Rust Iterration over Vector and Ownership

Question

I had been learning Rust recently.

I stumbled across the following code:

pub fn to_bytes(values: &[u32]) -> Vec<u8> {
    for &(mut value) in values {
        //...
    }
}

I do not understand the & in the for loop. What exactly is happening here?

I figured out that if you iterate over a vector, using a for loop, you get a reference (found here):

let v = vec![1, 2, 3];
for value in &v {
    // value is a reference 
}

Then why do I need another & in the first snipped?

Answer 1

I figured out that if you iterate over a vector, using a for loop, you get a reference.

This is actually not true. It depends on your vector. If your vector is a reference, it yields a reference. Otherwise it yields an owned value.

Back to your first question:

pub fn to_bytes(values: &[u32]) -> Vec<u8> {
    for &(mut value) in values {
        // value will be of type u32
        //...
    }
}

This is called destructuring. Since values here is of type &[u32], the value it yields with the for loop is of type &u32. With the help of &, you dereference the pointer so the value variable here will be of type u32.

Alternatively you can also do the following, but your value will be of type &u32.

pub fn to_bytes(values: &[u32]) -> Vec<u8> {
    for mut value in values {
        // value will be of type &u32
        //...
    }
}