bug#40407: [PATCH] slow ENCODE_FILE and DECODE_FILE

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Mon, 6 Apr 2020 18:55:30 +0200
> Cc: hirofumi@mail.parknet.co.jp, handa@gnu.org, 40407@debbugs.gnu.org
> 
> 6 apr. 2020 kl. 18.33 skrev Eli Zaretskii <eliz@gnu.org>:
> 
> > 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.
> 
> There is nothing common between utf-7 and utf-8 at all (apart from a subset 
> of ASCII being encoded in the same way, and the fact that both encode the 
> Unicode repertoire).

By "in common" in this context I meant from the POV of internal
treating of the two encodings.

> > 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.
> 
> We could use 'raw-text' as well but that implies that any byte value could be 
> part of an utf-7[-imap] text, which is incorrect.
> In fact, utf-7-imap only uses codes 0x20-0x7e (utf-7 is allowed to use a few 
> C0 controls too, as mentioned).
> 
> Arguably the heuristics of define-coding-system-internal are somewhat 
> inscrutable. There seems to be leaks between layers -- ascii-compatible-p is 
> an end-to-end property and cannot really be set the way it is by that 
> function. But since it is, fixing it afterwards should be the correct way.

I prefer to wait for Handa-san's response, and meanwhile install the
least disruptive change, which just fixes the one aspect that got
broken.  Call me a coward, if you wish.