Comment by Lvl999Noob

You are misunderstanding the implementation of a trait with the compilation of that implementation. The library author still needs to implement the trait on their type. The only thing that changes is that the body of the `impl Debug for ...` block is compiled at first use and not when the compiler sees it. If the library author did not derive(Debug) on the type then trying to get a debug string out of it is still an error.