I'm not expert in Nim or Rust. Maybe when I say Rust is safer and faster is wrong. Write a Nim wrapper around Rust won't give a better quality library.
But many Nim lib is in early stage, and miss feature, less actively maintened compare to a Rust or C lib. If I program in Nim, I would prefer use a mature Rust/C lib, with full feature, actively maintened, industrially used. So why Nim lib don't just wrap a mature Rust/C lib, that allow save many development time.
