What does "Changing `byte-compile-dest-file'" mean?

What does "Changing `byte-compile-dest-file'" mean?

[Alan Mackenzie]

Hello, Emacs.

I'm trying to debug a failing bootstrap.  Central to the failure is the
error message:

    "Changing `byte-compile-dest-file' is obsolete (as of 23.2);
    set `byte-compile-dest-file-function' instead."

..  The error message comes from emacs-lisp/bytecomp.el L. 171.

But what does it mean?  In what respect is byte-compile-dest-file being
changed?  Is the message talking about the variable or the function of
that name?  (I'm guessing the function.)  Is the entity being altered in
some fashion, or is it getting replaced by something else?

What is causing the error?  It seems that if a function called
byte-compile-dest-file is already defined when bytecomp.el first gets
loaded, this triggers the error.  But what causes that circumstance?

There are no clues in the comments around that part of bytecomp.el.  Just
that it might have something to do with early versions of automake.
(Which we don't use, do we?)

Help!

Thanks in advance!

-- 
Alan Mackenzie (Nuremberg, Germany).

Re: What does "Changing `byte-compile-dest-file'" mean?

[Stefan Monnier]

> I'm trying to debug a failing bootstrap.  Central to the failure is the
> error message:
>
>     "Changing `byte-compile-dest-file' is obsolete (as of 23.2);
>     set `byte-compile-dest-file-function' instead."
>
> ..  The error message comes from emacs-lisp/bytecomp.el L. 171.

Hmmm... my crystal ball suggests that your bootstrap ends up loading
`bytecomp` recursively, i.e. a first load of `bytecomp` defines
`byte-compile-dest-file` and then before reaching the end of the file,
it recursive tries to load `bytecomp`, causing the warning to be emitted
(because the function is already defined by (featurep 'bytecomp) is
still nil).

> But what does it mean?  In what respect is byte-compile-dest-file being
> changed?

It's about to be redefined by the subsequent `defun`.  That function
used to be considered as something that tools/users were allowed to
redefine locally in order to save .elc files elsewhere than their
standard location.  This practice was made obsolete by introducing
`byte-compile-dest-file-function` instead.

The warning you're seeing should arguably be removed now.
Tho I don't know if it will solve your underlying problem.

> Just that it might have something to do with early versions of
> automake.  (Which we don't use, do we?)

Not anymore, no, indeed.


        Stefan