无法从 &amp;mut Box 向下转换<dyn trait>作为 &amp;mut dyn 任何？</dyn>

Question

I encountered this issue, and I'm wondering why it doesn't work. I managed to replicate the problem:

I'm trying to get back to the struct from &mut Box<dyn Trait> , and this is my go at it:

use std::any::Any;

trait A {}

struct B;

impl A for B {}

fn main() {
    let mut a: Box<dyn A> = Box::new(B);
    let a_ref: &mut Box<dyn A> = &mut a;

    match (a_ref as &mut dyn Any).downcast_mut::<B>() { // downcast here 
        Some(_b) => println!("found b"),
        None => println!("found nothing")
    }
}

But this does not find the struct behind it? Am I missing something?

Here is a link to the playground

I've tried to downcast to other things, such as Box<B> or &mut B but none if this works.

Answer 1

You can't just say my_trait_object as &dyn Any and expect to be able to downcast to the original type. It does not work by magic, you need the original type to coerce itself as dyn Any , that's the only way it works. That is why the solutions here need to pass through the trait object, to get at the original type, to get it as an Any : How to get a reference to a concrete type from a trait object?

What happens here is that a concrete type is coerced into a dyn Any , but that concrete type is Box<dyn A> . This code prints "found box":

use std::any::Any;

trait A {}
struct B;
impl A for B {}

fn main() {
    let mut a: Box<dyn A> = Box::new(B);
    let a_ref: &mut Box<dyn A> = &mut a;

    match (a_ref as &mut dyn Any).downcast_mut::<Box<dyn A>>() {
        Some(_) => println!("found box"),
        None => println!("found nothing"),
    }
}

There's more explanation in the linked answers that I encourage you to read.