It is only Arc and not Mutex that allocates and thus has "extra" pointer indirection. A Mutex can live perfectly well on the stack, as long as it outlives the t

It is only Arc and not Mutex that allocates and thus has "extra" pointer indirection. A Mutex can live perfectly well on the stack, as long as it outlives the threads accessing it. Arc has to allocate, because it is meant to outlive the function that created it, and the only place to safely do that is on the heap.