[kevin_thibedeau]

Nim threads don't share memory. Each has its own GC heap with data exchanged through messages. The lack of thread safety is irrelevant when they can't normally get to each others memory. This of course has performance implications for certain types of parallelism. Nim optimizes for a model where workers don't have to have high speed, high volume inter-communication. If you need shared memory it is up to you to add it along with whatever safety measures you need.

[pcwalton]

Does Nim now deep copy all messages between threads and start every thread with a fresh environment? If so, it's safe. But it is really problematic for any parallelism. (asm.js tried this and developers felt it was unacceptable.) I think you really can't get away from thread-safe GC in practice.