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: singleplayer_world.rs
    path of file: ./repos/veloren/voxygen/src/singleplayer
    
    the code of the file until where you have to start completion: use std::{
    fs,
    io::Read,
    path::{Path, PathBuf},
};

use common::{assets::ASSETS_PATH, consts::DAY_LENGTH_DEFAULT};
use serde::{Deserialize, Serialize};
use server::{FileOpts, GenOpts, DEFAULT_WORLD_MAP};
use tracing::error
    
    fn world_folder_name(&self) -> String {
        use chrono::{Datelike, Timelike};
        let now = chrono::Local::now().naive_local();
        let name = format!(
            "world-{}-{}-{}-{}_{}_{}_{}",
            now.year(),
            now.month(),
            now.day(),
            now.hour(),
            now.minute(),
            now.second(),
            now.timestamp_subsec_millis()
        );

        let mut test_name = name.clone();
        let mut i = 0;
        'fail: loop {
            for world in self.worlds.iter() {
                if world.path.ends_with(&test_name) {
                    test_name = name.clone();
                    test_name.push('_');
                    test_name.push_str(&i.to_string());
                    i += 1;
                    continue 'fail;
                }
            }
            break;
        }
        test_name
    }