error[E0375]: implementing the trait `CoerceUnsized` requires multiple coercions
  --> $DIR/E0375.rs:22:12
   |
22 | impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
   |            ^^^^^^^^^^^^^^^^^^^^^^^^ requires multiple coercions
   |
   = note: `CoerceUnsized` may only be implemented for a coercion between structures with one field being coerced
   = note: currently, 2 fields need coercions: b (T to U), c (U to T)

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0375"