Rust is used a lot. Elixir/Phoenix comes up often. You’re looking for something statically typed with string guarantees and as FP as possible to make unit testing easier.
Rust for sure. Builds become a problem eventually though. Elixir is much more interesting now that they added types (I think they're doing more with this still?). I quite like Gleam from a human (non-agent) perspective.
the only substantive difference I've noticed is between languages with a proof system and those without, where with a proof system is orders of magnitude better for agentic work.
In my experience, Go. It was designed to be idiot-proof. It's very verbose, not very expressive, compiles fast for quick feedback loops, strict autoformatter, plenty of examples in models' training data.