bug#46859: 28.0.50; [PATCH]: Add option to truncate long lines in xref.el

[Juri Linkov]

> We currently don't visit the file buffer if it's not already visited,
> parsing the line in a temp buffer instead. That approach resulted in a nice
> perf improvement.

Reusing visited files is a nice feature, but still needs a fix.

Test case: visit emacs/src/xdisp.c and type

  C-x p g expose_frame RET

See that not all lines from xdisp.c have font-lock highlighting.
After applying this patch, all xref output lines from xdisp.c
have font-lock faces:

diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index 18fdd963fb..6a5361f852 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -1691,7 +1701,10 @@ xref--collect-matches-1
       (let* ((beg-column (- (match-beginning 0) line-beg))
              (end-column (- (match-end 0) line-beg))
              (loc (xref-make-file-location file line beg-column))
-             (summary (buffer-substring line-beg line-end)))
+             (summary (progn
+                        (unless syntax-needed
+                          (font-lock-ensure line-beg line-end))
+                        (buffer-substring line-beg line-end))))
         (add-face-text-property beg-column end-column 'xref-match
                                 t summary)
         (push (xref-make-match summary loc (- end-column beg-column))