Why does applying & to a String return a &str?

Question

In this example code from the Rust documentation :

fn takes_str(s: &str) { }

let s = String::from("Hello");

takes_str(&s);

What exactly is going on behind the scenes that causes &s to become a &str instead of a &String ? The documentation seems to suggest that there's some dereferencing going on, but I thought * was for dereferencing, not & ?

Answer 1

What's going on here is called deref coercions . These allow references to types that implement the Deref trait to be used in place of references to other types. As your example shows, &String can be used anywhere a &str is required, because String implements Deref to str .