error[E0658]: the `linkage` attribute is experimental and not portable across platforms (see issue #29603)
  --> $DIR/linkage4.rs:11:1
   |
LL | #[linkage = "external"]
   | ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = 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`.