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`",
);

} ```

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 program.

```rust use num_enum::UnsafeFromPrimitive;

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

[repr(u8)]

enum Number { Zero, One, }

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

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.