bug#21771: 25.0.50; Can't customize whitespace-display-mappings

[Ivan Andrus <darthandrus@gmail.com>]

On Thu, Dec 31, 2015 at 7:17 PM, Glenn Morris <rgm@gnu.org> wrote:
> Ivan Andrus wrote:
>
>>> This might be related to bug #2689,
>>> http://debbugs.gnu.org/cgi/bugreport.cgi?bug=2689
>
>> -  :valid-regexp "\\`.\\'"
>> +  :valid-regexp "\\`\\(.\\|\n\\)\\'"
>
> Note that this reintroduces part of the patch for #2689, which I had to
> revert since it caused a worse problem (#3136) than the one it solved.
> (I haven't checked whether the same issue applies now to your patch, but
> you should.)

Thanks for bringing that to my attention.  My patch doesn’t cause 3136,
because it doesn’t really solve the problem.  It just works around it by
using "\n\n" to mean "\n".

It does make things a little weird because there is a difference between
when it starts out as a newline and when it’s changed to a newline.  That
is, with my patch, if you customize white `whitespace-display-mappings’,
you’ll see it displayed as "\n\n".  If you change it to (say) "d" it
will be displayed as "d\n".  It saves 'd' fine, but if you customize it
again it will display as "d".  Changing this to "\n" will cause it to
display as "\n" and it will save "\n".  Customizing again will show "\n\n"
again.

But given that the alternative causes #3136, maybe it’s not so bad.

-Ivan