Macro join_internal

macro join_internal {
    (
        // Accumulate a token for each future that has been expanded: "_ _ _".
        current_position: [
            $($underscores:tt)*
        ]
        // Accumulate Futures and their positions in the tuple: `_0th ()   _1st ( _ ) …`.
        futures_and_positions: [
            $($acc:tt)*
        ]
        // Munch one future.
        munching: [
            $current:tt
            $($rest:tt)*
        ]
    ) => { ... },
    (
        current_position: $_:tt
        futures_and_positions: [
            $(
                $fut_expr:tt ( $($pos:tt)* )
            )*
        ]
        // Nothing left to munch.
        munching: []
    ) => { ... },
}

🔬This is a nightly-only experimental API. (future_join #91642)

To be able to name the i-th future in the tuple (say we want the .4-th), the following trick will be used: let (_, _, _, _, it, ..) = tuple; In order to do that, we need to generate a i-long repetition of _, for each i-th fut. Hence the recursive muncher approach.