Can this be

```rust
let mut new_env = env.move_iter().map(|(k, v)| {
    let new_v = if "PATH" == k {
       format!("{};{};{}", *v, lib_path, aux_path)
   } else {
        v
   };
   (k, new_v)
}).collect();
```

instead of a manual loop? (It doesn't really matter particularly.)