What Are Enums with Data (Tagged Unions) in Rust?

Rust enums with data allow each variant to hold unique fields, enabling a single type to represent multiple distinct states with different associated information.

Enums with data, often called tagged unions, are Rust enumerations where each variant can hold its own unique set of fields. This allows a single type to represent multiple distinct states, each carrying different information, which the compiler enforces via pattern matching.

enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}

let m = Message::Move { x: 10, y: 20 };

match m {
    Message::Quit => println!("Quit"),
    Message::Move { x, y } => println!("Move to {}, {}", x, y),
    Message::Write(text) => println!("Text: {}", text),
    Message::ChangeColor(r, g, b) => println!("Color: {}, {}, {}", r, g, b),
}

An enum with data is like a box that can hold different types of items, but only one at a time. Each item type (variant) has its own specific contents, and the box tells you exactly which type is inside so you know how to handle it. This is useful when a value can be one of several things, like a network message that is either a 'login' request, a 'logout' request, or a 'chat' message, each requiring different data.