xref-hack.el

  (defun ross/xref-find-definitions-etags-fallback (identifier)
    "Find the definition of the identifier at point.
With prefix argument or when there's no identifier at point,
prompt for it.

If the primary xref-backend fails, fall back to the etags backend."
    (interactive (list (xref--read-identifier "Find definitions of: ")))
    (condition-case nil
        (xref-find-definitions identifier)
      (user-error
       (let ((xref-backend-functions '(etags--xref-backend t)))
         (xref--find-definitions identifier nil)))))

;; Without this, we get an interactive prompt
  (add-to-list 'xref-prompt-for-identifier 'ross/xref-find-definitions-etags-fallback t)