bug#63282: 30.0.50; Phase out `cl-old-struct-compat-mode`

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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Package: Emacs
Version: 30.0.50


`cl-old-struct-compat-mode` is a hideous hack I introduced back
in Emacs-26 when we switched CL structs from using plain vectors to
using the new `record` type.  It provides some limited amount
of backward compatibility with code using `cl-defstruct` that was
compiled with 23<Emacs<26, at the cost of installing an advice on
`type-of` which tends to impact the performance of most
method dispatches in a non-trivial way.

I honestly don't know how much it's still used, so I propose the patch
below which declares that compatibility functionality as being obsolete,
and additionally emits a warning message *at run time* when it's used.

We usually keep our obsolescence message to compile time, but since this
is meant to detect the use of old `.elc` files, the relevant
compile-time is long past :-(

I also count on the fact that we still have a fair bit of time until
Emacs-30 is released, to discover that the run-time warning is too
annoying.

Any objection (beside the need to add a etc/NEWS message, obviously)?


        Stefan



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diff --git a/lisp/emacs-lisp/cl-lib.el b/lisp/emacs-lisp/cl-lib.el
index 7fee780a735..28d42cb69ab 100644
--- a/lisp/emacs-lisp/cl-lib.el
+++ b/lisp/emacs-lisp/cl-lib.el
@@ -560,6 +560,7 @@ cl-old-struct-compat-mode
     (advice-add 'type-of :around #'cl--old-struct-type-of))
    (t
     (advice-remove 'type-of #'cl--old-struct-type-of))))
+(make-obsolete 'cl-old-struct-compat-mode nil "30.1")
 
 (defun cl-constantly (value)
   "Return a function that takes any number of arguments, but returns VALUE."
diff --git a/lisp/emacs-lisp/cl-preloaded.el b/lisp/emacs-lisp/cl-preloaded.el
index 9445093f143..da2780503b8 100644
--- a/lisp/emacs-lisp/cl-preloaded.el
+++ b/lisp/emacs-lisp/cl-preloaded.el
@@ -158,7 +158,9 @@ cl-struct-define
   (cl-check-type name (satisfies cl--struct-name-p))
   (unless type
     ;; Legacy defstruct, using tagged vectors.  Enable backward compatibility.
-    (cl-old-struct-compat-mode 1))
+    (with-suppressed-warnings ((obsolete cl-old-struct-compat-mode))
+      (message "cl-old-struct-compat-mode is obsolete!")
+      (cl-old-struct-compat-mode 1)))
   (if (eq type 'record)
       ;; Defstruct using record objects.
       (setq type nil))