bug#32034: 26.1; [PACTH] better xref-location-marker for imperfect file locations

[Eli Zaretskii]

> From: joaotavora@gmail.com (João Távora)
> Date: Mon, 02 Jul 2018 14:46:52 +0100
> Cc: dgutov@yandex.ru
> 
> The attached patch fixes 1. and 2.  It should probably go into emacs-26
> 
> Then, I'd like to know your opinions on 3., to go into master.
> 
> diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
> index b0bdd62ae9..d38328cccd 100644ppp
> --- a/lisp/progmodes/xref.el
> +++ b/lisp/progmodes/xref.el
> @@ -119,13 +119,19 @@ xref-make-file-location
>      (with-current-buffer
>          (or (get-file-buffer file)
>              (let ((find-file-suppress-same-file-warnings t))
> +              (unless (file-exists-p file)
> +                (error "File %s doesn't exist!" file))
>                (find-file-noselect file)))
>        (save-restriction
>          (widen)
>          (save-excursion
>            (goto-char (point-min))
>            (beginning-of-line line)
> -          (forward-char column)
> +          (ignore-errors (forward-char column))
> +          (unless (and (= (1+ (current-line)) line)
> +                       (= (current-column) column))
> +            (message "Intended xref location was line=%d, column=%d"
> +                     line column))
>            (point-marker))))))

What will the last hunk do when a file changed, but the identifier was
still found?  AFAIR, the etags back-end is capable of doing that
(because it searches the file in the vicinity of the line/column if
not found at the exact location), and it's a valuable feature, for
obvious reasons.