Rust Any part 3: we have upcasts

1. rfcs

   1 690 6,214 9.1 Markdown

   RFCs for changes to Rust

   

   And for extra context the RFc lays out the current design and future options: https://github.com/rust-lang/rfcs/blob/master/text/3324-dyn-...

2. Stream

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3. rust-playground

   2 116 1,343 9.4 Rust

   The Rust Playground

   

   the article isn't very good for anyone not already familiar with the problem

   > 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

4. rust-gc

   3 13 1,025 3.9 Rust

   Simple tracing (mark and sweep) garbage collector for Rust

   

   Then use “tracing GC as a library” like [1] or [2]. I’m not saying there’s no use for tracing GC ever. I’m saying it shouldn’t be a language level feature and it’s perfectly fine as an opt-in library.

   [1] https://github.com/Manishearth/rust-gc

   [2] https://github.com/oilpan-gc/cppgc

5. cppgc

   4 2 37 0.0 C++

   https://v8.dev/blog/oilpan-library

   

   Then use “tracing GC as a library” like [1] or [2]. I’m not saying there’s no use for tracing GC ever. I’m saying it shouldn’t be a language level feature and it’s perfectly fine as an opt-in library.

   [1] https://github.com/Manishearth/rust-gc

   [2] https://github.com/oilpan-gc/cppgc

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