Add a better error message for rust-lang#39364

There is a known bug in the implementation of mpsc channels in rust.
This adds a clearer error message when the bug occurs, so that developers don't lose too much time looking for the origin of the bug.
See rust-lang#39364

Author: lovasoa

library/std/src/sync/mpsc/shared.rs

@@ -248,7 +248,7 @@ impl<T> Packet<T> {
     // Returns true if blocking should proceed.
     fn decrement(&self, token: SignalToken) -> StartResult {
         unsafe {
-            assert_eq!(self.to_wake.load(Ordering::SeqCst), 0);
+            assert_eq!(self.to_wake.load(Ordering::SeqCst), 0, "This is a known bug in rust. See https://github.com/rust-lang/rust/issues/39364");
             let ptr = token.cast_to_usize();
             self.to_wake.store(ptr, Ordering::SeqCst);