bug#56347: Optimize/simplify STRING_SET_MULTIBYTE

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: 56347@debbugs.gnu.org
> Date: Sat, 02 Jul 2022 14:37:18 -0400
> 
> >> Yes.  In my book, STRING_SET_*IBYTE should basically not exist: a string
> >> is created as unibyte or multibyte and never changes after that.
> >
> > That's require a much larger change, I think.  It is not enough just
> > to add an assertion in one place, because that'd just cause
> > maintenance headaches for no real gain.
> 
> `grep STRING_SET_MULTIBYTE` suggests it's not nearly as hard you think.
> 
> The only other use is in `aset`.

That's not what I meant.  If you want to disallow changing the
representation of strings, STRING_SET_MULTIBYTE is just the tip of the
iceberg.

> >> And indeed because we only have a single copy of the two possible empty
> >> strings, they can't be changed between unibyte<->multibyte
> > I can create an empty string without those singletons any time.
> 
> My experience is that it takes a fair bit of work to do so.

I don't see how that matters here, sorry.