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:

  1. // Here, Rust infers a lifetime that is as short as possible.
  2. // The two references are then coerced to that lifetime.
  3. fn multiply<'a>(first: &'a i32, second: &'a i32) -> i32 {
  4.     first * second
  5. }
  7. // `<'a: 'b, 'b>` reads as lifetime `'a` is at least as long as `'b`.
  8. // Here, we take in an `&'a i32` and return a `&'b i32` as a result of coercion.
  9. fn choose_first<'a: 'b, 'b>(first: &'a i32, _: &'b i32) -> &'b i32 {
  10.     first
  11. }
  13. fn main() {
  14.     let first = 2; // Longer lifetime
  16.     {
  17.         let second = 3; // Shorter lifetime
  19.         println!("The product is {}", multiply(&first, &second));
  20.         println!("{} is the first", choose_first(&first, &second));
  21.     };
  22. }