Expected Answer (Rust 1.97.1)

Rust enums define a value that is exactly one of several variants, and each variant can hold its own data. match requires all possibilities to be covered, so adding a new variant forces relevant handling sites to be reconsidered at compile time.

Use if let when only one pattern matters; choose match when every case should be handled explicitly or a value must be produced for each variant.

enum Payment {
    Card { last_four: String },
    BankTransfer { reference: String },
    Cash,
}

fn label(payment: &Payment) -> String {
    match payment {
        Payment::Card { last_four } => format!("card ending {last_four}"),
        Payment::BankTransfer { reference } => format!("transfer {reference}"),
        Payment::Cash => String::from("cash"),
    }
}

fn main() {
    let payment = Payment::Cash;
    assert_eq!(label(&payment), "cash");
}

Why It Matters

Enums turn a set of meaningful states into a checked part of the type system. Exhaustive matching makes state-machine changes safer and reduces unhandled branches in business logic and protocol code.

Common Mistakes

• Using a wildcard arm too early: It can hide newly added variants from important handling code.
• Using multiple optional fields for mutually exclusive states: An enum makes invalid combinations unrepresentable.
• Moving data out of an enum when only inspection is needed: Match on a reference to avoid consuming the value.

Follow-up Questions

• What is Option<T>? (Answer: An enum with Some(T) and None variants.)
• How can a match arm bind only part of a value? (Answer: Use patterns such as .. to ignore unneeded fields.)