Milestones and labels can be used even when issues are disabled.

app/helpers/projects_helper.rb

@@ -146,7 +146,7 @@ module ProjectsHelper
       nav_tabs << feature if project.send :"#{feature}_enabled"
     end
 
-    if project.issues_enabled
+    if project.issues_enabled || project.merge_requests_enabled
       nav_tabs << [:milestones, :labels]
     end
 

app/views/projects/milestones/show.html.haml

@@ -60,11 +60,12 @@
       Participants
       %span.badge= @users.count
 
-  .pull-right
-    = link_to new_namespace_project_issue_path(@project.namespace, @project, issue: { milestone_id: @milestone.id }), class: "btn  btn-grouped", title: "New Issue" do
-      %i.fa.fa-plus
-      New Issue
-    = link_to 'Browse Issues', namespace_project_issues_path(@milestone.project.namespace, @milestone.project, milestone_id: @milestone.id), class: "btn  edit-milestone-link btn-grouped"
+  - if @project.issues_enabled
+    .pull-right
+      = link_to new_namespace_project_issue_path(@project.namespace, @project, issue: { milestone_id: @milestone.id }), class: "btn  btn-grouped", title: "New Issue" do
+        %i.fa.fa-plus
+        New Issue
+      = link_to 'Browse Issues', namespace_project_issues_path(@milestone.project.namespace, @milestone.project, milestone_id: @milestone.id), class: "btn  edit-milestone-link btn-grouped"
 
 .tab-content
   .tab-pane.active#tab-issues