>> FWIW, it's was project.el's problem, not xref's.
>
> I thought it's the xref's problem because Help says it's in xref.el.
> Typing 'C-h f project--read-regexp RET' shows:
>
>   project--read-regexp is a Lisp function in ‘xref.el’.
>
> But this is because I tried to override this function in the init file:
>
> ;; Redefine to use `minibuffer-history':
> (with-eval-after-load 'project
>   (defun project--read-regexp ()
>     (let ((sym (thing-at-point 'symbol t)))
>       (read-regexp "Find regexp"
>                    (and sym (regexp-quote sym))
>                    'minibuffer-history))))
>
> Oh, well, need to find another way to do this
> (no feature request for this :-)

Actually, can't do this without feature request,
and can't use it as is, because never use regexp
while searching in project, whereas it pollutes
the regexp history for commands where I really
use regexps, not just strings.

A simple patch could be a saver to use with e.g.
(setq project-regexp-history-variable 'minibuffer-history):