num_enum 0.3.1

Procedural macros to make inter-operation between primitives and enums easier.

num_enum

Procedural macros to make inter-operation between primitives and enums easier. This crate is no_std compatible.

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Turning an enum into a primitive

use num_enum::IntoPrimitive;

#[derive(IntoPrimitive)]
#[repr(u8)]
enum Number {
    Zero,
    One,
}

#[test]
fn convert() {
    let zero: u8 = Number::Zero.into();
    assert_eq!(zero, 0u8);
}

num_enum's IntoPrimitive is more type-safe than using as, because as will silently truncate - num_enum only derives From for exactly the discriminant type of the enum.

Turning a primitive into an enum with try_from

use num_enum::TryFromPrimitive;
use std::convert::TryInto;

#[derive(Debug, Eq, PartialEq, TryFromPrimitive)]
#[repr(u8)]
enum Number {
    Zero,
    One,
}

#[test]
fn convert() {
    let zero: Number = 0u8.try_into().unwrap();
    assert_eq!(zero, Ok(Number::Zero));

    let three: Result<Number, String> = 3u8.try_into();
    assert_eq!(three, Err("No value in enum Number for value 3".to_owned()));
}

Unsafely turning a primitive into an enum with from_unchecked

If you're really certain a conversion will succeed, and want to avoid a small amount of overhead, you can use unsafe code to do this conversion. Unless you have data showing that the match statement generated in the try_from above is a bottleneck for you, you should avoid doing this, as the unsafe code has potential to cause serious memory issues in your programme.

use num_enum::UnsafeFromPrimitive;

#[derive(Debug, Eq, PartialEq, UnsafeFromPrimitive)]
#[repr(u8)]
enum Number {
    Zero,
    One,
}

#[test]
fn convert() {
    unsafe {
        assert_eq!(
            Number::Zero,
            Number::from_unchecked(0_u8)
        );
        assert_eq!(
            Number::One,
            Number::from_unchecked(1_u8)
        );
    }
}

Optional features

Some enum values may be composed of complex expressions, for example:

enum Number {
    Zero = (0, 1).0,
    One = (0, 1).1,
}

To cut down on compile time, these are not supported by default, but if you enable the complex-expressions feature of your dependency on num_enum, these should start working.