Fix mpsc::SyncSender spinning behavior

library/std/src/sync/mpmc/mod.rs

@@ -43,7 +43,7 @@ mod zero;
 use crate::fmt;
 use crate::panic::{RefUnwindSafe, UnwindSafe};
 use crate::time::{Duration, Instant};
-use error::*;
+pub use error::*;
 
 /// Creates a channel of unbounded capacity.
 ///

library/std/src/sync/mpmc/utils.rs

@@ -139,6 +139,6 @@ impl Backoff {
     /// Returns `true` if quadratic backoff has completed and blocking the thread is advised.
     #[inline]
     pub fn is_completed(&self) -> bool {
-        self.step.get() > YIELD_LIMIT
+        self.step.get() > SPIN_LIMIT
     }
 }

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

@@ -738,6 +738,15 @@ impl<T> SyncSender<T> {
     pub fn try_send(&self, t: T) -> Result<(), TrySendError<T>> {
         self.inner.try_send(t)
     }
+
+    // Attempts to send for a value on this receiver, returning an error if the
+    // corresponding channel has hung up, or if it waits more than `timeout`.
+    //
+    // This method is currently private and only used for tests.
+    #[allow(unused)]
+    fn send_timeout(&self, t: T, timeout: Duration) -> Result<(), mpmc::SendTimeoutError<T>> {
+        self.inner.send_timeout(t, timeout)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

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

@@ -1,5 +1,6 @@
 use super::*;
 use crate::env;
+use crate::sync::mpmc::SendTimeoutError;
 use crate::thread;
 use crate::time::Duration;
 
@@ -41,6 +42,13 @@ fn recv_timeout() {
     assert_eq!(rx.recv_timeout(Duration::from_millis(1)), Ok(1));
 }
 
+#[test]
+fn send_timeout() {
+    let (tx, _rx) = sync_channel::<i32>(1);
+    assert_eq!(tx.send_timeout(1, Duration::from_millis(1)), Ok(()));
+    assert_eq!(tx.send_timeout(1, Duration::from_millis(1)), Err(SendTimeoutError::Timeout(1)));
+}
+
 #[test]
 fn smoke_threads() {
     let (tx, rx) = sync_channel::<i32>(0);