Mostly the ownership model, trying to remember which functions expect borrowed types or not, etc.

The error messages in rust are really good, so I can usually make the code work quickly, but I need to properly understand the reason behind the error in order to learn, so that’s when I get headaches

Rust is definitely a really cool language (as someone who has played with it just a little) but it’s quite headache inducing, at least for me at the moment.

MQTT is great! There are clients available in Python, JS, etc

I was trying to rank up in Codewars, and there was a 1kyu (hardest and worth the most points) kata only available in OCaml, so I learned it in order to solve.