error: unstable feature `feat_foo` is used without being enabled.
  --> $DIR/unstable-impl-cannot-use-feature.rs:26:5
   |
LL |     Bar::foo();
   |     ^^^
   |
   = help: The feature can be enabled by marking the current item with `#[unstable_feature_bound(feat_foo)]`
note: required for `Bar` to implement `Foo`
  --> $DIR/unstable-impl-cannot-use-feature.rs:20:6
   |
LL | #[unstable_feature_bound(feat_foo)]
   | ----------------------------------- unsatisfied trait bound introduced here
LL | impl Foo for Bar {
   |      ^^^     ^^^

error: aborting due to 1 previous error