Implicitly copyable iterators are mega confusing

[kmcallister]

For example, this code:

fn main() {
    let stream = "Hello, world!".chars().cycle();
    for _ in range(0u, 10) {
        let chunk: String = stream.take(3).collect();
        println!("{}", chunk);
    }
}

compiles and prints Hel ten times, rather than the expected Hel, lo,, wo, etc.

The fact that stream isn't mut is a clue that this code doesn't work as expected.

I guess the solution is stream.as_ref().take(3) but the "default" behavior here is really confusing.

[thestinger]

In a garbage collected language, adaptors would always take the wrapped iterator by-reference. It makes more sense that way, but it doesn't work well in Rust. Rust's iterator adaptors need to be able to take the wrapped iterator by-value, and the natural way to deal with it is only having by-value adaptors and then taking advantage of references being values (by_ref).

[thestinger]

I think it would be wrong to avoid Copy on types like this, because it's necessary for features like Cell. I don't really think there's a problem to be fixed here.

[kmcallister]

Sure, I agree with all that. I just think there's a usability / confusion issue here that we need to resolve. Opt-in Copy would do it, or maybe a lint of some kind?

[kmcallister]

Hm, I hadn't thought about putting these iterators in Cell. How about a #[warn_on_copy] attribute for types, and Cell would allow it?

[brson]

cc @aturon

[thestinger]

@kmcallister: It would end up causing warnings for typical iterator chains. I never liked that I had to use by-value for the parameters but I don't really see an alternative.

[aturon]

@kmcallister Opt-in Copy is on the way: see @pcwalton's PR, which is part of opt-in built-in traits.

I think that PR probably makes everything in std opt-in to Copy if it would've been Copy before, but we can audit iterators to not opt-in. Seems like it would address this problem?

[thestinger]

@aturon: It doesn't address the problem. It would be incorrect to leave out of a Copy implementation for style / performance reasons because it's used by types like Cell.

[thestinger]

It's hardly an issue specific to iterator types anyway...

[steveklabnik]

This code still acts the same today, even after opt-in Copy.