跨线程共享具有特征对象的结构作为属性

我有下面的代码。有了注释掉的部分，它就可以工作了。当我取消注释这些部分时，它不再编译。

如何调整注释部分以使它们工作，即我想让线程同时访问表达式树。

当我尝试时，编译器会开始出现有关线程安全的错误。

我阅读了 Rust 书籍并了解 C/C++，但还不了解有关 Rust 类型系统和语义的所有内容。

use std::thread;
use std::sync::Arc;

pub trait Expr {
    fn run(&self) -> i32;
}

pub struct ConstantExpr {
    n: i32,
}

impl ConstantExpr {
    pub fn new(n: i32) -> Self {
        Self { n }
    }
}

impl Expr for ConstantExpr {
    fn run(&self) -> i32 {
        self.n
    }
}

pub struct AddExpr {
    expr1: Box<Expr>,
    expr2: Box<Expr>,
}

impl AddExpr {
    pub fn new(expr1: Box<Expr>, expr2: Box<Expr>) -> Self {
        Self { expr1, expr2 }
    }
}

impl Expr for AddExpr {
    fn run(&self) -> i32 {
        self.expr1.run() + self.expr2.run()
    }
}

struct Container {
    x: i32,
    cached_expr: Arc<Expr>,
}

impl Container {
    fn new() -> Self {
        Self {
            x: 0,
            cached_expr: Arc::new(AddExpr::new(
                Box::new(ConstantExpr::new(10)),
                Box::new(ConstantExpr::new(1)),
            )),
        }
    }
}

fn main() {
    let container = Arc::new(Container::new());

    let container1 = Arc::clone(&container);

    /*
    let thread1 = thread::spawn(move || {
        println!("thread1: {}", container1.x);
        println!("thread1: {}", container1.cached_expr.run());
    });
    */

    println!("main: {}", container.x);
    println!("main: {}", container.cached_expr.run());

    //thread1.join().unwrap();
}

错误：

error[E0277]: the trait bound `Expr + 'static: std::marker::Send` is not satisfied
  --> src/main.rs:64:19
   |
64 |     let thread1 = thread::spawn(move || {
   |                   ^^^^^^^^^^^^^ `Expr + 'static` cannot be sent between threads safely
   |
   = help: the trait `std::marker::Send` is not implemented for `Expr + 'static`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Expr + 'static>`
   = note: required because it appears within the type `Container`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Container>`
   = note: required because it appears within the type `[closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc<Container>]`
   = note: required by `std::thread::spawn`

error[E0277]: the trait bound `Expr + 'static: std::marker::Sync` is not satisfied
  --> src/main.rs:64:19
   |
64 |     let thread1 = thread::spawn(move || {
   |                   ^^^^^^^^^^^^^ `Expr + 'static` cannot be shared between threads safely
   |
   = help: the trait `std::marker::Sync` is not implemented for `Expr + 'static`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Expr + 'static>`
   = note: required because it appears within the type `Container`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Arc<Container>`
   = note: required because it appears within the type `[closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc<Container>]`
   = note: required by `std::thread::spawn`

我发现错误消息非常简单：

• 特质std::marker::Send没有实施Expr + 'static
• 由于 impl 的要求而需要std::marker::Send for std::sync::Arc<Expr + 'static>
• 必需的，因为它出现在类型中Container
• 由于 impl 的要求而需要std::marker::Send for std::sync::Arc<Container>
• 必需的，因为它出现在类型中[closure@src/main.rs:64:33: 67:6 container1:std::sync::Arc<Container>]
• 要求由std::thread::spawn

你正试图移动你的Arc<Container>到另一个线程，但它包含一个Arc<Expr + 'static>，不能保证安全发送（Send) 或共享 (Sync）跨线程。

要么添加Send and Sync作为 Expr 的超特征：

pub trait Expr: Send + Sync { /* ... */ }

或者将它们作为特征边界添加到特征对象中：

pub struct AddExpr {
    expr1: Box<Expr + Send + Sync>,
    expr2: Box<Expr + Send + Sync>,
}

impl AddExpr {
    pub fn new(expr1: Box<Expr + Send + Sync>, expr2: Box<Expr + Send + Sync>) -> Self {
        Self { expr1, expr2 }
    }
}

struct Container {
    x: i32,
    cached_expr: Arc<Expr + Send + Sync>,
}

也可以看看：

• 如何跨线程共享引用？ https://stackoverflow.com/q/29540407/155423
• 多线程应用程序无法编译并出现错误链 https://stackoverflow.com/q/46491293/155423
• 有没有办法实现 ZipFile 的发送特征？ https://stackoverflow.com/q/45690958/155423
• 如何使用 Arc>> 在线程之间共享通用结构？ https://stackoverflow.com/q/40035731/155423