How can deserialization of polymorphic trait objects be added in Rust if at all?

Question

I'm trying to solve the problem of serializing and deserializing Box<SomeTrait>. I know that in the case of a closed type hierarchy, the recommended way is to use an enum and there are no issues with their serialization, but in my case using enums is an inappropriate solution.

At first I tried to use Serde as it is the de-facto Rust serialization mechanism. Serde is capable of serializing Box<X> but not in the case when X is a trait. The Serialize trait can’t be implemented for trait objects because it has generic methods. This particular issue can be solved by using erased-serde so serialization of Box<SomeTrait> can work.

The main problem is deserialization. To deserialize polymorphic type you need to have some type marker in serialized data. This marker should be deserialized first and after that used to dynamically get the function that will return Box<SomeTrait>.

std::any::TypeId could be used as a marker type, but the main problem is how to dynamically get the deserialization function. I do not consider the option of registering a function for each polymorphic type that should be called manually during application initialization.

I know two possible ways to do it:

1. Languages that have runtime reflection like C# can use it to get deserialization method.
2. In C++, the cereal library uses magic of static objects to register deserializer in a static map at the library initialization time.

But neither of these options is available in Rust. How can deserialization of polymorphic objects be added in Rust if at all?

Answer 1

All your libraries could provide a registration routine, guarded by std::sync::Once, that register some identifier into a common static mut, but obviously your program must call them all.

I've no idea if TypeId yields consistent values across recompiles with different dependencies.

Answer 2

A library to do this should be possible. To create such a library, we would create a bidirectional mapping from TypeId to type name before using the library, and then use that for serialization/deserialization with a type marker. It would be possible to have a function for registering types that are not owned by your package, and to provide a macro annotation that automatically does this for types declared in your package.

If there's a way to access a type ID in a macro, that would be a good way to instrument the mapping between TypeId and type name at compile time rather than runtime.