std: Tidy up some `unsafe impl`s for `sync`

This commit removes many unnecessary `unsafe impl` blocks as well as pushing the
needed implementations to the lowest level possible. I noticed that the bounds
for `RwLock` are a little off when reviewing #22574 and wanted to ensure that we
had our story straight on these implementations.

11 files changed, 41 insertions, 25 deletions

diff --git a/src/libstd/sync/condvar.rs b/src/libstd/sync/condvar.rs
index 52561d482c3..fa45f72b3a1 100644
--- a/src/libstd/sync/condvar.rs
+++ b/src/libstd/sync/condvar.rs
@@ -61,9 +61,6 @@ use sync::{mutex, MutexGuard, PoisonError};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Condvar { inner: Box<StaticCondvar> }
 
-unsafe impl Send for Condvar {}
-unsafe impl Sync for Condvar {}
-
 /// Statically allocated condition variables.
 ///
 /// This structure is identical to `Condvar` except that it is suitable for use
@@ -83,9 +80,6 @@ pub struct StaticCondvar {
     mutex: AtomicUsize,
 }
 
-unsafe impl Send for StaticCondvar {}
-unsafe impl Sync for StaticCondvar {}
-
 /// Constant initializer for a statically allocated condition variable.
 #[unstable(feature = "std_misc",
            reason = "may be merged with Condvar in the future")]
diff --git a/src/libstd/sync/mutex.rs b/src/libstd/sync/mutex.rs
index e77c4d2e5eb..a4129e315ff 100644
--- a/src/libstd/sync/mutex.rs
+++ b/src/libstd/sync/mutex.rs
@@ -152,8 +152,6 @@ pub struct StaticMutex {
     poison: poison::Flag,
 }
 
-unsafe impl Sync for StaticMutex {}
-
 /// An RAII implementation of a "scoped lock" of a mutex. When this structure is
 /// dropped (falls out of scope), the lock will be unlocked.
 ///
diff --git a/src/libstd/sync/once.rs b/src/libstd/sync/once.rs
index 97f985e21e8..d2054a1e819 100644
--- a/src/libstd/sync/once.rs
+++ b/src/libstd/sync/once.rs
@@ -13,10 +13,9 @@
 //! This primitive is meant to be used to run one-time initialization. An
 //! example use case would be for initializing an FFI library.
 
+use prelude::v1::*;
+
 use isize;
-use marker::Sync;
-use mem::drop;
-use ops::FnOnce;
 use sync::atomic::{AtomicIsize, Ordering, ATOMIC_ISIZE_INIT};
 use sync::{StaticMutex, MUTEX_INIT};
 
@@ -43,8 +42,6 @@ pub struct Once {
     lock_cnt: AtomicIsize,
 }
 
-unsafe impl Sync for Once {}
-
 /// Initialization value for static `Once` values.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub const ONCE_INIT: Once = Once {
diff --git a/src/libstd/sync/poison.rs b/src/libstd/sync/poison.rs
index 32c8150ba40..2587ff5238e 100644
--- a/src/libstd/sync/poison.rs
+++ b/src/libstd/sync/poison.rs
@@ -16,6 +16,12 @@ use fmt;
 use thread;
 
 pub struct Flag { failed: UnsafeCell<bool> }
+
+// This flag is only ever accessed with a lock previously held. Note that this
+// a totally private structure.
+unsafe impl Send for Flag {}
+unsafe impl Sync for Flag {}
+
 pub const FLAG_INIT: Flag = Flag { failed: UnsafeCell { value: false } };
 
 impl Flag {
diff --git a/src/libstd/sync/rwlock.rs b/src/libstd/sync/rwlock.rs
index cd833b17867..c32fd88ab24 100644
--- a/src/libstd/sync/rwlock.rs
+++ b/src/libstd/sync/rwlock.rs
@@ -97,9 +97,6 @@ pub struct StaticRwLock {
     poison: poison::Flag,
 }
 
-unsafe impl Send for StaticRwLock {}
-unsafe impl Sync for StaticRwLock {}
-
 /// Constant initialization for a statically-initialized rwlock.
 #[unstable(feature = "std_misc",
            reason = "may be merged with RwLock in the future")]
diff --git a/src/libstd/sys/unix/condvar.rs b/src/libstd/sys/unix/condvar.rs
index 3bc41473152..90dfebc4c45 100644
--- a/src/libstd/sys/unix/condvar.rs
+++ b/src/libstd/sys/unix/condvar.rs
@@ -8,10 +8,11 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use prelude::v1::*;
+
 use cell::UnsafeCell;
 use libc;
 use ptr;
-use std::option::Option::{Some, None};
 use sys::mutex::{self, Mutex};
 use sys::time;
 use sys::sync as ffi;
@@ -20,6 +21,9 @@ use num::{Int, NumCast};
 
 pub struct Condvar { inner: UnsafeCell<ffi::pthread_cond_t> }
 
+unsafe impl Send for Condvar {}
+unsafe impl Sync for Condvar {}
+
 pub const CONDVAR_INIT: Condvar = Condvar {
     inner: UnsafeCell { value: ffi::PTHREAD_COND_INITIALIZER },
 };
diff --git a/src/libstd/sys/unix/mutex.rs b/src/libstd/sys/unix/mutex.rs
index 9e1527aef20..f87c0339533 100644
--- a/src/libstd/sys/unix/mutex.rs
+++ b/src/libstd/sys/unix/mutex.rs
@@ -8,8 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use prelude::v1::*;
+
 use cell::UnsafeCell;
-use marker::Sync;
 use sys::sync as ffi;
 use sys_common::mutex;
 
