get_ownership.rs

fn main() {
    // This wont work if the T for Wrap::<T> comes from a generic 
    match (Wrap::<&str>::new()).status() {
        Status::Owned => println!("Owned"),
        Status::Shared => println!("Shared"),
        Status::Exclusive => println!("Exclusive"),
    }
}

enum Status {
    Owned,
    Shared,
    Exclusive,
}

struct Wrap<T: ?Sized>(std::marker::PhantomData<T>);

impl<T: ?Sized> Wrap<T> {
    pub fn new() -> Wrap<T> {
        Wrap(std::marker::PhantomData)
    }
}

trait GetStatus {
    fn status(self) -> Status;
}

impl<T: ?Sized> GetStatus for &Wrap<T> {
    fn status(self) -> Status {
        Status::Owned
    }
}

impl<T: ?Sized> GetStatus for Wrap<&T> {
    fn status(self) -> Status {
        Status::Shared
    }
}

impl<T: ?Sized> GetStatus for Wrap<&mut T> {
    fn status(self) -> Status {
        Status::Exclusive
    }
}