What does `type Item` in the `Iterator` trait mean

Question

The Iterator trait is defined as follows:

pub trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
}

What does type Item; mean? And how to call it?

Is the definition above equivalent to this one?

pub trait Iterator<T> {
    fn next(&mut self) -> Option<T>;
}

If it's the same why declare it that way? And if it's not the same then what's the difference?

Answer 1

TL;DR : The type Item; in Iterator is an associated type.

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Rust generics have both Input and Output types:

• Input types are those specified in the declaration of the trait ( trait X<T, U> has T and U as input types) plus Self (the concrete type for which the trait is being implemented)
• Output types are those specified in the definition of the trait via type X;

The RFC that introduced associated items is RFC 195: Associated Items . Specifically, its motivation section cites the benefits of having associated traits.

For me, the most important point is unicity : a single type is defined for any given implementation of the trait, which allows cleanly powering the Deref or Index trait for example. In a world where Deref or Index could yield many possible types, type inference would be even more complicated.