I'm a Rust fan and procedural macros are legitimately a cool feature, but the example seems much more simply solved via inheritance. You could do this without procedural macros by using a default method definition on the WritableTemplate trait, since WritableTemplate inherits from Template and should have the render method in scope.

This could just be a slightly contrived example to show the neat kinds of things you can do with procedural macros, though. If you needed a reference to the original struct definition, for example, procedural macros allow you to do the kinds of transformations at compile time that other languages need runtime reflection for.