repr_cast

A Rust procedural macro library that enhances fieldless enums with proper conversions between enum variants and their integer representation types.

Features

Bidirectional conversions: Convert enums to integers and integers back to enums
Type-safe: Uses TryFrom for fallible conversions from integers
Const-friendly: Generated methods work in const contexts
Ergonomic: Supports both explicit and implicit discriminants
Error handling: Provides descriptive error types for invalid conversions
Zero overhead: All conversions are inlined and compile to efficient code

Installation

Add this to your Cargo.toml:

[dependencies]
repr_cast = "0.1"

Usage

Simply add the #[repr_cast(T)] attribute to your fieldless enum, where T is the integer type you want to use:

use repr_cast::repr_cast;

#[derive(Debug, PartialEq)]
#[repr_cast(u8)]
enum Status {
    Pending = 0,
    Active = 1,
    Completed = 2,
}

Generated API

The macro generates the following for your enum:

#[repr(T)] - Ensures the enum has the specified memory representation
From<Enum> for T - Converts owned enum to integer
From<&Enum> for T - Converts enum reference to integer
TryFrom<T> for Enum - Converts owned integer to enum (returns EnumConversionError for invalid values)
TryFrom<&T> for Enum - Converts integer reference to enum
Enum::from_repr(value: T) -> Option<Enum> - Safe conversion from integer
Enum::as_repr(&self) -> T - Converts enum to integer
EnumConversionError - Error type for failed conversions

Examples

Basic conversion

use repr_cast::repr_cast;
use std::convert::TryFrom;

#[derive(Debug, PartialEq)]
#[repr_cast(u8)]
enum Status {
    Pending = 0,
    Active = 1,
    Completed = 2,
}

// Enum to integer
let status = Status::Active;
let value: u8 = status.into();  // or status.as_repr()
assert_eq!(value, 1);

// Integer to enum (fallible)
let status = Status::try_from(1).unwrap();
assert_eq!(status, Status::Active);

// Safe conversion
if let Some(status) = Status::from_repr(2) {
    println!("Got status: {:?}", status);
}

Signed integers

#[derive(Debug, PartialEq)]
#[repr_cast(i32)]
enum Priority {
    Low = -1,
    Normal = 0,
    High = 1,
    Critical = 100,
}

let priority = Priority::Low;
let value: i32 = priority.into();
assert_eq!(value, -1);

Reference conversions

#[derive(Debug, PartialEq)]
#[repr_cast(u8)]
enum Status {
    Pending = 0,
    Active = 1,
    Completed = 2,
}

let status = Status::Active;

// Convert owned enum (consumes the enum)
let value1: u8 = status.into();

// Convert reference (doesn't consume the enum)
let status = Status::Active;
let value2: u8 = (&status).into();
assert_eq!(value2, 1);

// Can use the enum after reference conversion
println!("Status: {:?}, Value: {}", status, value2);

// TryFrom works with references too
let code = 1u8;
let status = Status::try_from(&code).unwrap();
assert_eq!(status, Status::Active);

Implicit discriminants

#[derive(Debug, PartialEq)]
#[repr_cast(u16)]
enum Color {
    Red,    // 0
    Green,  // 1
    Blue,   // 2
}

assert_eq!(Color::Green.as_repr(), 1);

Const expressions

const ERROR_BASE: u16 = 400;
const SERVER_ERROR_BASE: u16 = 500;

#[derive(Debug, PartialEq)]
#[repr_cast(u16)]
enum ErrorCode {
    Success = 0,
    InvalidInput,                  // 1
    BadRequest = ERROR_BASE,       // 400
    Forbidden,                     // 401
    InternalError = SERVER_ERROR_BASE, // 500
    ServiceUnavailable,            // 501
}

assert_eq!(ErrorCode::BadRequest.as_repr(), ERROR_BASE);
assert_eq!(ErrorCode::try_from(500).unwrap(), ErrorCode::InternalError);

Const contexts

#[derive(Debug, PartialEq)]
#[repr_cast(u8)]
enum Flag {
    Off = 0,
    On = 1,
}

const FLAG_VALUE: u8 = Flag::On.as_repr();
const MAYBE_FLAG: Option<Flag> = Flag::from_repr(1);

Error handling

use std::convert::TryFrom;

#[derive(Debug, PartialEq)]
#[repr_cast(u8)]
enum Status {
    Pending = 0,
    Active = 1,
}

match Status::try_from(99) {
    Ok(status) => println!("Valid: {:?}", status),
    Err(err) => {
        println!("Error: {}", err);  // "unknown Status variant: 99"
        println!("Invalid value: {}", err.0);  // 99
    }
}

Supported Integer Types

The macro works with all Rust integer types:

Unsigned: u8, u16, u32, u64, u128, usize
Signed: i8, i16, i32, i64, i128, isize

Requirements

The enum must be fieldless (all variants must be unit variants)
The enum cannot have generics (currently)
All discriminant values must fit in the specified integer type

The macro provides clear error messages if these requirements are not met.

Comparison with `#[repr(i)]` / `#[repr(u)]`

Rust's built-in #[repr(i*)] and #[repr(u*)] attributes only control memory layout. They don't provide any conversion methods. This library:

✅ Includes #[repr(T)] automatically
✅ Adds safe conversion methods
✅ Implements standard traits (From, TryFrom)
✅ Supports both owned and reference conversions
✅ Provides const-compatible functions
✅ Handles complex discriminant expressions
✅ Includes proper error types with helpful messages

License

This project is dual-licensed under MIT or Apache-2.0, at your option.

Contributing

Contributions are welcome! Please feel free to submit issues or pull requests.

repr_cast 0.1.0