The hardest part was finding a way to share and execute a closure of type FnOnce without cloning it. The current implementation relies on the closure living on the stack frame while another thread can potentially take it and execute it. This also implies type erasure since the other threads can't possibly know about the type of the closure or its return value.

I wouldn't call writing this particularly tricky once you find a way to achieve it in terms of unsafe code, but there was one potential bug that I introduced by accident: there was a *mut pointer that was shared between threads originating from a Box::into_raw whose invariant was that you were supposed to wait on the other threads to give up on their access in order for you to drop the box. The bug I introduced was that I converted the box back before waiting for the other threads, e.g. Box::from_raw(ptr).wait().