error[E0038]: the trait `Foo` cannot be made into an object
  --> $DIR/arbitrary-self-types-not-object-safe.rs:40:33
   |
LL |     let x = Box::new(5usize) as Box<Foo>;
   |                                 ^^^^^^^^ the trait `Foo` cannot be made into an object
   |
   = note: method `foo` has a non-standard `self` type

error[E0038]: the trait `Foo` cannot be made into an object
  --> $DIR/arbitrary-self-types-not-object-safe.rs:40:13
   |
LL |     let x = Box::new(5usize) as Box<Foo>;
   |             ^^^^^^^^^^^^^^^^ the trait `Foo` cannot be made into an object
   |
   = note: method `foo` has a non-standard `self` type
   = note: required because of the requirements on the impl of `std::ops::CoerceUnsized<std::boxed::Box<dyn Foo>>` for `std::boxed::Box<usize>`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0038`.