Support for trait objects

[rbartlensky]

Can you wrap a trait object in a Gc? I would like to be able to write something like this. As a workaround, I am currently creating a Gc<Box<dyn MyTrait>>, but I don't really like the indirection. I am trying to use Gc the way you would use a Box or Rc with trait objects.

[Manishearth]

No, because we don't know how to trace it.

You can write such a thing in a wrapped struct with a custom implementation of Trace if you want.

[rbartlensky]

Correct me if I am wrong, but even if I create a wrapper, it will still contain some sort of pointer to MyTrait. I tried what you suggested, but I am not sure that I got it right.

pub struct DynWrapper<T: ?Sized + Trace + Finalize> {
    data: T,
}

impl<T: Trace> DynWrapper<T> {
    pub fn new(data: T) -> Self {
        DynWrapper { data  }
    }
}

But this doesn't work with trait objects, unless I have a */&/Box/Rc T instead of a plain T. I am trying to avoid the extra heap allocation, because Gc already does one.

[Manishearth]

No, you have to do this on a per-trait-object basis

pub struct MyTraitWrapper {
   inner: Box<Trait>
}

and your Trait needs to inherit from Trace (you can't do Box<Trait+Trace> sadly). You can also create a new trait that inherits from both traits.

If you want to avoid the heap allocation you can do the following:

pub struct MyTraitWrapper {
  inner: Trait

which is also a dynamically sized type and can be directly placed inside Gc. It's tricker to construct though, you'll likely need a constructor that directly returns Gc<MyTraitWrapper>

[rbartlensky]

Thank you for your help. I think I will stick with your first suggestion.

[andersk]

The original code actually just works in nightly Rust with the nightly crate feature, due to CoerceUnsized (67cc6e2).

[andersk]

With the nightly crate feature, #160 adds a conversion from Box<dyn Trait> to Gc<dyn Trait>. So I think trait objects are now supported as well as they can be.

The tracking issue for the required unstable Rust feature is

• DST/custom coercions rust-lang/rust#18598