尝试从标准正态分布生成一个随机数。需要将值乘以0.1以获得我正在寻找的数字范围。我尝试使用rand_dist的文档，您可以在这里找到：https://docs.rs/rand_distr/0.3.0/rand_distr/struct.StandardNormal.html
我的货物. toml如下：

[package]
name = "test_rng"
version = "0.1.0"
authors = ["Jack"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
rand = "0.7.3"
rand_distr = "0.3.0"

起始rust代码是上述rand_dist文档中提供的示例：

use rand::prelude::*;
use rand_distr::StandardNormal;

fn main() {
    let val: f64 = thread_rng().sample(StandardNormal);
    println!("{}", val);
}

当我运行这个程序时，它按预期工作，输出为：

C:\Users\Jack\Desktop\projects\software\rust\test_rng>cargo run
   Compiling test_rng v0.1.0 (C:\Users\Jack\Desktop\projects\software\rust\test_rng)
    Finished dev [unoptimized + debuginfo] target(s) in 2.11s
     Running `target\debug\test_rng.exe`
0.48398855288705356

C:\Users\Jack\Desktop\projects\software\rust\test_rng>

这就是我遇到问题的地方，当我在下面的代码中尝试将该数字乘以0.1时，我得到了结果错误：
一个三个三个一个
我尝试将0.1更改为0.1_f64，但出现了相同的错误。我尝试使用as f64将随机数转换为f64（它应该已经是），但结果如下：

fn main() {
    let val: f64 = 0.1 * thread_rng().sample(StandardNormal) as f64;
    println!("{}", val);
}

C:\Users\Jack\Desktop\projects\software\rust\test_rng>cargo run
   Compiling test_rng v0.1.0 (C:\Users\Jack\Desktop\projects\software\rust\test_rng)
error[E0282]: type annotations needed
 --> src\main.rs:5:39
  |
5 |     let val: f64 = 0.1 * thread_rng().sample(StandardNormal) as f64;
  |                                       ^^^^^^ cannot infer type for type parameter `T` declared on the associated function `sample`
  |
  = note: type must be known at this point
help: consider specifying the type arguments in the method call
  |
5 |     let val: f64 = 0.1 * thread_rng().sample::<T, D>(StandardNormal) as f64;
  |                                             ^^^^^^^^

error: aborting due to previous error

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

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

C:\Users\Jack\Desktop\projects\software\rust\test_rng>

认为这是一个优先级问题，所以我尝试将后半部分用括号括起来，但得到了同样的错误。
我可以通过使变量可变并将行分成如下两个操作来使它工作：

fn main() {
    let mut val: f64 = thread_rng().sample(StandardNormal);
    val *= 0.1;
    println!("{}", val);
}

C:\Users\Jack\Desktop\projects\software\rust\test_rng>cargo run
   Compiling test_rng v0.1.0 (C:\Users\Jack\Desktop\projects\software\rust\test_rng)
    Finished dev [unoptimized + debuginfo] target(s) in 1.62s
     Running `target\debug\test_rng.exe`
-0.034993448117065

C:\Users\Jack\Desktop\projects\software\rust\test_rng>

知道f64与随机数的输出相乘是怎么回事吗？