bug#15326: 24.3; Incorrect "variable not left unused" in destructuring cl-loop

bug#15326: 24.3; Incorrect "variable not left unused" in destructuring cl-loop

[Christopher Wellons]

The byte compiler emits an invalid warning about used "_" lexical
variables in cl-loop destructuring bindings. This was casually mentioned
in bug#8711, where destructuring-bind was fixed (presumably, since it's
fine now) but not loop.

Create a source file like so,

    ;;; -*- lexical-binding: t; -*-
    (require 'cl-lib)
    (cl-loop for (_ value) in '((a 1) (b 2))
             collect value)

Then compile,

    $ emacs -Q -batch -f batch-byte-compile tmp.el
    In toplevel form:
    tmp.el:3:1:Warning: variable `_' not left unused
    Wrote /tmp/tmp.elc

In the macro expansion, the _ is declared in a `let', then later set
with `setq', "using" it.


In GNU Emacs 24.3.1 (x86_64-pc-linux-gnu, GTK+ Version 3.8.2)
 of 2013-06-25 on sochi, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11204000
System Description:	Debian GNU/Linux unstable (sid)

Configured using:
 `configure '--build' 'x86_64-linux-gnu' '--build' 'x86_64-linux-gnu'
 '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib'
 '--localstatedir=/var/lib' '--infodir=/usr/share/info'
 '--mandir=/usr/share/man' '--with-pop=yes'
 '--enable-locallisppath=/etc/emacs24:/etc/emacs:/usr/local/share/emacs/24.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.3/site-lisp:/usr/share/emacs/site-lisp'
 '--with-crt-dir=/usr/lib/x86_64-linux-gnu' '--with-x=yes'
 '--with-x-toolkit=gtk3' '--with-toolkit-scroll-bars'
 'build_alias=x86_64-linux-gnu' 'CFLAGS=-g -O2 -fstack-protector
 --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall'
 'LDFLAGS=-Wl,-z,relro' 'CPPFLAGS=-D_FORTIFY_SOURCE=2''

bug#15326: 24.3; Incorrect "variable not left unused" in destructuring cl-loop

[Stefan Monnier]

> In the macro expansion, the _ is declared in a `let', then later set
> with `setq', "using" it.

A `setq' does not *use* a variable.  It just sets it, affecting
later uses.  If there's no later uses, the setq is just a no-op.

Fixing this will require someone to dig into the `cl-loop' macro.


        Stefan "who suspects it's not really fixed in
                destructuring-bind either"

bug#15326: 24.3; Incorrect "variable not left unused" in destructuring cl-loop

[Christopher Wellons]

Stefan Monnier <monnier@IRO.UMontreal.CA> writes:
> A `setq' does not *use* a variable.  It just sets it, affecting
> later uses.

I mean that the compiler counts the setq as a use even in this simple
expression:

    (let (_)
      (setq _ t))
    ;; Warning: variable `_' not left unused

> If there's no later uses, the setq is just a no-op.

Without bothering to dig into the bytecode for it, it seems that the
compiler is still emitting code for the setq:

    (defun foo ()
      (let (_)
        (setq _ t)))

    (defun bar ()
      (let (_)))

    (equal (byte-compile 'foo)
           (byte-compile 'bar))
    ;; => nil

Maybe I'm not understanding what you mean by "no-op" here.

bug#15326: 24.3; Incorrect "variable not left unused" in destructuring cl-loop

[Stefan Monnier]

I have just installed a patch into trunk which changes the code
generated by `cl-loop' such that this warning should not appear
any more.


        Stefan