IntoIter

Collections are iterated in Rust using the Iterator trait. It's a bit more complicated than Drop:

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

The new kid on the block here is type Item. This is declaring that every implementation of Iterator has an associated type called Item. In this case, this is the type that it can spit out when you call next.

The reason Iterator yields Option<Self::Item> is because the interface coalesces the has_next and get_next concepts. When you have the next value, you yield Some(value), and when you don't you yield None. This makes the API generally more ergonomic and safe to use and implement, while avoiding redundant checks and logic between has_next and get_next. Nice!

Sadly, Rust has nothing like a yield statement (yet), so we're going to have to implement the logic ourselves. Also, there's actually 3 different kinds of iterator each collection should endeavour to implement:

• IntoIter - T
• IterMut - &mut T
• Iter - &T

We actually already have all the tools to implement IntoIter using List's interface: just call pop over and over. As such, we'll just implement IntoIter as a newtype wrapper around List:

// Tuple structs are an alternative form of struct,
// useful for trivial wrappers around other types.
pub struct IntoIter<T>(List<T>);

impl<T> List<T> {
    pub fn into_iter(self) -> IntoIter<T> {
        IntoIter(self)
    }
}

impl<T> Iterator for IntoIter<T> {
    type Item = T;
    fn next(&mut self) -> Option<Self::Item> {
        // access fields of a tuple struct numerically
        self.0.pop()
    }
}

And let's write a test:

#[test]
fn into_iter() {
    let mut list = List::new();
    list.push(1); list.push(2); list.push(3);

    let mut iter = list.into_iter();
    assert_eq!(iter.next(), Some(3));
    assert_eq!(iter.next(), Some(2));
    assert_eq!(iter.next(), Some(1));
    assert_eq!(iter.next(), None);
}

> cargo test

     Running target/debug/lists-5c71138492ad4b4a

running 4 tests
test first::test::basics ... ok
test second::test::basics ... ok
test second::test::into_iter ... ok
test second::test::peek ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured

Nice!