Question

我已经掌握了Rust生命周期的基础知识以及如何使用迭代器，但仍然有一些麻烦来定义一个函数，它接受一个可迭代的元组并返回一个也在堆上分配的迭代元组。 / p>

（我知道＆＃34;可迭代＆＃34;并不代表Rust中的任何内容，但我仍然会使用它代替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

在遇到问题时创建MCVE非常有用，这是我鼓励所有学习者进行的事情。这是一个镜像代码的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，因此不支持您的函数签名。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))
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

我们按照建议行事并添加'static界限：

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

此错误是因为您有一个(usize, &'a (usize, f32))的迭代器，并且您正尝试将它们转换为(usize, &(i32, f32))。你无法转换这样的类型。

如何获取引用的IntoIterator并返回对修改后的值的引用的盒装迭代器？

1 个答案: