Enum variants with data: type-safe tagged unions

Developers model complex states with structs and boolean flags instead of using enum variants with data, leading to invalid state combinations.

Use enum variants with data to make invalid states unrepresentable:

// Bad: boolean flags allow invalid combinations
struct Connection {
    is_connected: bool,
    is_authenticated: bool, // meaningless if not connected
    address: Option<String>,
}

// Good: enum variants hold only relevant data
#[derive(Debug)]
enum Connection {
    Disconnected,
    Connected { address: String },
    Authenticated { address: String, user_id: u64 },
}

// Each variant holds exactly the data it needs
#[derive(Debug)]
enum Shape {
    Circle { radius: f64 },
    Rectangle { width: f64, height: f64 },
    Triangle { base: f64, height: f64 },
}

impl Shape {
    fn area(&self) -> f64 {
        match self {
            Shape::Circle { radius }            => std::f64::consts::PI * radius * radius,
            Shape::Rectangle { width, height }  => width * height,
            Shape::Triangle { base, height }    => 0.5 * base * height,
        }
    }
}

Enum variants with data implement the 'make impossible states impossible' principle. The type system enforces that data only exists in contexts where it is meaningful, eliminating entire classes of runtime errors.