Module copied 

First sanity check that the expression is OK.

#![deny(unused_must_use)]

use rayon::prelude::*;

let v: Vec<_> = (0..100).map(Some).collect();
let _ =
 v.par_iter().copied(); 

Now trigger the must_use.

#![deny(unused_must_use)]

use rayon::prelude::*;

let v: Vec<_> = (0..100).map(Some).collect();
 v.par_iter().copied();