Move occurs because `*arg` has type `String`, which does not implement the `Copy` trait

Question

Caveat: I'm new to Rust, so pardon any ignorance.

I have a function that accepts a reference to a vector. It then creates an iterator from that vector and does processing on its values.

Code:

fn new(args: &Vec<String>) -> Result<Config, &str> {
        let mut args_iter = args.iter(); //create iterator
        args_iter.next(); //skip the 0th item

        let query = match args_iter.next() {
            Some(arg) => *arg, //BREAKS
            None => return Err("no query"),
        };

        let file = match args_iter.next() {
            Some(arg) => arg.clone(), //WORKS
            None => return Err("no file"),
        };

        //more stuff
    }

Now I'm getting this error:

move occurs because `*arg` has type `String`, which does not implement the `Copy` trait

Which gets solved if I change *arg to arg.clone().

Can someone help me understand why? I thought that by creating an iterator inside the function, the function owns the iterator and should be able to mutate/move its values as it pleases?

"error[E0412]: cannot find type `Config` in this scope" => [mcve] — Stargateur, May 05 '21 at 07:51
@Stargateur this is useful, thank you. Will keep in mind for the future — ilmoi, May 06 '21 at 03:53

Masklinn · Accepted Answer · 2023-06-23T08:32:07.697

I thought that by creating an iterator inside the function, the function owns the iterator and should be able to mutate/move its values as it pleases?

It owns the iterator but the iterator may or may not own it’s values.

Here the iterator comes from an &Vec, so the current function does not own any of the data being iterated upon, which means the iterator only hands out &String: references owned by a caller.

Just don’t dereference the &String, there is no reason to in what little code you show.

That aside, your match could easily be replaced by .ok_or(msg)?.

And your output should probably be &'static str (or a Cow), as defined rustc thinks that there is a relationship between the inputs and the error message. Though I have to say I’d use an enum, whether contextual data is added or not. That allows finer error selection and implementing Error which can be useful.

If you don't dereference (but also don't clone) it breaks too. But I get your point about owning the vector but not the values inside the vector. Thanks for commenting. — ilmoi, May 06 '21 at 03:55

Move occurs because `*arg` has type `String`, which does not implement the `Copy` trait

1 Answers1