error[E0376]: the trait `CoerceUnsized` may only be implemented for a coercion between structures --> $DIR/E0376.rs:8:1 | LL | impl<T, U> CoerceUnsized<U> for Foo<T> {} | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ error: aborting due to previous error For more information about this error, try `rustc --explain E0376`.