Thanks for writing some code! I will try to spend some time to grok it: so is the idea basically that you are guaranteed there is only ever one copy of these unique IPs and there is absolutely no way for any more than one function to hold one? I have to say maybe HashSet was throwing me for a loop and if the important bit is having an object that can not be copied, then it shouldn’t matter what data structure you keep them in does it…

I don’t find a strong case for using Rust just to guarantee each thread gets its own IP (a mutex protected getter seems like it would do just fine for just that) but I guess it would guarantee that as long as you use this “uncopiable” object you could not, even if you wanted to, have two identical IPs in your program if I understand things correctly, which if your mutex getter had a bug would not protect you against.