Using par_iter and Rayon in Rust

Suppose I want to parallelise the code below which takes two Rust vectors and sums each element, returning a new vector in the same order as the originals:

a = b.iter().zip(c.iter()).map(|(aa, bb)|{aa + bb}).collect();

Using something like this:

a = b.par_iter().zip(c.par_iter()).map(|(aa, bb)|{aa + bb}).collect();

1. Do I need to use par_iter for the zip input as well (as shown)?
2. Can I be sure that the collected output vector will have the summed values in the same order as the input and will not be altered by threads ending at different times?

The answers to your questions are yes and yes. However, there's many details that are involved.

If you find calling c.par_iter() to be tedious, you can instead write b.par_iter().zip(&c), which will call c.par_iter() for you. This also applies to plain iterators and .iter().

Rayon also has a convenient shorthand for creating many zipped parallel iterators, which is especially useful if you need to zip more than two:

(&b, &c).into_par_iter().map(|(bb, cc)| bb + cc).collect()

And if you look at the docs for collect, you will find mention of collect_into_vec, which will work more efficiently than collect.

let mut a = Vec::new();
(&b, &c).into_par_iter().map(|(bb, cc)| bb + cc).collect_into_vec(&mut a);