fn ok_into<U>(self) -> OkInto<Self, U>
    where
        Self: Sized,
        Self::Ok: Into<U>,
    {
        assert_future::<Result<U, Self::Error>, _>(OkInto::new(self))
    }
Maps this future's [`Ok`](TryFuture::Ok) to a new type
using the [`Into`](std::convert::Into) trait.
futures-util/src/future/try_future/mod.rs