Add support for defining index var for task loops

test/integration/targets/loops/tasks/index_var_tasks.yml

@@ -0,0 +1,3 @@
+- name: check that index var exists inside included tasks file
+  assert:
+    that: my_idx == item|int

test/integration/targets/loops/tasks/main.yml

@@ -1,3 +1,7 @@
+#
+# loop_control/pause
+#
+
 - name: Measure time before
   shell: date +%s
   register: before
@@ -146,3 +150,29 @@
     that:
       - 'results10["results"][0]["ping"] == "Hello World"'
       - 'results10["results"][1]["ping"] == "Olá Mundo"'
+
+#
+# loop_control/index_var
+#
+
+- name: check that the index var is created and increments as expected
+  assert:
+    that: my_idx == item|int
+  with_sequence: start=0 count=3
+  loop_control:
+    index_var: my_idx
+
+- name: check that value of index var matches position of current item in source list
+  assert:
+    that: 'test_var.index(item) == my_idx'
+  vars:
+    test_var: ['a', 'b', 'c']
+  with_items: "{{ test_var }}"
+  loop_control:
+    index_var: my_idx
+
+- name: check index var with included tasks file
+  include_tasks: index_var_tasks.yml
+  with_sequence: start=0 count=3
+  loop_control:
+    index_var: my_idx