fn unit_error(self) -> UnitError<Self>
    where
        Self: Sized,
    {
        assert_future::<Result<Self::Output, ()>, _>(UnitError::new(self))
    }
Turns a [`Future<Output = T>`](Future) into a
[`TryFuture<Ok = T, Error = ()`>](futures_core::future::TryFuture).
futures-util/src/future/future/mod.rs