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_idents.rs:5:13
   |
LL |     let a = concat_idents!(X, Y_1);
   |             ^^^^^^^^^^^^^
   |
   = note: see issue #29599 <https://github.com/rust-lang/rust/issues/29599> for more information
   = help: add `#![feature(concat_idents)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

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_idents.rs:6:13
   |
LL |     let b = concat_idents!(X, Y_2);
   |             ^^^^^^^^^^^^^
   |
   = note: see issue #29599 <https://github.com/rust-lang/rust/issues/29599> for more information
   = help: add `#![feature(concat_idents)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: aborting due to 2 previous errors

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