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    use std::fmt;
    use std::future::Future;
    use std::pin::Pin;
    use std::sync::Arc;

    #[cfg(feature = "server")]
    use crate::body::{Body, HttpBody};
    #[cfg(all(feature = "http2", feature = "server"))]
    use crate::proto::h2::server::H2Stream;
    use crate::rt::Executor;
    #[cfg(feature = "server")]
    use crate::server::conn::spawn_all::{NewSvcTask, Watcher};
    #[cfg(feature = "server")]
    use crate::service::HttpService;

    #[cfg(feature = "server")]
    pub trait ConnStreamExec<F, B: HttpBody>: Clone {
        fn execute_h2stream(&mut self, fut: H2Stream<F, B>);
    }

    #[cfg(feature = "server")]
    pub trait NewSvcExec<I, N, S: HttpService<Body>, E, W: Watcher<I, S, E>>: Clone {
        fn execute_new_svc(&mut self, fut: NewSvcTask<I, N, S, E, W>);
    }

    pub(crate) type BoxSendFuture = Pin<Box<dyn Future<Output = ()> + Send>>;

    // Either the user provides an executor for background tasks, or we use
    // `tokio::spawn`.
    #[derive(Clone)]
    pub enum Exec {
        Default,
        Executor(Arc<dyn Executor<BoxSendFuture> + Send + Sync>),
    }

    // ===== impl Exec =====

    impl Exec {
        pub(crate) fn execute<F>(&self, fut: F)
        where
            F: Future<Output = ()> + Send + 'static,
        {
            match *self {
                Exec::Default => {
                    #[cfg(feature = "tcp")]
                    {
                        tokio::task::spawn(fut);
                    }
                    #[cfg(not(feature = "tcp"))]
                    {
                        // If no runtime, we need an executor!
                        panic!("executor must be set")
                    }
                }
                Exec::Executor(ref e) => {
                    e.execute(Box::pin(fut));
                }
            }
        }
    }

    impl fmt::Debug for Exec {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Exec").finish()
        }
    }

    #[cfg(feature = "server")]
    impl<F, B> ConnStreamExec<F, B> for Exec
    where
        H2Stream<F, B>: Future<Output = ()> + Send + 'static,
        B: HttpBody,
    {
        fn execute_h2stream(&mut self, fut: H2Stream<F, B>) {
            self.execute(fut)
        }
    }

    #[cfg(feature = "server")]
    impl<I, N, S, E, W> NewSvcExec<I, N, S, E, W> for Exec
    where
        NewSvcTask<I, N, S, E, W>: Future<Output = ()> + Send + 'static,
        S: HttpService<Body>,
        W: Watcher<I, S, E>,
    {
        fn execute_new_svc(&mut self, fut: NewSvcTask<I, N, S, E, W>) {
            self.execute(fut)
        }
    }

    // ==== impl Executor =====

    #[cfg(feature = "server")]
    impl<E, F, B> ConnStreamExec<F, B> for E
    where
        E: Executor<H2Stream<F, B>> + Clone,
        H2Stream<F, B>: Future<Output = ()>,
        B: HttpBody,
    {
        fn execute_h2stream(&mut self, fut: H2Stream<F, B>) {
            self.execute(fut)
        }
    }

    #[cfg(feature = "server")]
    impl<I, N, S, E, W> NewSvcExec<I, N, S, E, W> for E
    where
        E: Executor<NewSvcTask<I, N, S, E, W>> + Clone,
        NewSvcTask<I, N, S, E, W>: Future<Output = ()>,
        S: HttpService<Body>,
        W: Watcher<I, S, E>,
    {
        fn execute_new_svc(&mut self, fut: NewSvcTask<I, N, S, E, W>) {
            self.execute(fut)
        }
    }

    // If http2 is not enable, we just have a stub here, so that the trait bounds
    // that *would* have been needed are still checked. Why?
    //
    // Because enabling `http2` shouldn't suddenly add new trait bounds that cause
    // a compilation error.
    #[cfg(not(feature = "http2"))]
    #[allow(missing_debug_implementations)]
    pub struct H2Stream<F, B>(std::marker::PhantomData<(F, B)>);

    #[cfg(not(feature = "http2"))]
    impl<F, B, E> Future for H2Stream<F, B>
    where
        F: Future<Output = Result<http::Response<B>, E>>,
        B: crate::body::HttpBody,
        B::Error: Into<Box<dyn std::error::Error + Send + Sync>>,
        E: Into<Box<dyn std::error::Error + Send + Sync>>,
    {
        type Output = ();

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