Arc in Rust stands for Atomic Reference Counting. Arc in Nim stands for Automatic Reference Counting.
For a shared mutable variable you need to consider ownership: * which thread allocates the variable, which thread deallocates. For example if there is a master thread that create and destroy child threads, you can ensure that data lives long enough by having alloc/dealloc in the master thread. On the other hand, if data can be allocated and deallocated from any thread, you need atomic reference counting, which can be implemented in a couple dozens lines: <https://github.com/mratsim/weave/blob/71dc2d7/weave/cross_thread_com/flow_events.nim#L176-L201>= * can mutation be concurrent on the same data (and so need synchronization) or not (say parallel map on a sequence/vector)
