Nim's new ARC gc, which will be default soon-ish in 2.0, does not have a "runtime" as commonly understood.

It is fully deterministic, so it simply injects alloc/free calls in the generated code at compile time. It even has an option that can show you where the calls are injected.

https://nim-lang.org/blog/2020/10/15/introduction-to-arc-orc...