Run-time Dynamic type-casting (RTTI)

language design
25

Hi, I have hitting that issue as well (especially when working on shakacode/messagebus). Here is how it may look like (al least on my opinion): TraitObject has vtable for each type's impl - we can add typeid and next_sibling/prev_sibling to this vtable impl block. It will be possible to iterate through all dyn implementations for given type. And dynamic_cast function may look like (just draft):

pub unsafe fn dynamic_cast_ref<T: ?Sized + Unsized, U: ?Sized + Unsized>(tp: &T) -> Option<&U> {
    let target = TypeId::of::<U>();
    let to: TraitObject = mem::transmute(tp);
    let mut v = to.vtable;
    loop {
        let td: Option<&DynDeclType> = mem::transmute(v.offset(NEXT_SIBLING_OFFSET));
        let td = if let Some(val) = td {
            val
        } else {
            break;
        };

        if td.type_id == target {
            return Some(mem::transmute(TraitObject {data: to.data, vtable: td.vtable});
        } else {
            v = td.vtable;
        }
    }

    // and same for prev_sibling as well

    None
}