当U的From<T>可用时，将Vec<T>转换为Vec<U>

struct Person {
    name: String,
    age: u32,
}

struct Employee {
    name: String,
    position: String,
}

impl From<Person> for Employee {
    fn from(person: Person) -> Self {
        Employee {
            name: person.name,
            position: String::from("Employee"),
        }
    }
}

let people: Vec<Person> = vec![
    Person {
        name: String::from("John"),
        age: 25,
    },
    Person {
        name: String::from("Jane"),
        age: 30,
    },
];

let employees: Vec<Employee> = people.into_iter().map(Employee::from).collect();