Which trait bound should I use for a type that must implement the "iter" function

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I am trying to implement a struct composed of a generic field which type must implement the non-consuming "iter" method (see below).

struct Node<T> {
    key: T
}

impl<T> Node<T> where T: ?? {
    fn do_stuff(&self) {
        for e in self.key.iter() {
            /* ... */
        }
    }
}

fn main() {
    let n1 = Node { key: "foo".to_owned() };
    n1.do_stuff();
    
    let n2 = Node { key: vec![1, 2, 3] };
    n2.do_stuff();
}

Which trait bound should I associate with the parameter T?

rust traits generics
2 Answers

The trait bound you're looking for is &T: IntoIterator. By convention, types that provide a non-consuming iter() also provide an implementation of IntoIterator for &T. (Likewise, iter_mut() comes in pair with an impl of IntoIterator for &mut T.)

So, while you won't be able to literally call iter(), you'll be able to iterate over standard-library containers such as Vec, HashMap, etc., without consuming them:

impl<T> Node<T>
where
    for<'a> &'a T: IntoIterator,
{
    fn do_stuff(&self) {
        for e in &self.key { /* ... */ }
    }
}

Playground

Note that the above won't accept key: "foo".to_owned() simply because Rust strings are not iterable.

user_16008980

Which trait bound should I associate with the parameter T?

There is no such a trait in the standard library, the iter() is an inherent method – it doesn't come from a trait implementation. You could, of course, create your own trait (e.g., Iter) that specifies iter().

If you are working with a generic type and need to ensure that such type's values can be turned into an iterator, you may want to consider bounding the type to the IntoIterator trait.

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