Macro cfg_join 

macro_rules! cfg_join {
    ($a:expr, $b:expr $(,)?) => { ... };
    ($first:expr, $($rest:expr),+ $(,)?) => { ... };
}

Conditionally fork expressions into parallel tasks using rayon::join.

When the “parallel” feature is enabled, uses right-leaning nested rayon::join calls to execute all expressions concurrently. Otherwise, evaluates them sequentially.

Returns a right-nested pair tree: 2 items → (A, B), 3 items → (A, (B, C)), 4 items → (A, (B, (C, D))), etc. The shape is identical in both parallel and sequential mode.

(Unfortunately, we can’t flatten the result into a tuple of all items using just macro_rules)