error[E0377]: the trait `CoerceUnsized` may only be implemented for a coercion between structures
  --> $DIR/E0377.rs:12:1
   |
LL | impl<T, U> CoerceUnsized<Bar<U>> for Foo<T> where T: CoerceUnsized<U> {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: expected coercion between the same definition; expected `Foo`, found `Bar`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0377`.