error[E0658]: extern types are experimental (see issue #43467)
  --> $DIR/feature-gate-extern_types.rs:12:5
   |
LL |     type T; //~ ERROR extern types are experimental
   |     ^^^^^^^
   |
   = help: add #![feature(extern_types)] to the crate attributes to enable

error: aborting due to previous error

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