rust - Why can't I use `&Iterator<Item = &String>` as an iterator?

Question

I have the following function that's supposed to find and return the longest length of a String given an Iterator:

fn max_width(strings: &Iterator<Item = &String>) -> usize {
    let mut max_width = 0;
    for string in strings {
        if string.len() > max_width {
            max_width = string.len();
        }
    }
    return max_width;
}

However, the compiler gives me the following error:

error[E0277]: the trait bound `&std::iter::Iterator<Item=&std::string::String>: std::iter::Iterator` is not satisfied
 --> src/main.rs:3:19
  |
3 |     for string in strings {
  |                   ^^^^^^^ `&std::iter::Iterator<Item=&std::string::String>` is not an iterator; maybe try calling `.iter()` or a similar method
  |
  = help: the trait `std::iter::Iterator` is not implemented for `&std::iter::Iterator<Item=&std::string::String>`
  = note: required by `std::iter::IntoIterator::into_iter`

I'm new to Rust, and terribly confused by this, since I thought I was explicitly passing in an iterator. Calling strings.iter() tells me that it is not implemented, and calling strings.into_iter() sends me down a mutability rabbit-hole, and I certainly don't want to mutate the passed argument.

How can I iterate over my strings?

Answer 1

Your code fails because Iterator is not the same as &Iterator. You can fix this if you pass Iterator to your function, but since Iterator is a trait, the size cannot be determined (You can't know, what Iterator you are passing). The solution is to pass anything that implements Iterator:

fn max_width<'a>(strings: impl Iterator<Item = &'a String>) -> usize

playground

---

For more experienced Rust users:

The most generic way is probably this:

fn max_width<T: AsRef<str>>(strings: impl IntoIterator<Item = T>) -> usize {
    let mut max_width = 0;
    for string in strings {
        let string = string.as_ref();
        if string.len() > max_width {
            max_width = string.len();
        }
    }
    max_width
}

playground

However, you can also use

fn max_width<T: AsRef<str>>(strings: impl IntoIterator<Item = T>) -> usize {
    strings
        .into_iter()
        .map(|s| s.as_ref().len())
        .max()
        .unwrap_or(0)
}

playground