如何制作具有关联类型的特征的类型擦除版本？

Question

假设有一个集合特征，其项目具有关联类型：

trait CollectionItem {
    // ...
}

trait Collection {
    type Item: CollectionItem;
    
    fn get(&self, index: usize) -> Self::Item;
    // ...
}

Collection我可以以某种方式将其类型擦除为对和特征都使用动态调度的类型吗CollectionItem？即将其包装成如下所示：

struct DynCollection(Box<dyn Collection<Item=Box<dyn CollectionItem>>>);
impl DynCollection {
  fn get(&self, index: usize) -> Box<dyn CollectionItem> {
    // ... what to do here?
  }
}
impl <C: Collection> From<C> for DynCollection {
  fn from(c: C) -> Self {
    // ... what to do here?
  }
}

操场

Answer 1

您可以添加私有的、类型擦除的辅助特征：

trait DynCollectionCore {
    fn get_dyn(&self, index: usize) -> Box<dyn CollectionItem>;
}

impl<C> DynCollectionCore for C
where
    C: ?Sized + Collection,
    C::Item: 'static,
{
    fn get_dyn(&self, index: usize) -> Box<dyn CollectionItem> {
        Box::new(self.get(index))
    }
}

然后用它来构建一个包装类型：

struct DynCollection(Box<dyn DynCollectionCore>);

impl DynCollection {
    fn new<C>(inner: C) -> Self
    where
        C: Collection + 'static,
        C::Item: 'static,
    {
        Self(Box::new(inner))
    }
}

impl Collection for DynCollection {
    type Item = Box<dyn CollectionItem>;

    fn get(&self, index: usize) -> Box<dyn CollectionItem> {
        self.0.get_dyn(index)
    }
}

// note: something like this is also needed for `Box<dyn CollectionItem>:
//       CollectionItem` to be satisfied
impl<T: ?Sized + CollectionItem> CollectionItem for Box<T> {
    // ...
}