我正在用更小的Vec<f64>构造一个Vec<f64>，不太明白哪种方法是正确的。
以下是三次尝试，并对每一次尝试进行了评论：

fn main() {
    // this vector will be getting changed so is mutable
    let mut a:Vec<Vec<f64>> = vec![];

    // these won't be getting changed, they gets appended to `a`
    let b = vec![0.0, 1.0];
    let c = vec![2.0, 3.0];

    a.push(b);
    a.push(c);
    
    // and then the vector gets flattened into the form that I require:
    dbg!(a.concat());

    // however, if I want a single level vector constructed from other vectors
    let mut d:Vec<f64> = vec![];

    // b and c have to be mutable, why is that?
    // their contents don't change?
    // why can't they just be consumed?
    let mut e = vec![0.0, 1.0];
    let mut f = vec![2.0, 3.0];

    d.append(&mut e);
    d.append(&mut f);

    dbg!(d);

    // another method is to extend from slice
    // does this have performance problems compared to the other methods due
    // to the copying required?
    let mut g:Vec<f64> = vec![];
    
    let h = vec![0.0, 1.0];
    let i = vec![2.0, 3.0];

    g.extend_from_slice(&h);
    g.extend_from_slice(&i);

    dbg!(g);

}

输出：

[src/main.rs:15] a.concat() = [
    0.0,
    1.0,
    2.0,
    3.0,
]
[src/main.rs:28] d = [
    0.0,
    1.0,
    2.0,
    3.0,
]
[src/main.rs:46] g = [
    0.0,
    1.0,
    2.0,
    3.0,
]

我倾向于extend_from_slice，因为它向代码的读者传达向量h&i不会改变。
但我的问题是：复制是否会影响性能？是否有一种方法可以在追加时只使用数据，而不使向量e & f可变？