#![allow(clippy::redundant_clone)]
#![warn(rust_2018_idioms)]
#![cfg(feature = "sync")]

#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
use wasm_bindgen_test::wasm_bindgen_test as test;
#[cfg(all(target_family = "wasm", not(target_os = "wasi")))]
use wasm_bindgen_test::wasm_bindgen_test as maybe_tokio_test;

#[cfg(not(all(target_family = "wasm", not(target_os = "wasi"))))]
use tokio::test as maybe_tokio_test;

use std::fmt;
use std::panic;
use std::sync::Arc;
use tokio::sync::mpsc;
use tokio::sync::mpsc::error::{TryRecvError, TrySendError};
use tokio_test::*;

#[cfg(not(target_family = "wasm"))]
mod support {
    pub(crate) mod mpsc_stream;
}

#[allow(unused)]
trait AssertRefUnwindSafe: panic::RefUnwindSafe {}
impl<T> AssertRefUnwindSafe for mpsc::OwnedPermit<T> {}
impl<'a, T> AssertRefUnwindSafe for mpsc::Permit<'a, T> {}
impl<'a, T> AssertRefUnwindSafe for mpsc::PermitIterator<'a, T> {}
impl<T> AssertRefUnwindSafe for mpsc::Receiver<T> {}
impl<T> AssertRefUnwindSafe for mpsc::Sender<T> {}
impl<T> AssertRefUnwindSafe for mpsc::UnboundedReceiver<T> {}
impl<T> AssertRefUnwindSafe for mpsc::UnboundedSender<T> {}
impl<T> AssertRefUnwindSafe for mpsc::WeakSender<T> {}
impl<T> AssertRefUnwindSafe for mpsc::WeakUnboundedSender<T> {}

#[allow(unused)]
trait AssertUnwindSafe: panic::UnwindSafe {}
impl<T> AssertUnwindSafe for mpsc::OwnedPermit<T> {}
impl<'a, T> AssertUnwindSafe for mpsc::Permit<'a, T> {}
impl<'a, T> AssertUnwindSafe for mpsc::PermitIterator<'a, T> {}
impl<T> AssertUnwindSafe for mpsc::Receiver<T> {}
impl<T> AssertUnwindSafe for mpsc::Sender<T> {}
impl<T> AssertUnwindSafe for mpsc::UnboundedReceiver<T> {}
impl<T> AssertUnwindSafe for mpsc::UnboundedSender<T> {}
impl<T> AssertUnwindSafe for mpsc::WeakSender<T> {}
impl<T> AssertUnwindSafe for mpsc::WeakUnboundedSender<T> {}

#[maybe_tokio_test]
async fn send_recv_with_buffer() {
    let (tx, mut rx) = mpsc::channel::<i32>(16);

    // Using poll_ready / try_send
    // let permit assert_ready_ok!(tx.reserve());
    let permit = tx.reserve().await.unwrap();
    permit.send(1);

    // Without poll_ready
    tx.try_send(2).unwrap();

    drop(tx);

    let val = rx.recv().await;
    assert_eq!(val, Some(1));

    let val = rx.recv().await;
    assert_eq!(val, Some(2));

    let val = rx.recv().await;
    assert!(val.is_none());
}

#[tokio::test]
#[cfg(feature = "full")]
async fn reserve_disarm() {
    let (tx, mut rx) = mpsc::channel::<i32>(2);
    let tx1 = tx.clone();
    let tx2 = tx.clone();
    let tx3 = tx.clone();
    let tx4 = tx;

    // We should be able to `poll_ready` two handles without problem
    let permit1 = assert_ok!(tx1.reserve().await);
    let permit2 = assert_ok!(tx2.reserve().await);

    // But a third should not be ready
    let mut r3 = tokio_test::task::spawn(tx3.reserve());
    assert_pending!(r3.poll());

    let mut r4 = tokio_test::task::spawn(tx4.reserve());
    assert_pending!(r4.poll());

    // Using one of the reserved slots should allow a new handle to become ready
    permit1.send(1);

    // We also need to receive for the slot to be free
    assert!(!r3.is_woken());
    rx.recv().await.unwrap();
    // Now there's a free slot!
    assert!(r3.is_woken());
    assert!(!r4.is_woken());

    // Dropping a permit should also open up a slot
    drop(permit2);
    assert!(r4.is_woken());

    let mut r1 = tokio_test::task::spawn(tx1.reserve());
    assert_pending!(r1.poll());
}

#[tokio::test]
#[cfg(all(feature = "full", not(target_os = "wasi")))] // Wasi doesn't support threads
async fn send_recv_stream_with_buffer() {
    use tokio_stream::StreamExt;

    let (tx, rx) = support::mpsc_stream::channel_stream::<i32>(16);
    let mut rx = Box::pin(rx);

    tokio::spawn(async move {
        assert_ok!(tx.send(1).await);
        assert_ok!(tx.send(2).await);
    });

    assert_eq!(Some(1), rx.next().await);
    assert_eq!(Some(2), rx.next().await);
    assert_eq!(None, rx.next().await);
}

#[tokio::test]
#[cfg(feature = "full")]
async fn async_send_recv_with_buffer() {
    let (tx, mut rx) = mpsc::channel(16);

    tokio::spawn(async move {
        assert_ok!(tx.send(1).await);
        assert_ok!(tx.send(2).await);
    });

    assert_eq!(Some(1), rx.recv().await);
    assert_eq!(Some(2), rx.recv().await);
    assert_eq!(None, rx.recv().await);
}

#[tokio::test]
#[cfg(feature = "full")]
async fn async_send_recv_many_with_buffer() {
    let (tx, mut rx) = mpsc::channel(2);
    let mut buffer = Vec::<i32>::with_capacity(3);

    // With `limit=0` does not sleep, returns immediately
    assert_eq!(0, rx.recv_many(&mut buffer, 0).await);

    let handle = tokio::spawn(async move {
        assert_ok!(tx.send(1).await);
        assert_ok!(tx.send(2).await);
        assert_ok!(tx.send(7).await);
        assert_ok!(tx.send(0).await);
    });

    let limit = 3;
    let mut recv_count = 0usize;
    while recv_count < 4 {
        recv_count += rx.recv_many(&mut buffer, limit).await;
        assert_eq!(buffer.len(), recv_count);
    }

    assert_eq!(vec![1, 2, 7, 0], buffer);
    assert_eq!(0, rx.recv_many(&mut buffer, limit).await);
    handle.await.unwrap();
}

