How to specify a supertrait for the reference of a trait?

Question

I'd like to create a trait that enforces implementation of the Add trait for both the type and a reference to the type. That is, N + N and &N + &N should both be implemented if using the NumberTrait shown below.

use std::ops::Add;

// I think a supertrait needs to be added to NumberTrait,
// something like &Add<Output = Self>, but I don't know
// the correct syntax
pub trait NumberTrait: Sized + Add<Output = Self> {}

fn add_number<N: NumberTrait>(a: N, b: N) -> N {
    a + b
}

fn add_number_ref<N: NumberTrait>(a: &N, b: &N) -> N {
    a + b // compiler error occurs in this line: an implementation of `std::ops::Add` might be missing for `&N`
}

Answer 1

You can do something like this:

use std::ops::Add;

pub trait NumberTrait: Sized + Add<Output = Self>
where
    for<'a> &'a Self: Add<Output = Self>,
{
}

fn add_number<N: NumberTrait>(a: N, b: N) -> N
where
    for<'a> &'a N: Add<Output = N>,
{
    a + b
}

fn add_number_ref<N: NumberTrait>(a: &N, b: &N) -> N
where
    for<'a> &'a N: Add<Output = N>,
{
    a + b
}

But most likely, you don't need that constraint everywhere, and you could just put where clause on the add_number_ref function.

See: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b5bdd3633d22ea1e0873d431a6665f9d