vector - 如何在 Rust 中将值推送到 2D Vec？

Question

这是一个非常简单的 2D Vec 尝试。我正在尝试将一个元素添加到顶级 Vec 中的最后一个条目:

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

我收到这个错误:

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

我还尝试了 Some(ref v) 和 Some(ref mut v)，结果相同。我找不到任何专门描述此错误的文档。什么是正确的方法？

安answer to a similar question推荐一些更像 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 获取对最后一个元素的可变引用;无需更改模式。

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