std: add regression test for rust-lang#107466

Tests that messages are immediately dropped once the last receiver is destroyed.

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

@@ -1,5 +1,6 @@
 use super::*;
 use crate::env;
+use crate::rc::Rc;
 use crate::sync::mpmc::SendTimeoutError;
 use crate::thread;
 use crate::time::Duration;
@@ -656,3 +657,15 @@ fn issue_15761() {
         repro()
     }
 }
+
+#[test]
+fn drop_unreceived() {
+    let (tx, rx) = sync_channel::<Rc<()>>(1);
+    let msg = Rc::new(());
+    let weak = Rc::downgrade(&msg);
+    assert!(tx.send(msg).is_ok());
+    drop(rx);
+    // Messages should be dropped immediately when the last receiver is destroyed.
+    assert!(weak.upgrade().is_none());
+    drop(tx);
+}