[Rustonomicon] Show how to convert a trait object into concrete type

[Havvy]

Last night somebody asked how to convert a trait object where they knew the concrete type into the concrete type. I tried looking for the answer in the Rustonomicon, but alas, no answer was found. Should this be in there?

The answer has to do with transmuting into std::raw::TraitObject. Even if it's just a link to that type from the transmutation page, I think it's important to explicitly show that it is possible. One issue though, is that std::raw::TraitObject is currently unstable as per issue #27751.

[steveklabnik]

/cc @gankro

[Gankra]

I was under the impression that the lang team (cc @nikomatsakis -- can't ping lang myself) wasn't happy with std::raw::TraitObject, so I'm kinda iffy about talking about it. Especially because this is a really specific usecase that's kinda sketchy right now. IDK. Maybe @aturon's inheritance stuff is the way forward?

[steveklabnik]

That's what I was under the impression of too

On Sep 21, 2015, at 21:41, Alexis Beingessner notifications@github.com wrote:

I was under the impression that the lang team (cc @nikomatsakis -- can't ping lang myself) wasn't happy with std::raw::TraitObject, so I'm kinda iffy about talking about it. Especially because this is a really specific usecase that's kinda sketchy right now. IDK. Maybe @aturon's inheritance stuff is the way forward?

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[aturon]

Yes, we don't have a stable, recommended way to do this at the moment.

Ultimately, we plan to support "upcasting" in traits (so that if you have trait Foo: Any you can go from &Foo to &Any). That will allow you to inherit from Any to get the kind of downcasting you want. It shouldn't be too hard, but it's not on anyone's plate at the moment.

[steveklabnik]

Okay! Giving this a close, then.