Ideally you want `Id` to actually be parameterized on traits, not trait object types that happen to have the same syntax (now deprecated) as their corresponding

Ideally you want `Id` to actually be parameterized on traits, not trait object types that happen to have the same syntax (now deprecated) as their corresponding traits. Of course, that's not currently possible. But you may be interested in this repo for planning an RFC to eventually add such functionality: https://github.com/Centril/rfc-trait-parametric-polymorphism/ https://github.com/Centril/rfc-trait-parametric-polymorphism... After being created 8 months ago, it's sort of inactive, but hopefully that'll change once the compiler's feature-implementation backlog starts catching up – in particular, once "chalk in rustc" becomes a reality.