chunks_exact_mut threading lifetimes - multithreading

Question

The following code produces interesting results:
use std::thread;
fn main() {
let mut handles = Vec::new();
let mut v = vec![1, 2, 3, 4];
for x in v.chunks_exact_mut(2) {
let handle = thread::spawn(move || {
println!("Here's a vector: {:?}", x);
});
handles.push(handle);
}
for h in handles { h.join().unwrap(); }
}
error[E0597]: `v` does not live long enough
--> src/main.rs:7:14
|
7 | for x in v.chunks_exact_mut(2) {
| ^^^^^^^^^^^^^^^^^^^^^
| |
| borrowed value does not live long enough
| argument requires that `v` is borrowed for `'static`
...
16 | }
| - `v` dropped here while still borrowed
Why does 'chunking' v change the lifetime? Without 'chunking' v, the code performs correctly. So why now does it throw an error?

Answer 1

The problem here is that thread::spawn completely decouples the lifetime from the rest, as the thread might continue to run while main is already finished.
x is a reference to a variable in main, and there is no guarantee that main lives longer than the thread.
I think what you really want here is a threading helper library like rayon, as they already solve those problems internally:
use rayon::prelude::*;
fn main() {
let mut v = vec![1, 2, 3, 4];
v.par_chunks_exact_mut(2).for_each(|x: &mut [i32]| {
let thread_id = std::thread::current().id();
println!("{:?}: Here's a vector: {:?}", thread_id, x);
});
}
ThreadId(5): Here's a vector: [1, 2]
ThreadId(2): Here's a vector: [3, 4]

Answer 2

Without "chunking" v, you're iterating over Vec<i32>. This iterator produces items of type i32, so, no references, no lifetimes, the requirements for thread::spawn() (specifically the 'static requirement) are met.
chunks_exact_mut(), however, is defined on slices and gives iterator over &mut [T], so it does have a reference. And since this reference refers v, which exists in main(), it is not 'static and does not work.
You can observe the same problem if you'll iterate over &v instead of v, since that iterator gives you &i32s (playground):
error[E0597]: `v` does not live long enough
--> src/main.rs:7:14
|
7 | for x in &v {
| ^^
| |
| borrowed value does not live long enough
| argument requires that `v` is borrowed for `'static`
...
16 | }
| - `v` dropped here while still borrowed
Other than that, I'd recommend using rayon as #Finomnis suggested.