引用内部闭包的迭代器的智能构造函数

Question

考虑以下用于（大大简化的）迭代器的代码，其中引用了内部的闭包：

struct IteratorState<'a, T: 'a + Fn(i32) -> i32> {
    closure: &'a T,
}

impl<'a, T: 'a + Fn(i32) -> i32> Iterator for IteratorState<'a, T> {
    type Item = i32;

    fn next(&mut self) -> Option<i32> {
        None
    }
}

它编译，我可以直接构造IteratorStates。但是，我还需要一个智能构造函数来隐藏一些实现细节（MCVE 中未显示）。以下尝试无法编译：

fn mk_iter<'a, T: Fn(i32) -> i32>(closure: &'a T) -> impl Iterator<Item = i32> {
    IteratorState { closure }
}

The error is

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
  --> src/lib.rs:14:5
   |
14 |     IteratorState { closure }
   |     ^^^^^^^^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime 'a as defined on the function body at 13:1...
  --> src/lib.rs:13:1
   |
13 | fn mk_iter<'a, T: Fn(i32) -> i32>(closure: &'a T) -> impl Iterator<Item = i32> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...so that reference does not outlive borrowed content
  --> src/lib.rs:14:21
   |
14 |     IteratorState { closure }
   |                     ^^^^^^^
   = note: but, the lifetime must be valid for the static lifetime...
note: ...so that return value is valid for the call
  --> src/lib.rs:13:54
   |
13 | fn mk_iter<'a, T: Fn(i32) -> i32>(closure: &'a T) -> impl Iterator<Item = i32> {
   |                                                      ^^^^^^^^^^^^^^^^^^^^^^^^^

我想我明白问题是什么：不能保证构造的 IteratorState 不会比所包含的参考更有效（如果我弄错了请纠正我），但我不太确定如何要解决这个问题。

Answer 1

impl Trait 语法支持向 return 类型添加生命周期：

fn mk_iter<'a, T: Fn(i32) -> i32>(closure: &'a T) -> impl Iterator<Item = i32> + 'a {
      //                                                                  here ^^^^
    IteratorState {
        closure
    }
}

(link to playground)

引用内部闭包的迭代器的智能构造函数

A smart constructor for an iterator with a reference to a closure inside

lifetime

rust