Macro maybe_par_map_init 

macro_rules! maybe_par_map_init {
    ($slice:expr, $init:expr, $f:expr) => { ... };
}

Parallel or sequential map with a reusable mutable state produced by $init.

Lets a hot per-item body hoist its scratch buffers out of the loop: with rayon each worker thread gets its own state and reuses it across the items it steals, and without the feature a single state is reused for the whole iteration. The per-item body must fully overwrite whatever it reads from the state, so results stay independent of the thread count.