DispatchFromDyn: require additional checks by dingxiangfei2009 · Pull Request #149068 · rust-lang/rust (original) (raw)

Quote @theemathas

@dingxiangfei2009 Your comment in the code unfortunately does not answer my latest comment in this PR. My comment does not involve a double pointer indirection.

I don't think my text articulates my thoughts right. &Outer<T> -> &Outer<U> is not a double pointer indirection, true, and the point is that there are too many "indirection"s.

Maybe I should reword it. The pre-requisite of a valid DispatchFromDyn relation from a type A to a type B is that

• A and B are of the same shape, sharing the same concrete type constructor, excluding aliases and generics
• A and B are both generic over two distinct type parameter T and U respectively
• T: Unsize<U>
• Most importantly, the "downcast" of a value of B back to A is supported. Here we can give an exhaustive list of supported "downcasts" and we can add more as supported types grow.
  • A := &T from B := &U, or A := &mut T from B := &mut U
    * This works today by dispatching the call to the corresponding method after transparently discarding the metadata
  • A := Adt<T> from B := Adt<U> when T: DispatchFromDyn<U> and Adt<_> is reasonable
    * This works today by delegating the dispatch to the only field in A after transparently discarding the metadata

The last bullet point will answer the important question...

Should it?

... which is no, because we don't know how to "downcast" &Outer<U> back to &Outer<T> because that metadata is actually behind one layer of "indirection." The argument against &mut Outer<U> back to &mut Outer<T> case should run like this. The "downcast" mutation would need to leave the address unchanged and, in turn, necessitate mutation of the data behind this &mut borrow. The transformation is no longer transparent and actually would change type of the place behind that borrow. I don't see a way to support it in Rust.

The A := &T case, on the other hand, has no indirection because the metadata is directly attached to the &U pointer. So this "downcast" is simple for us to implement.

I hope that this would be a better answer to the earlier question. I am not sure how much this content could go into the documentation. Please advise.