diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index 855c86f08e745..7c80f695f681b 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -119,7 +119,7 @@ use heap::deallocate;
 /// ```
 #[unsafe_no_drop_flag]
 #[stable(feature = "rust1", since = "1.0.0")]
-pub struct Arc<T> {
+pub struct Arc<T: Send + Sync> {
     // FIXME #12808: strange name to try to avoid interfering with
     // field accesses of the contained type via Deref
     _ptr: NonZero<*mut ArcInner<T>>,
@@ -136,7 +136,7 @@ unsafe impl<T: Sync + Send> Sync for Arc<T> { }
 #[unsafe_no_drop_flag]
 #[unstable(feature = "alloc",
            reason = "Weak pointers may not belong in this module.")]
-pub struct Weak<T> {
+pub struct Weak<T: Send + Sync> {
     // FIXME #12808: strange name to try to avoid interfering with
     // field accesses of the contained type via Deref
     _ptr: NonZero<*mut ArcInner<T>>,
@@ -146,13 +146,13 @@ unsafe impl<T: Sync + Send> Send for Weak<T> { }
 unsafe impl<T: Sync + Send> Sync for Weak<T> { }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: fmt::Debug> fmt::Debug for Weak<T> {
+impl<T: fmt::Debug + Send + Sync> fmt::Debug for Weak<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "(Weak)")
     }
 }
 
-struct ArcInner<T> {
+struct ArcInner<T: Send + Sync> {
     strong: atomic::AtomicUsize,
     weak: atomic::AtomicUsize,
     data: T,
@@ -161,7 +161,7 @@ struct ArcInner<T> {
 unsafe impl<T: Sync + Send> Send for ArcInner<T> {}
 unsafe impl<T: Sync + Send> Sync for ArcInner<T> {}
 
-impl<T> Arc<T> {
+impl<T: Send + Sync> Arc<T> {
     /// Constructs a new `Arc<T>`.
     ///
     /// # Examples
@@ -205,7 +205,7 @@ impl<T> Arc<T> {
     }
 }
 
-impl<T> Arc<T> {
+impl<T: Send + Sync> Arc<T> {
     #[inline]
     fn inner(&self) -> &ArcInner<T> {
         // This unsafety is ok because while this arc is alive we're guaranteed
@@ -235,15 +235,15 @@ impl<T> Arc<T> {
 /// Get the number of weak references to this value.
 #[inline]
 #[unstable(feature = "alloc")]
-pub fn weak_count<T>(this: &Arc<T>) -> usize { this.inner().weak.load(SeqCst) - 1 }
+pub fn weak_count<T: Send + Sync>(this: &Arc<T>) -> usize { this.inner().weak.load(SeqCst) - 1 }
 
 /// Get the number of strong references to this value.
 #[inline]
 #[unstable(feature = "alloc")]
-pub fn strong_count<T>(this: &Arc<T>) -> usize { this.inner().strong.load(SeqCst) }
+pub fn strong_count<T: Send + Sync>(this: &Arc<T>) -> usize { this.inner().strong.load(SeqCst) }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Clone for Arc<T> {
+impl<T: Send + Sync> Clone for Arc<T> {
     /// Makes a clone of the `Arc<T>`.
     ///
     /// This increases the strong reference count.
@@ -277,7 +277,7 @@ impl<T> Clone for Arc<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Deref for Arc<T> {
+impl<T: Send + Sync> Deref for Arc<T> {
     type Target = T;
 
     #[inline]
@@ -324,7 +324,7 @@ impl<T: Send + Sync + Clone> Arc<T> {
 
 #[unsafe_destructor]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Drop for Arc<T> {
+impl<T: Send + Sync> Drop for Arc<T> {
     /// Drops the `Arc<T>`.
     ///
     /// This will decrement the strong reference count. If the strong reference
@@ -392,7 +392,7 @@ impl<T> Drop for Arc<T> {
 
 #[unstable(feature = "alloc",
            reason = "Weak pointers may not belong in this module.")]
-impl<T> Weak<T> {
+impl<T: Send + Sync> Weak<T> {
     /// Upgrades a weak reference to a strong reference.
     ///
     /// Upgrades the `Weak<T>` reference to an `Arc<T>`, if possible.
@@ -458,7 +458,7 @@ impl<T: Sync + Send> Clone for Weak<T> {
 
