task: make LocalSet non-Send (#2398)

This does not count as a breaking change as it fixes a
regression and a soundness bug. 

1 files changed, 5 insertions, 0 deletions

diff --git a/tokio/src/task/local.rs b/tokio/src/task/local.rs
index 551362a1..fce46707 100644
--- a/tokio/src/task/local.rs
+++ b/tokio/src/task/local.rs
@@ -7,6 +7,7 @@ use std::cell::{Cell, RefCell};
 use std::collections::VecDeque;
 use std::fmt;
 use std::future::Future;
+use std::marker::PhantomData;
 use std::pin::Pin;
 use std::sync::{Arc, Mutex};
 use std::task::Poll;
@@ -114,6 +115,9 @@ cfg_rt_util! {
 
         /// State available from thread-local
         context: Context,
+
+        /// This type should not be Send.
+        _not_send: PhantomData<*const ()>,
     }
 }
 
@@ -228,6 +232,7 @@ impl LocalSet {
                     waker: AtomicWaker::new(),
                 }),
             },
+            _not_send: PhantomData,
         }
     }