fn main() {
    let arr_of_arr = [[1, 2], [3, 4]];
    let res = arr_of_arr
        .iter()
        .flat_map(|arr| arr.iter())
        .collect::<Vec<i32>>();
}

error[E0277]: the trait bound `std::vec::Vec<i32>: std::iter::FromIterator<&{integer}>` is not satisfied
 --> src/main.rs:6:10
  |
6 |         .collect::<Vec<i32>>();
  |          ^^^^^^^ a collection of type `std::vec::Vec<i32>` cannot be built from an iterator over elements of type `&{integer}`
  |
  = help: the trait `std::iter::FromIterator<&{integer}>` is not implemented for `std::vec::Vec<i32>`

fn main() {
    let arr_of_arr = [[1, 2], [3, 4]];
    let res = arr_of_arr.iter()
        .flat_map(|arr| arr.iter())
        .cloned() // or `.map(|e| *e)` since `i32` are copyable
        .collect::<Vec<i32>>();
}