Auto merge of #597 - servo:void-vtable, r=fitzgen,emilio

Make vtables non-zero-size to fix a rustc warning.

```
warning: found non-foreign-function-safe member in struct marked #[repr(C)]: found zero-size struct in foreign module, consider adding a member to this struct
```

Emilio said on IRC:
> the empty vtable means that we don't care of figuring out the proper vtable layout, so we create an empty struct

Sounds like all that matters is to have a pointer, we don’t look at the data behind it. Using `c_void` seems appropriate, then.

1 files changed, 4 insertions, 6 deletions

diff --git a/src/codegen/mod.rs b/src/codegen/mod.rs
index 7db083e7..fb6c839d 100644
--- a/src/codegen/mod.rs
+++ b/src/codegen/mod.rs
@@ -706,17 +706,15 @@ impl<'a> CodeGenerator for Vtable<'a> {
         assert_eq!(item.id(), self.item_id);
         // For now, generate an empty struct, later we should generate function
         // pointers and whatnot.
-        let mut attributes = vec![attributes::repr("C")];
-
-        if ctx.options().derive_default {
-            attributes.push(attributes::derives(&["Default"]))
-        }
+        let attributes = vec![attributes::repr("C")];
 
         let vtable = aster::AstBuilder::new()
             .item()
             .pub_()
             .with_attrs(attributes)
-            .struct_(self.canonical_name(ctx))
+            .tuple_struct(self.canonical_name(ctx))
+            .field()
+            .build_ty(helpers::ast_ty::raw_type(ctx, "c_void"))
             .build();
         result.push(vtable);
     }