Why can't I convert i to usize in iterator function?

I want to print out every value from this vector and I know I can do it with .enumerate, but I’m curious what’s wrong with this:

fn main() {
  let vec = vec![1,2,3];
  for i in vec.iter() {
    println!("{}", vec[i as usize]);
  }
}

I’m new to Rust, so go easy on me.

Here’s the error I got from the faulty code:

error[E0277]: the type `[{integer}]` cannot be indexed by `&{integer}`
 --> src\main.rs:4:24
  |
4 |     println!("{}", vec[i]);
  |                        ^ slice indices are of type `usize` or ranges of `usize`
  |
  = help: the trait `SliceIndex<[{integer}]>` is not implemented for `&{integer}`
  = help: the trait `SliceIndex<[T]>` is implemented for `usize`
  = note: required because of the requirements on the impl of `Index<&{integer}>` for `Vec<{integer}>`

For more information about this error, try `rustc --explain E0277`.

>Solution :

i is the item being iterated and not the iterator itself.
You can use enumerate to get the iterator index variable

for (i,n) in vec.iter().enumerate()