rust - 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，其中包含对内部闭包的引用:

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
    }
}

它编译并且我可以直接构建IteratorState。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 不会比包含的引用更有效(如果我弄错了请纠正我)，但我不太确定如何修复它。

Answer 1

impl Trait 语法支持为返回类型添加生命周期:

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

( link to playground )