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

Let's have a look at the following example.

fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'a u32 {
    x
}

fn main() {
    let x = 12;
    let z: &u32 = {
        let y = 42;
        foo(&x, &y)
    };
}

Here, the explicit lifetimes are important. This compiles because the result of foo has the same lifetime as its first argument ('a), so it may outlive its second argument. This is expressed by the lifetime names in the signature of foo. If you switched the arguments in the call to foo the compiler would complain that y does not live long enough:

error[E0597]: `y` does not live long enough
  --> src/main.rs:10:5
   |
9  |         foo(&y, &x)
   |              - borrow occurs here
10 |     };
   |     ^ `y` dropped here while still borrowed
11 | }
   | - borrowed value needs to live until here