Function unordered_map_order 

pub fn unordered_map_order(hashes: &[u64]) -> Vec<u32>

The order a std::unordered_map iterates keys inserted in the order of hashes (each key distinct): libstdc++’s _Hashtable with cached hash codes, a new node at the front of its bucket (or of the whole list when its bucket is empty), and the unique-key rehash that relinks nodes front-first. The result lists insertion indices.