use real vtable type in the trait object representation

[thestinger]

There's no reason to be casting to and from an opaque type as it confuses LLVM and breaks devirtualization and other optimizations. I think it will involve some pain to have trans::meth spit out the vtable type without generating one though.

[reem]

Triage: This would be nice for downstream users of TraitObject who want to create type-erased data structures by keeping the vtable inline etc.

[steveklabnik]

Triage: no change. TraitObject still looks like

pub struct TraitObject {
    pub data: *mut (),
    pub vtable: *mut (),
}

[steveklabnik]

Triage: no real changes here, and I'm not aware of any plans to. Given no real comments since 2014, I'm going to give this one a close, as I don't think it's likely to ever be prioritized.

If anyone feels passionate about changing this, ping @rust-lang/compiler to see if they think it's still viable.

[eddyb]

This has nothing to do with TraitObject, although I'm not sure LLVM is limited by this anymore.