How do I iterate arguments of a macros in Rust?

Question

I am trying to create a macro that fills first elements of a vector with a fixed size. The rest of elements should be zero. I wrote:

const SIZE: usize = 3;

#[derive(Debug)]
struct MyVec {
    data: [f32; SIZE]
}

macro_rules! my_vec {
    ($($x:expr),*) => [{
        let mut data: [f32; SIZE] = [0.0; SIZE];
        for (i, e) in x.enumerate() {
            data[i] = e;
        }
        MyVec { data }
    }]
}

fn main() {
    let v = my_vec![1.0, 2.0];
    println!("{:?}", v);  // must be MyVec { data: [1.0, 2.0, 0.0] }
}

It looks like I try to iterate the arguments in a wrong way. How can I fix it?

Answer 1

You could do this,

const SIZE: usize = 3;

#[derive(Debug)]
struct MyVec {
    data: [f32; SIZE],
}

macro_rules! my_vec {
    ( $( $x:expr ),* ) => {
        {
            let mut data: [f32; SIZE] = [0.0; SIZE];
            let mut index = 0;
            $(
                #[allow(unused_assignments)]
                {
                    data[index] = $x;
                    index = index + 1;
                }
            )*
            MyVec { data }
        }
    };
}

fn main() {
    let v = my_vec![1.0, 2.0];
    println!("{:?}", v); // print MyVec { data: [1.0, 2.0, 0.0] }
}

The code inside the $(...)* will be expanded for each matches.