Expose loop_control.loop_var as ansible_loop_var (#54240)

* Expose the loop_var as ansible_loop_var

* Add docs

* fix assert test

* Indicate version added

docs/docsite/rst/reference_appendices/special_variables.rst

@@ -28,6 +28,9 @@ ansible_limit
 ansible_loop
     A dictionary/map containing extended loop information when enabled via ``loop_control.extended``
 
+ansible_loop_var
+    The name of the value provided to ``loop_control.loop_var``. Added in ``2.8``
+
 ansible_play_batch
     List of active hosts in the current play run limited by the serial, aka 'batch'. Failed/Unreachable hosts are not considered 'active'.
 

docs/docsite/rst/user_guide/playbooks_loops.rst

@@ -400,6 +400,14 @@ Variable                    Description
       loop_control:
         extended: yes
 
+.. versionadded:: 2.8
+
+As of Ansible 2.8 you can get the name of the value provided to ``loop_control.loop_var`` using the ``ansible_loop_var`` variable
+
+For role authors, writing roles that allow loops, instead of dictating the required ``loop_var`` value, you can gather the value via::
+
+    "{{ lookup('vars', ansible_loop_var) }}"
+
 .. _migrating_to_loop:
 
 Migrating from with_X to loop