Processing vec in parallel: how to do safely, or without using unstable features?

One can use an external library for this, e.g. simple_parallel (disclaimer, I wrote it) allows one to write:

extern crate simple_parallel;

let mut data = vec![1u32, 2, 3, 4, 5];

let mut pool = simple_parallel::Pool::new(4);

pool.for_(data.chunks_mut(3), |target| {
    // do stuff with `target`
})

The chunks and chunks_mut methods are the perfect way to split a vector/slice of Ts into equally sized chunks: they respectively return an iterator over elements of type &[T] and &mut [T].

Today the rayon crate is the de facto standard for this sort of thing:

use rayon::prelude::*;

fn main() {
    let mut data = vec![1, 2, 3];
    data.par_iter_mut()
        .enumerate()
        .for_each(|(i, x)| *x = 10 + i as u32);
    assert_eq!(vec![10, 11, 12], data);
}

Note that this is just one line different from the single-threaded version using standard iterators, which would replace par_iter_mut with iter_mut.

See also Writing a small ray tracer in Rust and Zig.