Why can't I do `dynamic_cast` in Rust as I do in C++?

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10

The clever trick that @alice refers to is this,

playground

// blanket impl for all sized types, this allows for a very large majority of use-cases
impl<T: Bar> AsBar for T {
    fn as_bar(&self) -> &dyn Bar { self }
}

// a helper-trait to do the conversion
trait AsBar {
    fn as_bar(&self) -> &dyn Bar;
}

// note that Bar requires `AsBar`, this is what allows you to call `as_bar`
// from a trait object of something that requires `Bar` as a super-trait
trait Bar: AsBar {
    fn bar_method(&self) {
        println!("this is bar");
    }
}

// no change here
trait Foo: Bar {
    fn foo_method(&self) {
        println!("this is foo");
    }
}

// now this works
fn foo(foo: &dyn Foo) -> &dyn Bar {
    foo.as_bar()
}
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