#[tokio::test]
#[cfg(feature = "full")]
async fn start_send_past_cap() {
    use std::future::Future;

    let mut t1 = tokio_test::task::spawn(());

    let (tx1, mut rx) = mpsc::channel(1);
    let tx2 = tx1.clone();

    assert_ok!(tx1.try_send(()));

    let mut r1 = Box::pin(tx1.reserve());
    t1.enter(|cx, _| assert_pending!(r1.as_mut().poll(cx)));

    {
        let mut r2 = tokio_test::task::spawn(tx2.reserve());
        assert_pending!(r2.poll());

        drop(r1);

        assert!(rx.recv().await.is_some());

        assert!(r2.is_woken());
        assert!(!t1.is_woken());
    }

    drop(tx1);
    drop(tx2);

    assert!(rx.recv().await.is_none());
}

#[test]
#[should_panic]
#[cfg(not(target_family = "wasm"))] // wasm currently doesn't support unwinding
fn buffer_gteq_one() {
    mpsc::channel::<i32>(0);
}

#[maybe_tokio_test]
async fn send_recv_unbounded() {
    let (tx, mut rx) = mpsc::unbounded_channel::<i32>();

    // Using `try_send`
    assert_ok!(tx.send(1));
    assert_ok!(tx.send(2));

    assert_eq!(rx.recv().await, Some(1));
    assert_eq!(rx.recv().await, Some(2));

    drop(tx);

    assert!(rx.recv().await.is_none());
}

#[maybe_tokio_test]
async fn send_recv_many_unbounded() {
    let (tx, mut rx) = mpsc::unbounded_channel::<i32>();

    let mut buffer: Vec<i32> = Vec::new();

    // With `limit=0` does not sleep, returns immediately
    rx.recv_many(&mut buffer, 0).await;
    assert_eq!(0, buffer.len());

    assert_ok!(tx.send(7));
    assert_ok!(tx.send(13));
    assert_ok!(tx.send(100));
    assert_ok!(tx.send(1002));

    rx.recv_many(&mut buffer, 0).await;
    assert_eq!(0, buffer.len());

    let mut count = 0;
    while count < 4 {
        count += rx.recv_many(&mut buffer, 1).await;
    }
    assert_eq!(count, 4);
    assert_eq!(vec![7, 13, 100, 1002], buffer);
    let final_capacity = buffer.capacity();
    assert!(final_capacity > 0);

    buffer.clear();

    assert_ok!(tx.send(5));
    assert_ok!(tx.send(6));
    assert_ok!(tx.send(7));
    assert_ok!(tx.send(2));

    // Re-use existing capacity
    count = rx.recv_many(&mut buffer, 32).await;

    assert_eq!(final_capacity, buffer.capacity());
    assert_eq!(count, 4);
    assert_eq!(vec![5, 6, 7, 2], buffer);

    drop(tx);

    // recv_many will immediately return zero if the channel
    // is closed and no more messages are waiting
    assert_eq!(0, rx.recv_many(&mut buffer, 4).await);
    assert!(rx.recv().await.is_none());
}

#[tokio::test]
#[cfg(feature = "full")]
async fn send_recv_many_bounded_capacity() {
    let mut buffer: Vec<String> = Vec::with_capacity(9);
    let limit = buffer.capacity();
    let (tx, mut rx) = mpsc::channel(100);

    let mut expected: Vec<String> = (0..limit)
        .map(|x: usize| format!("{x}"))
        .collect::<Vec<_>>();
    for x in expected.clone() {
        tx.send(x).await.unwrap()
    }
    tx.send("one more".to_string()).await.unwrap();

    // Here `recv_many` receives all but the last value;
    // the initial capacity is adequate, so the buffer does
    // not increase in side.
    assert_eq!(buffer.capacity(), rx.recv_many(&mut buffer, limit).await);
    assert_eq!(expected, buffer);
    assert_eq!(limit, buffer.capacity());

    // Receive up more values:
    assert_eq!(1, rx.recv_many(&mut buffer, limit).await);
    assert!(buffer.capacity() > limit);
    expected.push("one more".to_string());
    assert_eq!(expected, buffer);

    tokio::spawn(async move {
        tx.send("final".to_string()).await.unwrap();
    });

    // 'tx' is dropped, but `recv_many` is guaranteed not
    // to return 0 as the channel has outstanding permits
    assert_eq!(1, rx.recv_many(&mut buffer, limit).await);
    expected.push("final".to_string());
    assert_eq!(expected, buffer);
    // The channel is now closed and `recv_many` returns 0.
    assert_eq!(0, rx.recv_many(&mut buffer, limit).await);
    assert_eq!(expected, buffer);
}

#[tokio::test]
#[cfg(feature = "full")]
async fn send_recv_many_unbounded_capacity() {
    let mut buffer: Vec<String> = Vec::with_capacity(9); // capacity >= 9
    let limit = buffer.capacity();
    let (tx, mut rx) = mpsc::unbounded_channel();

    let mut expected: Vec<String> = (0..limit)
        .map(|x: usize| format!("{x}"))
        .collect::<Vec<_>>();
    for x in expected.clone() {
        tx.send(x).unwrap()
    }
    tx.send("one more".to_string()).unwrap();

    // Here `recv_many` receives all but the last value;
    // the initial capacity is adequate, so the buffer does
    // not increase in side.
    assert_eq!(buffer.capacity(), rx.recv_many(&mut buffer, limit).await);
    assert_eq!(expected, buffer);
    assert_eq!(limit, buffer.capacity());

    // Receive up more values:
    assert_eq!(1, rx.recv_many(&mut buffer, limit).await);
    assert!(buffer.capacity() > limit);
    expected.push("one more".to_string());
    assert_eq!(expected, buffer);

    tokio::spawn(async move {
        tx.send("final".to_string()).unwrap();
    });

    // 'tx' is dropped, but `recv_many` is guaranteed not
    // to return 0 as the channel has outstanding permits
    assert_eq!(1, rx.recv_many(&mut buffer, limit).await);
    expected.push("final".to_string());
    assert_eq!(expected, buffer);
    // The channel is now closed and `recv_many` returns 0.
    assert_eq!(0, rx.recv_many(&mut buffer, limit).await);
    assert_eq!(expected, buffer);
}

