Rust polymorphism

The article shows how to define a common interface in Rust with a trait and implement it for two structs.

An enum selects the implementation, which is stored in `Box<dyn TraitThing>` so the same `show()` call can dispatch to different backends at runtime.

In practice, this demonstrates trait objects and dynamic dispatch as a way to build one API over multiple concrete types.

It also touches `Debug`, `Default`, `tracing`, and `tracing_subscriber` to show a more realistic Rust polymorphism setup.