Implement `{Arc, Rc}::downcast`

[apasel422]

I think this is possible to do safely, as it is essentially the inverse operation of CoerceUnsized. See an example implementation at https://github.com/apasel422/ref_count/.

[Gankra]

For those who don't want to dig through some source:

impl Arc<Any> {
    pub fn downcast<T>(self) -> Result<Arc<T>, Self> where T: Any;
}

[glaebhoerl]

Could we do this generically (one impl, instead of one per smart pointer type) if we had HKT? (And if we don't?)

[seanmonstar]

Don't forget OIBITs... impl Arc<Any + send>, impl Arc<Any + Send + Sync>, etc...

[apasel422]

@seanmonstar Thanks for pointing that out. Do we need all combinatoric possibilities of {'static, Send, Sync}?

[Gankra]

I believe if proper trait object subtyping (or just coercions...?) was implemented, this would be a non-issue.

[mbrubeck]

<Rc<Any>>::downcast was implemented in rust-lang/rust#44273.

The Arc method hasn't been implemented yet because it needs extra Send or Sync bounds to be useful.

[bluss]

What do you think of simply implementing all combinations, instead of us guessing up front which ones are useful? That would be: Arc<Any>, Arc<Any + Send>, Arc<Any + Sync>, Arc<Any + Send + Sync>.

[Centril]

This is implemented for both types, so I'm closing this.