bug#66332: Xref mode-line

[Juri Linkov]

>>>> grep-mode shows the number of matches on the mode-line.
>>>> Could xref do the same?
>>> Certainly.
>>>
>>> This should be especially useful later, when we make it update
>>> asynchronously.
>> Should I look into the implementation, or you already know how it could work?
>
> I might not get around to this until after the next few weeks, so you're
> welcome to take initiative.

Ok, here is the initial version based on grep/compilation variables:

diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index b7bfb192d87..dbf013bdba2 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -626,6 +626,18 @@ xref-pop-to-location
 (defconst xref-buffer-name "*xref*"
   "The name of the buffer to show xrefs.")
 
+(defvar-local xref-num-matches-found 0)
+
+(defvar xref-num-matches-face 'compilation-info
+  "Face name to show the number of matches on the mode line.")
+
+(defconst xref-mode-line-matches
+  `(" [" (:propertize (:eval (int-to-string xref-num-matches-found))
+                      face ,xref-num-matches-face
+                      help-echo "Number of matches so far")
+    "]"))
+(put 'xref-mode-line-matches 'risky-local-variable t)
+
 (defface xref-file-header '((t :inherit compilation-info))
   "Face used to highlight file header in the xref buffer."
   :version "27.1")
@@ -1235,6 +1229,8 @@ xref--show-xref-buffer
       (xref--ensure-default-directory dd (current-buffer))
       (xref--xref-buffer-mode)
       (xref--show-common-initialize xref-alist fetcher alist)
+      (setq xref-num-matches-found (length xrefs))
+      (setq mode-line-process (list xref-mode-line-matches))
       (pop-to-buffer (current-buffer))
       (setq buf (current-buffer)))
     (xref--auto-jump-first buf (assoc-default 'auto-jump alist))