std::sync::Arc of trait in Rust

Question

I am trying to implement library for making TCP servers.

This is very simplified code with a problem:

#![crate_name="http_server2"]
#![crate_type="lib"]

use std::io::{TcpListener, Listener, Acceptor, TcpStream, IoResult, Reader, Writer};
use std::ops::Fn;
use std::sync::Arc;


pub trait Handler: Sized + Send {
    fn do_it(s: TcpStream) -> IoResult<()>;
}

fn serve(handler: Arc<Handler + Sized>) -> IoResult<()>
{
    let listener = TcpListener::bind("127.0.0.1", 1234);

    for stream in try!(listener.listen()).incoming() {
        let stream = try!(stream);
        let handler = handler.clone();
        spawn(proc() {
            handler.do_it(stream);
        });
    }
    Ok(())
}

Compiler totally ignores my specifications of Handler + Sized . If I implement structure with trait Handler and try to call serve with this structure, such advice about size will be ignored too ( http://is.gd/OWs22i ).

<anon>:13:1: 25:2 error: the trait `core::kinds::Sized` is not implemented for the type `Handler+'static+Sized`
<anon>:13 fn serve(handler: Arc<Handler + Sized>) -> IoResult<()>
<anon>:14 {
<anon>:15     let listener = TcpListener::bind("127.0.0.1", 1234);
<anon>:16 
<anon>:17     for stream in try!(listener.listen()).incoming() {
<anon>:18         let stream = try!(stream);
          ...
<anon>:13:1: 25:2 note: the trait `core::kinds::Sized` must be implemented because it is required by `alloc::arc::Arc`
<anon>:13 fn serve(handler: Arc<Handler + Sized>) -> IoResult<()>
<anon>:14 {
<anon>:15     let listener = TcpListener::bind("127.0.0.1", 1234);
<anon>:16 
<anon>:17     for stream in try!(listener.listen()).incoming() {
<anon>:18         let stream = try!(stream);
          ...
error: aborting due to previous error

How can I implement one template function with multithreading that will accept different handlers?

Answer 1

As I said in my comment above,

use std::io::{TcpListener, Listener, Acceptor, TcpStream, IoResult, Writer};
use std::sync::Arc;

pub trait Handler: Sized + Send {
    fn do_it(&self, s: TcpStream) -> IoResult<()>;
}

fn serve<T: Handler + Sized + Send + Sync>(handler: Arc<T>) -> IoResult<()> {
    let listener = TcpListener::bind("127.0.0.1", 1234);
    for stream in try!(listener.listen()).incoming() {
        let stream = try!(stream);
        let handler = handler.clone();
        spawn(proc() { 
            let _ = handler.do_it(stream); 
        });
    }
    Ok(())
}

struct Hello {
    x: u32,
}

impl Handler for Hello {
    fn do_it(&self, mut s: TcpStream) -> IoResult<()> { s.write_le_u32(self.x) }
}

fn main() { 
    let s = Arc::new(Hello{x: 123,}); 
    let _ = serve(s); 
}

compiles fine. (playpen)

---

Changes

1. Make do_it take &self .
2. Make serve generic, by adding a type parameter with the constraints you want.
3. Make the impl of Handler for Hello in do_it not discard the result of the write (remove ; ).
4. Clarify with let _ = ... that we intentionally discard a result.

You will not be able to execute it in the playpen though ( application terminated abnormally with signal 31 (Bad system call) ), as the playpen forbids IO (network IO in this case). It runs fine on my local box though.