我正在尝试Vec使用递归函数打印 a 的唯一连续子数组，如下所示：

use std::collections::HashSet;

fn recurse<'a>(nums: &'a [i32], already_printed: &'a mut HashSet<&'a [i32]>) {
    if !already_printed.contains(nums) {
        println!("{:#?}", nums);
    }

    already_printed.insert(nums);

    if nums.len() >= 2 {
        recurse(&nums[0..nums.len() - 1], already_printed);
        recurse(&nums[1..nums.len()], already_printed);
    }
}

pub fn main() {
    let k = vec![1, 2, 3, 4, 5];
    let mut already_printed: HashSet<&[i32]> = HashSet::new();
    recurse(&k[0..], &mut already_printed);
}

当然，正如有经验的 Rustaceans 可能已经猜到的那样，编译失败并出现以下错误：

error[E0499]: cannot borrow `*already_printed` as mutable more than once at a time
  --> src/main.rs:12:39
   |
3  | fn recurse<'a>(nums: &'a [i32], already_printed: &'a mut HashSet<&'a [i32]>) {
   |            -- lifetime `'a` defined here
...
11 |         recurse(&nums[0..nums.len() - 1], already_printed);
   |         --------------------------------------------------
   |         |                                 |
   |         |                                 first mutable borrow occurs here
   |         argument requires that `*already_printed` is borrowed for `'a`
12 |         recurse(&nums[1..nums.len()], already_printed);
   |                                       ^^^^^^^^^^^^^^^ second mutable borrow occurs here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0499`.

我从非常有用的错误中理解为什么编译器拒绝编译它。但是，一般来说，实现采用可变引用的递归函数的解决方法是什么，如上述代码所示？

我能想到的一种可能的解决方法是使用内部可变性模式à la RefCell：

use std::cell::RefCell;
use std::collections::HashSet;

fn recurse<'a>(nums: &'a [i32], already_printed: &'a RefCell<HashSet<&'a [i32]>>) {
    if !already_printed.borrow().contains(nums) {
        println!("{:#?}", nums);
    }

    already_printed.borrow_mut().insert(nums);

    if nums.len() >= 2 {
        recurse(&nums[0..nums.len() - 1], already_printed);
        recurse(&nums[1..nums.len()], already_printed);
    }
}

pub fn main() {
    let k = vec![1, 2, 3, 4, 5];
    let already_printed: HashSet<&[i32]> = HashSet::new();
    let ref_cell: RefCell<HashSet<&[i32]>> = RefCell::new(already_printed);
    recurse(&k[0..], &ref_cell);
}

虽然这有效，但这似乎放弃了编译时借用检查器提供的安全轨道。是否有不同的规范方法可以在编译时借用检查器通过的同时进行上述递归函数调用？

回答

fn recurse<'a, 'b>(nums: &'a [i32], already_printed: &'b mut HashSet<&'a [i32]>) {
// I just changed this lifetime -----------------------^