Is it possible to create a RefCell<Any>?

Question

Is it possible to create something of type RefCell<Any> in Rust? I tried the following:

fn test2<T : Any>(x : T) -> RefCell<Any>{
    return RefCell::new(x) as RefCell<Any>
}

But I get the following error:

error: the trait `core::marker::Sized` is not implemented for the type `core::any::Any + 'static` [E0277]
<anon>:8 fn test2<T : Any>(x : T) -> RefCell<Any>{

The documentation for RefCell includes the following

pub struct RefCell<T> where T: ?Sized {
    // some fields omitted
}

Which leads me to believe (along with the answer from this question) that such a thing is possible. I also tried:

fn test1<T : Any>(x : T) -> Box<Any>{
    return Box::new(x) as Box<Any>
}

which works just fine. Both Box and RefCell seem to have similar bounds, so I'm not quite sure what I'm missing here. Any help would be much appreciated. I have this in a Rust Playground, if that is helpful.

Answer 1

Box has the trait std::ops::CoerceUnsized, which allows to cast to Box<Any>. RefCell does not, so you can't.

Of course you can do this:

let x = RefCell::new( String::new() );
let x = &x as &RefCell<Any>;

So you can have a RefCell<Any>, but cannot construct one or coerce to one, only coerce references.

Answer 2

RefCell<Any> is an unsized type; you can’t have an actual instance of an unsized type—how much stack space will it take? This is the same as the way in which you can’t return Any but must rather return Box<Any>; so also you can’t return RefCell<Any> but must return something like Rc<RefCell<Any>>. The likes of RefCell<Box<Any>> would work fine too, because a RefCell of a sized type is itself sized.

Demonstration.