如果都是vector,可以用vector自带的append方法 或者 Extend trait 或者slice的concat方法

fn main() {
    let mut a = vec![1, 2, 3];
    let mut b = vec![4, 5, 6];

    a.append(&mut b);

    assert_eq!(a, [1, 2, 3, 4, 5, 6]);
    assert_eq!(b, []);
}
或者
fn main() {
    let mut a = vec![1, 2, 3];
    let b = vec![4, 5, 6];

    a.extend(b);
    assert_eq!(a, [1, 2, 3, 4, 5, 6]);
}
或者

fn main() {
    let mut a = vec![1, 2, 3];
    let b = vec![4, 5, 6];

    [a, b].concat()
    assert_eq!(a, [1, 2, 3, 4, 5, 6]);
}

用extend_from_slice

fn cat<T: Clone>(a: &[T], b: &[T]) -> Vec<T> {
    let mut v = Vec::with_capacity(a.len() + b.len());
    v.extend_from_slice(a);
    v.extend_from_slice(b);
    v
}

或者

fn join<T: Clone>(a: &[T], b: &[T]) -> Vec<T> {
    a.iter().cloned().chain(b.iter().cloned()).collect()
}

或者
fn join<T: Clone>(a: &[T], b: &[T]) -> Vec<T> {
    let mut v = a.to_vec();
    v.extend_from_slice(b);
    v
}