Convert between a reference to a trait and a struct that implements that trait in Rust

Question

I have a trait and a struct that implements that trait:

trait A {
    fn something(&self);
}

struct B {
    some_field: i32,
}

impl A for B {
    fn something(&self) {}
}

There is a part of my code where I have an &A, and I know it is an instance of B, I would like to cast &A to &B. How can I accomplish this? So far, I have tried first casting to *const A and then casting to *const B, but I can't figure out how to go from *const B to &B.

After playing around with some things, I think this is correct?

fn some_func(a: &dyn A) {
    let a_ptr = a as *const A;
    let b_ptr = a_ptr as *const B;
    unsafe { let b = &*b_ptr as &B; }
}

"After playing around with some things, I think this is correct?" this is not really how I will call this kind of thing in rust. But yes this will "compile" — Stargateur, Sep 09 '18 at 19:53
Maybe if you added a bit of the code that produces the `A` we could find a way around the problem. — Aankhen, Sep 09 '18 at 20:12
If you have an `&A`, and you *know* it is an instance of `B`, why can't you prove that it is -- i.e., use `&B`? — trent, Sep 09 '18 at 21:37
@trentcl my code would be provable with tagged unions, but Rust doesn't have such functionality. I could probably use an enum, but that seems like a less fun way to write the code. — Max, Sep 10 '18 at 02:17

score 1 · Accepted Answer · answered Oct 07 '18 at 19:17

One way to do this safely is by adding a method fn any(&self) -> &Any to your trait, which carries type information and makes it possible to perform a checked downcast to an arbitrary type (in this case B).

use std::any::Any;

trait A {
    fn something(&self);
    fn any(&self) -> &Any;
}

struct B {
    some_field: i32,
}

impl A for B {
    fn something(&self) {}
    fn any(&self) -> &Any {
        self
    }
}

fn some_func(a: &dyn A) {
    let b: &B = a.any().downcast_ref().unwrap();
}

Convert between a reference to a trait and a struct that implements that trait in Rust

1 Answers1