error[E0038]: the trait `Foo` is not dyn compatible
  --> $DIR/dyn-compatibility.rs:14:41
   |
LL |     let i = Box::new(42_u32) as Box<dyn Foo>;
   |                                         ^^^ `Foo` is not dyn compatible
   |
note: for a trait to be dyn compatible it needs to allow building a vtable
      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
  --> $DIR/dyn-compatibility.rs:4:22
   |
LL | trait Foo {
   |       --- this trait is not dyn compatible...
LL |     fn baz(&self) -> impl Debug;
   |                      ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
   = help: consider moving `baz` to another trait
   = help: only type `u32` implements `Foo`; consider using it directly instead.

error: aborting due to 1 previous error

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