Remove support for destructive splicing in elisp

* src/lread.c (read1): Don't handle destructive splicing in
backquote expressions (e.g. ",.<identifier>").  (Bug#19790)
(syms_of_lread): Remove Qcomma_dot.
* src/print.c (print_object): Don't check for Qcomma_dot.
* test/src/eval-tests.el
(eval-tests-19790-backquote-comma-dot-substitution): New test.
* etc/NEWS: Announce it.

diff --git a/etc/NEWS b/etc/NEWS
index 09394432aa84ef4ce2fb982d188b7765bc85e31a..0a4ada3cc6cd68ee3aed5ed46192a9b3e8b04a6d 100644 (file)
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2005,6 +2005,19 @@ immediately.  Type 'M-x so-long-commentary' for full documentation.
 \f
 * Incompatible Lisp Changes in Emacs 27.1
 
+---
+** Incomplete destructive splicing support has been removed.
+Support for Common Lisp style destructive splicing (",.") was
+incomplete and broken for a long time.  It has now been removed.
+
+This means that backquote substitution now works for identifiers
+starting with a period (".").  Consider the following example:
+
+    (let ((.foo 42)) `,.foo)
+
+In the past, this would have incorrectly evaluated to '(\,\. foo)',
+but will now instead evaluate to '42'.
+
 ---
 ** The REGEXP in 'magic-mode-alist' is now matched case-sensitively.
 Likewise for 'magic-fallback-mode-alist'.

diff --git a/src/lread.c b/src/lread.c
index 151731a81d9d4aea676d05333acb2d7f3dc49651..5000b38a0154655d069644c66c37c7100bc6ce9e 100644 (file)
--- a/src/lread.c
+++ b/src/lread.c
@@ -3310,8 +3310,6 @@ read1 (Lisp_Object readcharfun, int *pch, bool first_in_list)
 
            if (ch == '@')
              comma_type = Qcomma_at;
-           else if (ch == '.')
-             comma_type = Qcomma_dot;
            else
              {
                if (ch >= 0) UNREAD (ch);
@@ -5080,7 +5078,6 @@ this variable will become obsolete.  */);
   DEFSYM (Qbackquote, "`");
   DEFSYM (Qcomma, ",");
   DEFSYM (Qcomma_at, ",@");
-  DEFSYM (Qcomma_dot, ",.");
 
   DEFSYM (Qinhibit_file_name_operation, "inhibit-file-name-operation");
   DEFSYM (Qascii_character, "ascii-character");

diff --git a/src/print.c b/src/print.c
index 7e5aed828772a6c6479122ac9e4ece75c673ff9d..77ddd93efbad99dfebb56b9fb0a29c0a53badfd1 100644 (file)
--- a/src/print.c
+++ b/src/print.c
@@ -2076,8 +2076,7 @@ print_object (Lisp_Object obj, Lisp_Object printcharfun, bool escapeflag)
       else if (print_quoted && CONSP (XCDR (obj)) && NILP (XCDR (XCDR (obj)))
               && new_backquote_output
               && (EQ (XCAR (obj), Qcomma)
-                  || EQ (XCAR (obj), Qcomma_at)
-                  || EQ (XCAR (obj), Qcomma_dot)))
+                  || EQ (XCAR (obj), Qcomma_at)))
        {
          print_object (XCAR (obj), printcharfun, false);
          new_backquote_output--;

diff --git a/test/src/eval-tests.el b/test/src/eval-tests.el
index 48295b81fa3c7f744cd0c7ae36088e1f5b3804d6..7a8eae82cf98bcead8724af8b6f2f335c0c52930 100644 (file)
--- a/test/src/eval-tests.el
+++ b/test/src/eval-tests.el
@@ -169,4 +169,11 @@ are found on the stack and therefore not garbage collected."
   "Remove the Lisp reference to the byte-compiled object."
   (setf (symbol-function #'eval-tests-33014-func) nil))
 
+(defun eval-tests-19790-backquote-comma-dot-substitution ()
+  "Regression test for Bug#19790.
+Don't handle destructive splicing in backquote expressions (like
+in Common Lisp).  Instead, make sure substitution in backquote
+expressions works for identifiers starting with period."
+  (should (equal (let ((.x 'identity)) (eval `(,.x 'ok))) 'ok)))
+
 ;;; eval-tests.el ends here