error[E0658]: unless otherwise specified, attributes with the prefix `rustc_` are reserved for internal compiler diagnostics (see issue #29642)
  --> $DIR/expand-to-unstable-2.rs:18:10
   |
LL | #[derive(Unstable)]
   |          ^^^^^^^^
   |
   = help: add #![feature(rustc_attrs)] to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.