Fix cargo clippy for latest rust version

ballista/cache/src/backend/policy/lru/mod.rs

@@ -83,8 +83,8 @@ where
         Self {
             max_num: capacity,
             current_num: 0,
-            _key_marker: PhantomData::default(),
-            _value_marker: PhantomData::default(),
+            _key_marker: PhantomData,
+            _value_marker: PhantomData,
         }
     }
 }

ballista/scheduler/src/cluster/memory.rs

@@ -98,7 +98,7 @@ impl ClusterState for InMemoryClusterState {
         let mut guard = self.task_slots.lock().await;
 
         for (executor_id, num_slots) in increments {
-            if let Some(mut data) = guard.get_mut(&executor_id) {
+            if let Some(data) = guard.get_mut(&executor_id) {
                 data.slots += num_slots;
             }
         }

ballista/scheduler/src/cluster/mod.rs

@@ -513,7 +513,7 @@ pub(crate) async fn bind_task_round_robin(
                 }
                 // Since the slots is a vector with descending order, and the total available slots is larger than 0,
                 // we are sure the available slot number at idx_slot is larger than 1
-                let mut slot = &mut slots[idx_slot];
+                let slot = &mut slots[idx_slot];
                 let executor_id = slot.executor_id.clone();
                 let task_id = *task_id_gen;
                 *task_id_gen += 1;

ballista/scheduler/src/state/execution_graph_dot.rs

@@ -69,7 +69,7 @@ impl<'a> ExecutionGraphDot<'a> {
             writeln!(&mut dot, "}}")?;
             Ok(dot)
         } else {
-            Err(fmt::Error::default())
+            Err(fmt::Error)
         }
     }