error[E0375]: implementing the trait `CoerceUnsized` requires multiple coercions
  --> $DIR/E0375.rs:10:12
   |
LL | 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 1 previous error

For more information about this error, try `rustc --explain E0375`.