What's the difference between ref and & when assigning a variable from a reference?

Question

What is wrong with this code?

fn example() {
    let vec = vec![1, 2, 3];
    let &_y = &vec;
}

Answer 1

The same symbol (&) is doing two different things when used on the right-end and left-end side of a binding. The left-hand side works like a pattern matching, so:

let x = (y, z); // x contains a tuple with value (y, z)
let (a, b) = x  // x is destructured into (a, b), so now
                // a has value y and b has value z

In the same way

let x = &y; // x is a reference to y
let &z = x; // this is like let &z= &y, so we want z to be y
            // this is equivalent to let z = *x

A ref binding on the left side is saying "pattern match by reference, not by value". So these two statements are equivalent:

let ref y = vec;
let y = &vec;

although in a let, the second one is more idiomatic.

You can see more examples on the pointers/ref chapter on rust by example