No way to downcast Arc<dyn Array> to Arc<T: Array>

[scovich]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

arrow-rs makes heavy use of ArrayRef (= Arc<dyn Array>), and it's impossible to impl Array for !Any types because Array::as_any is a required method. But for some reason we have Array: Send + Sync instead of Array: Any + Send + Sync, so there's no way to Arc::downcast an ArrayRef to e.g. Arc<StructArray>.

Describe the solution you'd like

trait Array: std::any::Any + ... { ... }

Which allows

let array: ArrayRef = ...;
let struct_array: Arc<StructArray> = Arc::downcast(array).map_err(...)?;

One big problem tho -- the blanket impl<T: Array> Array for &T means Array cannot require Any (because Any: 'static). I'm not sure how to work around that.

Describe alternatives you've considered

As far as I can tell, with today's code you have to downcast to &StructArray, clone it, and allocate a new Arc:

let array: ArrayRef = ...;
let struct_array = array.as_struct_opt().ok_or_else(...)?;
let struct_array = Arc::new(struct_array.clone());

Sure, cloning an array is probably fairly inexpensive, but there's no way it's as cheap as an arc clone.