Code shared from the Rust Playground

playground.rs

// Trait to be delegated
trait Work {
    fn do_work(&self) -> u64;
}

// Delegate trait, this would be in the std
// `Trait` refers to the trait we would delegate to
// We could imagine a variant of Delegate that would not accept a Trait 
// parameter and stand for delegation of inherent impl.
trait Delegate<Trait: ?Sized> {
    // The type to which we will delegate
    type Inner;

    fn inner(&self) -> &Self::Inner;

    fn inner_mut(&mut self) -> &mut Self::Inner;

    fn into_inner(self) -> Self::Inner;

    fn from_inner(delegate: Self::Inner) -> Self;
}

// Struct to which we will delegate: will stand for our Inner type
struct Worker {
    x: u64,
    y: u64,
}

// Implementation of our target trait
impl Work for Worker {
    fn do_work(&self) -> u64 {
        self.x + self.y
    }
}

// Struct that will delegate its Work implementation
struct Boss {
    worker: Worker,
}

// Declare that we delegate our Work implementation
impl Delegate<Work> for Boss {
    type Inner = Worker;

    fn inner(&self) -> &Self::Inner {
        &self.worker
    }

    fn inner_mut(&mut self) -> &mut Self::Inner {
        &mut self.worker
    }

    fn into_inner(self) -> Self::Inner {
        self.worker
    }

    fn from_inner(delegate: Self::Inner) -> Self {
        Boss { worker: delegate }
    }
}

// This would be generated by the compiler
impl<T: Work, U: Delegate<Work, Inner = T>> Work for U {
    fn do_work(&self) -> u64 {
        self.inner().do_work()
    }
}

fn main() {
    let boss = Boss {
        worker: Worker { x: 12, y: 30 },
    };
    println!("Here are my results: {}", boss.do_work());
}