GNU bug report logs - #12369
24.2.50; cl-loop: variable not left unused

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

From: Christopher Schmidt <christopher <at> ch.ristopher.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 24.2.50; cl-loop: variable not left unused
Date: Thu,  6 Sep 2012 14:06:05 +0100 (BST)

severity: minor

    ;; -*- lexical-binding: t -*-

    (require 'cl-lib)
    (cl-loop for (rms . emacs) in nil)

Byte compile this piece of code.

    Warning: Unused lexical variable `rms'

Attempting to fix this warning by renaming rms to _rms results in
another warning.

    Warning: variable `_rms' not left unused

GNU Emacs 24.2.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.10) of
2012-09-06.

        Christopher

Message #8 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: bug-gnu-emacs <at> gnu.org
Subject: Re: bug#12369: 24.2.50; cl-loop: variable not left unused
Date: Thu, 06 Sep 2012 11:43:04 -0400

>     ;; -*- lexical-binding: t -*-
>     (require 'cl-lib)
>     (cl-loop for (rms . emacs) in nil)
> Byte compile this piece of code.
>     Warning: Unused lexical variable `rms'

Yes, first bug: `emacs' is not listed as unused.

> Attempting to fix this warning by renaming rms to _rms results in
> another warning.
>     Warning: variable `_rms' not left unused

Yup, second bug.

This is all due to the code generated by cl-loop which was optimized to
use `set' in the loop instead of `let'.  This made sense for dynamically
scoped code where `let' is a bit slower, but for lexically-scoped code,
it's the exact opposite, so we should change the generated code from:

   (identity
    (catch '--cl-block-nil--
      (let* ((--cl-var-- nil)
             (emacs nil) (rms nil))
        (while (consp --cl-var--)
          (setq emacs (car --cl-var--) rms (pop emacs))
          (setq --cl-var-- (cdr --cl-var--)))
        nil)))
to
   (identity
    (catch '--cl-block-nil--
      (let* ((--cl-var-- nil))
        (while (consp --cl-var--)
          (let* ((tmp (car --cl-var--))
                 (emacs (car tmp))
                 (rms (cdr tmp)))
            (setq --cl-var-- (cdr --cl-var--))))
        nil)))


-- Stefan

Message #11 received at 12369 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 12369 <at> debbugs.gnu.org
Subject: Re: bug#12369: 24.2.50; cl-loop: variable not left unused
Date: Tue, 01 Jun 2021 09:18:29 +0200

Stefan Monnier <monnier <at> iro.umontreal.ca> writes:

>>     ;; -*- lexical-binding: t -*-
>>     (require 'cl-lib)
>>     (cl-loop for (rms . emacs) in nil)
>> Byte compile this piece of code.
>>     Warning: Unused lexical variable `rms'
>
> Yes, first bug: `emacs' is not listed as unused.

This problem is still present in Emacs 28.

>> Attempting to fix this warning by renaming rms to _rms results in
>> another warning.
>>     Warning: variable `_rms' not left unused
>
> Yup, second bug.

This one is fixed, though.

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

Message #16 received at 12369 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Lars Ingebrigtsen <larsi <at> gnus.org>
Cc: 12369 <at> debbugs.gnu.org
Subject: Re: bug#12369: 24.2.50; cl-loop: variable not left unused
Date: Sat, 05 Jun 2021 16:08:40 -0400

>>>     ;; -*- lexical-binding: t -*-
>>>     (require 'cl-lib)
>>>     (cl-loop for (rms . emacs) in nil)
>>> Byte compile this piece of code.
>>>     Warning: Unused lexical variable `rms'
>> Yes, first bug: `emacs' is not listed as unused.
> This problem is still present in Emacs 28.

Indeed, because it *is* used internally by the code expanded from the
destructuring-bind.

>>> Attempting to fix this warning by renaming rms to _rms results in
>>> another warning.
>>>     Warning: variable `_rms' not left unused
>> Yup, second bug.
> This one is fixed, though.

Hmm... I wonder what caused this change.


        Stefan

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