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: mod.rs
    path of file: ./repos/diem/storage/jellyfish-merkle/src/tree_cache
    
    the code of the file until where you have to start completion: // Copyright (c) The Diem Core Contributors
// SPDX-License-Identifier: Apache-2.0

//! A transaction can have multiple operations on state. For example, it might update values
//! for a few existing keys. Imagine that we have the following tree.
//!
//! ```text
//!                 root0
//!                 /    \
//!                /      \
//!  key1 => value11        key2 => value21
//! ```
//!
//! The next transaction updates `key1`'s value to `value12` and `key2`'s value to `value22`.
//! Let's assume we update key2 first. Then the tree becomes:
//!
//! ```text
//!                   (on disk)              (in memory)
//!                     root0                  root1'
//!                    /     \                /     \
//!                   /   ___ \ _____________/       \
//!                  /  _/     \                      \
//!                 / _/       
    
    pub fn freeze(&mut self) {
        let root_node_key = self.get_root_node_key();
        let root_hash = self
            .get_node(root_node_key)
            .unwrap_or_else(|_| unreachable!("Root node with key {:?} must exist", root_node_key))
            .hash();
        self.frozen_cache.root_hashes.push(root_hash);
        let node_stats = NodeStats {
            new_nodes: self.node_cache.len(),
            new_leaves: self.num_new_leaves,
            stale_nodes: self.stale_node_index_cache.len(),
            stale_leaves: self.num_stale_leaves,
        };
        self.frozen_cache.node_stats.push(node_stats);
        self.frozen_cache.node_cache.extend(self.node_cache.drain());
        let stale_since_version = self.next_version;
        self.frozen_cache
            .stale_node_index_cache
            .extend(
                self.stale_node_index_cache
                    .drain()
                    .map(|node_key| StaleNodeIndex {
                        stale_since_version,
                        node_key,
                    }),
            );

        // Clean up
        self.num_stale_leaves = 0;
        self.num_new_leaves = 0;

        // Prepare for the next version after freezing
        self.next_version += 1;
    }