如何获取引用的 IntoIterator 并返回对修改值的引用的装箱迭代器？

Question

我已经掌握了 Rust 生命周期的基础知识以及如何在其中使用迭代器，但仍然难以定义一个函数，该函数接受一个可迭代的元组并返回一个也分配在堆上的可迭代元组。

（我知道“可迭代”在 Rust 中没有任何意义，但我仍然会使用它而不是 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)

以下是错误：

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

遇到问题时创建MCVE 非常有用，我鼓励所有学习者都这样做。这是一个反映您的代码的此类 MCVE：

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>`

您正在接受参考，然后根据参考创建一个全新的值。这意味着返回类型不再是引用，因此您的函数签名不会得到支持。你需要改变它：

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

这将解锁另一个错误，因为编译器对 I::IntoIter 的具体类型知之甚少：

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))
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

我们按照它的建议做并添加'static 绑定：

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))
}

另见：

• 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

这个错误是因为你有一个(usize, &'a (usize, f32)) 的迭代器，并且你试图将它们变成(usize, &(i32, f32))。你不能像这样转换类型。