Shared via Rust Playground

playground.rs

macro_rules! halve {
    (@count $mac:tt $xs:tt [] $acc:tt)
        => { halve!(@split $mac $xs [] $acc) };
    (@count $mac:tt $xs:tt [$head:expr] $acc:tt)
        => { halve!(@split $mac $xs [] ($acc)) };
    (@count $mac:tt $xs:tt [$head:expr, $($tail:expr),+] $acc:tt)
        => { halve!(@count $mac $xs [$($tail),+] ($acc)) };
    
    (@split [$mac:ident!($($arg:expr),*)] $left:tt $right:tt ())
        => { ($mac!($($arg,)* $right), $mac!($($arg,)* $left)) };
    (@split $mac:tt [$head:expr, $($tail:expr),+] [$($right:expr),*] (($acc:tt)))
        => { halve!(@split $mac [$($tail),+] [$($right,)* $head] $acc) };

    ($mac:ident!($($arg:expr),*): $($x:expr),*)
        => { halve!(@count [$mac!($($arg),*)] [$($x),*] [$($x),*] ()) }
}

fn main() {
    halve!(println!("{:?}"): 1, 2, 3, 4, 5, 6);
}