Add trailing zeros for discriminators with tests

src/utils.rs

@@ -22,7 +22,32 @@ impl User for serenity::User {
         if self.discriminator == 0 {
             return self.name.clone();
         }
-        format!("{}#{}", self.name, self.discriminator)
+        format!("{}#{:0>4}", self.name, self.discriminator)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::serenity::User;
+    use crate::utils::User as UserTrait;
+
+    fn test_user(name: &str, discriminator: u16) -> User {
+        let mut user = User::default();
+        user.name = name.to_owned();
+        user.discriminator = discriminator;
+        user
+    }
+
+    #[test]
+    fn test_legacy_username() {
+        let user = test_user("test", 123);
+        assert_eq!(user.username(), "test#0123");
+    }
+
+    #[test]
+    fn test_new_username() {
+        let user = test_user("test", 0);
+        assert_eq!(user.username(), "test");
     }
 }