dyn Trait<T> + 'a

main.rs

trait Provider<T> {
    fn get(&self) -> T;
}

struct Owned<T>(T);

impl<T: Clone> Provider<T> for Owned<T> {
    fn get(&self) -> T {
        self.0.clone()
    }
}

struct Borrowed<'a, T>(&'a T);

impl<'a, T: Clone> Provider<T> for Borrowed<'a, T> {
    fn get(&self) -> T {
        self.0.clone()
    }
}

fn select_provider<'a, T: Clone + 'static>(owned: T, borrowed: &'a T) -> Box<dyn Provider<T> + 'a> {
    let args: Vec<_> = std::env::args().collect();
    if let Some("owned") = args.get(1).map(|x| x.as_str()) {
        Box::new(Owned(owned))
    } else {
        Box::new(Borrowed(borrowed))
    }
}

fn main() {
    let owned_msg = String::from("Hello, owned world!");
    let borrowed_msg = String::from("Hello, borrowed world!");
    let provider = select_provider(owned_msg, &borrowed_msg);
    println!("{}", provider.get());
}