Keep cross project reference logic in GroupLabel#to_reference

app/models/group_label.rb

@@ -15,8 +15,20 @@ class GroupLabel < Label
   #
   # Returns a String
   #
-  def to_reference(from_project = nil, format: :id)
+  def to_reference(source_project = nil, target_project = nil, format: :id)
     format_reference = label_format_reference(format)
-    "#{self.class.reference_prefix}#{format_reference}"
+    reference = "#{self.class.reference_prefix}#{format_reference}"
+
+    if cross_project_reference?(source_project, target_project)
+      source_project.to_reference + reference
+    else
+      reference
+    end
+  end
+
+  private
+
+  def cross_project_reference?(source_project, target_project)
+    source_project && target_project && source_project != target_project
   end
 end

lib/gitlab/gfm/reference_rewriter.rb

@@ -76,15 +76,7 @@ module Gitlab
         if referable.respond_to?(:project)
           referable.to_reference(target_project)
         else
-          to_reference(referable, target_project)
-        end
-      end
-
-      def to_reference(referable, target_project)
-        if @source_project != target_project
-          @source_project.to_reference + referable.to_reference
-        else
-          referable.to_reference
+          referable.to_reference(@source_project, target_project)
         end
       end