I’m trying to do this:

use std::any::Any;

trait A: Any + 'static {}

#[derive(Debug)]
struct S {
    i: i64,
}

impl A for S {}

fn main() {
    let a: Box<dyn Any> = Box::new(S { i: 1 });
    let b: Box<dyn A> = Box::new(S { i: 2 });
    let c: Box<S> = Box::new(S { i: 3 });
    let d = S { i: 4 };
    let e = &d;
    println!(
        "{:?} {:?}",
        (*a).downcast_ref::<S>(), // works
        (*b).downcast_ref::<S>(), // does not work
    );
}

I get the following compilation error:

error[E0599]: no method named `downcast_ref` found for trait object `(dyn A + 'static)` in the current scope
  --> src/main.rs:18:14
   |
18 |         (*b).downcast_ref::<S>(),
   |              ^^^^^^^^^^^^ method not found in `dyn A`

Which is strange, because A explicitly requires Any. First I tried without being explicit about it, but it didn’t work that way either.

I also tried this:

(&*b as &dyn Any).downcast_ref::<S>()

But in this case I get the following error:

error[E0658]: cannot cast `dyn A` to `dyn Any`, trait upcasting coercion is experimental
  --> src/main.rs:19:10
   |
19 |         (&*b as &dyn Any).downcast_ref::<S>(),
   |          ^^^
   |

Box has a downcast() method, but it eats the box, while I want to work on a reference. My original use case has b as a &Box<dyn A>, in which case the above code compiles but doesn’t work because it converts the Box to Any instead of the underlying value. The equivalent code, &**b as &dyn Any, gives the same compile error.

Is there any way to downcast a &Box<dyn A> to a &S?