When is it useful to define multiple lifetimes in a struct?

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After staying up way too late, I was able to come up with an example case where the lifetimes matter. Here is the code:

static ZERO: i32 = 0;

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

fn get_x_or_zero_ref<'a, 'b>(x: &'a i32, y: &'b i32) -> &'a i32 {
    if *x > *y {
        return x
    } else {
        return &ZERO
    }
}

fn main() {
    let x = 1;
    let v;
    {
        let y = 2;
        let f = Foo { x: &x, y: &y };
        v = get_x_or_zero_ref(&f.x, &f.y);
    }
    println!("{}", *v);
}

If you were to change the definition of Foo to this:

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

Then the code won’t compile.

Basically, if you want to use the fields of the struct on any function that requires it’s parameters to have different lifetimes, then the fields of the struct must have different lifetimes as well.

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