Auto merge of #22573 - nwin:impl-debug-rwlock-weak, r=Manishearth

Implements `Debug`  for `RwLock` and `arc::Weak` in the same way it is implemented for `rc::Weak` (basically copy & paste).

The lack of this implementation prevents the automatic implementation of `Debug` for structs containing members of these types.

src/liballoc/arc.rs

@@ -139,6 +139,13 @@ pub struct Weak<T> {
 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> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "(Weak)")
+    }
+}
+
 struct ArcInner<T> {
     strong: atomic::AtomicUsize,
     weak: atomic::AtomicUsize,

src/libstd/sync/mutex.rs

@@ -15,6 +15,7 @@ use marker;
 use ops::{Deref, DerefMut};
 use sync::poison::{self, TryLockError, TryLockResult, LockResult};
 use sys_common::mutex as sys;
+use fmt;
 
 /// A mutual exclusion primitive useful for protecting shared data
 ///
@@ -250,6 +251,19 @@ impl<T: Send> Drop for Mutex<T> {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: fmt::Debug + Send + '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),
+            Err(TryLockError::Poisoned(err)) => {
+                write!(f, "Mutex {{ data: Poisoned({:?}) }}", **err.get_ref())
+            },
+            Err(TryLockError::WouldBlock) => write!(f, "Mutex {{ <locked> }}")
+        }
+    }
+}
+
 struct Dummy(UnsafeCell<()>);
 unsafe impl Sync for Dummy {}
 static DUMMY: Dummy = Dummy(UnsafeCell { value: () });

src/libstd/sync/rwlock.rs

@@ -15,6 +15,7 @@ use marker;
 use ops::{Deref, DerefMut};
 use sync::poison::{self, LockResult, TryLockError, TryLockResult};
 use sys_common::rwlock as sys;
+use fmt;
 
 /// A reader-writer lock
 ///
@@ -255,6 +256,19 @@ impl<T> Drop for RwLock<T> {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: fmt::Debug + Send + Sync> 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),
+            Err(TryLockError::Poisoned(err)) => {
+                write!(f, "RwLock {{ data: Poisoned({:?}) }}", **err.get_ref())
+            },
+            Err(TryLockError::WouldBlock) => write!(f, "RwLock {{ <locked> }}")
+        }
+    }
+}
+
 struct Dummy(UnsafeCell<()>);
 unsafe impl Sync for Dummy {}
 static DUMMY: Dummy = Dummy(UnsafeCell { value: () });