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:

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// 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));
    };
}
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