Yep, that was it, changing &primes.to_vec() --> &primes fixed it.
When I wrote this two years ago, it wouldn't compile without the.to_vec() for some reason.
Rust now uses a smidgen less memory than Crystal for these inputs: ~11GB vs ~11.7 GB max.
Thanks for the quick response. I've made the change in the gist.
Happy Holidays! 