Crate newtype_tools 

Newtype Tools

A lightweight library (~400 lines of code with minimum dependencies) designed to make the newtype idiom more ergonomic to use.

Motivation

Instead of trying to be everything or deriving dozens of unused trait implementations, this crate provides unique, simple, yet powerful tools for your newtypes.

The crate focuses on three main areas to make newtype usage more enjoyable:

1. Conversions between types.
2. Operations on newtypes.
3. Iteration over newtypes ranges.

Usage

Adding the crate to your project:

cargo add newtype-tools

Examples

Conversion between types:

use newtype_tools::Newtype;

#[derive(Newtype)]
#[newtype(
    from(Oranges, with = "|oranges| Apples(oranges.0 as u64 * 2)"),
    into(Oranges, with = "|apples| Oranges((apples.0 / 2) as u32)")
)]
struct Apples(u64);
struct Oranges(u32);

let apples = Apples(42);
assert_eq!(apples.0, 42);

let oranges = Oranges::from(apples);
assert_eq!(oranges.0, 21);

Operations on newtypes:

use newtype_tools::Newtype;

#[derive(Debug, Newtype)]
#[newtype(
    partial_eq(Oranges, with = "|apples, oranges| apples.0 == oranges.0 as u64 * 2")
)]
struct Apples(u64);
#[derive(Debug)]
struct Oranges(u32);

let apples = Apples(42);
let oranges = Oranges(21);
assert_eq!(apples, oranges);

Iterations over newtype ranges:

use newtype_tools::Newtype;

#[derive(Debug, Newtype)]
#[newtype(from(usize, with = |u| Apples(u as u64)))]
#[newtype(range_iter(usize))]
struct Apples(u64);

for apple in Apples::range_iter(Apples(0)..Apples(42)) {
    println!("{apple:?}");
}

This will become even more ergonomic once the Step trait is stabilized.

Alternatives

1. nutype – An impressive 12.5k lines of code, with 7.5k lines in proc-macros alone. After trying to extend it, I realized it would be faster to simply write a new crate.
2. phantom_newtype – Provides 19 trait implementations out of the box, but lacks a mechanism for providing custom trait implementations.
3. newtype_derive – Outdated and relies on the legacy custom_derive! declarative macro.
4. newtype-derive-2018 – Less outdated, but it’s 2026.

References

1. Rust newtype idiom.
2. Rust Step trait.

Derive Macros

Newtype

Expands the derive macro into a TokenStream.