在Rust struct中用自引用封装顺序初始化状态

我试图定义一个 struct ，它可以作为RefCell中包含的Vec的迭代器:

use std::slice::Iter;
use std::cell::Ref;
use std::cell::RefCell;

struct HoldsVecInRefCell {
    vec_in_refcell: RefCell<Vec<i32>>,
}

// TODO: struct HoldsVecInRefCellIter implementing Iterator ...

impl HoldsVecInRefCell {
    fn new() -> HoldsVecInRefCell {
        HoldsVecInRefCell { vec_in_refcell: RefCell::new(Vec::new()) }
    }

    fn add_int(&self, i: i32) {
        self.vec_in_refcell.borrow_mut().push(i);
    }

    fn iter(&self) -> HoldsVecInRefCellIter {
        // TODO ...
    }
}

fn main() {
    let holds_vec = HoldsVecInRefCell::new();
    holds_vec.add_int(1);
    holds_vec.add_int(2);
    holds_vec.add_int(3);

    let mut vec_iter = holds_vec.iter();  // Under the hood: run-time borrow check

    for i in vec_iter {
        println!("{}", i);
    }
}

相比之下，vec_iter可以在main()中在线初始化，如下所示(故意冗长):

// Elided: lifetime parameter of Ref
let vec_ref: Ref<Vec<i32>> = holds_vec.vec_in_refcell.borrow();
// Elided: lifetime parameter of Iter
let mut vec_iter: Iter<i32> = vec_ref.iter();

当第二个容器从第一个容器派生(并保存从第一个容器获得的引用)时，有没有办法定义一个实现Iterator的 struct ，该 struct 同时保存Ref(保持不变的RefCellborrow 活动)和Iter(保持next()的迭代器状态，而不是滚动我自己的Vec迭代器或任何其他容器)？

我try 了几种方法来实现这一点，但都与借阅判断器相冲突.如果我把两个状态都作为裸 struct 成员，比如

struct HoldsVecInRefCellIter<'a> {
    vec_ref: Ref<'a, Vec<i32>>,
    vec_iter: Iter<'a, i32>,
}

然后我不能用HoldsVecInRefCellIter { ... }语法同时初始化这两个字段(例如，参见Does Rust have syntax for initializing a struct field with an earlier field?).如果我try 用类似

struct HoldsVecInRefCellIter<'a> {
    vec_ref: Ref<'a, Vec<i32>>,
    vec_iter: Option<Iter<'a, i32>>,
}

// ...

impl HoldsVecInRefCell {
    // ...

    fn iter(&self) -> HoldsVecInRefCellIter {
        let mut new_iter = HoldsVecInRefCellIter { vec_ref: self.vec_in_refcell.borrow(), vec_iter: None };
        new_iter.vec_iter = new_iter.vec_ref.iter();
        new_iter
    }
}

then I incur a mutable self-borrow of the struct that prevents returning it from iter(). This self-borrowing of a struct can also happen if you try to store a reference to one part of a struct in the struct itself (Why can't I store a value and a reference to that value in the same struct?), which would prevent safely moving instances of the struct. By comparison, it seems like a struct like HoldsVecInRefCellIter, if you could complete initialization, would do the correct thing when moved, since all references internally are to data elsewhere that outlives this struct.

有一些技巧可以避免使用Rc创建自引用(参见第https://internals.rust-lang.org/t/self-referencing-structs/418/3页的示例)，但如果您想存储现有的Iterator struct ，而该 struct 的实现是为了保存对底层容器的直接引用，而不是Rc，我不知道如何应用这些技巧.

作为来自C++的 rust 新手，这感觉起来像是一个经常出现的问题("我在代码块中有一些复杂的状态初始化逻辑，我想抽象掉那个逻辑，并将结果状态保存在 struct 中以供使用)).

Related Question: Returning iterator of a Vec in a RefCell