Backward compatibility with pre-existing struct instances.scratch/record

* lisp/emacs-lisp/cl-lib.el (cl--old-struct-type-of): New function.
(cl-old-struct-compat-mode): New minor mode.

* lisp/emacs-lisp/cl-macs.el (cl-defstruct): Pass `record' to
cl-struct-define to signal use of record objects.

* lisp/emacs-lisp/cl-preloaded.el (cl--struct-get-class,
cl-struct-define): Enable legacy defstruct compatibility.

* test/lisp/emacs-lisp/cl-lib-tests.el (cl-lib-old-struct,
old-struct): New tests.

* doc/lispref/elisp.texi, doc/lispref/records.texi: Document
`old-struct-compat'.

1 files changed, 23 insertions, 0 deletions

diff --git a/test/lisp/emacs-lisp/cl-lib-tests.el b/test/lisp/emacs-lisp/cl-lib-tests.el
index 26b19e93e42..98c4bd92de6 100644
--- a/test/lisp/emacs-lisp/cl-lib-tests.el
+++ b/test/lisp/emacs-lisp/cl-lib-tests.el
@@ -500,4 +500,27 @@
     (should (eq (type-of x) 'foo))
     (should (eql (foo-x x) 42))))
 
+(ert-deftest old-struct ()
+  (cl-defstruct foo x)
+  (let ((x [cl-struct-foo])
+        (saved cl-old-struct-compat-mode))
+    (cl-old-struct-compat-mode -1)
+    (should (eq (type-of x) 'vector))
+
+    (cl-old-struct-compat-mode 1)
+    (setq cl-struct-foo (cl--struct-get-class 'foo))
+    (setf (symbol-function 'cl-struct-foo) :quick-object-witness-check)
+    (should (eq (type-of x) 'foo))
+    (should (eq (type-of [foo]) 'vector))
+
+    (cl-old-struct-compat-mode (if saved 1 -1))))
+
+(ert-deftest cl-lib-old-struct ()
+  (let ((saved cl-old-struct-compat-mode))
+    (cl-old-struct-compat-mode -1)
+    (cl-struct-define 'foo "" 'cl-structure-object nil nil nil
+                      'cl-struct-foo-tags 'cl-struct-foo t)
+    (should cl-old-struct-compat-mode)
+    (cl-old-struct-compat-mode (if saved 1 -1))))
+
 ;;; cl-lib.el ends here