The Arc type is Rust’s thread-safe smart pointer, and like other pointer types in Rust (e.g., Box), one can use the as keyword1 to cast an Arc into an Arc, as long as MyTrait is dyn compatible. What about casting it back from Arc to Arc? Well, here the plot thickens a bit. We can’t use the as keyword here, because that operation is inherently unsafe. An arbitrary Arc might not hold a MyStruct in it, but some other object that implements MyTrait, and trying to cast it into the wrong type would result in undefined behaviour. But the fact that something is unsafe doesn’t mean it should be impossible, or even undesirable. An unsized coercion if you want the fancy term. ↩