[RustBook] [Solved] WebServer Multithreaded tutorial compilation errors


#1

Hi,

I’m trying to follow the web server tutorial of 2018 rust book edition. i’m getting the following errors while trying to apply the Function boxing trick :

error[E0411]: expected trait, found self type `Self`
--> src/lib.rs:47:31
|
47 |     fn call_box(self: Box<dyn Self>);
|                               ^^^^ `Self` is only available in traits and impls

error[E0404]: expected trait, found type parameter `F`
--> src/lib.rs:51:31
|
51 |     fn call_box(self: Box<dyn F>) {
|                               ^ did you mean `Fn`?

for the following code :

trait FnBox {
    fn call_box(self: Box<dyn Self>);
}

impl<F: FnOnce() -> ()> FnBox for F {
    fn call_box(self: Box<dyn F>) {
        (*self)()
    }
}

Here is the complete file :

use std::sync::mpsc;
use std::sync::Arc;
use std::sync::Mutex;
use std::thread;


pub struct ThreadPool {
    workers: Vec<Worker>,
    sender: mpsc::Sender<Job>,
}

impl ThreadPool {
    /// Create a new ThreadPool.
    ///
    /// The size is the number of threads in the pool.
    ///
    /// # Panics
    ///
    /// The `new` function will panic if the size is zero.
    pub fn new(size: usize) -> ThreadPool {
        assert!(size > 0);

        let (sender, receiver) = mpsc::channel();

        let receiver = Arc::new(Mutex::new(receiver));

        let mut workers = Vec::with_capacity(size);

        for id in 0..size {
            workers.push(Worker::new(id, Arc::clone(&receiver)));
        }

        ThreadPool { workers, sender }
    }

    pub fn execute<F>(&self, f: F)
    where
        F: FnOnce() + Send + 'static,
    {
        let job = Box::new(f);

        self.sender.send(job).unwrap();
    }
}

trait FnBox {
    fn call_box(self: Box<dyn Self>);
}

impl<F: FnOnce() -> ()> FnBox for F {
    fn call_box(self: Box<dyn F>) {
        (*self)()
    }
}

type Job = Box<dyn FnBox + Send + 'static>;

struct Worker {
    id: usize,
    thread: thread::JoinHandle<()>,
}

impl Worker {
    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
        let thread = thread::spawn(move || loop {
            let job = receiver.lock().unwrap().recv().unwrap();

            println!("Worker {} got a job; executing.", id);

            job.call_box();
        });

        Worker { id, thread }
    }
}

I understand what the errors are saying but i don’t see why the tutorial code generates those. Am i using the wrong rust verstion (stable 1.28) or have i made a dumb mistake i can’t see ?

Thanks in advance


#2
fn call_box(self: Box<dyn Self>);

That should be Box<Self>, not Box<dyn Self>; same for the impl instead of Box<dyn F>.


#3

Thanks a lot that solved my problem. The github master version of the book was correct.

To be honest i’m still far away from being able to understand the code though…