Keyword enum

A type that can be any one of several variants.

Enums in Rust are similar to those of other compiled languages like C, but have important differences that make them considerably more powerful. What Rust calls enums are more commonly known as Algebraic Data Types if you’re coming from a functional programming background. The important detail is that each enum variant can have data to go along with it.

enum SimpleEnum {
    FirstVariant,
    SecondVariant,
    ThirdVariant,
}

enum Location {
    Unknown,
    Anonymous,
    Known(Coord),
}

enum ComplexEnum {
    Nothing,
    Something(u32),
    LotsOfThings {
        usual_struct_stuff: bool,
        blah: String,
    }
}

enum EmptyEnum { }

The first enum shown is the usual kind of enum you’d find in a C-style language. The second shows off a hypothetical example of something storing location data, with Coord being any other type that’s needed, for example a struct. The third example demonstrates the kind of data a variant can store, ranging from nothing, to a tuple, to an anonymous struct.

Instantiating enum variants involves explicitly using the enum’s name as its namespace, followed by one of its variants. SimpleEnum::SecondVariant would be an example from above. When data follows along with a variant, such as with rust’s built-in Option type, the data is added as the type describes, for example Option::Some(123). The same follows with struct-like variants, with things looking like ComplexEnum::LotsOfThings { usual_struct_stuff: true, blah: "hello!".to_string(), }. Empty Enums are similar to ! in that they cannot be instantiated at all, and are used mainly to mess with the type system in interesting ways.

For more information, take a look at the Rust Book or the Reference