Possibly as a result of some recent ivy-mode update, Emacs crashes quite
promptly when I use it. I've tried going back a couple of days in Emacs
revisions, and the problem is also there.

Here are a few xbacktrace-s (I think 3-5 are from an earlier revision,
but 1-2 should be from this one). Let me know if 'bt full' is needed.
I'll be doing 'git bisect' next.

highlight-tail-mode features in some of them, too, but it's fairly
stable when not in conjunction with ivy-mode. E.g., it's currently
enabled in this email buffer.