bug#47712: 27.1; Provide `string-display-width` function, which takes properties into account, `substring-width`

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  47712@debbugs.gnu.org
> Date: Tue, 13 Apr 2021 10:01:12 +0200
> 
> Daniel Mendler <mail@daniel-mendler.de> writes:
> 
> > Note that I would like to have an implementation of
> > `substring-width`/`string-display-width`/`substring-display-width`
> > which does not allocate, since this reduces the GC pressure when
> > formatting many items. How do you think about this?
> >
> > Regarding images, different fonts, maybe a
> > `string-property-pixel-width` would be useful too. But for the use
> > cases I have in mind (formatting monospaced text) computing columns is
> > sufficient.
> 
> Sure; I think a non-allocating function to count monospaced string width
> would be useful.

I won't mount the barricades over this, but please keep in mind that
even with fixed-pitch fonts this will not always produce correct
results, because on GUI frames the "double-width" characters not
always take exactly two columns (it depends on the font).  So this
function will only work reliably on text-mode (a.k.a. TTY) frames and
for Latin characters on GUI frames.

If you still think we should have such a semi-broken function, at
least include some warning in its doc string, so that users wouldn't
be unpleasantly surprised.