You can view generic parameters as "inputs", they are controlled by the consumer of the type/trait (you construct a `Vec<String>` because you want to store a bu

You can view generic parameters as "inputs", they are controlled by the consumer of the type/trait (you construct a `Vec<String>` because you want to store a bunch of `String` values). Associated types are "outputs", they are controlled by the impl (`<Vec<String> as IntoIterator>::into_iter()` will always return a `std::vec::IntoIter<String>`, because that is how the trait is implemented for `Vec<T>`). You can often use generic parameters for "outputs" as well (you could have a world where you had `impl<T> IntoIterator<IntoIter<T>> for Vec<T>`). However, this prevents the compiler from inferring the "output" type since there is no longer a guarantee that it is unique for a given set of "inputs".