rust 二维数组类型推导错误：rustc E0282

Question

我想通过下标访问Vec<Vec<Point>>这样的二维数组map，但是编译器报错了意思是无法推导类型？
我想不通，map[x][y]的类型不就是Point吗？
这是报错内容

error[E0282]: type annotations needed
  --> src/main.rs:34:15
   |
34 |         match map[x][y] {
   |               ^^^^^^ cannot infer type
   |
   = note: type must be known at this point

以下是源码，删除了无关紧要的内容

use rand::prelude::*;

#[derive(Debug, Clone, Copy)]
enum Point {
    Landmine,
    Num(u8),
}

fn init_map(height: u8, weight: u8, mines: u8) {
    let mut rng = thread_rng();
    let mut map: Vec<Vec<Point>> = vec![vec![Point::Num(0); weight.into()]; height.into()];
    // let mut map: [[Point; 30]; 16] = [[Point::Num(0); 30]; 16];

    let mut num = 0;
    while num < mines {
        let x = rng.gen_range(0..height).into();
        let y = rng.gen_range(0..weight).into();

        match map[x][y] {
            Point::Landmine => {}
            Point::Num(_) => {}
        }
    }

}

Answer 1

在 error index 里有这么一句话：

This error indicates that type inference did not result in one unique possible type, and extra information is required. In most cases this can be provided by adding a type annotation. Sometimes you need to specify a generic type parameter manually.

尝试对程序中做出以下修改 play：

        println!("{:?}", map[x]);
        // match map[x][y] {
        //     Point::Landmine => {}
        //     Point::Num(_) => {}
        // }

我们发现报错指向的位置变化了：

   Compiling playground v0.0.1 (/playground)
error[E0282]: type annotations needed
  --> src/main.rs:16:13
   |
16 |         let x = rng.gen_range(0..height).into();
   |             ^ consider giving `x` a type

error: aborting due to previous error

For more information about this error, try `rustc --explain E0282`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

因此，该段程序的报错是因为 rng.get_range 的返回值需要指定泛型。给 x, y 加上类型标注，或者
通过 turbo fish 语法 指定返回值的类型 rng.gen_range::<usize>::(0..height)，均可修复这个问题。

修复play