Add xref--etags-backend to xref-backing-functions using add-hook

* lisp/progmodes/xref.el (xref-backend-functions): Move the
default value into a separate `add-hook' call (bug#21964).

* lisp/progmodes/elisp-mode.el (emacs-lisp-mode):
Don't declare the xref-backend-functions variable.
It doesn't make any difference.

2 files changed, 5 insertions, 4 deletions

diff --git a/lisp/progmodes/elisp-mode.el b/lisp/progmodes/elisp-mode.el
index bf0f4b0c9b9..7e55c8e14bc 100644
--- a/lisp/progmodes/elisp-mode.el
+++ b/lisp/progmodes/elisp-mode.el
@@ -228,7 +228,6 @@ Blank lines separate paragraphs.  Semicolons start comments.
 
 \\{emacs-lisp-mode-map}"
   :group 'lisp
-  (defvar xref-backend-functions)
   (defvar project-library-roots-function)
   (lisp-mode-variables nil nil 'elisp)
   (add-hook 'after-load-functions #'elisp--font-lock-flush-elisp-buffers)
diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index dcaffa4bbfb..c5518907aec 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -201,15 +201,17 @@ LENGTH is the match length, in characters."
 
 ;;; API
 
-;; We make the etags backend the default for now, until something
-;; better comes along.
-(defvar xref-backend-functions (list #'xref--etags-backend)
+(defvar xref-backend-functions nil
   "Special hook to find the xref backend for the current context.
 Each functions on this hook is called in turn with no arguments
 and should return either nil to mean that it is not applicable,
 or an xref backend, which is a value to be used to dispatch the
 generic functions.")
 
+;; We make the etags backend the default for now, until something
+;; better comes along.
+(add-hook 'xref-backend-functions #'xref--etags-backend)
+
 ;;;###autoload
 (defun xref-find-backend ()
   (run-hook-with-args-until-success 'xref-backend-functions))