GNU bug report logs - #50720
unnamed &rest broken

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Mattias Engdegård <mattiase <at> acm.org>
To: bug-gnu-emacs <at> gnu.org
Subject: unnamed &rest broken
Date: Tue, 21 Sep 2021 13:15:54 +0200

The interpreter and compiler allow &rest to be used without a variable name following but the generated byte code is completely broken:

(funcall (byte-compile (lambda (&rest) 'ta)))

crashes, and

(defun boo (a &rest)
  (if a a (list 1 2 3 4)))

(boo 'hiss)
=> hiss        ; interpreted
=> (1 2 3 4)   ; compiled

The reason is that the compiler generates code from the argument variable list but the byte-code interpreter will only look at the signature code which was generated from the actual signature:

(byte-compile (lambda (&rest) 'ta))
=> #[128 "\300\207" [ta] 1 "..."]

The 128 indicates zero positional parameters and a &rest argument, and the 1 is the maximum stack size required which is wrong; 2 stack slots are needed and that's what we get if naming the argument:

(byte-compile (lambda (&rest _r) 'ta))
=> #[128 "\300\207" [ta] 2 "..."]

In the `boo` case above, it is clear that the compiler doesn't expect any &rest param to have been pushed at all so the stack offsets are wrong.

Now, either we fix this bug or we stop pretending that unnamed &rest arguments work at all and signal an error, because it's clear from the above that they can't have seen much use.

Message #10 received at 50720 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Mattias Engdegård <mattiase <at> acm.org>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 50720 <at> debbugs.gnu.org, 50268 <at> debbugs.gnu.org
Subject: Re: bug#50268: 28.0.50; Assertion warning during native compilation
Date: Tue, 21 Sep 2021 18:11:34 +0200

Mattias Engdegård <mattiase <at> acm.org> writes:

> Now, either we fix this bug or we stop pretending that unnamed &rest
> arguments work at all and signal an error, because it's clear from the
> above that they can't have seen much use.

I did a quick grep through core and GNU ELPA, and I couldn't find any
instances of it being used, so I'd be fine with either solution (i.e.,
either fixing the bug or signalling an error).

Perhaps Eli has an opinion (added to the CCs).

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

Message #13 received at 50720 <at> debbugs.gnu.org (full text, mbox):

From: Mattias Engdegård <mattiase <at> acm.org>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 50720 <at> debbugs.gnu.org,
 Noam Postavsky <npostavs <at> gmail.com>, 50268 <at> debbugs.gnu.org
Subject: Re: bug#50268: 28.0.50; Assertion warning during native compilation
Date: Tue, 21 Sep 2021 18:22:11 +0200

21 sep. 2021 kl. 18.11 skrev Lars Ingebrigtsen <larsi <at> gnus.org>:

> I did a quick grep through core and GNU ELPA, and I couldn't find any
> instances of it being used, so I'd be fine with either solution (i.e.,
> either fixing the bug or signalling an error).

I know how to fix it but there's no really elegant way of doing it, and unless a convincing case can be made for permitting anonymous &rest, I'd favour disallowing it.

Unless I'm mistaken it was Noam who added it (1d47d777ef24c0be9153b0a1c8ba21918fa1025a), so let's ask him.

Message #16 received at 50720 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: mattiase <at> acm.org, 50720 <at> debbugs.gnu.org, 50268 <at> debbugs.gnu.org
Subject: Re: bug#50268: 28.0.50; Assertion warning during native compilation
Date: Tue, 21 Sep 2021 19:26:19 +0300

> From: Lars Ingebrigtsen <larsi <at> gnus.org>
> Cc: 50720 <at> debbugs.gnu.org,  50268 <at> debbugs.gnu.org, Eli Zaretskii <eliz <at> gnu.org>
> Date: Tue, 21 Sep 2021 18:11:34 +0200
> 
> Mattias Engdegård <mattiase <at> acm.org> writes:
> 
> > Now, either we fix this bug or we stop pretending that unnamed &rest
> > arguments work at all and signal an error, because it's clear from the
> > above that they can't have seen much use.
> 
> I did a quick grep through core and GNU ELPA, and I couldn't find any
> instances of it being used, so I'd be fine with either solution (i.e.,
> either fixing the bug or signalling an error).
> 
> Perhaps Eli has an opinion (added to the CCs).

I think I'd like to at least see the prototype of the fix to have an
opinion.

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