Rollup merge of #22596 - alexcrichton:fix-some-impls, r=huonw

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.

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 },
 };

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 {

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 {