Automatically implement traits of enclosed type for Rust newtypes (tuple structs with one field)

Question

In Rust, tuple structs with only one field can be created like the following:

struct Centimeters(i32);

I want to do basic arithmetic with Centimeters without extracting their "inner" values every time with pattern matching, and without implementing the Add, Sub, ... traits and overloading operators.

What I want to do is:

let a = Centimeters(100);
let b = Centimeters(200);
assert_eq!(a + a, b);

Answer 1

is there a way to do it without extracting their "inner" values every time with pattern matching, and without implementing the Add, Sub, ... traits and overloading operators?

No, the only way is to implement the traits manually. Rust doesn't have an equivalent to the Haskell's GHC extension GeneralizedNewtypeDeriving which allows deriving on wrapper types to automatically implement any type class/trait that the wrapped type implements (and with the current set-up of Rust's #[derive] as a simple AST transformation, implementing it like Haskell is essentially impossible.)

To abbreviate the process, you could use a macro:

use std::ops::{Add, Sub};

macro_rules! obvious_impl {
    (impl $trait_: ident for $type_: ident { fn $method: ident }) => {
        impl $trait_<$type_> for $type_ {
            type Output = $type_;

            fn $method(self, $type_(b): $type_) -> $type_ {
                let $type_(a) = self;
                $type_(a.$method(&b))
            }
        }
    }
}

#[derive(Eq, PartialEq, Ord, PartialOrd, Clone, Debug)]
pub struct Centimeters(i32);

obvious_impl! { impl Add for Centimeters { fn add } }
obvious_impl! { impl Sub for Centimeters { fn sub } }

#[derive(Eq, PartialEq, Ord, PartialOrd, Clone, Debug)]
pub struct Inches(i32);

obvious_impl! { impl Add for Inches { fn add } }
obvious_impl! { impl Sub for Inches { fn sub } }

fn main() {
    let a = Centimeters(100);
    let b = Centimeters(200);
    let c = Inches(10);
    let d = Inches(20);
    println!("{:?} {:?}", a + b, c + d); // Centimeters(300) Inches(30)
    // error:
    // a + c;
}

playpen

I emulated the normal impl syntax in the macro to make it obvious what is happening just by looking at the macro invocation (i.e. reducing the need to look at the macro definition), and also to maintain Rust's natural searchability: if you're looking for traits on Centimeters just grep for for Centimeters and you'll find these macro invocations along with the normal impls.

If you are accessing the contents of the Centimeters type a lot, you could consider using a proper struct with a field to define the wrapper:

struct Centimeters { amt: i32 }

This allows you to write self.amt instead of having to do the pattern matching. You can also define a function like fn cm(x: i32) -> Centimeters { Centimeters { amt: x } }, called like cm(100), to avoid the verbosity of constructing a full struct.

You can also access the inner values of a tuple struct using the .0, .1 syntax.