Why are explicit lifetimes needed in Rust?

Question

I was reading the lifetimes chapter of the Rust book, and I came across this example for a named/explicit lifetime:

struct Foo<\'a> {
    x: &\'a i         


        

Answer 1

The lifetime annotation in the following structure:

struct Foo<'a> {
    x: &'a i32,
}

specifies that a Foo instance shouldn't outlive the reference it contains (x field).

The example you came across in the Rust book doesn't illustrate this because f and y variables go out of scope at the same time.

A better example would be this:

fn main() {
    let f : Foo;
    {
        let n = 5;  // variable that is invalid outside this block
        let y = &n;
        f = Foo { x: y };
    };
    println!("{}", f.x);
}

Now, f really outlives the variable pointed to by f.x.