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

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.