RealTimeRendering | Website pages for realtimerendering.com | Web Site library

put in the same position as the Rust runtime, I would be able to look at the Normal implementer I have in hand at a moment, say Sphere and say I will look in Sphere's vtable

Except there's nothing the Rust runtime can do here.

In fact, Rust don't have the runtime in the sense of "something executing the code". Rust runtime only performs the setup and cleanup task, but as long as the control flow is somewhere inside your main function, you're on your own (and in no_std environments, even this won't exist). So, every dynamic dispatch must be baked into the type, by placing the vtable pointer next to data pointer - see this documentation for a bit more details.

But, since the generics are, as you've already stated, monomorphized, there won't be one fn material for every implementation of Normal: there will be an unknown, potentially infinite family of these functions, one for each type implementing Material. Note the "unknown, potentially infinite" bit: since you can't leak private parts, if the Normal trait is public, the Material trait must be public too - and then nothing will prevent the user of your code to add another implementation of Material, not known to your code, which simply cannot be baked into the vtable of dyn Normal.

That's why generic methods are not object-safe. They can't fit into the trait object vtable, since we don't know them all when trait object is created.