bug#40407: [PATCH] slow ENCODE_FILE and DECODE_FILE

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Mon, 6 Apr 2020 17:56:27 +0200
> Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
>         Kenichi Handa <handa@gnu.org>, 40407@debbugs.gnu.org
> 
> > Kenichi, why was coding-type of UTF-7 systems set to 'utf-8'?
> > Wouldn't it be better to set it to 'utf-16'?  Or is there some
> > subtlety here that we should be aware of?  Do you have any comments on
> > the patch below?
> 
> There is no reason why utf-7[-imap] should have utf-8 as coding-type, is 
> there?

I think it might be just some convenience thing: utf-7 and utf-8 have
something in common that made it convenient to treat them the same in
the internal routines.  Or maybe it's just an accident.

> utf-16 is definitely wrong (utf-7* are encoded in ASCII).

Why do you think the ASCII encoding contradicts the utf-16
coding-type?

> What about the patch below instead?

I don't think 'charset' is the right type for this encoding (any
reason why you've chosen it?), but I will let Handa-san comment.
Defining coding-systems is a black art which I don't think I ever
mastered.