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Answer: Where structs give you a way of grouping together related fields and data, enums give you a way of saying a value is one of a possible set of values. (S1) The enum keyword allows the creation of a type which may be one of a few different variants. (S2)

The IP address example

• Any IP address can be either a version four or a version six address, but not both at the same time. (S1)
• That property of IP addresses makes the enum data structure appropriate because an enum value can only be one of its variants. (S1)
• Note that the variants of the enum are namespaced under its identifier, and we use a double colon to separate the two. (S1)
• IpAddrKind::V4 and IpAddrKind::V6 are of the same type: IpAddrKind. (S1)

Variants and aliases

• Any variant which is valid as a struct is also valid in an enum. (S2)
• If you use a type alias, you can refer to each enum variant via its alias. (S2)
• This might be useful if the enum's name is too long or too generic, and you want to rename it. (S2)
• The most common place you'll see this is in impl blocks using the Self alias. (S2)

Claims

• id: C1 claim: An enum value can only be one of its variants. source: S1 evidence: "structure appropriate because an enum value can only be one of its variants."
• id: C2 claim: Enum variants are namespaced under the enum and use a double colon. source: S1 evidence: "use a double colon to separate the two."
• id: C3 claim: Any variant valid as a struct is valid in an enum. source: S2 evidence: "Any variant which is valid as a struct is also valid in"

Sources

• [S1] Defining an Enum - The Rust Programming Language. The Rust Programming Language. https://doc.rust-lang.org/book/ch06-01-defining-an-enum.html . License: MIT or Apache-2.0 (rust-lang/book; Copyright (c) 2010 The Rust Project Developers; https://github.com/rust-lang/book/blob/main/LICENSE-MIT). Accessed 2026-11-04.
• [S2] Enums - Rust By Example. Rust by Example. https://doc.rust-lang.org/rust-by-example/custom_types/enum.html . License: MIT or Apache-2.0 (rust-lang/rust-by-example; Copyright (c) 2014 Jorge Aparicio; https://github.com/rust-lang/rust-by-example/blob/master/LICENSE-MIT). Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_j1a58wns,
  author = {{iinventedlight}},
  title = {{Rust enums: one of a set of possible variants}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/rust-enums/},
  note = {Permanent ID iil:note:j1a58wns. License: CC-BY-4.0.},
}

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