bug#12079: 23.3; erroneous byte-compile-file warning

bug#12079: 23.3; erroneous byte-compile-file warning

[Winston]

GNU Emacs 23.3.1 (amd64-portbld-freebsd9.0, GTK+ Version 2.24.6) of 2011-12-05

It looks like byte-compile-file is issuing a bogus warning for
(set-buffer) inside (save-excursion).

1) The internal description of (save-excursion) says it saves and
   restores point, mark, and the current buffer (just as it's always
   done).  Here's the relevant part of the description (with -->).

    save-excursion is a special form in `C source code'.

    (save-excursion &rest BODY)

    Save point, mark, and current buffer; execute BODY; restore those things.
    Executes BODY just like `progn'.
--> The values of point, mark and the current buffer are restored
    even in case of abnormal exit (throw or error).
    The state of activation of the mark is also restored.

2) This trivial example, run from buffer *scratch*, agrees with that
   description, that the current buffer gets restored:

    (progn
      (save-excursion
        (set-buffer "foo")      ; I'd already created this
        (barf-if-buffer-read-only)
        )
      (current-buffer)
    )
    #<buffer *scratch*>

3) However, when I write that code to foo.el and run byte-compile-file
   on it, I get the following error:

    Compiling file /home/wbe/foo.el at Sat Jul 28 09:07:28 2012
    Entering directory `/home/wbe/'
--> foo.el:1:1:Warning: `save-excursion' defeated by `set-buffer'

It looks to me like the Warning message is untrue.
 -WBE

[The rest is from (report-emacs-bug).  I removed some clearly irrelevant
parts.]


In GNU Emacs 23.3.1 (amd64-portbld-freebsd9.0, GTK+ Version 2.24.6)
 of 2011-12-05 on jail-15
Windowing system distributor `The X.Org Foundation', version 11.0.10707000
configured using `configure  '--localstatedir=/var' '--with-x-toolkit=gtk' '--x-libraries=/usr/local/lib' '--x-includes=/usr/local/include' '--prefix=/usr/local' '--mandir=/usr/local/man' '--infodir=/usr/local/info/' '--build=amd64-portbld-freebsd9.0' 'build_alias=amd64-portbld-freebsd9.0' 'CC=cc' 'CFLAGS=-O2 -pipe -fno-strict-aliasing' 'LDFLAGS= -L/usr/local/lib -rpath=/usr/lib:/usr/local/lib' 'CPPFLAGS=-I/usr/local/include' 'CPP=cpp''

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bug#12079: 23.3; erroneous byte-compile-file warning

[Glenn Morris]

Winston wrote:

>     (progn
>       (save-excursion
>         (set-buffer "foo")      ; I'd already created this
>         (barf-if-buffer-read-only)
>         )
>       (current-buffer)
>     )
>     #<buffer *scratch*>
>
> 3) However, when I write that code to foo.el and run byte-compile-file
>    on it, I get the following error:
>
>     Compiling file /home/wbe/foo.el at Sat Jul 28 09:07:28 2012
>     Entering directory `/home/wbe/'
> --> foo.el:1:1:Warning: `save-excursion' defeated by `set-buffer'
>
> It looks to me like the Warning message is untrue.

In an example such as you show, you should use with-current-buffer
instead of save-excursion+set-buffer. I think this has been discussed
several times on this list and/or emacs-devel.

The compilation warning is because save-excursion is pointless in the
code as shown. Eg it doesn't preserve point in buffer foo (unless foo
happened to be current beforehand).

bug#12079: 23.3; erroneous byte-compile-file warning

[Winston]

I originally submitted (in part):
>>     (progn
>>       (save-excursion
>>         (set-buffer "foo")      ; I'd already created this
>>         (barf-if-buffer-read-only)
>>         )
>>       (current-buffer)
>>     )
>>     #<buffer *scratch*>
>>
>> 3) However, when I write that code to foo.el and run byte-compile-file
>>    on it, I get the following error:
>>
>>     Compiling file /home/wbe/foo.el at Sat Jul 28 09:07:28 2012
>>     Entering directory `/home/wbe/'
>> --> foo.el:1:1:Warning: `save-excursion' defeated by `set-buffer'
>>
>> It looks to me like the Warning message is untrue.

to which Glenn replied:
> In an example such as you show, you should use with-current-buffer
> instead of save-excursion+set-buffer.

OK.  Back when the code I was compiling was written, with-current-buffer
didn't exist.  After reading the description of with-current-buffer,
yes, it looks like it would be as good or better than save-excursion if
I feel like rewriting the code.

> I think this has been discussed several times on this list and/or
> emacs-devel.

I wouldn't know.  I'm not on that list.

> The compilation warning is because save-excursion is pointless in the
> code as shown.

I disagree.  When the body finishes, save-excursion changes back to the
buffer that was current when it was called.  Perhaps it's useless in a
trivial example like the one above, but "return to whatever the current
buffer is after making changes in some other buffer" can be useful in
subroutines.  Restoring the buffer is certainly not "pointless".

I think what you're trying to say is that using save-excursion instead
of with-current-buffer when the body begins with set-buffer is
"pointless" because save-excursion does nothing more than what
with-current-buffer would do.  That's a fair point, but that's NOT what
the warning message says.  The warning claims that save-excursion won't
work.

> Eg it doesn't preserve point in buffer foo (unless foo happened to be
> current beforehand).

Irrelevant.  You've agreed that save-excursion works exactly as the
documentation claims, restoring on exit the buffer, point, and mark as
of the time it was called, so the clear English meaning of the warning
is false -- no part of save-excursion's functionality was "defeated" by
calling set-buffer.

I recommend the current misleading warning be replaced with a suggestion
that using with-current-buffer would be more efficient, which is
apparently what was intended.
 -WBE

bug#12079: 23.3; erroneous byte-compile-file warning

[Alp Aker]

On Mon, Jul 30, 2012 at 12:42 AM, Winston <wbe@psr.com> wrote:
> I recommend the current misleading warning be replaced with a suggestion
> that using with-current-buffer would be more efficient, which is
> apparently what was intended.

The warning message was changed in just that way over a year ago.  If
you upgrade to v24.1 the warning you'll see is "Use
`with-current-buffer' rather than save-excursion+set-buffer".

bug#12079: 23.3; erroneous byte-compile-file warning

[Winston]

> The warning message was changed in just that way over a year ago.  If
> you upgrade to v24.1 the warning you'll see is "Use
> `with-current-buffer' rather than save-excursion+set-buffer".

Sounds good.  bug#12079 is resolved, then.
 -WBE