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/matrix-rust-sdk/crates/matrix-sdk/src/encryption/backups
    
    the code of the file until where you have to start completion: // Copyright 2023 The Matrix.org Foundation C.I.C.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitation
    
    pub async fn download_room_key(&self, room_id: &RoomId, session_id: &str) -> Result<(), Error> {
        let olm_machine = self.client.olm_machine().await;
        let olm_machine = olm_machine.as_ref().ok_or(Error::NoOlmMachine)?;

        let backup_keys = olm_machine.store().load_backup_keys().await?;

        if let Some(decryption_key) = backup_keys.decryption_key {
            if let Some(version) = backup_keys.backup_version {
                let request = get_backup_keys_for_session::v3::Request::new(
                    version,
                    room_id.to_owned(),
                    session_id.to_owned(),
                );
                let response = self.client.send(request, Default::default()).await?;

                // Transform response to standard format (map of room ID -> room key).
                let response = get_backup_keys::v3::Response::new(BTreeMap::from([(
                    room_id.to_owned(),
                    RoomKeyBackup::new(BTreeMap::from([(
                        session_id.to_owned(),
                        response.key_data,
                    )])),
                )]));

                self.handle_downloaded_room_keys(response, decryption_key, olm_machine).await?;
            }
        }

        Ok(())
    }