如何在Rust中实现子序列迭代器？

我想要实现一个能生成输入序列所有**子序列（subsequences）**的迭代器，示例如下：

subsequences "abc"
["","a","b","ab","c","ac","bc","abc"]

subsequences [1,2]
[[],[1],[2],[1,2]]

subsequences [1,2,3]
[[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]

subsequences [1,2,3,4]
[[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3],[4],[1,4],[2,4],[1,2,4],[3,4],[1,3,4],[2,3,4],[1,2,3,4]]

Haskell有个非常简洁的实现：

subsequences            :: [a] -> [[a]]
subsequences xs         =  [] : nonEmptySubsequences xs

nonEmptySubsequences         :: [a] -> [[a]]
nonEmptySubsequences []      =  []
nonEmptySubsequences (x:xs)  =  [x] : foldr f [] (nonEmptySubsequences xs)
  where f ys r = ys : (x : ys) : r

但我无法在Rust中复现该实现。我希望它能有如下签名，以此避免不必要的内存分配，从而处理极长序列：

fn subsequences<A: Copy>(xs: &[A]) -> impl Iterator<Item=impl Iterator<Item=A>>;

恳请提供相关指导。

内容的提问来源于stack exchange，提问作者user1002430