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.

5 files changed, 8 insertions, 16 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")]