Enumerations
Syntax
Enumeration :
enum
IDENTIFIER GenericParams? WhereClause?{
EnumItems?}
EnumItems :
EnumItem (,
EnumItem )*,
?EnumItem :
OuterAttribute* Visibility?
IDENTIFIER ( EnumItemTuple | EnumItemStruct | EnumItemDiscriminant )?EnumItemTuple :
(
TupleFields?)
EnumItemStruct :
{
StructFields?}
EnumItemDiscriminant :
=
Expression
An enumeration, also referred to as an enum, is a simultaneous definition of a nominal enumerated type as well as a set of constructors, that can be used to create or pattern-match values of the corresponding enumerated type.
Enumerations are declared with the keyword enum
.
An example of an enum
item and its use:
#![allow(unused)] fn main() { enum Animal { Dog, Cat, } let mut a: Animal = Animal::Dog; a = Animal::Cat; }
Enum constructors can have either named or unnamed fields:
#![allow(unused)] fn main() { enum Animal { Dog(String, f64), Cat { name: String, weight: f64 }, } let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2); a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 }; }
In this example, Cat
is a struct-like enum variant, whereas Dog
is simply
called an enum variant. Each enum instance has a discriminant which is an
integer associated to it that is used to determine which variant it holds. An
opaque reference to this discriminant can be obtained with the
mem::discriminant
function.
Custom Discriminant Values for Fieldless Enumerations
If there is no data attached to any of the variants of an enumeration, then the discriminant can be directly chosen and accessed.
These enumerations can be cast to integer types with the as
operator by a
numeric cast. The enumeration can optionally specify which integer each
discriminant gets by following the variant name with =
followed by a constant
expression. If the first variant in the declaration is unspecified, then it is
set to zero. For every other unspecified discriminant, it is set to one higher
than the previous variant in the declaration.
#![allow(unused)] fn main() { enum Foo { Bar, // 0 Baz = 123, // 123 Quux, // 124 } let baz_discriminant = Foo::Baz as u32; assert_eq!(baz_discriminant, 123); }
Under the default representation, the specified discriminant is interpreted as
an isize
value although the compiler is allowed to use a smaller type in the
actual memory layout. The size and thus acceptable values can be changed by
using a primitive representation or the C
representation.
It is an error when two variants share the same discriminant.
#![allow(unused)] fn main() { enum SharedDiscriminantError { SharedA = 1, SharedB = 1 } enum SharedDiscriminantError2 { Zero, // 0 One, // 1 OneToo = 1 // 1 (collision with previous!) } }
It is also an error to have an unspecified discriminant where the previous discriminant is the maximum value for the size of the discriminant.
#![allow(unused)] fn main() { #[repr(u8)] enum OverflowingDiscriminantError { Max = 255, MaxPlusOne // Would be 256, but that overflows the enum. } #[repr(u8)] enum OverflowingDiscriminantError2 { MaxMinusOne = 254, // 254 Max, // 255 MaxPlusOne // Would be 256, but that overflows the enum. } }
Zero-variant Enums
Enums with zero variants are known as zero-variant enums. As they have no valid values, they cannot be instantiated.
#![allow(unused)] fn main() { enum ZeroVariants {} }
Zero-variant enums are equivalent to the never type, but they cannot be coerced into other types.
#![allow(unused)] fn main() { enum ZeroVariants {} let x: ZeroVariants = panic!(); let y: u32 = x; // mismatched type error }
Variant visibility
Enum variants syntactically allow a Visibility annotation, but this is rejected when the enum is validated. This allows items to be parsed with a unified syntax across different contexts where they are used.
#![allow(unused)] fn main() { macro_rules! mac_variant { ($vis:vis $name:ident) => { enum $name { $vis Unit, $vis Tuple(u8, u16), $vis Struct { f: u8 }, } } } // Empty `vis` is allowed. mac_variant! { E } // This is allowed, since it is removed before being validated. #[cfg(FALSE)] enum E { pub U, pub(crate) T(u8), pub(super) T { f: String } } }