Rust Auto-dereferencing Deref coercions


Given two types T and U, &T will coerce (implicitly convert) to &U if and only if T implements Deref<Target=U>

This allows us to do things like this:

fn foo(a: &[i32]) {
    // code

fn bar(s: &str) {
    // code

let v = vec![1, 2, 3];
foo(&v); // &Vec<i32> coerces into &[i32] because Vec<T> impls Deref<Target=[T]>

let s = "Hello world".to_string();
let rc = Rc::new(s);
// This works because Rc<T> impls Deref<Target=T> ∴ &Rc<String> coerces into 
// &String which coerces into &str. This happens as much as needed at compile time.