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'.

2 files changed, 17 insertions, 1 deletions

diff --git a/doc/lispref/elisp.texi b/doc/lispref/elisp.texi
index 0f7efb6f187..3a348aae98e 100644
--- a/doc/lispref/elisp.texi
+++ b/doc/lispref/elisp.texi
@@ -423,6 +423,7 @@ Sequences, Arrays, and Vectors
 Records
 
 * Record Functions::        Functions for records.
+* Backward Compatibility::  Compatibility for cl-defstruct.
 
 Hash Tables
 
diff --git a/doc/lispref/records.texi b/doc/lispref/records.texi
index 822fd2bf36e..9a5d900cfc9 100644
--- a/doc/lispref/records.texi
+++ b/doc/lispref/records.texi
@@ -26,7 +26,8 @@ evaluating it is the same record.  This does not evaluate or even
 examine the slots.  @xref{Self-Evaluating Forms}.
 
 @menu
-* Record Functions::      Functions for records.
+* Record Functions::        Functions for records.
+* Backward Compatibility::  Compatibility for cl-defstruct.
 @end menu
 
 @node Record Functions
@@ -98,3 +99,17 @@ the copied record, are also visible in the original record.
 @end group
 @end example
 @end defun
+
+@node Backward Compatibility
+@section Backward Compatibility
+
+  Code compiled with older versions of @code{cl-defstruct} that
+doesn't use records may run into problems when used in a new Emacs.
+To alleviate this, Emacs detects when an old @code{cl-defstruct} is
+used, and enables a mode in which @code{type-of} handles old struct
+objects as if they were records.
+
+@defun cl-old-struct-compat-mode arg
+If @var{arg} is positive, enable backward compatibility with old-style
+structs.
+@end defun