Simultaneous mutable access to arbitrary indices of a large vector that are guaranteed to be disjoint

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You can sort indices_to_update and extract mutable references by calling split_*_mut. 

let len = big_vector_of_elements.len();

while has_things_to_do() {
    let mut tail = big_vector_of_elements.as_mut_slice();

    let mut indices_to_update = compute_indices();
    // I assumed compute_indices() returns unsorted vector
    // to highlight the importance of sorted order
    indices_to_update.sort();

    let mut elems = Vec::new();

    for idx in indices_to_update {
        // cut prefix, so big_vector[idx] will be tail[0]
        tail = tail.split_at_mut(idx - (len - tail.len())).1;

        // extract tail[0]
        let (elem, new_tail) = tail.split_first_mut().unwrap();
        elems.push(elem);

        tail = new_tail;
    }
}

Double check everything in this code; I didn’t test it. Then you can call elems.par_iter(...) or whatever.

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