Not protecting Lisp objects from GC

[Gerd Möllmann]

I've just noticed the latest commit in the igc branch, which adds to the
comment of DEFVAR_LISP_NOPRO, and together with what I've seen in other
discussions, I feel I have to say something.

I'm talking about the old GC, and it is _bad_ to not protect GC'able
objects because

- they are protected from GC by being referenced from other objects that
  we '"know" are protected from GC

- because a GC "cannot happen"

And there are _no_ exceptions.

I'm speaking as a software engineer with decades of experience in
commercial software projects, and also from my experience as Emacs'
maintainer. Back then, even local variables had to be protected from GC
with a macro named GCPRO. And that was used in many places making
exactly the above assumptions.

I called that "aggressive use of GCPRO".

Aggressive because it's unmaintainable. I cannot possibly describe what
a debugging nightmare it was when the assumptions were no longer true
and some GCPRO was now missing because of innocent changes kilometers
away in a call tree.

And now I see the same assumptions being made in other circumstances,
and it seems nothing has ever been learned.

Anyone, please stop doing this. It's *BAD*.