bug#26960: 26.0.50; Complaints about unused variable in cl-destructuring-bind

bug#26960: 26.0.50; Complaints about unused variable in cl-destructuring-bind

[Philipp Stephani]

Create a file with the following contents (here /tmp/destr.el):

;;; -*- lexical-binding: t; -*-
(require 'cl-lib)
(cl-destructuring-bind (&whole a b &rest _) '(1 2)
  (print (list a b)))

Then run

emacs -Q -batch -f batch-byte-compile /tmp/destr.el

The byte compiler will complain:

In toplevel form:
/tmp/destr.el:3:1:Warning: variable ‘_’ not left unused

even though the variable is clearly unused.


In GNU Emacs 26.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.10.8)
 of 2017-05-17 built on localhost
Repository revision: f7c07930b581b1bcfdfb1874b6883233516bdf11
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description:	Ubuntu 14.04 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
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Memory information:
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bug#26960: 26.0.50; Complaints about unused variable in cl-destructuring-bind

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

> I can reproduce this in Emacs 25.1, but not in Emacs 29.  That is,
> there's no warning for `foo'.  There's still a warning for `bar', but
> that's correct, isn't it?

Yes.

> `ignore' is just a normal function, so _ is used there.

It's not 100% normal, but yes.

> However, this example from a merged bug report is giving a warning:
>
>> ;;; -*- lexical-binding: t; -*-
>> (require 'cl-lib)
>> (cl-destructuring-bind (&whole a b &rest _) '(1 2)
>>   (print (list a b)))
>
> And that seems like a bug?

Indeed.  If you look at the macroexpanded code, you see that
`cl-destructuring-bind` uses:

    (let* ((_ '(1 2))
           (a _)
           (b (if _ (pop _)
                (signal 'wrong-number-of-arguments
                        (list '(&whole a b &rest _) (length _))))))
      (print (list a b)))

so, you can see that the problem is that `cl-destructuring-bind` tries
to avoid using gensym and "abuses" the &rest var as the "iterator"
variable while parsing the list.

It might have been useful back in the dynbinding days because `setq`
might have been a bit cheaper than `let`, but with lexical scoping
`let` byte-compiles to virtually nothing.


        Stefan

bug#26960: 26.0.50; Complaints about unused variable in cl-destructuring-bind

[Stefan Monnier via Bug reports for GNU Emacs, the Swiss army knife of text editors]

> I had a quick peek at `cl--do-arglist' (which I guess is what's creating
> this form), and...  I think I'll leave this for somebody else to fix.  🫠

I did the same some years ago, FWIW :-)


        Stefan