Function for_each_chunk 

pub fn for_each_chunk<'l, T, F>(items: &'l [T], f: F)
where
    T: Sync,
    F: Fn(&'l [T]) + Send + Sync,

Like for_each, but calls f once per chunk instead of once per item. A slice too small to be chunked is passed to f as a whole on the calling thread, and an empty slice doesn’t call f. Uses the same chunk sizing as for_each.