Future incompatibility report: Future breakage diagnostic:
warning: use of unstable library feature 'rustc_encodable_decodable': derive macro for `rustc-serialize`; should not be used in new code
  --> $DIR/rustc-decodable-issue-123156.rs:10:10
   |
LL | #[derive(RustcDecodable)]
   |          ^^^^^^^^^^^^^^
   |
   = 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>