How to take an IntoIterator of references and return a boxed Iterator of references to the modified values?

Question

I've grasped the basics of Rust lifetimes and how to work with iterators in it, but still have troubles to define a function that takes an iterable of tuples and return an iterable of tuples also allocated on the heap.

(I know that "iterable" doesn't mean anything in Rust but I will still use it instead of IntoInterator)

use std::iter::{once, repeat};

fn foo<'a, I>(costs: I) -> Box<Iterator<Item = &'a (i32, f32)>>
where
    I: IntoIterator<Item = &'a (usize, f32)>,
{
    let preliminary_examination_costs = once(10.0).chain(repeat(20.0));
    let id_assignment_costs = once(10.0).chain(repeat(20.0));
    let batch_fixed = once(10.0).chain(repeat(0.0));
    let temp: Vec<(usize, &(i32, f32))> = costs.into_iter().enumerate().collect();
    temp.sort_by_key(|&(_, &(n, cost))| n);
    Box::new(temp.into_iter().map(|(i, &(n, cost))| {
        (
            i,
            cost + preliminary_examination_costs.next().unwrap()
                + id_assignment_costs.next().unwrap() + batch_fixed.next().unwrap(),
        )
    }))
}

(playground)

Here's the errors:

error[E0277]: the trait bound `std::vec::Vec<(usize, &(i32, f32))>: std::iter::FromIterator<(usize, &'a (usize, f32))>` is not satisfied
  --> src/main.rs:10:73
   |
10 |     let temp: Vec<(usize, &(i32, f32))> = costs.into_iter().enumerate().collect();
   |                                                                         ^^^^^^^ a collection of type `std::vec::Vec<(usize, &(i32, f32))>` cannot be built from an iterator over elements of type `(usize, &'a (usize, f32))`
   |
   = help: the trait `std::iter::FromIterator<(usize, &'a (usize, f32))>` is not implemented for `std::vec::Vec<(usize, &(i32, f32))>`

error[E0271]: type mismatch resolving `<[closure@src/main.rs:12:35: 18:6 preliminary_examination_costs:_, id_assignment_costs:_, batch_fixed:_] as std::ops::FnOnce<((usize, &(i32, f32)),)>>::Output == &(i32, f32)`
  --> src/main.rs:12:5
   |
12 | /     Box::new(temp.into_iter().map(|(i, &(n, cost))| {
13 | |         (
14 | |             i,
15 | |             cost + preliminary_examination_costs.next().unwrap()
16 | |                 + id_assignment_costs.next().unwrap() + batch_fixed.next().unwrap(),
17 | |         )
18 | |     }))
   | |_______^ expected tuple, found &(i32, f32)
   |
   = note: expected type `(usize, f32)`
              found type `&(i32, f32)`
   = note: required because of the requirements on the impl of `std::iter::Iterator` for `std::iter::Map<std::vec::IntoIter<(usize, &(i32, f32))>, [closure@src/main.rs:12:35: 18:6 preliminary_examination_costs:_, id_assignment_costs:_, batch_fixed:_]>`
   = note: required for the cast to the object type `std::iter::Iterator<Item=&(i32, f32)>`

Answer 1

It's very useful to create a MCVE when encountering a problem, and it's something I encourage all learners to undertake. Here's one such MCVE that mirrors your code:

fn foo<'a, I>(costs: I) -> Box<Iterator<Item = &'a i32>>
where
    I: IntoIterator<Item = &'a i32>,
{
    Box::new(costs.into_iter().map(|i| i + 1))
}

error[E0271]: type mismatch resolving `<[closure@src/main.rs:5:36: 5:45] as std::ops::FnOnce<(&'a i32,)>>::Output == &i32`
 --> src/main.rs:5:5
  |
5 |     Box::new(costs.into_iter().map(|i| i + 1))
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected i32, found &i32
  |
  = note: expected type `i32`
             found type `&i32`
  = note: required because of the requirements on the impl of `std::iter::Iterator` for `std::iter::Map<<I as std::iter::IntoIterator>::IntoIter, [closure@src/main.rs:5:36: 5:45]>`
  = note: required for the cast to the object type `std::iter::Iterator<Item=&i32>`

You are accepting a reference then creating a brand new value based on the reference. That means that the return type is no longer a reference, thus your function signature is not upheld. You need to change it:

fn foo<'a, I>(costs: I) -> Box<Iterator<Item = i32>>

This will unlock another error because the compiler doesn't know enough about the concrete type of I::IntoIter:

error[E0310]: the associated type `<I as std::iter::IntoIterator>::IntoIter` may not live long enough
 --> src/main.rs:6:5
  |
6 |     Box::new(costs.into_iter().map(|i| i + 1))
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: consider adding an explicit lifetime bound `<I as std::iter::IntoIterator>::IntoIter: 'static`...
note: ...so that the type `std::iter::Map<<I as std::iter::IntoIterator>::IntoIter, [closure@src/main.rs:6:36: 6:45]>` will meet its required lifetime bounds
 --> src/main.rs:6:5
  |
6 |     Box::new(costs.into_iter().map(|i| i + 1))
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

We do as it suggests and add the 'static bound:

fn foo<'a, I>(costs: I) -> Box<Iterator<Item = i32>>
where
    I: IntoIterator<Item = &'a i32>,
    I::IntoIter: 'static,
{
    Box::new(costs.into_iter().map(|i| i + 1))
}

See also:

• Is there any way to return a reference to a variable created in a function?

---

error[E0277]: the trait bound `std::vec::Vec<(usize, &(i32, f32))>: std::iter::FromIterator<(usize, &'a (usize, f32))>` is not satisfied

This error is because you have an iterator of (usize, &'a (usize, f32)) and you are trying to make them into (usize, &(i32, f32)). You can't convert types like this.