bug#44486: 27.1; C-@ chars corrupt elisp buffer

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: larsi@gnus.org,  thievol@posteo.net,  handa@gnu.org,
>   schwab@linux-m68k.org,  44486@debbugs.gnu.org
> Date: Sat, 14 Nov 2020 12:55:51 -0500
> 
> >> AFAIK `prefer-utf-8` is only ever used for files which are known to
> >> contain text and should almost always contain UTF-8 text.
> > For those, we should use utf-8, not prefer-utf-8.
> 
> No, `utf-8` should be used when other coding systems should be
> considered as errors (i.e. not "almost always" but "always")

Why?

> whereas `prefer-utf-8` is for use when utf-8 is the most likely one
> and other coding systems should be tried only when there's some
> evidence that the file actually doesn't use utf-8.
> 
> `prefer-utf-8` was introduced specifically for `.el` files (and I don't
> know of any other use of that encoding so far).

Maybe that was the history, but the reality is different.
prefer-utf-8 is the same as 'undecided' with coding-systems'
priorities tampered to prefer UTF-8.

> If `utf-8` is preferable over `prefer-utf-8` for this usage I think
> the problem is in `prefer-utf-8` since it was introduced
> specifically for that.

The implementation doesn't support your POV.

> >> I believe if there's a NUL byte in such a files but it otherwise doesn't
> >> contain any invalid UTF-8 byte sequence, it will result in better
> >> behavior if we treat it as UFT-8 than as binary.
> > We treat null bytes as the _single_ telltale sign of a binary file.
> 
> A .el file should *never* be a binary file.

We are not talking about .el files, we are talking about _any_ file
read using prefer-utf-8.

For .el files, we can always bind inhibit-null-byte-detection to t
when we load or visit such files.

> > If we disable that in coding-systems that are supposed to _detect_
> > encoding, we will never be able to detect binary files.
> 
> In which scenario would it be beneficial to detect a `.el` file as being
> binary instead of utf-8?

I'm not talking about .el files.  The coding-system's applicability is
wider than that.