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

[João Távora]

Hi maintainers,

Observed this when working on Eglot, an Emacs LSP client.  Some LSP
servers, when asked about symbol definitions, make the mistake of
sending an non-existent or outdated path and location.

On the other hand, because xrefs come in potentially large numbers, a
sane xref backend will not check if a file exists before making
xref-file-location objects (neither should it check if the file contains
the purported location of the definition).  This can only reasonably be
done when the user decides to visit such a location.

So, currently, if the user chooses to visit a xref-file-location
pointing to a non-existing file, that file buffer is created
automatically and the command will probably error with an cryptic "End
of buffer" because of the `forward-char' call in the corresponding
xref-location-marker method.  The new empty buffer isn't even shown
which would at least help the user recognize the problem.

So:

1. xref-location-marker should check for `file-readable-p' before trying
to open the file, and if that fails issues an error ("File %s can't be
found.").

2. if the file is found, xref-location-marker should detect if the
location is indeed available in the file, and if it isn't, issue a
message.  In that case it should return a marker to the nearest possible
location.

3. Number 2. could turn out to be brittle and annoying if we have
changed the file in the meantime (but probably not more so than jumping
to a wrong location).  So we could have a "hint" field in
xref-file-location (or a xref-hinted-file-location) that helps in
looking around the landing point for, say, a regexp, and puts point
there.  Historically, this technique is successfully used in SLIME.  We
could also reasonably default that field to the identifier being looked
for.

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))))))
 
 (cl-defmethod xref-location-group ((l xref-file-location))

João