num_enum

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

Turning an enum into a primitive

```rust 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

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

[derive(Debug, Eq, PartialEq, TryFromPrimitive)]

[repr(u8)]

enum Number { Zero, One, }

fn main() { let zero = Number::tryfrom(0u8); asserteq!(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.)

```rust 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::tryfrom(0u8); asserteq!(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.)

```rust 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::tryfrom(0u8); asserteq!(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):

```rust use num_enum::FromPrimitive;

[derive(Debug, Eq, PartialEq, FromPrimitive)]

[repr(u8)]

enum Number { Zero, #[num_enum(default)] NonZero, }

fn main() { asserteq!( Number::Zero, unsafe { Number::from(0u8) }, ); asserteq!( Number::NonZero, unsafe { Number::from(1u8) }, ); } ```

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.

```rust use num_enum::UnsafeFromPrimitive;

[derive(Debug, Eq, PartialEq, UnsafeFromPrimitive)]

[repr(u8)]

enum Number { Zero, One, }

fn main() { asserteq!( unsafe { Number::fromunchecked(0u8) }, Number::Zero, ); asserteq!( unsafe { Number::fromunchecked(1u8) }, Number::One, ); }

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

Optional features

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

rust 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.