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    cfg_rt! {
        pub(crate) use crate::runtime::spawn_blocking;
        pub(crate) use crate::task::JoinHandle;
    }

    cfg_not_rt! {
        use std::fmt;
        use std::future::Future;
        use std::pin::Pin;
        use std::task::{Context, Poll};

        pub(crate) fn spawn_blocking<F, R>(_f: F) -> JoinHandle<R>
        where
            F: FnOnce() -> R + Send + 'static,
            R: Send + 'static,
        {
            assert_send_sync::<JoinHandle<std::cell::Cell<()>>>();
            panic!("requires the `rt` Tokio feature flag")

        }

        pub(crate) struct JoinHandle<R> {
            _p: std::marker::PhantomData<R>,
        }

        unsafe impl<T: Send> Send for JoinHandle<T> {}
        unsafe impl<T: Send> Sync for JoinHandle<T> {}

        impl<R> Future for JoinHandle<R> {
            type Output = Result<R, std::io::Error>;

            fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
                unreachable!()
            }
        }

        impl<T> fmt::Debug for JoinHandle<T>
        where
            T: fmt::Debug,
        {
            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
                fmt.debug_struct("JoinHandle").finish()
            }
        }

        fn assert_send_sync<T: Send + Sync>() {
        }
    }