bug#34950: 27.0.50; Silently and unexpectedly falling back to `no-conversion`

[Stefan Monnier]

Package: Emacs
Version: 27.0.50


I've seen a few reports now where users are puzzled by \NNN thingies in
their buffers (typically because Emacs ended up deciding that the file
should be opened with `no-conversion` instead of utf-8) and have no idea
where they come from or how to fix them.

I think we should try and change the auto-detection of the
coding-system, so as to keep track of the reason for the choice (where
possible), and then offer a command that uses this info to explain
what happened (e.g. "inhibit-nul-byte-detection set
and found a NUL char at position <foo>").

For extra bonus points we could change some major modes (e.g. prog-mode
and text-mode) to automatically warn the user when the buffer is in
unibyte mode and suggest running that new command.


        Stefan