Fix clippy::new-without-default for HashArena

Author: konstin

src/internal/arena.rs

@@ -150,9 +150,7 @@ impl<T: Hash + Eq + fmt::Debug> fmt::Debug for HashArena<T> {
 
 impl<T: Hash + Eq> HashArena<T> {
     pub fn new() -> Self {
-        HashArena {
-            data: FnvIndexSet::default(),
-        }
+        Self::default()
     }
 
     pub fn alloc(&mut self, value: T) -> Id<T> {
@@ -161,6 +159,14 @@ impl<T: Hash + Eq> HashArena<T> {
     }
 }
 
+impl<T: Hash + Eq> Default for HashArena<T> {
+    fn default() -> Self {
+        Self {
+            data: FnvIndexSet::default(),
+        }
+    }
+}
+
 impl<T: Hash + Eq> Index<Id<T>> for HashArena<T> {
     type Output = T;
     fn index(&self, id: Id<T>) -> &T {