Subject: bug#42522: Emacs 27: 100% CPU when setting mode-name with ligatures

From: Eli Zaretskii <[email-masked]>
Date: Sat, 25 Jul 2020 09:24:47 +0300

> From: Evan Moses <[email-masked]>
> Date: Fri, 24 Jul 2020 14:35:50 -0700
> 
> To reproduce, evaluate this
> 
> (set-char-table-range composition-function-table 47 '([".\\(?:\\(?:\\*\\*\\|//\\|==\\)\\|[*/=>]\\)" 0
> font-shape-gstring]))
> 
> And then evaluate 
> 
> (setq mode-name "test//")
> 
> Emacs will become unresponsive and consume 100% CPU.

It doesn't hang here.  Could this be macOS-specific?  Or maybe the
problem happens only with some fonts?  Does it happen in "emacs -Q"?

In any case, did you really mean for Emacs to produce ligatures on the
mode line?  More generally, what kind of ligatures are you trying to
produce with this setup?