The anzu package was mentioned recently in the mailing list.  Emacs now
includes the lazy-count feature, which is the main purpose of anzu, but
it's still missing one nice feature from that package, namely the
highlighting and count of matches as one types a regexp or string to
replace in `query-replace{-regexp}'.  See the below patch for a
(probably not yet mature) stab at this.

I've also added lazy highlighting to the `isearch-edit-string' command.

I didn't introduce any new customization variables.  Lazy highlighting
in both `query-replace{-regexp}' and `isearch-edit-string' is controlled
by `isearch-lazy-highlight' (therefore is on by default) and lazy count
next to the minibuffer prompt is controlled by `isearch-lazy-count'
(therefore off by default).

I believe the problems pointed out in a previous iteration of this in
https://lists.gnu.org/archive/html/emacs-devel/2021-05/msg01196.html are
now solved.