#discriminant #enums #ordinal

macro enum_discriminant

Procedural macro to add functions on enum types to get discrimnant value from variant or create unit variant from discriminant value

1 unstable release

0.1.0 Apr 10, 2025

#1146 in Rust patterns

Download history 100/week @ 2025-04-06 23/week @ 2025-04-13 3/week @ 2025-04-20

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MIT license

9KB
76 lines

enum_discriminant

Procedural macro for enum types to convert between variant and discriminant and vice versa.

Added functions

When the procedural macro is used on an enum, you can get the discriminant, i.e., the numeric value or ordinal, of a variant with the function discrimnant(). The other way around, variants can be created with the function from_discrimnant().

use enum_discriminant::discriminant;

#[discriminant(u8)]
#[derive(Debug, PartialEq)]
enum MyEnum {
    Zero,
    Two = 2,
    Three
}

assert_eq!(0, MyEnum::Zero.discriminant());
assert_eq!(2, MyEnum::Two.discriminant());
assert_eq!(3, MyEnum::Three.discriminant());

assert_eq!(Some(MyEnum::Zero), MyEnum::from_discriminant(0));
assert_eq!(None, MyEnum::from_discriminant(1));
assert_eq!(Some(MyEnum::Two), MyEnum::from_discriminant(2));
assert_eq!(Some(MyEnum::Three), MyEnum::from_discriminant(3));

The macro requires you to specify an integer type and will behave the same as #[repr()].

It is possible to get the discriminant of all types of enum variants, including tuples and struct variants. However, tuple and struct variants cannot be created from the discriminant since it also requires the member values of the variant.

use enum_discriminant::discriminant;

#[discriminant(u8)]
#[derive(Debug, PartialEq)]
enum MyEnum {
    Unit,
    Tuple(u8, String),
    Struct{ a: u8, b: String }
}

assert_eq!(0, MyEnum::Unit.discriminant());
assert_eq!(1, MyEnum::Tuple(17, "blargh".to_string()).discriminant());
assert_eq!(2, MyEnum::Struct{ a: 17, b: "blargh".to_string() }.discriminant());

assert_eq!(Some(MyEnum::Unit), MyEnum::from_discriminant(0));
assert_eq!(None, MyEnum::from_discriminant(1));
assert_eq!(None, MyEnum::from_discriminant(2));

Alternatives

There are similar, popular crates, including:

All of these alternatives only support unit type enum variants. There are likely many other similar crates with overlapping or identical functionality as this one.

Dependencies

~200–630KB
~15K SLoC