error[E0658]: `concat_idents` is not stable enough for use and is subject to change
  --> $DIR/feature-gate-concat_idents3.rs:7:20
   |
LL |     assert_eq!(10, concat_idents!(X, Y_1));
   |                    ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/29599
   = help: add #![feature(concat_idents)] to the crate attributes to enable

error[E0658]: `concat_idents` is not stable enough for use and is subject to change
  --> $DIR/feature-gate-concat_idents3.rs:8:20
   |
LL |     assert_eq!(20, concat_idents!(X, Y_2));
   |                    ^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: for more information, see https://github.com/rust-lang/rust/issues/29599
   = help: add #![feature(concat_idents)] to the crate attributes to enable

error: aborting due to 2 previous errors

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