error[E0376]: the trait `CoerceUnsized` may only be implemented for a coercion between structures
  --> $DIR/E0376.rs:18:1
   |
LL | impl<T, U> CoerceUnsized<U> for Foo<T> {} //~ ERROR E0376
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0376"