diff --git a/src/libstd/sync/mutex.rs b/src/libstd/sync/mutex.rs
index 6f0febd61e8..7b5076acbca 100644
--- a/src/libstd/sync/mutex.rs
+++ b/src/libstd/sync/mutex.rs
@@ -121,6 +121,8 @@ pub struct Mutex<T> {
     data: UnsafeCell<T>,
 }
 
+// these are the only places where `T: Send` matters; all other
+// functionality works fine on a single thread.
 unsafe impl<T: Send> Send for Mutex<T> { }
 
 unsafe impl<T: Send> Sync for Mutex<T> { }
@@ -179,7 +181,7 @@ impl<'a, T> !marker::Send for MutexGuard<'a, T> {}
     poison: poison::FLAG_INIT,
 };
 
-impl<T: Send> Mutex<T> {
+impl<T> Mutex<T> {
     /// Creates a new mutex in an unlocked state ready for use.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new(t: T) -> Mutex<T> {
@@ -242,7 +244,7 @@ pub fn is_poisoned(&self) -> bool {
 
 #[unsafe_destructor]
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: Send> Drop for Mutex<T> {
+impl<T> Drop for Mutex<T> {
     fn drop(&mut self) {
         // This is actually safe b/c we know that there is no further usage of
         // this mutex (it's up to the user to arrange for a mutex to get
@@ -252,7 +254,7 @@ fn drop(&mut self) {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: fmt::Debug + Send + 'static> fmt::Debug for Mutex<T> {
+impl<T: fmt::Debug + 'static> fmt::Debug for Mutex<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self.try_lock() {
             Ok(guard) => write!(f, "Mutex {{ data: {:?} }}", *guard),
diff --git a/src/libstd/sync/rwlock.rs b/src/libstd/sync/rwlock.rs
index e9ff6c0bf9d..d32eae15a1b 100644
--- a/src/libstd/sync/rwlock.rs
+++ b/src/libstd/sync/rwlock.rs
@@ -129,7 +129,7 @@ pub struct RwLockWriteGuard<'a, T: 'a> {
 
 impl<'a, T> !marker::Send for RwLockWriteGuard<'a, T> {}
 
-impl<T: Send + Sync> RwLock<T> {
+impl<T> RwLock<T> {
     /// Creates a new instance of an `RwLock<T>` which is unlocked.
     ///
     /// # Examples
@@ -257,7 +257,7 @@ fn drop(&mut self) {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: fmt::Debug + Send + Sync> fmt::Debug for RwLock<T> {
+impl<T: fmt::Debug> fmt::Debug for RwLock<T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self.try_read() {
             Ok(guard) => write!(f, "RwLock {{ data: {:?} }}", *guard),