#[maybe_tokio_test]
async fn recv_many_with_non_empty_buffer_bounded_rx_closed_and_idle() {
    let (_tx, mut rx) = mpsc::channel::<i32>(1);

    let mut buffer: Vec<i32> = vec![1];

    rx.close();

    assert_eq!(0, rx.recv_many(&mut buffer, 1).await);
    assert_eq!(vec![1], buffer);
}

#[maybe_tokio_test]
async fn recv_many_with_non_empty_buffer_unbounded_rx_closed_and_idle() {
    let (_tx, mut rx) = mpsc::unbounded_channel::<i32>();

    let mut buffer: Vec<i32> = vec![1];

    rx.close();

    assert_eq!(0, rx.recv_many(&mut buffer, 1).await);
    assert_eq!(vec![1], buffer);
}

#[tokio::test]
#[cfg(feature = "full")]
async fn async_send_recv_unbounded() {
    let (tx, mut rx) = mpsc::unbounded_channel();

    tokio::spawn(async move {
        assert_ok!(tx.send(1));
        assert_ok!(tx.send(2));
    });

    assert_eq!(Some(1), rx.recv().await);
    assert_eq!(Some(2), rx.recv().await);
    assert_eq!(None, rx.recv().await);
}

#[tokio::test]
#[cfg(all(feature = "full", not(target_os = "wasi")))] // Wasi doesn't support threads
async fn send_recv_stream_unbounded() {
    use tokio_stream::StreamExt;

    let (tx, rx) = support::mpsc_stream::unbounded_channel_stream::<i32>();

    let mut rx = Box::pin(rx);

    tokio::spawn(async move {
        assert_ok!(tx.send(1));
        assert_ok!(tx.send(2));
    });

    assert_eq!(Some(1), rx.next().await);
    assert_eq!(Some(2), rx.next().await);
    assert_eq!(None, rx.next().await);
}

#[maybe_tokio_test]
async fn no_t_bounds_buffer() {
    struct NoImpls;

    let (tx, mut rx) = mpsc::channel(100);

    // sender should be Debug even though T isn't Debug
    is_debug(&tx);
    // same with Receiver
    is_debug(&rx);
    // and sender should be Clone even though T isn't Clone
    assert!(tx.clone().try_send(NoImpls).is_ok());

    assert!(rx.recv().await.is_some());
}

#[maybe_tokio_test]
async fn no_t_bounds_unbounded() {
    struct NoImpls;

    let (tx, mut rx) = mpsc::unbounded_channel();

    // sender should be Debug even though T isn't Debug
    is_debug(&tx);
    // same with Receiver
    is_debug(&rx);
    // and sender should be Clone even though T isn't Clone
    assert!(tx.clone().send(NoImpls).is_ok());

    assert!(rx.recv().await.is_some());
}

#[tokio::test]
#[cfg(feature = "full")]
async fn send_recv_buffer_limited() {
    let (tx, mut rx) = mpsc::channel::<i32>(1);

    // Reserve capacity
    let p1 = assert_ok!(tx.reserve().await);

    // Send first message
    p1.send(1);

    // Not ready
    let mut p2 = tokio_test::task::spawn(tx.reserve());
    assert_pending!(p2.poll());

    // Take the value
    assert!(rx.recv().await.is_some());

    // Notified
    assert!(p2.is_woken());

    // Trying to send fails
    assert_err!(tx.try_send(1337));

    // Send second
    let permit = assert_ready_ok!(p2.poll());
    permit.send(2);

    assert!(rx.recv().await.is_some());
}

#[maybe_tokio_test]
async fn recv_close_gets_none_idle() {
    let (tx, mut rx) = mpsc::channel::<i32>(10);

    rx.close();

    assert!(rx.recv().await.is_none());

    assert_err!(tx.send(1).await);
}

#[tokio::test]
#[cfg(feature = "full")]
async fn recv_close_gets_none_reserved() {
    let (tx1, mut rx) = mpsc::channel::<i32>(1);
    let tx2 = tx1.clone();

    let permit1 = assert_ok!(tx1.reserve().await);
    let mut permit2 = tokio_test::task::spawn(tx2.reserve());
    assert_pending!(permit2.poll());

    rx.close();

    assert!(permit2.is_woken());
    assert_ready_err!(permit2.poll());

    {
        let mut recv = tokio_test::task::spawn(rx.recv());
        assert_pending!(recv.poll());

        permit1.send(123);
        assert!(recv.is_woken());

        let v = assert_ready!(recv.poll());
        assert_eq!(v, Some(123));
    }

    assert!(rx.recv().await.is_none());
}

#[maybe_tokio_test]
async fn tx_close_gets_none() {
    let (_, mut rx) = mpsc::channel::<i32>(10);
    assert!(rx.recv().await.is_none());
}

#[maybe_tokio_test]
async fn try_send_fail() {
    let (tx, mut rx) = mpsc::channel(1);

    tx.try_send("hello").unwrap();

    // This should fail
    match assert_err!(tx.try_send("fail")) {
        TrySendError::Full(..) => {}
        _ => panic!(),
    }

    assert_eq!(rx.recv().await, Some("hello"));

    assert_ok!(tx.try_send("goodbye"));
    drop(tx);

    assert_eq!(rx.recv().await, Some("goodbye"));
    assert!(rx.recv().await.is_none());
}

#[maybe_tokio_test]
async fn try_send_fail_with_try_recv() {
    let (tx, mut rx) = mpsc::channel(1);

    tx.try_send("hello").unwrap();

    // This should fail
    match assert_err!(tx.try_send("fail")) {
        TrySendError::Full(..) => {}
        _ => panic!(),
    }

    assert_eq!(rx.try_recv(), Ok("hello"));

    assert_ok!(tx.try_send("goodbye"));
    drop(tx);

    assert_eq!(rx.try_recv(), Ok("goodbye"));
    assert_eq!(rx.try_recv(), Err(TryRecvError::Disconnected));
}

#[maybe_tokio_test]
async fn reserve_many_above_cap() {
    const MAX_PERMITS: usize = tokio::sync::Semaphore::MAX_PERMITS;
    let (tx, _rx) = mpsc::channel::<()>(1);

    assert_err!(tx.reserve_many(2).await);
    assert_err!(tx.reserve_many(MAX_PERMITS + 1).await);
    assert_err!(tx.reserve_many(usize::MAX).await);
}

