test passthru_derive fails with use of unstable library feature `rustc_encodable_decodable`

[jayvdb]

cargo test fails with two errors atm, one is

error: use of unstable library feature `rustc_encodable_decodable`: derive macro for `rustc-serialize`; should not be used in new code
  --> tests/passthru_derive.rs:13:1
   |
13 | / macro_attr! {
14 | |     #[derive(Clone, Hash, RustcEncodable, RustcDecodable, PartialEq, Eq, PartialOrd, Ord, Debug, Default, Copy)]
15 | |     pub struct Dummy(u32);
16 | | }
   | |_^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #64266 <https://github.com/rust-lang/rust/issues/64266>
   = note: `#[deny(soft_unstable)]` on by default
   = note: this error originates in the macro `macro_attr_impl` which comes from the expansion of the macro `macro_attr` (in Nightly builds, run with -Z macro-backtrace for more info)

https://crates.io/crates/rustc-serialize says

NOTE: This crate is deprecated in favor of serde. No new feature development will happen in this crate, although bug fixes proposed through PRs will still be merged. It is very highly recommended by the Rust Library Team that you use serde, not this crate.

Adding #[allow(soft_unstable)] to tests/passthru_derive.rs does not fix the problem.