I just started learning rust and I am confused about how the ampersand placement affects the program in the following examples. I understand that ‘&’ indicates a reference to a variable, but I’m not sure how these examples differ.

Example 1, no ampersand:

let arr = [1,2,3];

for n in arr {
    println!("number {}", n);
}

Example 2, on the array definition:

let arr = &[1,2,3];

for n in arr {
    println!("number {}", n);
}

Example 3, in the for loop:

let arr = [1,2,3];

for n in &arr {
    println!("number {}", n);
}

All three of these examples produce the same output:

number 1
number 2
number 3

Is there any difference in how this code executes, or in its efficiency or memory consumption?