immutable.rs

use std::ops::DerefMut;
use std::ops::Deref;

struct List<T> {
    car: T,
    cdr: Option<Box<List<T>>>,
}


fn main() {
    let mut head = Box::new(List {car: 0i8, cdr: None});
    for i in range(1, 42i8) {
        head = Box::new(List {car: i, cdr: Some(head)})
    }

    {
        let mut now = head.deref();
        loop {
            match now.cdr {
                None => break,
                Some(ref x) => {
                    println!("{}", x.car);
                    now = x.deref();
                }
            }
        }
    }
}

// succues compile and run.

mutable.rs

use std::ops::DerefMut;
use std::ops::Deref;

struct List<T> {
    car: T,
    cdr: Option<Box<List<T>>>,
}


fn main() {
    let mut head = Box::new(List {car: 0i8, cdr: None});
    for i in range(1, 42i8) {
        head = Box::new(List {car: i, cdr: Some(head)})
    }

    {
        let mut now = head.deref_mut();
        loop {
            match now.cdr {
                None => break,
                Some(ref mut x) => {
                    println!("{}", x.car);
                    now = x.deref_mut();
                }
            }
        }
    }
}

// test.rs:21:22: 21:31 error: cannot borrow `now.cdr.0` as mutable more than once at a time
// test.rs:21                 Some(ref mut x) => {
//                                 ^~~~~~~~~
// test.rs:21:22: 21:31 note: previous borrow of `now.cdr.0` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `now.cdr.0` until the borrow ends
// test.rs:21                 Some(ref mut x) => {
//                                 ^~~~~~~~~
// test.rs:27:6: 27:6 note: previous borrow ends here
// test.rs:16     {
// ...
// test.rs:27     }
//                ^
// test.rs:23:21: 23:40 error: cannot assign to `now` because it is borrowed
// test.rs:23                     now = x.deref_mut();
//                                ^~~~~~~~~~~~~~~~~~~
// test.rs:21:22: 21:31 note: borrow of `now` occurs here
// test.rs:21                 Some(ref mut x) => {
//                                 ^~~~~~~~~
// error: aborting due to 2 previous errors