bug#69832: 30.0.50; Should `subr-primitive-p` apply to special-forms?

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>> - If we introduce `subr-function-p`, then `subr-primitive-p` is only
>>   "useful" at one place any more, and we can trivially rewrite the code to
>>   avoid it, so we could get rid of it.
> I don't see why we should get rid of subr-primitive-p.

Because there's no point keeping it if all its users could just as well
use `subr-function-p` or `subr-native-elisp-p` instead.

The information provided by the current semantics of `subr-primitive-p`
is "the source code was written in C", and that kind of information is
extremely rarely useful because ELisp code can't really act on that
information.  The only counter example is indeed when that ELisp code is
trying to jump to the source code.

> We can leave it alone, used in that single place where it's useful,
> and let 3rd party packages use it if they want.

It'll mostly lead to 3rd party users either wondering which one they
should use, or picking one arbitrarily without knowing the consequences.
Choice is good when the various alternatives have each their own
strengths and weaknesses, but here it's just extra complexity with no benefit.

If we introduce `subr-function-p` then we should mark `sur-primitive-p`
as obsolete.  And the only thing we gained in the process is churn (it
won't avoid regressions because the rare few users will likely just
blindly replace the old one with the new one).

>> - These functions are used very rarely, the majority is in core files,
>>   and the rest is mostly used to generate human-facing descriptions
>>   so the risk of breakage is low and the kind of breakage is likely to
>>   have a low impact.
> Yes, but I've heard these famous last words one or two times too
> many...

We make backward incompatible changes all the time in Emacs, and the
vast majority of them turns out fine.

I searched for `subr-primitive-p` in Emacs, GNU ELPA, NonGNU ELPA, and
Melpa before making my suggestion.


        Stefan