error[E0658]: use of unstable library feature 'concat_idents': `concat_idents` is not stable enough for use and is subject to change
  --> $DIR/feature-gate-concat_idents3.rs:5:20
   |
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]: use of unstable library feature 'concat_idents': `concat_idents` is not stable enough for use and is subject to change
  --> $DIR/feature-gate-concat_idents3.rs:6:20
   |
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`.