From 4fc2adae4fcc76fa8ff1169fed3db6d7c9100c4a Mon Sep 17 00:00:00 2001
From: Alice Ryhl <alice@ryhl.io>
Date: Sun, 12 Apr 2020 23:55:37 +0200
Subject: [PATCH] task: make LocalSet non-Send (#2398)

This does not count as a breaking change as it fixes a
regression and a soundness bug.
---
 tokio/src/task/local.rs        |  5 +++++
 tokio/tests/async_send_sync.rs | 39 ++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/tokio/src/task/local.rs b/tokio/src/task/local.rs
index 551362a13..fce467079 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,
         }
     }
 
diff --git a/tokio/tests/async_send_sync.rs b/tokio/tests/async_send_sync.rs
index f50b41a08..1fea19c2a 100644
--- a/tokio/tests/async_send_sync.rs
+++ b/tokio/tests/async_send_sync.rs
@@ -41,6 +41,44 @@ macro_rules! into_todo {
         x
     }};
 }
+macro_rules! assert_value {
+    ($type:ty: Send & Sync) => {
+        #[allow(unreachable_code)]
+        #[allow(unused_variables)]
+        const _: fn() = || {
+            let f: $type = todo!();
+            require_send(&f);
+            require_sync(&f);
+        };
+    };
+    ($type:ty: !Send & Sync) => {
+        #[allow(unreachable_code)]
+        #[allow(unused_variables)]
+        const _: fn() = || {
+            let f: $type = todo!();
+            AmbiguousIfSend::some_item(&f);
+            require_sync(&f);
+        };
+    };
+    ($type:ty: Send & !Sync) => {
+        #[allow(unreachable_code)]
+        #[allow(unused_variables)]
+        const _: fn() = || {
+            let f: $type = todo!();
+            require_send(&f);
+            AmbiguousIfSync::some_item(&f);
+        };
+    };
+    ($type:ty: !Send & !Sync) => {
+        #[allow(unreachable_code)]
+        #[allow(unused_variables)]
+        const _: fn() = || {
+            let f: $type = todo!();
+            AmbiguousIfSend::some_item(&f);
+            AmbiguousIfSync::some_item(&f);
+        };
+    };
+}
 macro_rules! async_assert_fn {
     ($($f:ident $(< $($generic:ty),* > )? )::+($($arg:ty),*): Send & Sync) => {
         #[allow(unreachable_code)]
@@ -206,6 +244,7 @@ async_assert_fn!(tokio::task::LocalKey<Rc<u32>>::scope(_, Rc<u32>, BoxFutureSync
 async_assert_fn!(tokio::task::LocalKey<Rc<u32>>::scope(_, Rc<u32>, BoxFutureSend<()>): !Send & !Sync);
 async_assert_fn!(tokio::task::LocalKey<Rc<u32>>::scope(_, Rc<u32>, BoxFuture<()>): !Send & !Sync);
 async_assert_fn!(tokio::task::LocalSet::run_until(_, BoxFutureSync<()>): !Send & !Sync);
+assert_value!(tokio::task::LocalSet: !Send & !Sync);
 
 async_assert_fn!(tokio::time::advance(Duration): Send & Sync);
 async_assert_fn!(tokio::time::delay_for(Duration): Send & Sync);