#[test]
fn try_reserve_many_zero() {
    let (tx, rx) = mpsc::channel::<()>(1);

    // Succeeds when not closed.
    assert!(assert_ok!(tx.try_reserve_many(0)).next().is_none());

    // Even when channel is full.
    tx.try_send(()).unwrap();
    assert!(assert_ok!(tx.try_reserve_many(0)).next().is_none());

    drop(rx);

    // Closed error when closed.
    assert_eq!(
        assert_err!(tx.try_reserve_many(0)),
        TrySendError::Closed(())
    );
}

#[maybe_tokio_test]
async fn reserve_many_zero() {
    let (tx, rx) = mpsc::channel::<()>(1);

    // Succeeds when not closed.
    assert!(assert_ok!(tx.reserve_many(0).await).next().is_none());

    // Even when channel is full.
    tx.send(()).await.unwrap();
    assert!(assert_ok!(tx.reserve_many(0).await).next().is_none());

    drop(rx);

    // Closed error when closed.
    assert_err!(tx.reserve_many(0).await);
}

#[maybe_tokio_test]
async fn try_reserve_many_edge_cases() {
    const MAX_PERMITS: usize = tokio::sync::Semaphore::MAX_PERMITS;

    let (tx, rx) = mpsc::channel::<()>(1);

    let mut permit = assert_ok!(tx.try_reserve_many(0));
    assert!(permit.next().is_none());

    let permit = tx.try_reserve_many(MAX_PERMITS + 1);
    match assert_err!(permit) {
        TrySendError::Full(..) => {}
        _ => panic!(),
    }

    let permit = tx.try_reserve_many(usize::MAX);
    match assert_err!(permit) {
        TrySendError::Full(..) => {}
        _ => panic!(),
    }

    // Dropping the receiver should close the channel
    drop(rx);
    assert_err!(tx.reserve_many(0).await);
}

#[maybe_tokio_test]
async fn try_reserve_fails() {
    let (tx, mut rx) = mpsc::channel(1);

    let permit = tx.try_reserve().unwrap();

    // This should fail
    match assert_err!(tx.try_reserve()) {
        TrySendError::Full(()) => {}
        _ => panic!(),
    }

    permit.send("foo");

    assert_eq!(rx.recv().await, Some("foo"));

    // Dropping permit releases the slot.
    let permit = tx.try_reserve().unwrap();
    drop(permit);

    let _permit = tx.try_reserve().unwrap();
}

#[maybe_tokio_test]
async fn reserve_many_and_send() {
    let (tx, mut rx) = mpsc::channel(100);
    for i in 0..100 {
        for permit in assert_ok!(tx.reserve_many(i).await) {
            permit.send("foo");
            assert_eq!(rx.recv().await, Some("foo"));
        }
        assert_eq!(rx.try_recv(), Err(TryRecvError::Empty));
    }
}
#[maybe_tokio_test]
async fn try_reserve_many_and_send() {
    let (tx, mut rx) = mpsc::channel(100);
    for i in 0..100 {
        for permit in assert_ok!(tx.try_reserve_many(i)) {
            permit.send("foo");
            assert_eq!(rx.recv().await, Some("foo"));
        }
        assert_eq!(rx.try_recv(), Err(TryRecvError::Empty));
    }
}

#[maybe_tokio_test]
async fn reserve_many_on_closed_channel() {
    let (tx, rx) = mpsc::channel::<()>(100);
    drop(rx);
    assert_err!(tx.reserve_many(10).await);
}

#[maybe_tokio_test]
async fn try_reserve_many_on_closed_channel() {
    let (tx, rx) = mpsc::channel::<usize>(100);
    drop(rx);
    match assert_err!(tx.try_reserve_many(10)) {
        TrySendError::Closed(()) => {}
        _ => panic!(),
    };
}

#[maybe_tokio_test]
#[cfg_attr(miri, ignore)] // Too slow on miri.
async fn try_reserve_many_full() {
    // Reserve n capacity and send k messages
    for n in 1..100 {
        for k in 0..n {
            let (tx, mut rx) = mpsc::channel::<usize>(n);
            let permits = assert_ok!(tx.try_reserve_many(n));

            assert_eq!(permits.len(), n);
            assert_eq!(tx.capacity(), 0);

            match assert_err!(tx.try_reserve_many(1)) {
                TrySendError::Full(..) => {}
                _ => panic!(),
            };

            for permit in permits.take(k) {
                permit.send(0);
            }
            // We only used k permits on the n reserved
            assert_eq!(tx.capacity(), n - k);

            // We can reserve more permits
            assert_ok!(tx.try_reserve_many(1));

            // But not more than the current capacity
            match assert_err!(tx.try_reserve_many(n - k + 1)) {
                TrySendError::Full(..) => {}
                _ => panic!(),
            };

            for _i in 0..k {
                assert_eq!(rx.recv().await, Some(0));
            }

            // Now that we've received everything, capacity should be back to n
            assert_eq!(tx.capacity(), n);
        }
    }
}

#[tokio::test]
#[cfg(feature = "full")]
async fn drop_permit_releases_permit() {
    // poll_ready reserves capacity, ensure that the capacity is released if tx
    // is dropped w/o sending a value.
    let (tx1, _rx) = mpsc::channel::<i32>(1);
    let tx2 = tx1.clone();

    let permit = assert_ok!(tx1.reserve().await);

    let mut reserve2 = tokio_test::task::spawn(tx2.reserve());
    assert_pending!(reserve2.poll());

    drop(permit);

    assert!(reserve2.is_woken());
    assert_ready_ok!(reserve2.poll());
}

#[maybe_tokio_test]
async fn drop_permit_iterator_releases_permits() {
    // poll_ready reserves capacity, ensure that the capacity is released if tx
    // is dropped w/o sending a value.
    for n in 1..100 {
        let (tx1, _rx) = mpsc::channel::<i32>(n);
        let tx2 = tx1.clone();

        let permits = assert_ok!(tx1.reserve_many(n).await);

        let mut reserve2 = tokio_test::task::spawn(tx2.reserve_many(n));
        assert_pending!(reserve2.poll());

        drop(permits);

        assert!(reserve2.is_woken());

        let permits = assert_ready_ok!(reserve2.poll());
        drop(permits);

        assert_eq!(tx1.capacity(), n);
    }
}

#[test]
fn dropping_last_permit_wakes_closed_receiver() {
    let (tx, mut rx) = mpsc::channel::<()>(100);

    let permit = tx.try_reserve().unwrap();
    rx.close();

    let mut recv = tokio_test::task::spawn(rx.recv());
    assert_pending!(recv.poll());
    drop(permit);
    assert!(recv.is_woken());
    assert_ready!(recv.poll());
}

