Implement conversion from Box to Rc / Arc.

[Ms2ger]

Initially proposed in #29580.

[Ms2ger]

Though the implementation there wasn't too interesting; it would be more useful to have an implementation that works on unsized types/

[apasel422]

How would this work? The backing storage for Arc<T> and Rc<T> is two usizes bigger than that of Box<T>. Or would this not be a "free" conversion?

[Ms2ger]

I guess it would need to reallocate, yes.

[alexcrichton]

I think that this issue is more appropriate in rust-lang/rfcs (e.g. feature requests), could you move over there instead?

[Mark-Simulacrum]

Closing in favor of #40475.

[snuk182]

Doesn't #40475 cover the boxed arrays conversions only?

[Mark-Simulacrum]

From the RFC: From<Box<T: ?Sized>> for Rc<T> is to be added. I think that covers it?