bug#50777: Dropping EIEIO from xref (for performance)

[Dmitry Gutov <dgutov@yandex.ru>]

On 24.09.2021 18:32, Dmitry Gutov wrote:
> One area of bigger concern is whether code compiled against the new 
> version of Xref would work with the old one without recompiling (and 
> vice versa). I guess for that to work we need to disable inlining on 
> xref-item's accessors, at least. Maybe there's something else I'm missing.

OK, so I've done some testing with ivy-xref (fixing it to stop using 
with-slots), and (:noinline t) indeed seems necessary for Emacs using 
previous version of xref to run code from ivy-xref.elc compiled with the 
newer version without errors.

But since I tested this in Emacs 27,

   (void-function make-closure)

was a much bigger problem. This is apparently how lambdas are compiled now.

So this is a +1 minor reason to release the new version together with 
Emacs 28: less need to worry about :noinline. Though we could use it 
anyway, to avoid tying byte code to a particular implementation: the 
performance seems unchanged.