bug#65464: Emacs 29.1 - VHDL mode missing updates…

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Eli Zaretskii <eliz@gnu.org>,  Reto Zimmermann <reto@gnu.org>,
>   65464@debbugs.gnu.org,  Stefan Kangas <stefankangas@gmail.com>
> Date: Mon, 11 Sep 2023 18:17:18 -0400
> 
> > In other words, if someone is unable to run vhdl-mode.elc from Emacs 29 on
> > Emacs 28, then we're very sorry but do not consider it a defect that needs
> > to be remedied.
> 
> Actually, I do think we should try and detect the problem (and ideally
> emit a message/error and maybe even fallback to loading the
> non-compiled file).
> 
> We do have the necessary info in the `.elc` file (the byte 4th byte
> (0-based numbering) of a `.elc` file holds the major version number of
> the Emacs on which it was compiled (with some caveat for non-released
> Emacs versions, such as 30.0.NN emitting `.elc` files that say "version
> 29")).
> 
> We just need to make use of that info.

Make use how?  In most cases the byte-code is compatible both backward
and forward, so displaying a warning would be an annoyance that 99% of
time is unjustified.