error[E0658]: the `linkage` attribute is experimental and not portable across platforms
  --> $DIR/feature-gate-linkage.rs:2:5
   |
LL |     #[linkage = "extern_weak"] static foo: isize;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/29603
   = help: add `#![feature(linkage)]` to the crate attributes to enable

error: aborting due to previous error

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