The Hashtable Problem: A Litmus Test for External Impl Proposals

I think the best answer is instead to add a level of indirection, not unlike we have with the buildhasher already.

If I use a CustomizedBinaryHeap<MyCustomStrategy, T> (where BinaryHeap<T> = CustomizedBinaryHeap<DefaultOrdStrategy, T>), then you can put any trait impl you want on your local MyCustomStrategy type. So even if the T is some external type, that's fine.

That way the instances carry around the necessarily "impls" without needing all these extra features for adding new ways to specify things.