如何将值推送到Rust中的2D Vec？

Question

这是一个非常简单的2D Vec尝试。我正在尝试在顶级Vec的最后一个条目中添加一个元素：

fn main() {
    let mut vec_2d = vec![vec![]];
    if let Some(v) = vec_2d.last() {
        v.push(1);
    }
    println!("{:?}", vec_2d);
}

我收到此错误：

error[E0596]: cannot borrow `*v` as mutable, as it is behind a `&` reference
 --> src/main.rs:4:9
  |
3 |     if let Some(v) = vec_2d.last() {
  |                 - help: consider changing this to be a mutable reference: `&mut std::vec::Vec<i32>`
4 |         v.push(1);
  |         ^ `v` is a `&` reference, so the data it refers to cannot be borrowed as mutable

我也尝试过Some(ref v)和Some(ref mut v)，结果相同。我找不到任何具体描述此错误的文档。这里的正确方法是什么？

answer to a similar question推荐更像Some(&mut v)的东西。然后我得到这些错误：

error[E0308]: mismatched types
 --> src/main.rs:3:17
  |
3 |     if let Some(&mut v) = vec_2d.last() {
  |                 ^^^^^^ types differ in mutability
  |
  = note: expected type `&std::vec::Vec<_>`
             found type `&mut _`
  = help: did you mean `mut v: &&std::vec::Vec<_>`?

如果我尝试Some(&ref mut v)我得到：

error[E0596]: cannot borrow data in a `&` reference as mutable
 --> src/main.rs:3:18
  |
3 |     if let Some(&ref mut v) = vec_2d.last() {
  |                  ^^^^^^^^^ cannot borrow as mutable

Answer 1

使用last_mut获取最后一个元素的可变引用;无需改变模式。

fn main() {
    let mut vec_2d = vec![vec![]];
    if let Some(v) = vec_2d.last_mut() {
        v.push(1);
    }
    println!("{:?}", vec_2d);
}

Answer 2

针对这种特殊情况的（更）更优雅的解决方案是：

fn main() {
    let vec_2d = vec![vec![1i32]];

    println!("{:?}", vec_2d);
}