为 GATified 借贷迭代器实现 chain()

Question

Iterator::chain()我正在尝试为 GATified 借贷迭代器实现类似操作：

#![feature(generic_associated_types)]

pub trait LendingIterator {
    type Item<'a> where Self: 'a;

    fn next(&mut self) -> Option<Self::Item<'_>>;
}

为此，与普通 Iterator::chain() 类似，我需要一个Chain包含两个链式迭代器和 impl 的结构LendingIterator。

我遇到的问题是我需要指定迭代器类型A并B具有匹配的Item泛型关联类型，所以我尝试了以下方法：

pub struct Chain<A, B> {
    a: Option<A>,
    b: Option<B>,
}

impl<A, B> LendingIterator for Chain<A, B>
    where
        A: LendingIterator,
        B: for<'a> LendingIterator<Item<'a>=A::Item<'a>>
{
    type Item<'a> where Self: 'a = A::Item<'a>;

    fn next(&mut self) -> Option<Self::Item<'_>> {
        todo!()
    }
}

但我得到：

error[E0311]: the parameter type `B` may not live long enough
  --> src\lib.rs:63:12
   |
63 | impl<A, B> LendingIterator for Chain<A, B> where A: LendingIterator, B: for<'a> LendingIterator<Item<'a>=A::Item<'a>> {
   |         -  ^^^^^^^^^^^^^^^ ...so that the type `B` will meet its required lifetime bounds...
   |         |
   |         help: consider adding an explicit lifetime bound...: `B: 'a`
   |
note: ...that is required by this bound
   |        ^^^^ ...so that the type `B` will meet its required lifetime bounds

当然，编译器的建议似乎不起作用。

如何添加B具有足够长生命周期的约束？

Answer 1

事实证明这是 Rust 的 GAT 实现中的一个错误。看看这个：

https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/.E2.9C.94.20GAT.20LendingIterator.3A.3Achain.20Issue

https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Understanding.20the.20motivations.20for.20GATs/near/269863728