是否可以通过渠道发送闭包？

Question

我想通过渠道发送闭包：

use std::thread;
use std::sync::mpsc;

#[derive(Debug)]
struct Test {
    s1: String,
    s2: String,
}

fn main() {
    let t = Test {
        s1: "Hello".to_string(),
        s2: "Hello".to_string(),
    };
    let (tx, rx) = mpsc::channel::<FnOnce(&mut Test)>();
    thread::spawn(move || {
        let mut test = t;
        let f = rx.recv().unwrap();
        f(&mut test);
        println!("{:?}", test);
    });
    tx.send(move |t: &mut Test| {
        let s = "test".to_string();
        t.s1 = s;
    });
}

(playground)

我收到一堆错误：

error[E0277]: the trait bound `for<'r> std::ops::FnOnce(&'r mut Test): std::marker::Sized` is not satisfied
  --> src/main.rs:15:20
   |
15 |     let (tx, rx) = mpsc::channel::<FnOnce(&mut Test)>();
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `for<'r> std::ops::FnOnce(&'r mut Test)` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `for<'r> std::ops::FnOnce(&'r mut Test)`
   = note: required by `std::sync::mpsc::channel`

error[E0277]: the trait bound `for<'r> std::ops::FnOnce(&'r mut Test): std::marker::Sized` is not satisfied
  --> src/main.rs:15:20
   |
15 |     let (tx, rx) = mpsc::channel::<FnOnce(&mut Test)>();
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `for<'r> std::ops::FnOnce(&'r mut Test)` does not have a constant size known at compile-time
   |
   = help: the trait `std::marker::Sized` is not implemented for `for<'r> std::ops::FnOnce(&'r mut Test)`
   = note: required by `std::sync::mpsc::Sender`

error[E0599]: no method named `recv` found for type `std::sync::mpsc::Receiver<for<'r> std::ops::FnOnce(&'r mut Test)>` in the current scope
  --> src/main.rs:18:20
   |
18 |         let f = rx.recv().unwrap();
   |                    ^^^^
   |
   = note: the method `recv` exists but the following trait bounds were not satisfied:
           `for<'r> std::ops::FnOnce(&'r mut Test) : std::marker::Sized`

error[E0599]: no method named `send` found for type `std::sync::mpsc::Sender<for<'r> std::ops::FnOnce(&'r mut Test)>` in the current scope
  --> src/main.rs:22:8
   |
22 |     tx.send(move |t: &mut Test| {
   |        ^^^^
   |
   = note: the method `send` exists but the following trait bounds were not satisfied:
           `for<'r> std::ops::FnOnce(&'r mut Test) : std::marker::Sized`

FnOnce 似乎无法发送，但我不明白为什么。

Answer 1

是的。您的代码存在一些问题。

首先FnOnce是一个trait，所以不能直接使用。特征必须要么是对具体类型的约束，要么是某种间接类型的约束。由于您要将闭包发送到其他地方，因此您需要 Box<FnOnce(...)> 之类的东西。

其次，您不能使用Box<FnOnce(...)>，因为由于对象安全规则，您实际上不能通过间接调用FnOnce。

（顺便说一句，您也不想使用FnOnce<...> 语法，这在技术上是不稳定的；请改用FnOnce(...)。）

要解决这个问题，您可以切换到Fn 或FnMut 或使用尚未稳定的FnBox trait。我走这条路是因为它可能具有您想要的语义，并且可能在不久的将来稳定下来。如果您对此感到不舒服，则需要适当地修改您的闭包。

以下是我和 Manishearth 的共同努力（他指出我错过了+ Send 约束）：

// NOTE: Requires a nightly compiler, as of Rust 1.0.

#![feature(core)]
use std::boxed::FnBox;
use std::thread;
use std::sync::mpsc;

#[derive(Debug)]
struct Test {
    s1: String,
    s2: String,
}

type ClosureType = Box<FnBox(&mut Test) + Send>;

fn main() {
    let t = Test { s1: "Hello".to_string(), s2: "Hello".to_string() };
    let (tx, rx) = mpsc::channel::<ClosureType>();

    thread::spawn(move || {
        let mut test = t;
        let f = rx.recv().unwrap();
        f.call_box((&mut test,));
        println!("{:?}", test);
    });

    tx.send(Box::new(move |t: &mut Test| {
        let s = "test".to_string();
        t.s1 = s;
    })).unwrap();

    // To give the output time to show up:
    thread::sleep_ms(100);
}

Answer 2

接受的答案没有详细说明，但如果您不使用FnOnce，您可以通过通道将闭包发送到线程，即使在稳定状态下也是如此：

use std::thread;
use std::sync::mpsc;

struct RawFunc {
    data: Box<Fn() + Send + 'static>,
}

impl RawFunc {
    fn new<T>(data: T) -> RawFunc
    where
        T: Fn() + Send + 'static,
    {
        return RawFunc {
            data: Box::new(data),
        };
    }

    fn invoke(self) {
        (self.data)()
    }
}

fn main() {
    // Local
    let x = RawFunc::new(move || {
        println!("Hello world");
    });
    x.invoke();

    // Via channel
    let (sx, rx) = mpsc::channel::<RawFunc>();
    sx.send(RawFunc::new(move || {
        println!("Hello world 2");
    })).unwrap();
    let output = rx.recv().unwrap();
    output.invoke();

    // In a thread
    let guard = thread::spawn(move || {
        let output = rx.recv().unwrap();
        output.invoke();
    });

    sx.send(RawFunc::new(move || {
        println!("Hello world 3!");
    })).unwrap();

    guard.join().unwrap();

    // Passing arbitrary data to a thread
    let (sx, rx) = mpsc::channel::<RawFunc>();
    let guard = thread::spawn(move || {
        let output = rx.recv().unwrap();
        output.invoke();
    });

    let foo = String::from("Hello World 4");
    sx.send(RawFunc::new(move || {
        println!("Some moved data: {:?}", foo);
    })).unwrap();

    guard.join().unwrap();
}