aldanor / derive-new

derive simple constructor functions for Rust structs

A custom derive implementation for #[derive(new)]

A derive(new) attribute creates a new constructor function for the annotated type. That function takes an argument for each field in the type giving a trivial constructor. This is useful since as your type evolves you can make the constructor non-trivial (and add or remove fields) without changing client code (i.e., without breaking backwards compatibility). It is also the most succinct way to initialise a struct or an enum.

Implementation uses macros 1.1 custom derive (which works in stable Rust from 1.15 onwards).

Examples

Cargo.toml:

[dependencies]
derive-new = "0.4"

Include the macro:

#[macro_use]
extern crate derive_new;

Generating constructor for a simple struct:

#[derive(new)]
struct Bar {
    a: i32,
    b: String,
}

let _ = Bar::new(42, "Hello".to_owned());

Default values can be specified either via #[new(default)] attribute which removes the argument from the constructor and populates the field with Default::default(), or via #[new(value = "..")] which initializes the field with a given expression:

#[derive(new)]
struct Foo {
    x: bool,
    #[new(value = "42")]
    y: i32,
    #[new(default)]
    z: Vec<String>,
}

let _ = Foo::new(true);

Generic types are supported; in particular, PhantomData<T> fields will be not included in the argument list and will be intialized automatically:

use std::marker::PhantomData;

#[derive(new)]
struct Generic<'a, T: Default, P> {
    x: &'a str,
    y: PhantomData<P>,
    #[new(default)]
    z: T,
}

let _ = Generic::<i32, u8>::new("Hello");

For enums, one constructor method is generated for each variant, with the type name being converted to snake case; otherwise, all features supported for structs work for enum variants as well:

#[derive(new)]
struct Enum {
    FirstVariant,
    SecondVariant(bool, #[new(default)] u8),
    ThirdVariant { x: i32, #[new(value = "vec![1]")] y: Vec<u8> }
}

let _ = Enum::new_first_variant();
let _ = Enum::new_second_variant(true);
let _ = Enum::new_third_variant(42);