This has been implemented by dtolnay.

The concept is quite clever ans is explained in the README:

How does it work?

 

We use the inventory crate to produce a registry of impls of your trait, which is built on the ctor crate to hook up initialization functions that insert into the registry. The first Box<dyn Trait> deserialization will perform the work of iterating the registry and building a map of tags to deserialization functions. Subsequent deserializations find the right deserialization function in that map. The erased-serde crate is also involved, to do this all in a way that does not break object safety.

To summarize, every implementation of the trait declared as [de]serializable is registered at compile-time, and this is resolved at runtime in case of [de]serialization of a trait object.

This has been implemented by dtolnay.

The concept is quite clever ans is explained in the README:

How does it work?

We use the inventory crate to produce a registry of impls of your trait, which is built on the ctor crate to hook up initialization functions that insert into the registry. The first Box<dyn Trait> deserialization will perform the work of iterating the registry and building a map of tags to deserialization functions. Subsequent deserializations find the right deserialization function in that map. The erased-serde crate is also involved, to do this all in a way that does not break object safety.

To summarize, every implementation of the trait declared as [de]serializable is registered at compile-time, and this is resolved at runtime in case of [de]serialization of a trait object.