error[E0119]: conflicting implementations of trait `Foo` for type `&_`
  --> $DIR/feature-gate-with_negative_coherence.rs:5:1
   |
LL | impl<T: std::ops::DerefMut> Foo for T { }
   | ------------------------------------- first implementation here
LL |
LL | impl<T> Foo for &T { }
   | ^^^^^^^^^^^^^^^^^^ conflicting implementation for `&_`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0119`.