Src/main.rs

use std::rc::Rc;
use std::sync::Mutex;
use std::thread;

fn main() {
let counter = Rc::new(Mutex::new(0));
let mut handles = vec![];

for _ in 0..10 {
let counter = Rc::clone(&counter);
let handle = thread::spawn(move || {
let mut num = counter.lock().unwrap();

*num += 1;
});
handles.push(handle);
}

for handle in handles {
handle.join().unwrap();
}

println!(“Result: {}”, *counter.lock().unwrap());
}
Listing 16-14: Attempting to use Rc<T> to allow multiple threads to own the Mutex<T>
Once again, we compile and get… different errors! The compiler is teaching us a lot.

$ cargo run
Compiling shared-state v0.1.0 (file:///projects/shared-state)
error[E0277]: `Rc<Mutex<i32>>` cannot be sent between threads safely
–> src/main.rs:11:36
|
11 | let handle = thread::spawn(move || {
| ————- ^——
| | |
| ______________________|_____________within this `{closure@src/main.rs:11:36: 11:43}`
| | |
| | required by a bound introduced by this call
12 | | let mut num = counter.lock().unwrap();
13 | |
14 | | *num += 1;
15 | | });
| |_________^ `Rc<Mutex<i32>>` cannot be sent between threads safely
|
= help: within `{closure@src/main.rs:11:36: 11:43}`, the trait `Send` is not implemented for `Rc<Mutex<i32>>`
note: required because it’s used within this closure
–> src/main.rs:11:36
|
11 | let handle = thread::spawn(move || {
| ^^^^^^^
note: required by a bound in `spawn`
–> /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/thread/mod.rs:728:1

For more information about this error, try `rustc –explain E0277`.
error: could not compile `shared-state` (bin “shared-state”) due to 1 previous error

$ cargo run

Compiling shared-state v0.1.0 (file:///projects/shared-state)

error[E0382]: borrow of moved value: counter

–> src/main.rs:21:29

|

5 | let counter = Mutex::new(0);

| ——- move occurs because counter has type Mutex<i32>, which does not implement the Copy trait

…

8 | for _ in 0..10 {

| ————– inside of this loop

9 | let handle = thread::spawn(move || {

| ——- value moved into closure here, in previous iteration of loop

…

21 | println!(“Result: {}”, *counter.lock().unwrap());

| ^^^^^^^ value borrowed here after move

|

help: consider moving the expression out of the loop so it is only moved once

|

8 ~ let mut value = counter.lock();

9 ~ for _ in 0..10 {

10 | let handle = thread::spawn(move || {

11 ~ let mut num = value.unwrap();

|

For more information about this error, try rustc --explain E0382.

error: could not compile shared-state (bin “shared-state”) due to 1 previous error