Function walk_fields 

fn walk_fields<'tcx>(
    tcx: TyCtxt<'tcx>,
    current_ty: Ty<'tcx>,
    field_indices: Vec<(usize, Ty<'tcx>)>,
    fields: &[String],
) -> Option<(Vec<(usize, Ty<'tcx>)>, Ty<'tcx>)>

Walk a list of field names from current_ty, appending (index, ty) pairs.