 #[unsafe_destructor]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Drop for Weak<T> {
+impl<T: Send + Sync> Drop for Weak<T> {
     /// Drops the `Weak<T>`.
     ///
     /// This will decrement the weak reference count.
@@ -503,7 +503,7 @@ impl<T> Drop for Weak<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: PartialEq> PartialEq for Arc<T> {
+impl<T: PartialEq + Send + Sync> PartialEq for Arc<T> {
     /// Equality for two `Arc<T>`s.
     ///
     /// Two `Arc<T>`s are equal if their inner value are equal.
@@ -535,7 +535,7 @@ impl<T: PartialEq> PartialEq for Arc<T> {
     fn ne(&self, other: &Arc<T>) -> bool { *(*self) != *(*other) }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: PartialOrd> PartialOrd for Arc<T> {
+impl<T: PartialOrd + Send + Sync> PartialOrd for Arc<T> {
     /// Partial comparison for two `Arc<T>`s.
     ///
     /// The two are compared by calling `partial_cmp()` on their inner values.
@@ -614,21 +614,21 @@ impl<T: PartialOrd> PartialOrd for Arc<T> {
     fn ge(&self, other: &Arc<T>) -> bool { *(*self) >= *(*other) }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Ord> Ord for Arc<T> {
+impl<T: Ord + Send + Sync> Ord for Arc<T> {
     fn cmp(&self, other: &Arc<T>) -> Ordering { (**self).cmp(&**other) }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Eq> Eq for Arc<T> {}
+impl<T: Eq + Send + Sync> Eq for Arc<T> {}
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: fmt::Display> fmt::Display for Arc<T> {
+impl<T: fmt::Display + Send + Sync> fmt::Display for Arc<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Display::fmt(&**self, f)
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: fmt::Debug> fmt::Debug for Arc<T> {
+impl<T: fmt::Debug + Send + Sync> fmt::Debug for Arc<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Debug::fmt(&**self, f)
     }
@@ -641,7 +641,7 @@ impl<T: Default + Sync + Send> Default for Arc<T> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Hash> Hash for Arc<T> {
+impl<T: Hash + Send + Sync> Hash for Arc<T> {
     fn hash<H: Hasher>(&self, state: &mut H) {
         (**self).hash(state)
     }
diff --git a/src/libcollections/borrow.rs b/src/libcollections/borrow.rs
index 2fe769b73f5cc..b84c2d2ec2bb2 100644
--- a/src/libcollections/borrow.rs
+++ b/src/libcollections/borrow.rs
@@ -16,7 +16,7 @@ use core::clone::Clone;
 use core::cmp::{Eq, Ord, Ordering, PartialEq, PartialOrd};
 use core::convert::AsRef;
 use core::hash::{Hash, Hasher};
-use core::marker::Sized;
+use core::marker::{Send, Sized, Sync};
 use core::ops::Deref;
 use core::option::Option;
 
@@ -115,7 +115,7 @@ impl<T> Borrow<T> for rc::Rc<T> {
     fn borrow(&self) -> &T { &**self }
 }
 
-impl<T> Borrow<T> for arc::Arc<T> {
+impl<T: Send + Sync> Borrow<T> for arc::Arc<T> {
     fn borrow(&self) -> &T { &**self }
 }
 
diff --git a/src/libserialize/serialize.rs b/src/libserialize/serialize.rs
index 81b5d4c5818b6..9036f892b103d 100644
--- a/src/libserialize/serialize.rs
+++ b/src/libserialize/serialize.rs
@@ -612,7 +612,7 @@ impl<T: Decodable> Decodable for RefCell<T> {
     }
 }
 
-impl<T:Encodable> Encodable for Arc<T> {
+impl<T:Sync+Send+Encodable> Encodable for Arc<T> {
     fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
         (**self).encode(s)
     }
diff --git a/src/libstd/io/lazy.rs b/src/libstd/io/lazy.rs
index df280dab37d46..7b05f8ac86c57 100644
--- a/src/libstd/io/lazy.rs
+++ b/src/libstd/io/lazy.rs
@@ -15,13 +15,13 @@ use cell::UnsafeCell;
 use rt;
 use sync::{StaticMutex, Arc};
 
-pub struct Lazy<T> {
+pub struct Lazy<T: Send + Sync> {
     pub lock: StaticMutex,
     pub ptr: UnsafeCell<*mut Arc<T>>,
     pub init: fn() -> Arc<T>,
 }
 
-unsafe impl<T> Sync for Lazy<T> {}
+unsafe impl<T: Send + Sync> Sync for Lazy<T> {}
 
 macro_rules! lazy_init {
     ($init:expr) => (::io::lazy::Lazy {
diff --git a/src/libstd/sync/mpsc/mod.rs b/src/libstd/sync/mpsc/mod.rs
index e14f32865fa10..19dbf87774f6b 100644
--- a/src/libstd/sync/mpsc/mod.rs
+++ b/src/libstd/sync/mpsc/mod.rs
@@ -373,12 +373,10 @@ unsafe impl<T: Send> Send for Sender<T> { }
 /// owned by one task, but it can be cloned to send to other tasks.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct SyncSender<T: Send> {
-    inner: Arc<UnsafeCell<sync::Packet<T>>>,
+    inner: Arc<SyncUnsafeCell<sync::Packet<T>>>,
 }
 
