[PATCH] Raise an error when detecting old-style backquotes.

[Philipp Stephani]

They have been deprecated for a decade now.

* src/lread.c (Fload): Don’t use record_unwind_protect to warn about
old-style backquotes any more.  They now generate a hard error.
(read1): Signal an error when detecting old-style backquotes.  Remove
unused label.
(syms_of_lread): Remove unused internal variable
‘lread--old-style-backquotes’.
(load_error_old_style_backquotes): Rename from
‘load_warn_oldstyle_backquotes’.  Signal an error.

* test/src/lread-tests.el (lread-tests--old-style-backquotes): Adapt
unit test.
---
 src/lread.c             | 32 ++++++--------------------------
 test/src/lread-tests.el | 10 +++++-----
 2 files changed, 11 insertions(+), 31 deletions(-)

diff --git a/src/lread.c b/src/lread.c
index 6bc93b1481..44953baa77 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -1003,14 +1003,11 @@ load_error_handler (Lisp_Object data)
   return Qnil;
 }
 
-static void
-load_warn_old_style_backquotes (Lisp_Object file)
+static _Noreturn void
+load_error_old_style_backquotes (void)
 {
-  if (!NILP (Vlread_old_style_backquotes))
-    {
-      AUTO_STRING (format, "Loading `%s': old-style backquotes detected!");
-      CALLN (Fmessage, format, file);
-    }
+  AUTO_STRING (format, "Loading `%s': old-style backquotes detected!");
+  xsignal1 (Qerror, CALLN (Fformat_message, format, Vload_file_name));
 }
 
 static void
@@ -1282,10 +1279,6 @@ 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);
@@ -3178,10 +3171,7 @@ 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 == ' ')
-         {
-           Vlread_old_style_backquotes = Qt;
-           goto default_label;
-         }
+          load_error_old_style_backquotes ();
        else
          {
            Lisp_Object value;
@@ -3232,10 +3222,7 @@ read1 (Lisp_Object readcharfun, int *pch, bool 
first_in_list)
            return list2 (comma_type, value);
          }
        else
-         {
-           Vlread_old_style_backquotes = Qt;
-           goto default_label;
-         }
+          load_error_old_style_backquotes ();
       }
     case '?':
       {
@@ -3423,7 +3410,6 @@ 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;
@@ -4996,12 +4982,6 @@ 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/src/lread-tests.el b/test/src/lread-tests.el
index ac730b4f00..241cebb6b1 100644
--- a/test/src/lread-tests.el
+++ b/test/src/lread-tests.el
@@ -173,13 +173,13 @@ lread-tests--last-message
     (should (string-suffix-p "/somelib.el" (caar load-history)))))
 
 (ert-deftest lread-tests--old-style-backquotes ()
-  "Check that loading warns about old-style backquotes."
+  "Check that loading doesn’t accept 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!")))))
+    (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!")))))))
 
 (ert-deftest lread-lread--substitute-object-in-subtree ()
   (let ((x (cons 0 1)))
-- 
2.14.2