Macro open_enum

macro_rules! open_enum {
    (
        $(#[$a:meta])*
        $v:vis enum $name:ident : $storage:ty {
            $(#![$implattr:meta])*
            $(
                $(#[$vattr:meta])*
                $variant:ident = $value:expr,
            )*
        }
    ) => { ... };
}

This macro creates an underlying struct that behaves like an enum, without the undefined behavior of trying to match with a value outside of the enum. The actual object created is a #[repr(transparent)] struct with a pub const value. See comment in example below for an example of the underlying structure.

This macro implements the following traits: Copy, Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd.

An example usage case for this macro is for protocols, when you want to use an enum as a field in a struct that represents a specific type, like u16 or u32. You are also able to convert to/from bytes with this typed enum.

Examples

use open_enum::open_enum;
open_enum! {
    #[expect(dead_code)] // This will apply to the generated struct defn
    pub enum ExampleEnumName: u32 {
        #![expect(missing_docs)] // This will apply to all subfields of the enum
        THIS_IS_AN_ENUM = 32,
    }
}
// Expands to:
//
// #[repr(transparent)]
// #[expect(dead_code)]
// struct ExampleEnumName(u32);
//
// #[expect(missing_docs)]
// impl ExampleEnumName {
//     pub const THIS_IS_AN_ENUM: ExampleEnumName = ExampleEnumName(0)
// }

// To access an element in ExampleEnumName
let example_enum = ExampleEnumName::THIS_IS_AN_ENUM;

assert_eq!(example_enum.0, 32);

let number: u32 = example_enum.0; // enum value is type u32