Add override for xref-backend-references

Which will take the provided identifier and only search for the last
segment (base name or method name), to synchronize with
xref-backend-identifier-at-point.

README.md

@@ -4,9 +4,10 @@ Robe is a code assistance tool that uses a Ruby REPL subprocess with
 your application or gem code loaded, to provide information about
 loaded classes and modules, and where each method is defined.
 
-Generally, you'll want to start with `M-x inf-ruby-console-auto`.
-If there's no Ruby console running, most interactive commands provided
-by Robe will offer to launch it automatically.
+Generally, you'll want to start with `M-x inf-ruby-console-auto` or
+with `M-x robe-start` (which will offer to call the former function as
+well if there is no Ruby console running). Some interactive commands
+provided by Robe will also offer to launch it first.
 
 The exceptions are code completion and eldoc, which only work if the
 server is already running. To launch it, type `M-x robe-start`.

robe.el

@@ -1346,7 +1346,7 @@ Only works with Rails, see e.g. `rinari-console'."
                (robe-complete-symbol-p (car bounds)))
       (buffer-substring (car bounds) (cdr bounds)))))
 
-(defun robe--xref-backend () 'robe)
+(defun robe--xref-backend () (and (robe-running-p) 'robe))
 
 (cl-defmethod xref-backend-identifier-completion-table ((_backend (eql 'robe)))
   (let ((context (robe-context))
@@ -1379,8 +1379,6 @@ Only works with Rails, see e.g. `rinari-console'."
     (cond
      (at-pt
       (or (robe--xref-variable-definition identifier (nth 3 context))
-          ;; FIXME: Probably move this check to `robe--xref-backend'.
-          (ignore (robe-start))
           (append
            (robe--xref-method-definitions identifier context)
            (robe--xref-module-definitions identifier (nth 1 context)))))
@@ -1431,6 +1429,11 @@ Only works with Rails, see e.g. `rinari-console'."
       (list
        (xref-make name var)))))
 
+;; XXX: Search across the full LOAD_PATH as well?
+(cl-defmethod xref-backend-references ((backend (eql 'robe)) identifier)
+  ;; FIXME: Do more filtering by scope, to remove definite non-matches.
+  (cl-call-next-method backend (car (last (string-split identifier "::\\|#\\|\\.")))))
+
 (cl-defmethod xref-location-marker ((var robe--variable))
   (save-excursion
     (save-restriction
@@ -1490,8 +1493,6 @@ Only works with Rails, see e.g. `rinari-console'."
         (file-relative-name file dir)
       file)))
 
-;; TODO: `xref-backend-references' across LOAD_PATH.
-
 (defvar robe-mode-map
   (let ((map (make-sparse-keymap)))
     (define-key map (kbd "C-c C-d") 'robe-doc)