#[test]
fn dropping_last_owned_permit_wakes_closed_receiver() {
    let (tx, mut rx) = mpsc::channel::<()>(100);

    let permit = tx.try_reserve_owned().unwrap();
    rx.close();

    let mut recv = tokio_test::task::spawn(rx.recv());
    assert_pending!(recv.poll());
    drop(permit);
    assert!(recv.is_woken());
    assert_ready!(recv.poll());
}

#[test]
fn dropping_last_permit_iterator_wakes_closed_receiver() {
    let (tx, mut rx) = mpsc::channel::<()>(100);

    let permits = tx.try_reserve_many(1).unwrap();
    rx.close();

    let mut recv = tokio_test::task::spawn(rx.recv());
    assert_pending!(recv.poll());
    drop(permits);
    assert!(recv.is_woken());
    assert_ready!(recv.poll());
}

#[test]
fn sending_last_permit_wakes_closed_receiver() {
    let (tx, mut rx) = mpsc::channel::<()>(100);

    let permit = tx.try_reserve().unwrap();
    rx.close();

    let mut recv = tokio_test::task::spawn(rx.recv());
    assert_pending!(recv.poll());
    permit.send(());
    assert!(recv.is_woken());
    assert_ready!(recv.poll());
}

#[test]
fn sending_last_owned_permit_wakes_closed_receiver() {
    let (tx, mut rx) = mpsc::channel::<()>(100);

    let permit = tx.try_reserve_owned().unwrap();
    rx.close();

    let mut recv = tokio_test::task::spawn(rx.recv());
    assert_pending!(recv.poll());
    permit.send(());
    assert!(recv.is_woken());
    assert_ready!(recv.poll());
}

#[test]
fn releasing_last_owned_permit_wakes_closed_receiver() {
    let (tx, mut rx) = mpsc::channel::<()>(100);

    let permit = tx.try_reserve_owned().unwrap();
    rx.close();

    let mut recv = tokio_test::task::spawn(rx.recv());
    assert_pending!(recv.poll());
    let inert_sender = permit.release();
    assert!(recv.is_woken());
    assert_ready!(recv.poll());
    drop(inert_sender);
}

#[maybe_tokio_test]
async fn dropping_rx_closes_channel() {
    let (tx, rx) = mpsc::channel(100);

    let msg = Arc::new(());
    assert_ok!(tx.try_send(msg.clone()));

    drop(rx);
    assert_err!(tx.reserve().await);
    assert_err!(tx.reserve_many(10).await);
    assert_eq!(1, Arc::strong_count(&msg));
}

#[test]
fn dropping_rx_closes_channel_for_try() {
    let (tx, rx) = mpsc::channel(100);

    let msg = Arc::new(());
    tx.try_send(msg.clone()).unwrap();

    drop(rx);

    assert!(matches!(
        tx.try_send(msg.clone()),
        Err(TrySendError::Closed(_))
    ));
    assert!(matches!(tx.try_reserve(), Err(TrySendError::Closed(_))));
    assert!(matches!(
        tx.try_reserve_owned(),
        Err(TrySendError::Closed(_))
    ));

    assert_eq!(1, Arc::strong_count(&msg));
}

#[test]
fn unconsumed_messages_are_dropped() {
    let msg = Arc::new(());

    let (tx, rx) = mpsc::channel(100);

    tx.try_send(msg.clone()).unwrap();

    assert_eq!(2, Arc::strong_count(&msg));

    drop((tx, rx));

    assert_eq!(1, Arc::strong_count(&msg));
}

#[test]
#[cfg(all(feature = "full", not(target_os = "wasi")))] // Wasi doesn't support threads
fn blocking_recv() {
    let (tx, mut rx) = mpsc::channel::<u8>(1);

    let sync_code = std::thread::spawn(move || {
        assert_eq!(Some(10), rx.blocking_recv());
    });

    tokio::runtime::Runtime::new()
        .unwrap()
        .block_on(async move {
            let _ = tx.send(10).await;
        });
    sync_code.join().unwrap()
}

#[tokio::test]
#[should_panic]
#[cfg(not(target_family = "wasm"))] // wasm currently doesn't support unwinding
async fn blocking_recv_async() {
    let (_tx, mut rx) = mpsc::channel::<()>(1);
    let _ = rx.blocking_recv();
}

#[test]
#[cfg(all(feature = "full", not(target_os = "wasi")))] // Wasi doesn't support threads
fn blocking_send() {
    let (tx, mut rx) = mpsc::channel::<u8>(1);

    let sync_code = std::thread::spawn(move || {
        tx.blocking_send(10).unwrap();
    });

    tokio::runtime::Runtime::new()
        .unwrap()
        .block_on(async move {
            assert_eq!(Some(10), rx.recv().await);
        });
    sync_code.join().unwrap()
}

#[tokio::test]
#[should_panic]
#[cfg(not(target_family = "wasm"))] // wasm currently doesn't support unwinding
async fn blocking_send_async() {
    let (tx, _rx) = mpsc::channel::<()>(1);
    let _ = tx.blocking_send(());
}

#[tokio::test]
#[cfg(feature = "full")]
async fn ready_close_cancel_bounded() {
    let (tx, mut rx) = mpsc::channel::<()>(100);
    let _tx2 = tx.clone();

    let permit = assert_ok!(tx.reserve().await);

    rx.close();

    let mut recv = tokio_test::task::spawn(rx.recv());
    assert_pending!(recv.poll());

    drop(permit);

    assert!(recv.is_woken());
    let val = assert_ready!(recv.poll());
    assert!(val.is_none());
}

