Coercion
A longer lifetime can be coerced into a shorter one so that it works inside a scope it normally wouldn't work in. This comes in the form of inferred coercion by the Rust compiler, and also in the form of declaring a lifetime difference:
// Here, Rust infers a lifetime that is as short as possible. // The two references are then coerced to that lifetime. fn multiply<'a>(first: &'a i32, second: &'a i32) -> i32 { first * second } // `<'a: 'b, 'b>` reads as lifetime `'a` is at least as long as `'b`. // Here, we take in an `&'a i32` and return a `&'b i32` as a result of coercion. fn choose_first<'a: 'b, 'b>(first: &'a i32, _: &'b i32) -> &'b i32 { first } fn main() { let first = 2; // Longer lifetime { let second = 3; // Shorter lifetime println!("The product is {}", multiply(&first, &second)); println!("{} is the first", choose_first(&first, &second)); }; }