Answer by solstice333 for When is it appropriate to use an associated type versus a generic type?

Associated types can be used to tell the compiler "these two types between these two implementations are the same". Here's a double dispatch example that compiles, and is almost similar to how the standard library relates iterator to sum types:

trait MySum {    type Item;    fn sum<I>(iter: I)    where        I: MyIter<Item = Self::Item>;}trait MyIter {    type Item;    fn next(&self) {}    fn sum<S>(self)    where        S: MySum<Item = Self::Item>;}struct MyU32;impl MySum for MyU32 {    type Item = MyU32;    fn sum<I>(iter: I)    where        I: MyIter<Item = Self::Item>,    {        iter.next()    }}struct MyVec;impl MyIter for MyVec {    type Item = MyU32;    fn sum<S>(self)    where        S: MySum<Item = Self::Item>,    {        S::sum::<Self>(self)    }}fn main() {}

Also, https://blog.thomasheartman.com/posts/on-generics-and-associated-types has some good information on this as well:

In short, use generics when you want to type A to be able to implement a trait any number of times for different type parameters, such as in the case of the From trait.

Use associated types if it makes sense for a type to only implement the trait once, such as with Iterator and Deref.