bug#33618: 27.0.50; ada-mode breaks M-x grep

[Eli Zaretskii]

> Date: Wed, 05 Dec 2018 16:37:28 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: stephen_leake@member.fsf.org, 33618@debbugs.gnu.org
> 
> > The patch I use changes compile.el so the var is looked up from the
> > buffer from which the compilation is launched (e.g. an ada-mode buffer)
> > and then stashed into the compilation buffer (for later use).
> 
> What will that do if I invoke, e.g., "M-x recompile" from a source
> buffer other than the one from which I invoked the previous "M-x compile"?
> And what if we have multiple compilation buffers?

And, btw, isn't it wrong for a mode to set the value of a defcustom?

Maybe we should have a separate variable for this purpose, one that
isn't a defcustom.  A buffer-local value of a defcustom is going to
surprise users, I think.