Enum

An enum is a closed set of named variants.

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enum Deliver {
    To
    Reply
    Forward
}

Use it as a field type, optionally with a default:

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struct Message {
    deliver: Deliver = default   // first variant, `To`
}

Variants are bare identifiers. Attaching data to a variant (Encrypt(EncryptionAlgorithm)) is a proposal, not yet part of the language.