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 12:49:53 -0400
> 
> >> --- a/src/composite.c
> >> +++ b/src/composite.c
> >> @@ -1879,11 +1879,7 @@ Otherwise (for terminal display), FONT-OBJECT must be a terminal ID, a
> >>  	  for (i = SBYTES (string) - 1; i >= 0; i--)
> >>  	    if (!ASCII_CHAR_P (SREF (string, i)))
> >>  	      error ("Attempt to shape unibyte text");
> >> -	  /* STRING is a pure-ASCII string, so we can convert it (or,
> >> -	     rather, its copy) to multibyte and use that thereafter.  */
> >> -	  Lisp_Object string_copy = Fconcat (1, &string);
> >> -	  STRING_SET_MULTIBYTE (string_copy);
> >> -	  string = string_copy;
> >> +	  /* STRING is a pure-ASCII string, so we can treat it as multibyte.  */
> >
> > Did you actually try your change in the situations where this problem
> > pops up?  AFAIR, the code makes a copy of the string for good reasons:
> > the rest of handling of the string down the line barfs if we keep a
> > multibyte string here.
> 
> Of course, if we really do need a multibyte copy of the string, I can
> change the patch to call `Fstring_to_multibyte` instead of `Fconcat`.

Why not call make_multibyte_string directly?