Downcasting an Arc<T> into Arc<U> where trait T: U

1

Is this possible? I've tried doing Arc<T + U> but this comes up with errors about U not being an auto-trait. Doing arc_t as Arc<U> comes up with errors about it not being a trivial cast. Really, I just want to share the control block and refcounts between the instances. I can change pretty much everything at the T layer, but changing the Arc<U> boundary of that crate isn't so easy (but is possible).

As for the use case, U is used in the low-level machinery (and from another crate which uses Arc<U> due to Send being required and U being something that really should be shared between the components), but T adds methods that are used a bit higher up (at this level, we know there is information locally, but there is also now some additional I/O that needs to go on). And there are still multiple implementations of T, so an enum doesn't work here either. But, if there's just a better way of doing it, I could start down that road as well.

The U library: Utils / rust-ghostflow ยท GitLab (U is HostingService)
The T is added in this MR in a CLI layer on top of that: https://gitlab.kitware.com/utils/rust-ghostflow/merge_requests/206 (T is LocalService which is basically a HostingService with some added things used because we know there's a specific repository on-disk that is being worked on)

2

A coworker was able to figure it out:

use std::sync::Arc;

trait U {
    fn u(&self);
}

trait T: U {
    fn as_arc_u(self: Arc<Self>) -> Arc<dyn U>;
}

struct A;
impl U for A {
    fn u(&self) {
        println!("u")
    }
}
impl T for A {
    fn as_arc_u(self: Arc<Self>) -> Arc<dyn U> {
        self
    }
}

fn f(a: Arc<dyn T>) -> Arc<dyn U> {
    a.as_arc_u()
}

fn main() {
    let a: Arc<dyn T> = Arc::new(A);
    a.clone().as_arc_u().u();
    f(a).u();
}

I had seen references to the same thing when searching on here for a solution with Option<dyn U> instead of Arc<dyn U> and the obvious default implementation, but hadn't tried to do it without the Option.

3

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