toml crate如何构造类依赖结构

[database]
host = "localhost"
port = 3306
username = "admin"
password = "password"

[server]
address = "127.0.0.1"
port = 8080
database_url = "mysql://{{database.username}}:{{database.password}}@{{database.host}}:{{database.port}}/mydb"

use toml::Value;

fn main() {
    let toml_str = r#"
        [database]
        host = "localhost"
        port = 3306
        username = "admin"
        password = "password"

        [server]
        address = "127.0.0.1"
        port = 8080
        database_url = "mysql://{{database.username}}:{{database.password}}@{{database.host}}:{{database.port}}/mydb"
    "#;

    let value = toml::from_str(toml_str).unwrap();
    let database_url = value["server"]["database_url"].as_str().unwrap();

    println!("Database URL: {}", database_url);
}