Update to latest nightly

src/step.rs

@@ -141,6 +141,7 @@ impl<'a, 'tcx> EvalContext<'a, 'tcx> {
                 self.deallocate_local(old_val)?;
             }
 
+            // Just a borrowck thing
             EndRegion(..) => {}
 
             // Defined to do nothing. These are added by optimization passes, to avoid changing the