You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: tracked_data.rs
    path of file: ./repos/valence/crates/valence_entity/src
    
    the code of the file until where you have to start completion: use bevy_ecs::prelude::*;
use tracing::warn;
use valence_protocol::Encode;

/// Cache for all the tracked data of an entity. Used for the
/// [`EntityTrackerUpdateS2c`][packet] packet.
///
/// [packet]: valenc
    
    pub fn append_update_value(&mut self, index: u8, type_id: u8, value: impl Encode) {
        debug_assert!(
            index != 0xff,
            "index of 0xff is reserved for the terminator"
        );

        self.update_data.pop(); // Remove terminator.

        self.update_data.extend_from_slice(&[index, type_id]);
        if let Err(e) = value.encode(&mut self.update_data) {
            warn!("failed to encode updated tracked data: {e:#}");
        }

        self.update_data.push(0xff); // Add terminator.
    }