-unsafe impl<T: Send> Send for SyncSender<T> {}
-
-impl<T> !Sync for SyncSender<T> {}
+impl<T: Send> !Sync for SyncSender<T> {}
 
 /// An error returned from the `send` function on channels.
 ///
@@ -434,11 +432,33 @@ pub enum TrySendError<T> {
 }
 
 enum Flavor<T:Send> {
-    Oneshot(Arc<UnsafeCell<oneshot::Packet<T>>>),
-    Stream(Arc<UnsafeCell<stream::Packet<T>>>),
-    Shared(Arc<UnsafeCell<shared::Packet<T>>>),
-    Sync(Arc<UnsafeCell<sync::Packet<T>>>),
+    Oneshot(Arc<SyncUnsafeCell<oneshot::Packet<T>>>),
+    Stream(Arc<SyncUnsafeCell<stream::Packet<T>>>),
+    Shared(Arc<SyncUnsafeCell<shared::Packet<T>>>),
+    Sync(Arc<SyncUnsafeCell<sync::Packet<T>>>),
+}
+
+// the channel impls are free and fast with mutability and sharing, so
+// we have to hack around this with this type.
+struct SyncUnsafeCell<T> {
+    data: UnsafeCell<T>
 }
+impl<T> SyncUnsafeCell<T> {
+    pub fn new(x: T) -> SyncUnsafeCell<T> {
+        SyncUnsafeCell {
+            data: UnsafeCell::new(x)
+        }
+    }
+}
+impl<T> ::ops::Deref for SyncUnsafeCell<T> {
+    type Target = UnsafeCell<T>;
+    fn deref(&self) -> &UnsafeCell<T> {
+        &self.data
+    }
+}
+
+unsafe impl<T: Send> Send for SyncUnsafeCell<T> {}
+unsafe impl<T: Send> Sync for SyncUnsafeCell<T> {}
 
 #[doc(hidden)]
 trait UnsafeFlavor<T:Send> {
@@ -489,7 +509,7 @@ impl<T:Send> UnsafeFlavor<T> for Receiver<T> {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn channel<T: Send>() -> (Sender<T>, Receiver<T>) {
-    let a = Arc::new(UnsafeCell::new(oneshot::Packet::new()));
+    let a = Arc::new(SyncUnsafeCell::new(oneshot::Packet::new()));
     (Sender::new(Flavor::Oneshot(a.clone())), Receiver::new(Flavor::Oneshot(a)))
 }
 
@@ -529,7 +549,7 @@ pub fn channel<T: Send>() -> (Sender<T>, Receiver<T>) {
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn sync_channel<T: Send>(bound: usize) -> (SyncSender<T>, Receiver<T>) {
-    let a = Arc::new(UnsafeCell::new(sync::Packet::new(bound)));
+    let a = Arc::new(SyncUnsafeCell::new(sync::Packet::new(bound)));
     (SyncSender::new(a.clone()), Receiver::new(Flavor::Sync(a)))
 }
 
@@ -576,12 +596,12 @@ impl<T: Send> Sender<T> {
         let (new_inner, ret) = match *unsafe { self.inner() } {
             Flavor::Oneshot(ref p) => {
                 unsafe {
-                    let p = p.get();
+                    let p = p.data.get();
                     if !(*p).sent() {
                         return (*p).send(t).map_err(SendError);
                     } else {
                         let a =
-                            Arc::new(UnsafeCell::new(stream::Packet::new()));
+                            Arc::new(SyncUnsafeCell::new(stream::Packet::new()));
                         let rx = Receiver::new(Flavor::Stream(a.clone()));
                         match (*p).upgrade(rx) {
                             oneshot::UpSuccess => {
@@ -623,7 +643,7 @@ impl<T: Send> Clone for Sender<T> {
     fn clone(&self) -> Sender<T> {
         let (packet, sleeper, guard) = match *unsafe { self.inner() } {
             Flavor::Oneshot(ref p) => {
-                let a = Arc::new(UnsafeCell::new(shared::Packet::new()));
+                let a = Arc::new(SyncUnsafeCell::new(shared::Packet::new()));
                 unsafe {
                     let guard = (*a.get()).postinit_lock();
                     let rx = Receiver::new(Flavor::Shared(a.clone()));
@@ -635,7 +655,7 @@ impl<T: Send> Clone for Sender<T> {
                 }
             }
             Flavor::Stream(ref p) => {
-                let a = Arc::new(UnsafeCell::new(shared::Packet::new()));
+                let a = Arc::new(SyncUnsafeCell::new(shared::Packet::new()));
                 unsafe {
                     let guard = (*a.get()).postinit_lock();
                     let rx = Receiver::new(Flavor::Shared(a.clone()));
@@ -681,7 +701,7 @@ impl<T: Send> Drop for Sender<T> {
 ////////////////////////////////////////////////////////////////////////////////
 
