let mut need_base: Vec<_> = repeat(true).take(field_tys.len()).collect();

        let numbered_fields = fields.iter().map(|field| {
            let opt_pos =
                field_tys.iter().position(|field_ty|
                                          field_ty.name == field.ident.node.name);
            let result = match opt_pos {
                Some(i) => {
                    need_base[i] = false;
                    (i, &*field.expr)
                }
                None => {
                    tcx.sess.span_bug(field.span,
                                      "Couldn't find field in struct type")
                }
            };
            result
        }).collect::<Vec<_>>();