我很困惑为什么我在使用config-rs执行cargo run时会收到Err(missing field "web3_node_provider")错误。似乎在s.try_deserialize()时失败：

use config::{Config, ConfigError, Environment, File};
use serde::Deserialize;

#[derive(Debug, Deserialize)]
#[allow(unused)]
struct Web3NodeProvider {
    ethereum_mainnet_node_url_http: String,
    alchemy_api_key: String,
}

#[derive(Debug, Deserialize)]
#[allow(unused)]
pub struct Settings {
    web3_node_provider: Web3NodeProvider,
}

impl Settings {
    pub fn new() -> Result<Self, ConfigError> {
        let s = Config::builder()
            .add_source(File::with_name("config/default"))
            .add_source(File::with_name("config/local").required(false))
            .add_source(Environment::with_prefix("app"))
            .build()?;
        s.try_deserialize()
    }
}

fn main() {
    let settings = Settings::new();
    println!("{:?}", settings);
}

我在config-rs中基本上遵循了hierarchy example，所以我确信我只是误解了一些基本的东西或者遗漏了一些东西，我可以使用"Web3NodeProvider.url"，但不能使用"web3_node_provider.ethereum_mainnet_node_url_http"。
default.toml

[Web3NodeProvider]
ethereum_mainnet_node_url_http = "https://eth-mainnet.g.alchemy.com/v2/"
alchemy_api_key = "alchemy-api-key"

local.toml

[Web3NodeProvider]
alchemy_api_key = "randomapikey"