rust - What does "cannot move out of index of" mean?

Question

I am playing with Rust, and I'm trying to access the first command line argument with this code:

use std::env;

fn main() {
    let args: Vec<_> = env::args().collect();
    let dir = args[1];
}

And I get this error:

error[E0507]: cannot move out of indexed content
 --> src/main.rs:5:15
  |
5 |     let dir = args[1];
  |         ---   ^^^^^^^ cannot move out of indexed content
  |         |
  |         hint: to prevent move, use `ref dir` or `ref mut dir`

Or in later versions of Rust:

error[E0507]: cannot move out of index of `std::vec::Vec<std::string::String>`
 --> src/main.rs:5:15
  |
5 |     let dir = args[1];
  |               ^^^^^^^
  |               |
  |               move occurs because value has type `std::string::String`, which does not implement the `Copy` trait
  |               help: consider borrowing here: `&args[1]`

If I change it to let ref dir, it compiles, but I don't grok what's going on. Could someone explain what "indexed content" means?

Answer 1

When you use an index operator ([]) you get the actual object at index location. You do not get a reference, pointer or copy. Since you try to bind that object with a let binding, Rust immediately tries to move (or copy, if the Copy trait is implemented).

In your example, env::args() is an iterator of Strings which is then collected into a Vec<String>. This is an owned vector of owned strings, and owned strings are not automatically copyable.

You can use a let ref binding, but the more idiomatic alternative is to take a reference to the indexed object (note the & symbol):

use std::env;

fn main() {
    let args: Vec<_> = env::args().collect();
    let ref dir = &args[1];
    //            ^
}

Implicitly moving out of a Vec is not allowed as it would leave it in an invalid state — one element is moved out, the others are not. If you have a mutable Vec, you can use a method like Vec::remove to take a single value out:

use std::env;

fn main() {
    let mut args: Vec<_> = env::args().collect();
    let dir = args.remove(1);
}

See also:

• What is the return type of the indexing operation?

---

For your particular problem, you can also just use Iterator::nth:

use std::env;

fn main() {
    let dir = env::args().nth(1).expect("Missing argument");
}