error[E0658]: the `#[marker]` attribute is an experimental feature
  --> $DIR/feature-gate-marker_trait_attr.rs:3:1
   |
LL | #[marker] trait ExplicitMarker {}
   | ^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/29864
   = help: add `#![feature(marker_trait_attr)]` to the crate attributes to enable

error: aborting due to previous error

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