Create owned value on demand

There is an existing value a, and I want to get a reference to either a or a new value b created on demand, depending on some condition. The code below won't compile. I would like to know what is the idiomatic way of doing so in Rust.

fn main() {    let condition = false;    let a: String = "a".to_string();    let r: &String = if condition {&a    } else {        let b: String = "b".to_string();&b    };}

New example (in response to @PitaJ):

struct S(i32);fn main() {    let condition = false;    let a: S = S(0);    let r: &S = if condition {&a    } else {        let b: S = S(1);&b    };}