bug#30539: 26.0; `char-displayable-p' is much slower in Emacs 25 and 26

[Noam Postavsky]

Drew Adams <drew.adams@oracle.com> writes:

> I couldn't try your code with Emacs 24.5 because `pcase-dolist'

pcase-dolist is not autoloaded in 24.5, but is available after loading
pcase.el.

> and `inhibit-compacting-font-caches' are both undefined.

Of course setting inhibit-compacting-font-caches has no effect in 24.5,
I meant that remark just for 25+.

> But everything else you say checks out, including the effect
> of (setq inhibit-compacting-font-caches t).
>
> Is this a bug that is likely to get fixed?

Unfortunately, no, I don't think so (at least not soon).  My
understanding is that this inhibit-compacting-font-caches variable is
due to several mysterious font bugs with different users needing
different settings to work around them, and there isn't anyone who has a
good idea of how to sort it out.

> Another question is whether this bug should/will affect all
> users or only some?  If the latter then I can let users
> decide whether to test `char-displayable-p' (I have an
> option for that anyway) or whether to bind
> `inhibit-compacting-font-caches to `t'.  If only some users
> are affected by the bug, do we know why?  Does it have to
> do with the fonts they have installed, for example?

Well, as I mentioned, I don't see it on my GNU/Linux box, so it's not
universal.  I would guess the fonts installed is the main factor.