Add comment to explain why we change the layout for Projection

src/librustc_mir/interpret/operand.rs

@@ -475,6 +475,8 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> InterpretCx<'a, 'mir, 'tcx, M>
                 PlaceBase::Local(mir::RETURN_PLACE) => return err!(ReadFromReturnPointer),
                 PlaceBase::Local(local) => {
                     // FIXME use place_projection.is_empty() when is available
+                    // Do not use the layout passed in as argument if the base we are looking at
+                    // here is not the entire place.
                     let layout = if let Place::Base(_) = mir_place {
                         layout
                     } else {