#enum #conversion #safe #ffi #derive

num_enum

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

14 releases

0.5.1 Aug 5, 2020
0.5.0 May 9, 2020
0.4.3 Mar 17, 2020
0.4.2 Oct 4, 2019
0.1.1 Nov 12, 2018

#4 in Rust patterns

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num_enum

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

crates.io Documentation Build Status

Turning an enum into a primitive

use num_enum::IntoPrimitive;

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

fn main() {
    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.

Attempting to turn a primitive into an enum with try_from

use num_enum::TryFromPrimitive;
use std::convert::TryFrom;

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

fn main() {
    let zero = Number::try_from(0u8);
    assert_eq!(zero, Ok(Number::Zero));

    let three = Number::try_from(3u8);
    assert_eq!(
        three.unwrap_err().to_string(),
        "No discriminant in enum `Number` matches the value `3`",
    );
}

Variant alternatives

Sometimes a single enum variant might be representable by multiple numeric values.

The #[num_enum(alternatives = [..])] attribute allows you to define additional value alternatives for individual variants.

(The behavior of IntoPrimitive is unaffected by this attribute, it will always return the canonical value.)

use num_enum::TryFromPrimitive;
use std::convert::TryFrom;

#[derive(Debug, Eq, PartialEq, TryFromPrimitive)]
#[repr(u8)]
enum Number {
    Zero = 0,
    #[num_enum(alternatives = [2])]
    OneOrTwo = 1,
}

fn main() {
    let zero = Number::try_from(0u8);
    assert_eq!(zero, Ok(Number::Zero));

    let one = Number::try_from(1u8);
    assert_eq!(one, Ok(Number::OneOrTwo));

    let two = Number::try_from(2u8);
    assert_eq!(two, Ok(Number::OneOrTwo));

    let three = Number::try_from(3u8);
    assert_eq!(
        three.unwrap_err().to_string(),
        "No discriminant in enum `Number` matches the value `3`",
    );
}

Default variant

Sometimes it is desirable to have an Other variant in an enum that acts as a kind of a wildcard matching all the value not yet covered by other variants.

The #[num_enum(default)] attribute allows you to mark variant as the default.

(The behavior of IntoPrimitive is unaffected by this attribute, it will always return the canonical value.)

use num_enum::TryFromPrimitive;
use std::convert::TryFrom;

#[derive(Debug, Eq, PartialEq, TryFromPrimitive)]
#[repr(u8)]
enum Number {
    Zero = 0,
    #[num_enum(default)]
    NonZero = 1,
}

fn main() {
    let zero = Number::try_from(0u8);
    assert_eq!(zero, Ok(Number::Zero));

    let one = Number::try_from(1u8);
    assert_eq!(one, Ok(Number::NonZero));

    let two = Number::try_from(2u8);
    assert_eq!(two, Ok(Number::NonZero));
}

Safely turning a primitive into an exhaustive enum with from_primitive

If your enum has all possible primitive values covered by the use of a variant marked #[num_enum(default)], you can derive FromPrimitive for it (which auto-implement stdlib's From):

use num_enum::FromPrimitive;

#[derive(Debug, Eq, PartialEq, FromPrimitive)]
#[repr(u8)]
enum Number {
    Zero,
    #[num_enum(default)]
    NonZero,
}

fn main() {
    assert_eq!(
        Number::Zero,
        unsafe { Number::from(0_u8) },
    );
    assert_eq!(
        Number::NonZero,
        unsafe { Number::from(1_u8) },
    );
}

Unsafely turning a primitive into an enum with from_unchecked

If you're really certain a conversion will succeed (and have not made use of #[num_enum(default)] or #[num_enum(alternatives = [..])] for any of its variants), 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 program.

use num_enum::UnsafeFromPrimitive;

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

fn main() {
    assert_eq!(
        unsafe { Number::from_unchecked(0_u8) },
        Number::Zero,
    );
    assert_eq!(
        unsafe { Number::from_unchecked(1_u8) },
        Number::One,
    );
}

unsafe fn undefined_behavior() {
    let _ = Number::from_unchecked(2); // 2 is not a valid discriminant!
}

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.

Dependencies

~0.4–0.9MB
~23K SLoC