rust - 为什么在 nightly Rust 1.29 中工作的生成器在 nightly 1.34.0 中出错？

Question

我正在使用 Rust 生成器，它在 nightly 1.29.0 上运行良好:

#![feature(generators, generator_trait)]

use std::ops::{Generator, GeneratorState};

fn main() {
    let mut generator: Box<dyn Generator<Yield = u64, Return = &str>> = Box::new(move || {
        yield 1;
        return "foo";
    });

    unsafe {
        match generator.resume() {
            GeneratorState::Yielded(1) => {
                println!("Yielded");
            }
            _ => panic!("unexpected return from resume"),
        }
        match generator.resume() {
            GeneratorState::Complete("foo") => {
                println!("Completed");
            }
            _ => panic!("unexpected return from resume"),
        }
    }
}

但是，nightly 版本 1.34.0 要求将 Generator 包装在 Pin 中，但是当我将 Generator 包装到 Pin 中时，代码不再有效。

#![feature(generators, generator_trait)]

use std::ops::{Generator, GeneratorState};
use std::pin::Pin;

fn main() {
    let mut generator: Box<dyn Generator<Yield = u64, Return = &str>> = Box::new(move || {
        yield 1;
        return "foo";
    });

    match Pin::new(&mut generator).resume() {
        GeneratorState::Yielded(1) => {
            println!("Yielded");
        }
        _ => panic!("unexpected return from resume"),
    }
    match Pin::new(&mut generator).resume() {
        GeneratorState::Complete("foo") => {
            println!("Completed");
        }
        _ => panic!("unexpected return from resume"),
    }
}

并且代码给出了以下错误。

error[E0599]: no method named `resume` found for type `std::pin::Pin<&mut std::boxed::Box<dyn std::ops::Generator<Yield = u64, Return = &str>>>` in the current scope
  --> src/main.rs:12:36
   |
12 |     match Pin::new(&mut generator).resume() {
   |                                    ^^^^^^
   |
   = note: the method `resume` exists but the following trait bounds were not satisfied:
           `std::boxed::Box<dyn std::ops::Generator<Yield = u64, Return = &str>> : std::ops::Generator`

当我删除生成器的类型时，它工作正常:

let mut generator = Box::new(move || {
        yield 1;
        return "foo"
});

我有几个问题:

• 为什么将 Generator 包裹在 Pin 中会在最新版本中出现错误？
• 对生成器使用 Pin 有什么用(因为它可以防止指针的值被移动)，我不明白 Pin 之间的关系和生成器?
• 为什么当我们删除 generator 变量的类型时代码仍然有效？
• 为什么 resume 方法不再不安全？

Answer 1

Why does generator code that was working in nightly Rust 1.29 give an error in Nightly 1.34.0?

因为这是一个每晚，不稳定的特性，它可能随时改变，它已经改变了。

另见:

• What is the stabilization process?

Why does wrapping the Generator in Pin give the error in the latest version?

如错误消息所述，Box<dyn Generator<Yield = u64, Return = &str>>没有实现 Generator .

另见:

• What makes something a "trait object"?

What's the use of using the Pin for the generator?

Why is the resume method no longer unsafe?

resume方法是不安全的，因为如果生成器有一个自引用变量并且它在对 resume 的两次调用之间移动，则可能会引入内存不安全。 . Pin阻止移动，从而消除不安全性。这是在仅公开安全接口(interface)的 API 中包装不安全逻辑的典型示例。

另见:

• What are the use cases of the newly proposed Pin type?
• How to use the Pin struct with self-referential structures?

Why does the code work when we remove the type for generator variable?

因为它不再是特征对象 (Box<dyn Generator>)，而只是一个盒装混凝土生成器。

另见:

• Lazy sequence generation in Rust

---

我会这样写

#![feature(generators, generator_trait)]

use std::ops::{Generator, GeneratorState};

fn main() {
    let mut generator = Box::pin(move || {
        yield 1;
        "foo"
    });

    match generator.as_mut().resume() {
        GeneratorState::Yielded(1) => println!("Yielded"),
        _ => panic!("unexpected return from resume"),
    }
    match generator.as_mut().resume() {
        GeneratorState::Complete("foo") => println!("Completed"),
        _ => panic!("unexpected return from resume"),
    }
}