For loop permanently borrows immutable vector?

[trevor]

Hey folks, I'm learning about the Rust language. One simple thing I'm trying to do is iterate over a Vec, in a for loop, and then access the Vec after the for loop.

let x = vec![5,6,17,2];

for val in x {
    println!("{}", val);
}
println!("{}", x[0]);

The above code yields the following error:

borrow of moved value: x value borrowed here after move

I figured by cloning the Vec with x.clone(), I could work around this.

let x = vec![5,6,17,2];

for val in x.clone() { // edited this line
    println!("{}", val);
}
println!("{}", x[0]);

This does work, but I'm confused at why the initial example fails. Why does x get borrowed by the for loop, but never given back to the parent scope?

[OptimisticPeach]

for loops work with the IntoIterator trait. Vec's IntoIterator impl consumes the vec and iterates items by value. What you're looking for is

for x in vec.iter() {

}

Which iterates over items by reference.

Notice that every iterator automatically implements IntoIterator.

[trevor]

Perfect explanation. Thanks for helping me understand this better.

[steffahn]

One can also write

for val in &x {
    println!("{}", val);
}

by the way, because &Vec<T> has it's own IntoIterator implementation. (Just like with explicit .iter(), val is a &i32 this way.)

[H2CO3]

@trevor

Why does x get borrowed by the for loop, but never given back to the parent scope?

To address the question itself: no, x is not borrowed. It is moved. Moving is the opposite of borrowing – it's destructive.