Iteratively building a singly linked list in Rust

Question

(Rust noob here) I would like to create a singly linked list in Rust, by holding a reference to the tail and writing into it. Here is the algorithm in C++:

#include <memory>
#include <string>

struct CharList {
    char c;
    std::unique_ptr<CharList> next;
    explicit CharList(char c) : c(c) {}
};

CharList make_list(const std::string &s) {
    CharList res{'a'};
    auto *tail = &res.next;
    for (char c : s) {
        *tail = std::make_unique<CharList>(c);
        tail = &(*tail)->next;
    }
    return res;
}

This is my attempt in Rust (Rust playground link)

struct CharList {
    c: char,
    next: Option<Box<CharList>>,
}

fn make(s:&str) -> CharList {
    let mut result = CharList{
        c: 'a',
        next: None
    };
    {
        let mut tail = &mut result.next;
        for c in s.chars() {
            let cl = CharList {
                c: c,
                next: None
            };
            mem::replace(tail, Some(Box::new(cl)))
            let mut next = &mut tail.as_mut().unwrap().next;
            tail = next
        }
    }
    result
}

The resulting error is:

error[E0499]: cannot borrow `*tail` as mutable more than once at a time
  --> src/main.rs:21:26
   |
21 |             mem::replace(tail, Some(Box::new(cl)));
   |                          ^^^^ second mutable borrow occurs here
22 |             let next = &mut tail.as_mut().unwrap().next;
   |                             ---- first mutable borrow occurs here
...
25 |     }
   |     - first borrow ends here

I find it confusing that the first borrow is reported to occur after the second borrow, but I suppose that is because they occur in different iterations of the loop.

What is the right way to implement this algorithm in Rust?

Due to borrowing rules, linked lists are actually surprisingly hard to do in rust. In fact, there is a rather comprehensive [introduction to rust](https://cglab.ca/~abeinges/blah/too-many-lists/book/) built solely on them. You might find it interesting. — Michail, Nov 18 '18 at 08:54
TL;DR the duplicate: `let next = &mut {tail}.as_mut().unwrap().next;` — Shepmaster, Nov 18 '18 at 13:30
How [I'd write the entire function](https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&gist=aeae36d2af68b325454e368a358a3c53) — Shepmaster, Nov 18 '18 at 14:50

Iteratively building a singly linked list in Rust

0 Answers0