error[E0374]: the trait `CoerceUnsized` may only be implemented for a coercion between structures with one field being coerced, none found
  --> $DIR/E0374.rs:18:1
   |
LL | / impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> //~ ERROR E0374
LL | |     where T: CoerceUnsized<U> {}
   | |________________________________^

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0374"