Rust 所有权 - 由于“位于共享引用后面”而无法移动

Question

I am learning Rust and practicing on Linked Lists, whereby I want to get to a solution that doesn't involve a .clone() at the end.

Here's the smallest useful MVP code I can come up with ( Rust Playground ):

#![allow(unused)]
fn main() {
    let v: Option<Box<ListNode>> = None;

    println!("{:?}", v);
    let result2 = get_nth_node(v, 3);
    println!("  --> {:?}", result2);
}

#[derive(PartialEq, Eq, Clone, Debug)]
pub struct ListNode {
    pub val: i32,
    pub next: Option<Box<ListNode>>
}

fn get_nth_node(head: Option<Box<ListNode>>, n:usize) -> Option<Box<ListNode>> {
    if head.is_none() {
        return None;
    }

    let mut count = 1;
    let mut ptr: &Option<Box<ListNode>> = &head;
    while count < n {
        count += 1;
        match ptr {
            None                    => return None,
            Some(v) => ptr = &v.next
        }
    }
    return (*ptr);
}

This yields an error on moving and borrowing:

error[E0507]: cannot move out of `*ptr` which is behind a shared reference
  --> src/main.rs:30:12
   |
30 |     return (*ptr);
   |            ^^^^^^
   |            |
   |            move occurs because `*ptr` has type `std::option::Option<std::boxed::Box<ListNode>>`, which does not implement the `Copy` trait
   |            help: consider borrowing the `Option`'s content: `(*ptr).as_ref()`

I understand why changing the function to return (*ptr).clone() works, but it seems superfluous.

My two main questions: 1. What specifically is the shared reference that *ptr is behind? 2. Is there a better way, conceptually, to work with this in Rust?

Answer 1

What specifically is the shared reference that *ptr is behind?

ptr is a shared (immutable) reference:

 let mut ptr: &Option<Box<ListNode>> = &head;

*ptr is behind that reference.

Is there a better way, conceptually, to work with this in Rust?

Well you could just not work with references since you're taking the list by value and are therefore consuming it:

fn get_nth_node(mut head: Option<Box<ListNode>>, n:usize) -> Option<Box<ListNode>> {
    let mut count = 1;
    while count < n {
        count += 1;
        match head {
            None                    => return None,
            Some(v) => head = v.next
        }
    }
    return head;
}

Alternatively, you could work with refs (that would probably be a better idea really):

fn get_nth_node(mut head: Option<&ListNode>, n:usize) -> Option<&ListNode> {
    let mut count = 1;
    while count < n {
        count += 1;
        match head {
            None                    => return None,
            Some(v) => head = v.next.as_ref().map(Box::as_ref)
        }
    }
    head
}