Convert Box<dyn T> to Box<dyn Any>

[samuelpilz]

@RustyYato

There is just enough type information to call trait methods and to clean up the value, but not enough to actually use the type itself at runtime. This information is injected into the trait object when the type is coerced to a trait object.

I understand that this is the case. However, what is the reason to exlude the type-id from the vtable?

The thread linked above gives another use case:

trait A {}
trait B: A {}

fn upcast(x: Box<dyn B>) -> Box<dyn A> {
    panic!("this is not implementable??")
}

For me, the functionality of upcast is useful in some cases. It is frustrating to me that I am unable to comprehend why this is not possible to implement: All information should be there to construct a trait-object of type A from one of type B.