But even in Cpp, you need to eventually iterate over the underlying iterator to get the results of the map. So, if you want to debug print it, you still need to

But even in Cpp, you need to eventually iterate over the underlying iterator to get the results of the map. So, if you want to debug print it, you still need to iterate over it regardless. Regarding the mutability and collect issues you raised, I think I just explained poorly. What I meant is that the author could've done something like this https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e58b99ff86a5f26e98c023ed39f596b8 https://play.rust-lang.org/?version=stable&mode=debug&editio... However, debug printing "on the fly" (without collection first) would definitely be more involved with interior mutability and require a wrapper type, for the reasons you outlined, but not impossible.