bug#6083: Regression bug in auto-coding-alist

[Chong Yidong]

Juri Linkov <juri@jurta.org> writes:

>   ;; By default, Emacs thinks .arc is an archive extension.
>   ;; This makes it normal.
>   (dolist (coding auto-coding-alist)
>     (when (and (string-match "[Aa][Rr][Cc]\\\\|" (car coding))
>                (or (eq (cdr coding) 'no-conversion)
>                    (eq (cdr coding) 'no-conversion-multibyte)))
>       (setcar coding (replace-match "" nil nil (car coding)))))
>
> This code fails with
>
>   (error "Attempt to modify read-only object")
>
> A similar bug#4917 http://debbugs.gnu.org/4917
> for `auto-coding-regexp-alist' was fixed 2009-11-13
> by using purecopy only for car or each item,
> not the whole list.
>
> To make the code above to not fail, purecopy should be removed
> from `auto-coding-alist'.

Thanks, I've fixed it in the branch.