The trait `A` is not implemented for the type `A`

Question

I am trying to use a trait that has a function that takes a closure as argument, and then use it on a trait object.

trait A {
    fn f<P>(&self, p: P) where P: Fn() -> ();
}

struct B {
    a: Box<A>
}

impl B {
    fn c(&self) {
        self.a.f(|| {});
    }
}

This snippet generates the following error:

the trait `A` is not implemented for the type `A` [E0277]

The version of rustc is rustc 1.0.0-beta.3 (5241bf9c3 2015-04-25) (built 2015-04-25).

Answer 1

The problem is that method f is not object-safe because it is generic, and hence it can't be called on a trait object. You will have to force its users to pass boxed closure:

trait A {
    fn f(&self, p: Box<Fn() -> ()>);
}

I wonder why Rust allows Box<A> in the first place, I would expect an error there. And this particular error is really misleading. I would file a bug about this.

Alternatively, you can discard trait objects in favor of regular bounded generics, though it is not always possible.