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&l...
