also-doesnt-work.rs

macro_rules! Q(($T:ty, $f:expr, $name:ident) => {
    struct $name {
        value: $T
    }

    impl $name {
        pure fn add(&self, other: &$name) -> $name {
            $name { value: self.value + other.value }
        }
    }
})

Q!(i8, 8, Q8)

fn main() {
    let a = Q8 { value: 4 };
    let b = Q8 { value: 8 };
    
    io::println(fmt!("Q8: %?", &a.add(&b)))
}

doesnt-work.rs

macro_rules! Q(($T:ty, $f:expr, $name:ident) => (
struct $name {
    value: $T
}

impl $name: Add<$name, $name> {
    pure fn add(&self, other: &$name) -> $name {
        $name { value: self.value + other.value }
    }
}
))

Q!(i8, 8, Q8)

fn main() {

    let a = Q8 { value: 4 };
    let b = Q8 { value: 8 };
    
    io::println(fmt!("Q8: %?", a + b))

}

error

test.rs:67:31: 67:36 error: binary operation + cannot be applied to type `Q8`
test.rs:67     io::println(fmt!("Q8: %?", a + b))

works.rs

macro_rules! Q(($T:ty, $f:expr, $name:ident) => (
struct $name {
    value: $T
}
))
    
impl Q8: Add<Q8, Q8> {
    pure fn add(&self, other: &Q8) -> Q8 {
        Q8 { value: self.value + other.value }
    }
}

Q!(i8, 8, Q8)

fn main() {

    let a = Q8 { value: 4 };
    let b = Q8 { value: 8 };
    
    io::println(fmt!("Q8: %?", a + b))

}