nit: use default hashmap

src/bin/process.rs

@@ -336,7 +336,7 @@ fn main() {
         for user in conversation.messages.iter().map(|m| m.user) {
             user_convo
                 .entry(user)
-                .or_insert_with(HashMap::new)
+                .or_default()
                 .insert(conversation.hash, conversation);
         }
     }