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