Rust Any part 3: we have upcasts (opens in new tab)

It's some of the other stuff that gets OOP its bad rap.

Garbage collection, common in many OOP languages, enables having arbitrary object graphs. The programmer doesn't need to think hard about who has a reference to who, and how long these references live. This leads to performance-defeating pointer chasing, and fuzzy, not-well-thought-of object lifetimes, which in turn lead to accidental memory leaks.

Additionally, references in major languages are unconditionally mutable. Having long-lived mutable references from arbitrary places makes it hard to track object states. It makes easier to end up in unexpected states, and accidentally violate invariants from subroutine code, which means bugs.

Also, class-like functionality conflates data-layout, privacy/encapsulation, API, namespacing and method dispatch into a single thing. Untangling that knot and having control over these features separately might make a better design.

Object-Oriented Programming is a spectrum with varying degrees of abuse.

Consider Linux. Time was, metaprogramming was BAD, C was all you needed, we didn't need dynamic CPU schedulers, we didn't need multiple LSMs, and we sure as hell didn't need ABI stability. Now we have forms of all of these things (for the last, see CO-RE), because as it turns out, they're actually good.

In Python, well, turns out multiprocessing isn't all you need, and a free-threaded Python has transitioned from impossible and unwanted to exciting and imminent.

In transfer encodings, "front end" people thought that JSON was all you needed. Schemas? Binary encodings? Versioning? References? All for those loser XML weenies. Well, who's the weenie now?

And in Rust? Well, let's see. Turns out monomorphization isn't all you need. Turns out that it is, in fact, occasionally useful to unify an object and its behavior in a runtime-polymorphic way. I expect yeet_expr to go through eventually too. Others are trying to stabilize (i.e. ossify) the notionally unstable ABI, just like they did to poor C++, which is now stuck with runtime pessimization because somebody is too lazy to recompile a binary from 2010.

As surely as water will wet and fire will burn, the gods of Java with stupidity and complexity return.

This is not completly correct. It's true you won't e.g. be able to implement `Foo` and specify `Bar`'s method in it, however it does mean that e.g. `dyn Foo` will support `Bar`'s methods.

> Despite this it's still accurate to say that this enables a supertype/subtype relationship in certain contexts

It does enable something that looks like subtyping because you're allowed to use something that implements `Foo` when something that implements `Bar` is expected. However this is not called subtyping in Rust terms; subtyping in Rust exists but is unrelated to traits (it's only affected by lifetimes)

In an interface/class hierarchy A < B < C, C can be an abstract class that only implements B’s methods and not A’s.

Should those people be learning Any in the first place initially as an introduction to the language? How similar to `unsafe` is `Any` in terms of "try to avoid it"?

Yeah, you can do the same trick if you care about types that implement all of A, B and C.

> What I'd like to do is have some base trait object and query it to see if it supports other interfaces, but also not have to have stub "I don't actually implement this" trait impls for the unsupported ones.

Currently this is not possible, though it might be possible in the future once Rust gets specialization. With that you would basically be able to write the "I don't actually implement this" stub automatically. The catch however would be that this can only work for `'static` types, since such specialization for non-`'static` types is unsound.

> I believe I understand that in rust this has not historically been possible because rust doesn't have inheritance, so, there can be only one vtable for a type, rather than an chain like you might have in c++.

The issue is kinda the opposite. In Rust types can implement any number of traits (even infinite!), so they would require an potentially an infinite amount of vtables if implemented like in C++, which is just not possible. So instead this is splitted from types and moved to trait objects, which allow to carry a vtable for one specific trait.

https://play.rust-lang.org/?version=beta&mode=debug&edition=... (ninja edited a few times with some improvements)

With a bit of API massaging, this could be improved quite a bit in terms of ergonomics. The challenge is that traits often require concrete structs if you want them to participate in dynamic typing.

Basically you can now make something like `Arc<dyn QueryInterface>` work, where multiple traits are available through a `query_interface`:

    fn main() {
        let x = Arc::new(X);
        let y = Arc::new(Y);
        let any = [x as Arc<dyn QueryInterface>, y as _];
        for any in any {
            if let Some(a) = any.query_interface::<dyn A>() {
                a.a();
            }
            if let Some(b) = any.query_interface::<dyn B>() {
                b.b();
            }
        }
    }

> What I'd like to do is have some base trait object and query it to see if it supports other interfaces

> rust doesn't have inheritance

rust traits and lifetimes have inheritance, kinda, (through rust types do not)

E.g. `trait A: Debug + Any` means anything implementing A also implements Debug and Any. This is a form of interference, but different to C++ inheritance.

This is why we speaking about upcasts when casting `&dyn A as &dyn Debug` and downcast when trying to turn a `&dyn A` into a specific type.

But because this inheritance only is about traits and the only runtime type identifying information rust has is the `Any::type_id()` the only dynamic cast related querying you can do is downcasting. (Upcasting is static and possible due to changes to the vtable which allow you to turn a `&dyn A` vtable into a `dyn Any`/`dyn Debug` vtable (in the example below).

The part of bout downcast form the article you can ignore, it's some nice convenient thing implicitly enabled by the upcasting feature due to how `downcast_ref` works.

This https://play.rust-lang.org/?version=beta&mode=debug&edition=... might help.

So I think what you want might not be supported. It also is very hard to make it work in rust as you would conceptually need a metadata table for each type with pointer to a `dyn` vtable for each object safe trait the type implements and then link it in every vtable. The issue is concrete rust traits can be unbound e.g. a type `C` might implement `T<bool>` and `T<T<bool>>` and `T<T<T<bool>>>` up to infinity :=) So as long as you don't have very clever pruning optimizations that isn't very practical and in general adds a hidden runtime cost in ways rust doesn't like. Like e.g. if we look at `dyn T` it e.g. also only contains runtime type information for up casting if `T: SomeTrait` and downcasting if `T: Any` and the reason up casting took like 5+ years is because there are many subtle overhead trait offs between various vtable representations for `C: A+B` which might also affect future features etc.

Importantly, note that the specifics of the vtable layout are not guaranteed, only the general property that the layout must be amenable to supporting this.

And for diamond patterns vtable entries are duplicated.

It begins:

> Let's say you want to have a wrapper around a boxed any that you can debug.