Implementing "move" thread semantics - multithreading

Question

I want to write a function to be called like this:
send("message","address");
Where some other thread that is doing
let k = recv("address");
println!("{}",k);
sees message.
In particular, the message may be large, and so I'd like "move" or "zero-copy" semantics for sending the message.
In C, the solution is something like:
Allocate messages on the heap
Have a global, threadsafe hashmap that maps "address" to some memory location
Write pointers into the memory location on send, and wake up the receiver using a semaphore
Read pointers out of the memory location on receive, and wait on a semaphore to process new messages
But according to another SO question, step #2 "sounds like a bad idea". So I'd like to see a more Rust-idiomatic way to approach this problem.

Answer 1

You get these sort of move semantics automatically, and get achieve light-weight moves by placing large values into a Box (i.e. allocate them on the heap). Using type ConcurrentHashMap<K, V> = Mutex<HashMap<K, V>>; as the threadsafe hashmap (there's various ways this could be improved), one might have:
use std::collections::{HashMap, RingBuf};
use std::sync::Mutex;
type ConcurrentHashMap<K, V> = Mutex<HashMap<K, V>>;
lazy_static! {
pub static ref MAP: ConcurrentHashMap<String, RingBuf<String>> = {
Mutex::new(HashMap::new())
}
}
fn send(message: String, address: String) {
MAP.lock()
// find the place this message goes
.entry(address)
.get()
// create a new RingBuf if this address was empty
.unwrap_or_else(|v| v.insert(RingBuf::new()))
// add the message on the back
.push_back(message)
}
fn recv(address: &str) -> Option<String> {
MAP.lock()
.get_mut(address)
// pull the message off the front
.and_then(|buf| buf.pop_front())
}
That code is using the lazy_static! macro to achieve a global hashmap (it may be better to use a local object that wraps an Arc<ConcurrentHashMap<...>, fwiw, since global state can make reasoning about program behaviour hard). It also uses RingBuf as a queue, so that messages bank up for a given address. If you only wish to support one message at a time, the type could be ConcurrentHashMap<String, String>, send could become MAP.lock().insert(address, message) and recv just MAP.lock().remove(address).
(NB. I haven't compiled this, so the types may not match up precisely.)