Add read and store steps for SliceElementNode

Author: owen-mc

go/ql/lib/semmle/go/dataflow/internal/ContainerFlow.qll

@@ -24,6 +24,13 @@ predicate containerStoreStep(Node node1, Node node2, Content c) {
       exists(Write w | w.writesElement(node2.(PostUpdateNode).getPreUpdateNode(), _, node1))
       or
       node1 = node2.(ImplicitVarargsSlice).getCallNode().getAnImplicitVarargsArgument()
+      or
+      // To model data flow from array elements of the base of a `SliceNode` to
+      // the `SliceNode` itself, we consider there to be a read step with array
+      // content from the base to the corresponding `SliceElementNode` and then
+      // a store step with array content from the `SliceelementNode` to the
+      // `SliceNode` itself.
+      node2 = node1.(SliceElementNode).getSliceNode()
     )
   )
   or
@@ -57,6 +64,13 @@ predicate containerReadStep(Node node1, Node node2, Content c) {
     )
     or
     node2.(RangeElementNode).getBase() = node1
+    or
+    // To model data flow from array elements of the base of a `SliceNode` to
+    // the `SliceNode` itself, we consider there to be a read step with array
+    // content from the base to the corresponding `SliceElementNode` and then
+    // a store step with array content from the `SliceelementNode` to the
+    // `SliceNode` itself.
+    node2.(SliceElementNode).getSliceNode().getBase() = node1
   )
   or
   c instanceof CollectionContent and