Code shared from the Rust Playground

playground.rs

#[allow(dead_code)]
#[derive(Debug)]
enum ConfigError {
    MissingKey(String),
    ParseError(std::num::ParseIntError),
}
impl From<std::num::ParseIntError> for ConfigError {
    fn from(err: std::num::ParseIntError) -> Self {
        ConfigError::ParseError(err)
    }
}
fn get_config_good(key: &str) -> Result<String, ConfigError> {
    let config: std::collections::HashMap<&str, &str> =
        [("port", "8080")].iter().cloned().collect();
    config
        .get(key)
        .ok_or_else(|| ConfigError::MissingKey(key.to_string()))
        .map(|s| s.to_string())
}
fn parse_port_good(port: &str) -> Result<u16, ConfigError> {
    port.parse::<u16>().map_err(ConfigError::from)
}
fn main() {
    match get_config_good("port") {
        Ok(port) => println!("Port: {:?}", &parse_port_good(&port)),
        Err(e) => eprintln!("Error: {:?}", e),
    }
}