error[E0374]: the trait `CoerceUnsized` may only be implemented for a coercion between structures
  --> $DIR/E0374.rs:8:1
   |
LL | / impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
LL | |     where T: CoerceUnsized<U> {}
   | |_____________________________^
   |
   = note: expected a single field to be coerced, none found

error: aborting due to previous error

For more information about this error, try `rustc --explain E0374`.