Equivalent?

Error on non-recursive

rustc: i686-pc-mingw32/stage2/test/collectionstest-i686-pc-mingw32.exe
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:864:14: 864:23 error: cannot borrow `cur#0` as mutable more than once at a time
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:864         Some(ref mut x) => {
                                                                                     ^~~~~~~~~
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:864:14: 864:23 note: previous borrow of `cur#0` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `cur#0` until the borrow ends
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:864         Some(ref mut x) => {
                                                                                     ^~~~~~~~~
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:874:2: 874:2 note: previous borrow ends here
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:860                              -> Option<&'r mut V> {
...
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:874 }
                                                                        ^
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:866:22: 866:39 error: cannot assign to `cur` because it is borrowed
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:866             Less => {cur = &mut x.left}
                                                                                             ^~~~~~~~~~~~~~~~~
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:864:14: 864:23 note: borrow of `cur` occurs here
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:864         Some(ref mut x) => {
                                                                                     ^~~~~~~~~
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:867:25: 867:43 error: cannot assign to `cur` because it is borrowed
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:867             Greater => {cur = &mut x.right},
                                                                                                ^~~~~~~~~~~~~~~~~~
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:864:14: 864:23 note: borrow of `cur` occurs here
C:\Users\Alexis\Documents\GitHub\rust\src\libcollections\treemap.rs:864         Some(ref mut x) => {
                                                                                     ^~~~~~~~~
error: aborting due to 3 previous errors

non-recursive

fn find_mut<'r, K: Ord, V>(node: &'r mut Option<Box<TreeNode<K, V>>>,
                            key: &K)
                             -> Option<&'r mut V> {
    let mut cur = node;
    loop {
      match *cur {
        Some(ref mut x) => {
          match key.cmp(&x.key) {
            Less => {cur = &mut x.left}
            Greater => {cur = &mut x.right},
            Equal => return Some(&mut x.value),
          }
        }
        None => return None
      }
    }
}

recursive

fn find_mut<'r, K: Ord, V>(node: &'r mut Option<Box<TreeNode<K, V>>>,
                                key: &K)
                             -> Option<&'r mut V> {
    match *node {
      Some(ref mut x) => {
        match key.cmp(&x.key) {
          Less => find_mut(&mut x.left, key),
          Greater => find_mut(&mut x.right, key),
          Equal => Some(&mut x.value),
        }
      }
      None => None
    }
}