Arc-Mutex yes, Arc-Box no.
It's like "I want a shared, immutable reference to something recursive, or unsized". The heap part makes no sense because it's already allocated there if you use an Arc : https://doc.rust-lang.org/std/sync/struct.Arc.html
> The type Arc<T> provides shared ownership of a value of type T, allocated in the heap.
Moreover, you don't even need the box for a dyn : https://gist.github.com/rust-play/f19567f8ad4cc00e3ef17ae6b3...
Yeah, looks like Arc-Box is kind of pointless, but isn't there some thread locality reason why people wrap `Box` in `Arc`? I remember reading something about it a while ago but maybe I'm misremembering.
`Arc<T>` places the refcounts immediately before `T` in memory. If you are desperate to have `T` and `T`'s refcounts be on different cachelines to reduce false sharing in some contrived scenario, `Arc<Box<T>>` would technically accomplish this. I think a more realistic optimization would be to pad the start of `T` however.
`Box<Box<T>>` and `Arc<Box<T>>` are thin pointers (size ≈ size_of::<usize>()) even when `Box<T>` is a fat pointer (size ≈ 2*size_of::<usize>() because `T` is `str`, `[u8]`, `dyn SomeTrait`, etc.). While various "thin" crates are typically saner alternatives for FFI or memory density (prefixing lengths/vtables in the pointed-at data instead of incurring double-indirection and double-allocation), these double boxes are a quick and dirty way of accomplishing guaranteed thin pointers with only `std` / `alloc`.
I would not call either of these use cases "extremely common" however.