Re: decode-coding-string gone awry?

[Stefan Monnier]

> Is it reasonable to operate with decode-coding-string on a multibyte
> string?  If that is nonsense, maybe we should make it get an error,
> to help people debug such problems.

I think it would indeed make sense to signal errors when decoding
a multibyte string or when encoding a unibyte string.

> If there are some few cases where decode-coding-string makes sense on
> a multibyte string, maybe we can make it get an error except in those
> few cases.

The problem I suspect is that it's pretty common for ASCII-only strings to
be arbitrarily marked unibyte or multibyte depending on the circumstance.
So we would have to check for the case where the string is ASCII-only before
signalling an error.

I'm actually running right now with an Emacs that does signal such errors.
I've changed the notion of "multibyte/unibyte" string by saying:
- [same as now] if size_byte < 0, it's UNIBYTE.
- [same as now] if size_byte > size, it's MULTIBYTE.
- [changed]     if size_byte == size, it's neither/both (ASCII-only).

Then I've changed several parts of the C code to try and set size_byte==size
whenever possible (instead of marking the string as unibyte).


        Stefan


PS: As of now, the only place where Emacs has signalled a bad
    encoding/decoding with the proposed error is in Gnus, though I haven't
    checked any further whether this error really is a bug in Gnus.