Rust Error Handling Patterns

use std::fmt;

#[derive(Debug)]
pub enum AppError {
    Io(std::io::Error),
    Parse(std::num::ParseIntError),
    Custom(String),
}

impl fmt::Display for AppError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            AppError::Io(e)     => write!(f, "IO error: {}", e),
            AppError::Parse(e)  => write!(f, "Parse error: {}", e),
            AppError::Custom(s) => write!(f, "Error: {}", s),
        }
    }
}

impl From<std::io::Error> for AppError {
    fn from(e: std::io::Error) -> Self { AppError::Io(e) }
}

impl From<std::num::ParseIntError> for AppError {
    fn from(e: std::num::ParseIntError) -> Self { AppError::Parse(e) }
}

fn read_port(s: &str) -> Result<u16, AppError> {
    let port: i32 = s.trim().parse()?;
    if port < 1 || port > 65535 {
        return Err(AppError::Custom(format!("Port {} out of range", port)));
    }
    Ok(port as u16)
}