separate ordered trait

crates/aptos-in-memory-cache/src/caches/s3fifo.rs

@@ -1,7 +1,7 @@
 // Copyright © Aptos Foundation
 // SPDX-License-Identifier: Apache-2.0
 
-use crate::Cache;
+use crate::{Cache, Ordered};
 use quick_cache::{sync::Cache as S3FIFOCache, Lifecycle, Weighter};
 use std::hash::{BuildHasher, Hash};
 
@@ -20,7 +20,16 @@ where
     fn insert(&mut self, key: K, value: V) {
         S3FIFOCache::insert(self, key, value);
     }
+}
 
+impl<K, V, We, B, L> Ordered<K, V> for S3FIFOCache<K, V, We, B, L>
+where
+    K: Eq + Hash + Clone + Send + Sync,
+    V: Clone + Send + Sync,
+    We: Weighter<K, V> + Clone + Send + Sync,
+    B: BuildHasher + Clone + Send + Sync,
+    L: Lifecycle<K, V> + Clone + Send + Sync,
+{
     fn first_key(&self) -> Option<K> {
         unimplemented!();
     }

crates/aptos-in-memory-cache/src/lib.rs

@@ -16,7 +16,9 @@ where
 
     /// Inserts a given key-value pair in cache. Panics if the insert fails.
     fn insert(&mut self, key: K, value: V);
+}
 
+pub trait Ordered<K, V>: Send + Sync {
     /// Returns the first key in the cache. Returns [`None`] if the cache is empty.
     fn first_key(&self) -> Option<K>;