Rust 何时取消引用

我正在浏览"铁 rust 书"网站，为即将到来的求职面试学习语言.在向量一章中，有两个代码示例:

fn main() {
    let v = vec![100, 32, 57];
    for i in &v {
        println!("{}", i);
    }
}

以及:

fn main() {
    let mut v = vec![100, 32, 57];
    for i in &mut v {
        *i += 50;
    }
}

现在我想知道，为什么对于第一个样本，当我们把对向量元素100的引用传递到:

println!("{}", i);

但在我们给向量的每个元素加50的例子中，我们必须先用*go 引用元素，然后再加50？

为什么我们不能做以下事情:

fn main() {
    let v = vec![100, 32, 57];
    for i in &v {
        println!("{}", *i); // why don't we have to dereference before we pass to println!?
    }
}

或者:

fn main() {
    let mut v = vec![100, 32, 57];
    for i in &mut v {
        i += 50; // why can't we just add directly to the reference like this?
    }
}

I must have misunderstood what I read, but I thought Rust was able to discern when you need to dereference or not automatically. I guess I don't understand why we need to dereference (or not dereference) in the two samples. The two examples I provided are commented with the specific bits of code I am wondering about.