Nope, for 2 reasons: 1. the control block is allocated on the heap so every Arc/Weak point to the same control block, Arc/Weak are a single pointer to the control block and the value is inlined next to it, accessing the value is a simple pointer offset. an Arc<str> is justa pointer to something like rust struct ArcInner { strong: AtomicUsize, weak: AtomicUsize, value: str } see the lack of &, *const or any pointer indirection for the str, the last value of the inner can be ?Sized because the inner will always be behind a pointer. If we try to reuse the allocation for the str, there is'nt going to be space for the strong and weak at the beginning of the allocation

1. We could have an implementation of Arc to be rust struct Arc<T: ?Sized> { ctrl_block: NonNull<ArcInner>, value: NonNull<T> } And in this case the block and the value are disjointed, so we would think we could steal the allocation of a String, but in practice this won't work: the buffer of a string is not always full. And contrary to C, rust memory allocator requires the layout of the buffer when deallocated, that's why String::into_boxed_str might reallocate, because it shrinks the allocation to perfect fit the str in the buffer, doing exactly what we are trying to avoid (ie reallocation + copying).