Function par_merge 

unsafe fn par_merge<T, F>(
    left: &mut [T],
    right: &mut [T],
    dest: *mut T,
    is_less: &F,
)
where
    T: Send,
    F: Fn(&T, &T) -> bool + Sync,

Merges slices left and right in parallel and stores the result into dest.

Safety

The dest pointer must have enough space to store the result.

Even if is_less panics at any point during the merge process, this function will fully copy all elements from left and right into dest (not necessarily in sorted order).