#[tokio::test]
#[cfg(feature = "full")]
async fn permit_available_not_acquired_close() {
    let (tx1, mut rx) = mpsc::channel::<()>(1);
    let tx2 = tx1.clone();

    let permit1 = assert_ok!(tx1.reserve().await);

    let mut permit2 = tokio_test::task::spawn(tx2.reserve());
    assert_pending!(permit2.poll());

    rx.close();

    drop(permit1);
    assert!(permit2.is_woken());

    drop(permit2);
    assert!(rx.recv().await.is_none());
}

#[test]
fn try_recv_bounded() {
    let (tx, mut rx) = mpsc::channel(5);

    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    assert!(tx.try_send("hello").is_err());

    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());

    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    assert_eq!(Ok("hello"), rx.try_recv());
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    assert!(tx.try_send("hello").is_err());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());

    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    tx.try_send("hello").unwrap();
    drop(tx);
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Ok("hello"), rx.try_recv());
    assert_eq!(Err(TryRecvError::Disconnected), rx.try_recv());
}

#[test]
fn try_recv_unbounded() {
    for num in 0..100 {
        let (tx, mut rx) = mpsc::unbounded_channel();

        for i in 0..num {
            tx.send(i).unwrap();
        }

        for i in 0..num {
            assert_eq!(rx.try_recv(), Ok(i));
        }

        assert_eq!(rx.try_recv(), Err(TryRecvError::Empty));
        drop(tx);
        assert_eq!(rx.try_recv(), Err(TryRecvError::Disconnected));
    }
}

#[test]
fn try_recv_after_receiver_close() {
    let (_tx, mut rx) = mpsc::channel::<()>(5);

    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());
    rx.close();
    assert_eq!(Err(TryRecvError::Disconnected), rx.try_recv());
}

#[test]
fn try_recv_after_receiver_close_with_permit() {
    let (tx, mut rx) = mpsc::channel::<()>(5);

    let permit = tx.try_reserve().unwrap();

    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());
    rx.close();
    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());
    drop(permit);
    assert_eq!(Err(TryRecvError::Disconnected), rx.try_recv());
}

#[test]
fn try_recv_close_while_empty_bounded() {
    let (tx, mut rx) = mpsc::channel::<()>(5);

    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());
    drop(tx);
    assert_eq!(Err(TryRecvError::Disconnected), rx.try_recv());
}

#[test]
fn try_recv_close_while_empty_unbounded() {
    let (tx, mut rx) = mpsc::unbounded_channel::<()>();

    assert_eq!(Err(TryRecvError::Empty), rx.try_recv());
    drop(tx);
    assert_eq!(Err(TryRecvError::Disconnected), rx.try_recv());
}

#[tokio::test(start_paused = true)]
#[cfg(feature = "full")]
async fn recv_timeout() {
    use tokio::sync::mpsc::error::SendTimeoutError::{Closed, Timeout};
    use tokio::time::Duration;

    let (tx, rx) = mpsc::channel(5);

    assert_eq!(tx.send_timeout(10, Duration::from_secs(1)).await, Ok(()));
    assert_eq!(tx.send_timeout(20, Duration::from_secs(1)).await, Ok(()));
    assert_eq!(tx.send_timeout(30, Duration::from_secs(1)).await, Ok(()));
    assert_eq!(tx.send_timeout(40, Duration::from_secs(1)).await, Ok(()));
    assert_eq!(tx.send_timeout(50, Duration::from_secs(1)).await, Ok(()));
    assert_eq!(
        tx.send_timeout(60, Duration::from_secs(1)).await,
        Err(Timeout(60))
    );

    drop(rx);
    assert_eq!(
        tx.send_timeout(70, Duration::from_secs(1)).await,
        Err(Closed(70))
    );
}

#[test]
#[should_panic = "there is no reactor running, must be called from the context of a Tokio 1.x runtime"]
#[cfg(not(target_family = "wasm"))] // wasm currently doesn't support unwinding
fn recv_timeout_panic() {
    use futures::future::FutureExt;
    use tokio::time::Duration;

    let (tx, _rx) = mpsc::channel(5);
    tx.send_timeout(10, Duration::from_secs(1)).now_or_never();
}

// Tests that channel `capacity` changes and `max_capacity` stays the same
#[tokio::test]
async fn test_tx_capacity() {
    let (tx, _rx) = mpsc::channel::<()>(10);
    // both capacities are same before
    assert_eq!(tx.capacity(), 10);
    assert_eq!(tx.max_capacity(), 10);

    let _permit = tx.reserve().await.unwrap();
    // after reserve, only capacity should drop by one
    assert_eq!(tx.capacity(), 9);
    assert_eq!(tx.max_capacity(), 10);

    tx.send(()).await.unwrap();
    // after send, capacity should drop by one again
    assert_eq!(tx.capacity(), 8);
    assert_eq!(tx.max_capacity(), 10);
}

#[tokio::test]
async fn test_rx_is_closed_when_calling_close_with_sender() {
    // is_closed should return true after calling close but still has a sender
    let (_tx, mut rx) = mpsc::channel::<()>(10);
    rx.close();

    assert!(rx.is_closed());
}

#[tokio::test]
async fn test_rx_is_closed_when_dropping_all_senders() {
    // is_closed should return true after dropping all senders
    let (tx, rx) = mpsc::channel::<()>(10);
    let another_tx = tx.clone();
    let task = tokio::spawn(async move {
        drop(another_tx);
    });

    drop(tx);
    let _ = task.await;

    assert!(rx.is_closed());
}

#[tokio::test]
async fn test_rx_is_not_closed_when_there_are_senders() {
    // is_closed should return false when there is a sender
    let (_tx, rx) = mpsc::channel::<()>(10);
    assert!(!rx.is_closed());
}

#[tokio::test]
async fn test_rx_is_not_closed_when_there_are_senders_and_buffer_filled() {
    // is_closed should return false when there is a sender, even if enough messages have been sent to fill the channel
    let (tx, rx) = mpsc::channel(10);
    for i in 0..10 {
        assert!(tx.send(i).await.is_ok());
    }
    assert!(!rx.is_closed());
}

#[tokio::test]
async fn test_rx_is_closed_when_there_are_no_senders_and_there_are_messages() {
    // is_closed should return true when there are messages in the buffer, but no senders
    let (tx, rx) = mpsc::channel(10);
    for i in 0..10 {
        assert!(tx.send(i).await.is_ok());
    }
    drop(tx);
    assert!(rx.is_closed());
}