 impl<T: Send> SyncSender<T> {
-    fn new(inner: Arc<UnsafeCell<sync::Packet<T>>>) -> SyncSender<T> {
+    fn new(inner: Arc<SyncUnsafeCell<sync::Packet<T>>>) -> SyncSender<T> {
         SyncSender { inner: inner }
     }
 
diff --git a/src/libstd/sync/mutex.rs b/src/libstd/sync/mutex.rs
index b24cfbb6899a9..6f63f2fe76803 100644
--- a/src/libstd/sync/mutex.rs
+++ b/src/libstd/sync/mutex.rs
@@ -170,6 +170,8 @@ pub struct MutexGuard<'a, T: 'a> {
     __poison: poison::Guard,
 }
 
+// It is undefined behaviour to unlock a mutex on a thread other than
+// the one that locked it.
 impl<'a, T> !marker::Send for MutexGuard<'a, T> {}
 
 /// Static initialization of a mutex. This constant can be used to initialize
diff --git a/src/libstd/sync/rwlock.rs b/src/libstd/sync/rwlock.rs
index a79ffaa0860e3..a614a4ab2fad7 100644
--- a/src/libstd/sync/rwlock.rs
+++ b/src/libstd/sync/rwlock.rs
@@ -60,7 +60,7 @@ use fmt;
 /// } // write lock is dropped here
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
-pub struct RwLock<T> {
+pub struct RwLock<T: Send + Sync> {
     inner: Box<StaticRwLock>,
     data: UnsafeCell<T>,
 }
@@ -251,7 +251,7 @@ impl<T: Send + Sync> RwLock<T> {
 
 #[unsafe_destructor]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Drop for RwLock<T> {
+impl<T: Send + Sync> Drop for RwLock<T> {
     fn drop(&mut self) {
         unsafe { self.inner.lock.destroy() }
     }
diff --git a/src/libstd/thread/mod.rs b/src/libstd/thread/mod.rs
index 1202b353317cd..656bce79a0231 100644
--- a/src/libstd/thread/mod.rs
+++ b/src/libstd/thread/mod.rs
@@ -292,7 +292,7 @@ impl Builder {
         let my_thread = Thread::new(name);
         let their_thread = my_thread.clone();
 
-        let my_packet = Packet(Arc::new(UnsafeCell::new(None)));
+        let my_packet = Packet(Arc::new(PacketInner { data: UnsafeCell::new(None) }));
         let their_packet = Packet(my_packet.0.clone());
 
         // Spawning a new OS thread guarantees that __morestack will never get
@@ -331,7 +331,7 @@ impl Builder {
                 }
             };
             unsafe {
-                *their_packet.0.get() = Some(match (output, try_result) {
+                *their_packet.0.data.get() = Some(match (output, try_result) {
                     (Some(data), Ok(_)) => Ok(data),
                     (None, Err(cause)) => Err(cause),
                     _ => unreachable!()
@@ -585,26 +585,30 @@ impl thread_info::NewThread for Thread {
 #[stable(feature = "rust1", since = "1.0.0")]
 pub type Result<T> = ::result::Result<T, Box<Any + Send + 'static>>;
 
-struct Packet<T>(Arc<UnsafeCell<Option<Result<T>>>>);
+struct Packet<T: Send>(Arc<PacketInner<T>>);
+// see also SyncUnsafeCell in sync/mspc.
+struct PacketInner<T: Send> {
+    data: UnsafeCell<Option<Result<T>>>
+}
 
-unsafe impl<T:Send> Send for Packet<T> {}
-unsafe impl<T> Sync for Packet<T> {}
+unsafe impl<T:Send> Send for PacketInner<T> {}
+unsafe impl<T> Sync for PacketInner<T> {}
 
 /// Inner representation for JoinHandle and JoinGuard
-struct JoinInner<T> {
+struct JoinInner<T: Send> {
     native: imp::rust_thread,
     thread: Thread,
     packet: Packet<T>,
     joined: bool,
 }
 
-impl<T> JoinInner<T> {
+impl<T: Send> JoinInner<T> {
     fn join(&mut self) -> Result<T> {
         assert!(!self.joined);
         unsafe { imp::join(self.native) };
         self.joined = true;
         unsafe {
-            (*self.packet.0.get()).take().unwrap()
+            (*self.packet.0.data.get()).take().unwrap()
         }
     }
 }