp_Object readcharfun, int *pch, bool first_in_list)
          row.  */
       FALLTHROUGH;
     default:
+    default_label:
       if (c <= 040) goto retry;
       if (c == NO_BREAK_SPACE)
         goto retry;
@@ -4982,6 +4996,12 @@ variables, this must be set in the first line of a file.  */);
                doc: /* List of buffers being read from by calls to `eval-buffer' and `eval-region'.  */);
   Veval_buffer_list = Qnil;
 
+  DEFVAR_LISP ("lread--old-style-backquotes", Vlread_old_style_backquotes,
+               doc: /* Set to non-nil when `read' encounters an old-style backquote.
+For internal use only.  */);
+  Vlread_old_style_backquotes = Qnil;
+  DEFSYM (Qlread_old_style_backquotes, "lread--old-style-backquotes");
+
   DEFVAR_LISP ("lread--unescaped-character-literals",
                Vlread_unescaped_character_literals,
                doc: /* List of deprecated unescaped character literals encountered by `read'.
diff --git a/test/lisp/emacs-lisp/bytecomp-tests.el b/test/lisp/emacs-lisp/bytecomp-tests.el
index 1f85c269780..30d2a4753cf 100644
--- a/test/lisp/emacs-lisp/bytecomp-tests.el
+++ b/test/lisp/emacs-lisp/bytecomp-tests.el
@@ -534,18 +534,23 @@ literals (Bug#20852)."
 
 (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-debug t)
-             (err (should-error (byte-compile-file source))))
+            (byte-compile-error-on-warn t)
+            (byte-compile-debug t)
+            (err (should-error (byte-compile-file source))))
         (should (equal (cdr err)
-                       '("Loading `nil': old-style backquotes detected!")))))))
+                       (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.")))))))
 
 
 (ert-deftest bytecomp-tests-function-put ()
   "Check `function-put' operates during compilation."
+  (should (boundp 'lread--old-style-backquotes))
   (bytecomp-tests--with-temp-file source
     (dolist (form '((function-put 'bytecomp-tests--foo 'foo 1)
                     (function-put 'bytecomp-tests--foo 'bar 2)
diff --git a/test/src/lread-tests.el b/test/src/lread-tests.el
index 3f41982eba1..ac730b4f005 100644
--- a/test/src/lread-tests.el
+++ b/test/src/lread-tests.el
@@ -173,13 +173,13 @@ literals (Bug#20852)."
     (should (string-suffix-p "/somelib.el" (caar load-history)))))
 
 (ert-deftest lread-tests--old-style-backquotes ()
-  "Check that loading doesn't accept 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)
-    (let ((data (should-error (load file-name nil :nomessage :nosuffix))))
-      (should (equal (cdr data)
-                     (list (concat (format-message "Loading `%s': " file-name)
-                                   "old-style backquotes detected!")))))))
+    (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!")))))
 
 (ert-deftest lread-lread--substitute-object-in-subtree ()
   (let ((x (cons 0 1)))