#[tokio::test]
async fn test_rx_is_closed_when_there_are_messages_and_close_is_called() {
    // is_closed should return true when there are messages in the buffer, and close is called
    let (tx, mut rx) = mpsc::channel(10);
    for i in 0..10 {
        assert!(tx.send(i).await.is_ok());
    }
    rx.close();
    assert!(rx.is_closed());
}

#[tokio::test]
async fn test_rx_is_not_closed_when_there_are_permits_but_not_senders() {
    // is_closed should return false when there is a permit (but no senders)
    let (tx, rx) = mpsc::channel::<()>(10);
    let _permit = tx.reserve_owned().await.expect("Failed to reserve permit");
    assert!(!rx.is_closed());
}

#[tokio::test]
async fn test_rx_is_empty_when_no_messages_were_sent() {
    let (_tx, rx) = mpsc::channel::<()>(10);
    assert!(rx.is_empty())
}

#[tokio::test]
async fn test_rx_is_not_empty_when_there_are_messages_in_the_buffer() {
    let (tx, rx) = mpsc::channel::<()>(10);
    assert!(tx.send(()).await.is_ok());
    assert!(!rx.is_empty())
}

#[tokio::test]
async fn test_rx_is_not_empty_when_the_buffer_is_full() {
    let (tx, rx) = mpsc::channel(10);
    for i in 0..10 {
        assert!(tx.send(i).await.is_ok());
    }
    assert!(!rx.is_empty())
}

#[tokio::test]
async fn test_rx_is_not_empty_when_all_but_one_messages_are_consumed() {
    let (tx, mut rx) = mpsc::channel(10);
    for i in 0..10 {
        assert!(tx.send(i).await.is_ok());
    }

    for _ in 0..9 {
        assert!(rx.recv().await.is_some());
    }

    assert!(!rx.is_empty())
}

#[tokio::test]
async fn test_rx_is_empty_when_all_messages_are_consumed() {
    let (tx, mut rx) = mpsc::channel(10);
    for i in 0..10 {
        assert!(tx.send(i).await.is_ok());
    }
    while rx.try_recv().is_ok() {}
    assert!(rx.is_empty())
}

#[tokio::test]
async fn test_rx_is_empty_all_senders_are_dropped_and_messages_consumed() {
    let (tx, mut rx) = mpsc::channel(10);
    for i in 0..10 {
        assert!(tx.send(i).await.is_ok());
    }
    drop(tx);

    for _ in 0..10 {
        assert!(rx.recv().await.is_some());
    }

    assert!(rx.is_empty())
}

#[tokio::test]
async fn test_rx_len_on_empty_channel() {
    let (_tx, rx) = mpsc::channel::<()>(100);
    assert_eq!(rx.len(), 0);
}

#[tokio::test]
async fn test_rx_len_on_empty_channel_without_senders() {
    // when all senders are dropped, a "closed" value is added to the end of the linked list.
    // here we test that the "closed" value does not change the len of the channel.

    let (tx, rx) = mpsc::channel::<()>(100);
    drop(tx);
    assert_eq!(rx.len(), 0);
}

#[tokio::test]
async fn test_rx_len_on_filled_channel() {
    let (tx, rx) = mpsc::channel(100);

    for i in 0..100 {
        assert!(tx.send(i).await.is_ok());
    }
    assert_eq!(rx.len(), 100);
}

#[tokio::test]
async fn test_rx_len_on_filled_channel_without_senders() {
    let (tx, rx) = mpsc::channel(100);

    for i in 0..100 {
        assert!(tx.send(i).await.is_ok());
    }
    drop(tx);
    assert_eq!(rx.len(), 100);
}

#[tokio::test]
async fn test_rx_len_when_consuming_all_messages() {
    let (tx, mut rx) = mpsc::channel(100);

    for i in 0..100 {
        assert!(tx.send(i).await.is_ok());
        assert_eq!(rx.len(), i + 1);
    }

    drop(tx);

    for i in (0..100).rev() {
        assert!(rx.recv().await.is_some());
        assert_eq!(rx.len(), i);
    }
}

#[tokio::test]
async fn test_rx_len_when_close_is_called() {
    let (tx, mut rx) = mpsc::channel(100);
    tx.send(()).await.unwrap();
    rx.close();

    assert_eq!(rx.len(), 1);
}

#[tokio::test]
async fn test_rx_len_when_close_is_called_before_dropping_sender() {
    let (tx, mut rx) = mpsc::channel(100);
    tx.send(()).await.unwrap();
    rx.close();
    drop(tx);

    assert_eq!(rx.len(), 1);
}

#[tokio::test]
async fn test_rx_len_when_close_is_called_after_dropping_sender() {
    let (tx, mut rx) = mpsc::channel(100);
    tx.send(()).await.unwrap();
    drop(tx);
    rx.close();

    assert_eq!(rx.len(), 1);
}

#[tokio::test]
async fn test_rx_unbounded_is_closed_when_calling_close_with_sender() {
    // is_closed should return true after calling close but still has a sender
    let (_tx, mut rx) = mpsc::unbounded_channel::<()>();
    rx.close();

    assert!(rx.is_closed());
}

#[tokio::test]
async fn test_rx_unbounded_is_closed_when_dropping_all_senders() {
    // is_closed should return true after dropping all senders
    let (tx, rx) = mpsc::unbounded_channel::<()>();
    let another_tx = tx.clone();
    let task = tokio::spawn(async move {
        drop(another_tx);
    });

    drop(tx);
    let _ = task.await;

    assert!(rx.is_closed());
}

#[tokio::test]
async fn test_rx_unbounded_is_not_closed_when_there_are_senders() {
    // is_closed should return false when there is a sender
    let (_tx, rx) = mpsc::unbounded_channel::<()>();
    assert!(!rx.is_closed());
}

#[tokio::test]
async fn test_rx_unbounded_is_closed_when_there_are_no_senders_and_there_are_messages() {
    // is_closed should return true when there are messages in the buffer, but no senders
    let (tx, rx) = mpsc::unbounded_channel();
    for i in 0..10 {
        assert!(tx.send(i).is_ok());
    }
    drop(tx);
    assert!(rx.is_closed());
}

