bug#56347: Optimize/simplify STRING_SET_MULTIBYTE

[Eli Zaretskii <eliz@gnu.org>]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: 56347@debbugs.gnu.org
> Date: Sat, 02 Jul 2022 14:00:41 -0400
> 
> >> No, I want to reduce the scope of semantics of the macro, e.g. so it can
> >> be implemented as a function rather than a macro and so it doesn't
> >> magically substitute empty_multibyte_string into a variable that held
> >> something else.
> > But the effect is that you disallow calling STRING_SET_MULTIBYTE on an
> > empty string, isn't it?
> 
> 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.

And I'm not even sure everyone will agree with such a radical change.
It should be discussed first.

> 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.