@@ -24,6 +25,7 @@ pub const MUTEX_INIT: Mutex = Mutex {
     inner: UnsafeCell { value: ffi::PTHREAD_MUTEX_INITIALIZER },
 };
 
+unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {}
 
 impl Mutex {
diff --git a/src/libstd/sys/unix/rwlock.rs b/src/libstd/sys/unix/rwlock.rs
index 54523e0076d..b857f4ab75f 100644
--- a/src/libstd/sys/unix/rwlock.rs
+++ b/src/libstd/sys/unix/rwlock.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use prelude::v1::*;
+
 use cell::UnsafeCell;
 use sys::sync as ffi;
 
@@ -17,6 +19,9 @@ pub const RWLOCK_INIT: RWLock = RWLock {
     inner: UnsafeCell { value: ffi::PTHREAD_RWLOCK_INITIALIZER },
 };
 
+unsafe impl Send for RWLock {}
+unsafe impl Sync for RWLock {}
+
 impl RWLock {
     #[inline]
     pub unsafe fn new() -> RWLock {
diff --git a/src/libstd/sys/windows/condvar.rs b/src/libstd/sys/windows/condvar.rs
index db8038006fd..071637e3a93 100644
--- a/src/libstd/sys/windows/condvar.rs
+++ b/src/libstd/sys/windows/condvar.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use prelude::v1::*;
+
 use cell::UnsafeCell;
 use libc::{self, DWORD};
 use os;
@@ -17,6 +19,9 @@ use time::Duration;
 
 pub struct Condvar { inner: UnsafeCell<ffi::CONDITION_VARIABLE> }
 
+unsafe impl Send for Condvar {}
+unsafe impl Sync for Condvar {}
+
 pub const CONDVAR_INIT: Condvar = Condvar {
     inner: UnsafeCell { value: ffi::CONDITION_VARIABLE_INIT }
 };
diff --git a/src/libstd/sys/windows/mutex.rs b/src/libstd/sys/windows/mutex.rs
index 75495efc7cb..0847f3b52bf 100644
--- a/src/libstd/sys/windows/mutex.rs
+++ b/src/libstd/sys/windows/mutex.rs
@@ -8,7 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use marker::Sync;
+use prelude::v1::*;
+
 use cell::UnsafeCell;
 use sys::sync as ffi;
 
@@ -18,6 +19,7 @@ pub const MUTEX_INIT: Mutex = Mutex {
     inner: UnsafeCell { value: ffi::SRWLOCK_INIT }
 };
 
+unsafe impl Send for Mutex {}
 unsafe impl Sync for Mutex {}
 
 #[inline]
@@ -27,14 +29,15 @@ pub unsafe fn raw(m: &Mutex) -> ffi::PSRWLOCK {
 
 // So you might be asking why we're using SRWLock instead of CriticalSection?
 //
-// 1. SRWLock is several times faster than CriticalSection according to benchmarks performed on both
-// Windows 8 and Windows 7.
+// 1. SRWLock is several times faster than CriticalSection according to
+//    benchmarks performed on both Windows 8 and Windows 7.
 //
-// 2. CriticalSection allows recursive locking while SRWLock deadlocks. The Unix implementation
-// deadlocks so consistency is preferred. See #19962 for more details.
+// 2. CriticalSection allows recursive locking while SRWLock deadlocks. The Unix
+//    implementation deadlocks so consistency is preferred. See #19962 for more
+//    details.
 //
-// 3. While CriticalSection is fair and SRWLock is not, the current Rust policy is there there are
-// no guarantees of fairness.
+// 3. While CriticalSection is fair and SRWLock is not, the current Rust policy
+//    is there there are no guarantees of fairness.
 
 impl Mutex {
     #[inline]
diff --git a/src/libstd/sys/windows/rwlock.rs b/src/libstd/sys/windows/rwlock.rs
index 76fe352ed77..009605535a0 100644
--- a/src/libstd/sys/windows/rwlock.rs
+++ b/src/libstd/sys/windows/rwlock.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use prelude::v1::*;
+
 use cell::UnsafeCell;
 use sys::sync as ffi;
 
@@ -17,6 +19,9 @@ pub const RWLOCK_INIT: RWLock = RWLock {
     inner: UnsafeCell { value: ffi::SRWLOCK_INIT }
 };
 
+unsafe impl Send for RWLock {}
+unsafe impl Sync for RWLock {}
+
 impl RWLock {
     #[inline]
     pub unsafe fn read(&self) {