error[E0725]: the feature `rustc_const_unstable` is not in the list of allowed features
  --> $DIR/allow-features-empty.rs:4:12
   |
LL | #![feature(rustc_const_unstable)]
   |            ^^^^^^^^^^^^^^^^^^^^

error[E0725]: the feature `lang_items` is not in the list of allowed features
  --> $DIR/allow-features-empty.rs:6:12
   |
LL | #![feature(lang_items)]
   |            ^^^^^^^^^^

error[E0725]: the feature `unknown_stdlib_feature` is not in the list of allowed features
  --> $DIR/allow-features-empty.rs:8:12
   |
LL | #![feature(unknown_stdlib_feature)]
   |            ^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0725`.