Shared via Rust Playground

playground.rs

trait Unpack<T> {
    fn unpack<I: Iterator<Item=T>>(i: I) -> Self;
}

macro_rules! impl_unpack {
    (@replace_ty   $_t:tt $ty:ty    ) => { $ty   };
    (@replace_expr $_t:tt $expr:expr) => { $expr };

    (@expand $($ident:ident,)* )    => {
        impl<T> Unpack<T> for (
            $( impl_unpack!{ @replace_ty [$ident] Option<T> } ),*
        )   {
            fn unpack<I: Iterator<Item=T>>(mut _i: I) -> Self {
                (
                    $( impl_unpack!{ @replace_expr [$ident] _i.next() } ),*
                )
            }
        }
    };
    
    () => {
        impl_unpack!{ @expand }
    };
    ($x:ident, ) => {
        impl_unpack!{ @expand $x, }
        impl_unpack!{ }
    };
    ($x:ident, $($rest:ident,)* ) => {
        impl_unpack!{ @expand $x, $($rest,)* }
        impl_unpack!{             $($rest,)* }
    }
}

impl_unpack!{ x, x, x, x, x, x, x, x, x, x, x, x, x, x, }

fn main() {
    let (a, b, c, d) = Unpack::unpack(vec![1, 2].into_iter());
    println!("{:?}, {:?}, {:?}, {:?}", a, b, c, d);
}