I don't want to rain on everyone's parade (both Nim and Rust), but regarding thread safety none of Nim or Rust or any language besides maybe Ada/Sparks and Haskell address what is needed to write concurrent data structure safely.
In both Rust or Nim the borrow checker or sink/lent annotations will ensure that you move ownership across threads instead of accidentally sharing or free-ing what you don't own. However to prove that your concurrent data structures are without races, deadlocks or livelocks you need formal verification. Rust does have a library for that <https://github.com/tokio-rs/loom> because a borrow checker is not enough contrary to the usual claims about "concurrency without data races". This is the second part of my RFC <https://github.com/nim-lang/RFCs/issues/222> Correct-by-construction thread synchronization which I started to research in <https://github.com/mratsim/weave/issues/18> and implement in <https://github.com/mratsim/weave/pull/127> (but no time to finish)
