Small speed note: Rust 1.7 just made custom hashing functions available. Rust uses a cryptographically secure hashing function by default, but for work like this, that's inappropriate. To use FNV instead:

1. add fnv to your Cargo.toml's dependencies section 2. The first few lines become:

  extern crate fnv;

  use std::collections::HashMap;
  use std::hash::BuildHasherDefault;
  use fnv::FnvHasher;
    
  type MyHasher = BuildHasherDefault<FnvHasher>;

2. and the hash creation line becomes

  let mut last_digit_pair_counts: HashMap<String, u64, MyHasher> = HashMap::default();

This is 17% faster on my two runs of your code. :)

Other notes:

  format!("{}-{}", last_digit0, last_digit1).to_string();

The .to_string() is redundant here, format! already gives you a String. That should remove a bunch of allocations.