Macro for making a struct with an automatic ::new()

struct_with_constructor.rs

macro_rules! as_item {
    ($i:item) => ($i);
}

macro_rules! struct_with_constructor {
    (
        $(#[$meta:meta])*
        struct $name:ident {
            $($arg:ident : $typ:ty),*;
            $(fn $fn_name:ident $args:tt $(-> $ret_ty:ty)* { $($fn_body:tt)* })*
        }
    ) => {
        $(#[$meta])*
        struct $name {
            $($arg : $typ),*
        }
        
        as_item! {
            impl $name {
                fn new($($arg : $typ),*) -> $name {
                    $name { $($arg : $arg),* }
                }
                $(fn $fn_name $args $(-> $ret_ty)* {$($fn_body)*})*
            }
        }
    }
}

struct_with_constructor!(
    #[derive(Copy, Clone, Debug)]
    struct Matrix {
        rows: i32,
        cols: i32;
        fn numel(self) -> i32 { self.rows * self.cols }
    }
);

fn main() {
    let m = Matrix::new(3, 4);
    println!("{}", m.numel());
    println!("{:?}", m);
}