error[E0658]: #[doc(include = "...")] is experimental
  --> $DIR/feature-gate-external_doc.rs:1:1
   |
LL | #[doc(include="asdf.md")]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/44732
   = help: add #![feature(external_doc)] to the crate attributes to enable

error: aborting due to previous error

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