use std::fmt;
use std::error::Error;
 
#[derive(Debug)]
enum ValidationError {
    EmptyField(String),
    InvalidFormat { field: String, expected: String },
}
 
impl fmt::Display for ValidationError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            ValidationError::EmptyField(field) => write!(f, "Field '{}' cannot be empty", field),
            ValidationError::InvalidFormat { field, expected } => {
                write!(f, "Field '{}' has invalid format. Expected: {}", field, expected)
            }
        }
    }
}
 
impl Error for ValidationError {}
 
fn validate_input(value: &str, field_name: &str) -> Result<(), ValidationError> {
    if value.is_empty() {
        return Err(ValidationError::EmptyField(field_name.to_string()));
    }
    if !value.contains('@') && field_name == "email" {
        return Err(ValidationError::InvalidFormat {
            field: field_name.to_string(),
            expected: "valid email address".to_string(),
        });
    }
    Ok(())
}
 
fn main() {
    if let Err(e) = validate_input("", "username") {
        eprintln!("Validation failed: {}", e);
    }
}