Use Emacs Lisp xref backend in Lisp Help buffers

* lisp/help-fns.el (help-fns--setup-xref-backend):
New function (bug#69462).
(describe-function, describe-variable): Use it here.

* lisp/emacs-lisp/cl-extra.el (cl-describe-type): And here.

* lisp/progmodes/elisp-mode.el (xref-backend-definitions):
Only infer namespace in emacs-lisp-mode (use 'any' otherwise).

lisp/emacs-lisp/cl-extra.el

@@ -733,6 +733,8 @@ PROPLIST is a list of the sort returned by `symbol-plist'.
 Call `cl--find-class' to get TYPE's propname `cl--class'"
   (cl--find-class type))
 
+(declare-function help-fns--setup-xref-backend "help-fns" ())
+
 ;;;###autoload
 (defun cl-describe-type (type &optional _buf _frame)
   "Display the documentation for type TYPE (a symbol)."
@@ -753,6 +755,7 @@ Call `cl--find-class' to get TYPE's propname `cl--class'"
             ;; cl-deftype).
             (user-error "Unknown type %S" type))))
       (with-current-buffer standard-output
+        (help-fns--setup-xref-backend)
         ;; Return the text we displayed.
         (buffer-string)))))
 

lisp/help-fns.el

@@ -262,6 +262,11 @@ interactive command."
                   fn))
     (list fn)))
 
+(defun help-fns--setup-xref-backend ()
+  (add-hook 'xref-backend-functions #'elisp--xref-backend nil t)
+  (setq-local project-vc-external-roots-function #'elisp-load-path-roots)
+  (setq-local semantic-symref-filepattern-alist '((help-mode "*.el"))))
+
 ;;;###autoload
 (defun describe-function (function)
   "Display the full documentation of FUNCTION (a symbol).
@@ -295,6 +300,8 @@ handling of autoloaded functions."
         (princ " is ")
         (describe-function-1 function)
         (with-current-buffer standard-output
+          (help-fns--setup-xref-backend)
+
           ;; Return the text we displayed.
           (buffer-string))))))
 
@@ -1510,6 +1517,7 @@ it is displayed along with the global value."
                     (delete-char 1)))))
 
 	    (with-current-buffer standard-output
+              (help-fns--setup-xref-backend)
 	      ;; Return the text we displayed.
 	      (buffer-string))))))))
 

lisp/progmodes/elisp-mode.el

@@ -1049,7 +1049,9 @@ namespace but with lower confidence."
   (let ((sym (intern-soft identifier)))
     (when sym
       (let* ((pos (get-text-property 0 'pos identifier))
-             (namespace (if pos
+             (namespace (if (and pos
+                                 ;; Reusing it in Help Mode.
+                                 (derived-mode-p 'emacs-lisp-mode))
                             (elisp--xref-infer-namespace pos)
                           'any))
              (defs (elisp--xref-find-definitions sym)))