Rust by Example

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));

};

}

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