feat(maitake-sync): impl Default for locks (#473)

This commit adds `Default` impls to `Mutex`, `RwLock`, and `spin::Mutex`
where `T: Default`, allowing locked data to be constructed through
`Default` without needing `Mutex::new(Default::default())` or similar.

An implementation of `Default` for `spin::RwLock` was not added, because
I already did that in #472.

maitake-sync/src/mutex.rs

@@ -242,7 +242,7 @@ impl<T: ?Sized> Mutex<T> {
     /// # Examples
     ///
     /// ```
-    /// let mut lock = maitake_sync::Mutex::new(0);
+    /// let mut lock = maitake_sync::spin::Mutex::new(0);
     /// *lock.get_mut() = 10;
     /// assert_eq!(*lock.try_lock().unwrap(), 10);
     /// ```
@@ -268,6 +268,12 @@ impl<T: ?Sized> Mutex<T> {
     }
 }
 
+impl<T: Default> Default for Mutex<T> {
+    fn default() -> Self {
+        Self::new(Default::default())
+    }
+}
+
 impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let Self { data: _, wait } = self;

maitake-sync/src/rwlock.rs

@@ -431,6 +431,12 @@ impl<T: ?Sized> RwLock<T> {
     }
 }
 
+impl<T: Default> Default for RwLock<T> {
+    fn default() -> Self {
+        Self::new(Default::default())
+    }
+}
+
 impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let Self { sem, data: _ } = self;

maitake-sync/src/spin/mutex.rs

@@ -169,6 +169,12 @@ impl<T> Mutex<T> {
     }
 }
 
+impl<T: Default> Default for Mutex<T> {
+    fn default() -> Self {
+        Self::new(Default::default())
+    }
+}
+
 unsafe impl<T: Send> Send for Mutex<T> {}
 unsafe impl<T: Send> Sync for Mutex<T> {}