Transpose `HashMap<K, Vec<T>>` to `Vec<HashMap<K, T>>`

[krakow10]

What would be the best way to achieve this with rayon? HashMap is small ~60 and Vec is large ~60000 and K is Copy. My first thought was to implement the IntoParallelIterator and Producer traits manually with a wrapper type and use a HashMap of rayon::vec::DrainProducers for splitting, but DrainProducer is not in the public interface. Am I missing something obvious? I'm also looking to solve this for transposing a struct-of-arrays to array-of-structs.

Edit:
I couldn't think of any other way to do it, but my I wrote an implementation and my idea works!

[cuviper]

I wonder how that idea compares to a very simplistic loop, something like:

let map_len = map_to_vec.len();
let vec_len = map_to_vec.values().next().map_or(0, Vec::len);
let mut vec_to_map = vec![];
vec_to_map.resize_with(vec_len, || HashMap::with_capacity(map_len));
for (key, vec) in map_to_vec {
    vec_to_map.par_iter_mut().zip(vec)
        .for_each(|(map, item)| map.insert(key, item));
}

This won't be ideal, since it has a serialization point at each key, but it also doesn't require any temporary allocations.

Your version could also be a little simpler by using intermediate Vec<(K, DrainProducer)>, i.e. assuming uniqueness rather than rehashing a new map at each split.