Overloading an operator for all structs with a trait in Rust

Question

I'm trying to implement C++-style expression templates in Rust using traits and operator overloading. I'm getting stuck trying to overload '+' and '*' for every expression template struct. The compiler complains about the Add and Mul trait implementations:

error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
  --> src/main.rs:32:6
   |
32 | impl<T: HasValue + Copy, O: HasValue + Copy> Add<O> for T {
   |      ^ type parameter `T` must be used as the type parameter for some local type
   |
   = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
   = note: only traits defined in the current crate can be implemented for a type parameter

That error would make sense if the type I was trying to implement a trait for was constructible without my crate, but the type is a generic that must implement the HasValue trait I defined.

Here's the code:

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

trait HasValue {
    fn get_value(&self) -> i32;
}

// Val

struct Val {
    value: i32,
}

impl HasValue for Val {
    fn get_value(&self) -> i32 {
        self.value
    }
}

// Add

struct AddOp<T1: HasValue + Copy, T2: HasValue + Copy> {
    lhs: T1,
    rhs: T2,
}

impl<T1: HasValue + Copy, T2: HasValue + Copy> HasValue for AddOp<T1, T2> {
    fn get_value(&self) -> i32 {
        self.lhs.get_value() + self.rhs.get_value()
    }
}

impl<T: HasValue + Copy, O: HasValue + Copy> Add<O> for T {
    type Output = AddOp<T, O>;
    fn add(&self, other: &O) -> AddOp<T, O> {
        AddOp {
            lhs: *self,
            rhs: *other,
        }
    }
}

// Mul

struct MulOp<T1: HasValue + Copy, T2: HasValue + Copy> {
    lhs: T1,
    rhs: T2,
}

impl<T1: HasValue + Copy, T2: HasValue + Copy> HasValue for MulOp<T1, T2> {
    fn get_value(&self) -> i32 {
        self.lhs.get_value() * self.rhs.get_value()
    }
}

impl<T: HasValue + Copy, O: HasValue + Copy> Mul<O> for T {
    type Output = MulOp<T, O>;
    fn mul(&self, other: &O) -> MulOp<T, O> {
        MulOp {
            lhs: *self,
            rhs: *other,
        }
    }
}

fn main() {
    let a = Val { value: 1 };
    let b = Val { value: 2 };
    let c = Val { value: 2 };

    let e = ((a + b) * c).get_value();

    print!("{}", e);
}

Thoughts?

Answer 1

Trying to define the trait Add for you custom types, you are doing this:

impl<T: HasValue + Copy, O: HasValue + Copy> Add<O> for T {
    type Output = AddOp<T, O>;
    fn add(&self, other: &O) -> AddOp<T, O> {
        AddOp {
            lhs: *self,
            rhs: *other,
        }
    }
}

But T: HasValue + Copy matches any type implementing trait HasValue, and this type may not be defined in your crate (for example if you implement HasValue for i32). As Add isn't defined in your crate either, Rust complains: for example by defining HasValue for i32, you would also re-define Add<i32> for i32!

My suggestion would be to wrap all your operation and values structs into a generic one, and implementing Add and Mul for it. This way, you implement Add and Mul only for a simple type defined in your crate, and the compiler is happy.

Something like that:

struct Calculus<T> {
    calc: T,
}

impl<T: HasValue + Copy> HasValue for Calculus<T> {
    fn get_value(&self) -> i32 {
        self.calc.get_value()
    }
}

impl<T, O> Add<Calculus<O>> for Calculus<T>
where
    T: HasValue + Copy,
    O: HasValue + Copy,
{
    type Output = Calculus<AddOp<T, O>>;
    fn add(self, other: Calculus<O>) -> Calculus<AddOp<T, O>> {
        Calculus {
            calc: AddOp {
                lhs: self.calc,
                rhs: other.calc,
            },
        }
    }
}

impl<T, O> Mul<Calculus<O>> for Calculus<T>
where
    T: HasValue + Copy,
    O: HasValue + Copy,
{
    type Output = Calculus<MulOp<T, O>>;
    fn mul(self, other: Calculus<O>) -> Calculus<MulOp<T, O>> {
        Calculus {
            calc: MulOp {
                lhs: self.calc,
                rhs: other.calc,
            },
        }
    }
}

Then you can just add a neat new() method for your Val type:

impl Val {
    fn new(n: i32) -> Calculus<Val> {
        Calculus {
            calc: Val { value: n },
        }
    }
}

and use the whole thing like this:

fn main() {
    let a = Val::new(1);
    let b = Val::new(2);
    let c = Val::new(3);

    let e = ((a + b) * c).get_value();

    print!("{}", e);
}

Playground