#[tokio::test]
async fn test_rx_unbounded_is_closed_when_there_are_messages_and_close_is_called() {
    // is_closed should return true when there are messages in the buffer, and close is called
    let (tx, mut rx) = mpsc::unbounded_channel();
    for i in 0..10 {
        assert!(tx.send(i).is_ok());
    }
    rx.close();
    assert!(rx.is_closed());
}

#[tokio::test]
async fn test_rx_unbounded_is_empty_when_no_messages_were_sent() {
    let (_tx, rx) = mpsc::unbounded_channel::<()>();
    assert!(rx.is_empty())
}

#[tokio::test]
async fn test_rx_unbounded_is_not_empty_when_there_are_messages_in_the_buffer() {
    let (tx, rx) = mpsc::unbounded_channel();
    assert!(tx.send(()).is_ok());
    assert!(!rx.is_empty())
}

#[tokio::test]
async fn test_rx_unbounded_is_not_empty_when_all_but_one_messages_are_consumed() {
    let (tx, mut rx) = mpsc::unbounded_channel();
    for i in 0..10 {
        assert!(tx.send(i).is_ok());
    }

    for _ in 0..9 {
        assert!(rx.recv().await.is_some());
    }

    assert!(!rx.is_empty())
}

#[tokio::test]
async fn test_rx_unbounded_is_empty_when_all_messages_are_consumed() {
    let (tx, mut rx) = mpsc::unbounded_channel();
    for i in 0..10 {
        assert!(tx.send(i).is_ok());
    }
    while rx.try_recv().is_ok() {}
    assert!(rx.is_empty())
}

#[tokio::test]
async fn test_rx_unbounded_is_empty_all_senders_are_dropped_and_messages_consumed() {
    let (tx, mut rx) = mpsc::unbounded_channel();
    for i in 0..10 {
        assert!(tx.send(i).is_ok());
    }
    drop(tx);

    for _ in 0..10 {
        assert!(rx.recv().await.is_some());
    }

    assert!(rx.is_empty())
}

#[tokio::test]
async fn test_rx_unbounded_len_on_empty_channel() {
    let (_tx, rx) = mpsc::unbounded_channel::<()>();
    assert_eq!(rx.len(), 0);
}

#[tokio::test]
async fn test_rx_unbounded_len_on_empty_channel_without_senders() {
    // when all senders are dropped, a "closed" value is added to the end of the linked list.
    // here we test that the "closed" value does not change the len of the channel.

    let (tx, rx) = mpsc::unbounded_channel::<()>();
    drop(tx);
    assert_eq!(rx.len(), 0);
}

#[tokio::test]
async fn test_rx_unbounded_len_with_multiple_messages() {
    let (tx, rx) = mpsc::unbounded_channel();

    for i in 0..100 {
        assert!(tx.send(i).is_ok());
    }
    assert_eq!(rx.len(), 100);
}

#[tokio::test]
async fn test_rx_unbounded_len_with_multiple_messages_and_dropped_senders() {
    let (tx, rx) = mpsc::unbounded_channel();

    for i in 0..100 {
        assert!(tx.send(i).is_ok());
    }
    drop(tx);
    assert_eq!(rx.len(), 100);
}

#[tokio::test]
async fn test_rx_unbounded_len_when_consuming_all_messages() {
    let (tx, mut rx) = mpsc::unbounded_channel();

    for i in 0..100 {
        assert!(tx.send(i).is_ok());
        assert_eq!(rx.len(), i + 1);
    }

    drop(tx);

    for i in (0..100).rev() {
        assert!(rx.recv().await.is_some());
        assert_eq!(rx.len(), i);
    }
}

#[tokio::test]
async fn test_rx_unbounded_len_when_close_is_called() {
    let (tx, mut rx) = mpsc::unbounded_channel();
    tx.send(()).unwrap();
    rx.close();

    assert_eq!(rx.len(), 1);
}

#[tokio::test]
async fn test_rx_unbounded_len_when_close_is_called_before_dropping_sender() {
    let (tx, mut rx) = mpsc::unbounded_channel();
    tx.send(()).unwrap();
    rx.close();
    drop(tx);

    assert_eq!(rx.len(), 1);
}

#[tokio::test]
async fn test_rx_unbounded_len_when_close_is_called_after_dropping_sender() {
    let (tx, mut rx) = mpsc::unbounded_channel();
    tx.send(()).unwrap();
    drop(tx);
    rx.close();

    assert_eq!(rx.len(), 1);
}

// Regression test for https://github.com/tokio-rs/tokio/issues/6602
#[tokio::test]
async fn test_is_empty_32_msgs() {
    let (sender, mut receiver) = mpsc::channel(33);

    for value in 1..257 {
        sender.send(value).await.unwrap();
        receiver.recv().await.unwrap();
        assert!(receiver.is_empty(), "{value}. len: {}", receiver.len());
    }
}

#[test]
#[cfg(not(panic = "abort"))]
fn drop_all_elements_during_panic() {
    use std::sync::atomic::AtomicUsize;
    use std::sync::atomic::Ordering::Relaxed;
    use tokio::sync::mpsc::UnboundedReceiver;
    use tokio::sync::mpsc::UnboundedSender;

    static COUNTER: AtomicUsize = AtomicUsize::new(0);

    struct A(bool);
    impl Drop for A {
        // cause a panic when inner value is `true`.
        fn drop(&mut self) {
            COUNTER.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
            if self.0 {
                panic!("panic!")
            }
        }
    }

    fn func(tx: UnboundedSender<A>, rx: UnboundedReceiver<A>) {
        tx.send(A(true)).unwrap();
        tx.send(A(false)).unwrap();
        tx.send(A(false)).unwrap();

        drop(rx);

        // `mpsc::Rx`'s drop is called and gets panicked while dropping the first value,
        // but will keep dropping following elements.
    }

    let (tx, rx) = mpsc::unbounded_channel();

    let _ = panic::catch_unwind(panic::AssertUnwindSafe(|| {
        func(tx.clone(), rx);
    }));

    // all A's destructor should be called at this point, even before `mpsc::Chan`'s
    // drop gets called.
    assert_eq!(COUNTER.load(Relaxed), 3);

    drop(tx);
    // `mpsc::Chan`'s drop is called, freeing the `Block` memory allocation.
}

fn is_debug<T: fmt::Debug>(_: &T) {}
