Tree bypass in Rust vs Borrow Checker

Question

I am trying to implement a tree structure in Rust, intersect it and modify it, and I am having problems verifying the loan. My setup is more or less:

#![feature(slicing_syntax)]

use std::collections::HashMap;

#[deriving(PartialEq, Eq, Hash)]
struct Id {
    id: int,  // let’s pretend it’s that
}

struct Node {
    children: HashMap<Id, Box<Node>>,
    decoration: String,
    // other fields
}

struct Tree {
   root: Box<Node>
}

impl Tree {
    /// Traverse the nodes along the specified path.
    /// Return the node at which traversal stops either because the path is exhausted
    /// or because there are no more nodes matching the path.
    /// Also return any remaining steps in the path that did not have matching nodes.
    fn traverse_path<'p>(&mut self, mut path: &'p [Id]) -> (&mut Box<Node>, &'p [Id]) {
        let mut node = &mut self.root;
        loop {
            match node.children.get_mut(&path[0]) {
                Some(child_node) => {
                    path = path[1..];
                    node = child_node;
                },
                None => {
                    break;
                }
            }
        }
        (node, path)
    }
}

I have mutable links here because I want to be able to mutate the node returned by the method. For example, a method addwould call traverse_pathand then add nodes for the rest of the path that did not have matching nodes.

This leads to the following errors:

s.rs:28:19: 28:32 error: cannot borrow `node.children` as mutable more than once at a time
s.rs:28             match node.children.get_mut(&path[0]) {
                          ^~~~~~~~~~~~~
s.rs:28:19: 28:32 note: previous borrow of `node.children` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `node.children` until the borrow ends
s.rs:28             match node.children.get_mut(&path[0]) {
                          ^~~~~~~~~~~~~
s.rs:39:6: 39:6 note: previous borrow ends here
s.rs:25     fn traverse_path<'p>(&mut self, mut path: &'p [Id]) -> (&mut Box<Node>, &'p [Id]) {
...
s.rs:39     }
            ^
s.rs:31:21: 31:38 error: cannot assign to `node` because it is borrowed
s.rs:31                     node = child_node;
                            ^~~~~~~~~~~~~~~~~
s.rs:28:19: 28:32 note: borrow of `node` occurs here
s.rs:28             match node.children.get_mut(&path[0]) {
                          ^~~~~~~~~~~~~
s.rs:38:10: 38:14 error: cannot borrow `*node` as mutable more than once at a time
s.rs:38         (node, path)
                 ^~~~
s.rs:28:19: 28:32 note: previous borrow of `node.children` occurs here; the mutable borrow prevents subsequent moves, borrows, or modification of `node.children` until the borrow ends
s.rs:28             match node.children.get_mut(&path[0]) {
                          ^~~~~~~~~~~~~
s.rs:39:6: 39:6 note: previous borrow ends here
s.rs:25     fn traverse_path<'p>(&mut self, mut path: &'p [Id]) -> (&mut Box<Node>, &'p [Id]) {
...
s.rs:39     }
            ^
error: aborting due to 3 previous errors

I understand why the note checking tool does not like this code, but I don’t know how to do it.

I also tried performing an alternative implementation using an iterator using the following code:

struct PathIter<'a> {
    path: &'a [Id],
    node: &'a mut Box<Node>
}
impl<'a> Iterator<Box<Node>> for PathIter<'a> {
    fn next(&mut self) -> Option<Box<Node>> {
        let child = self.node.get_child(&self.path[0]);
        if child.is_some() {
            self.path = self.path[1..];
            self.node = child.unwrap();
        }
        child
    }
}

The errors here turned out to be related to life:

src/http_prefix_tree.rs:147:27: 147:53 error: cannot infer an appropriate lifetime for autoref due to conflicting requirements
src/http_prefix_tree.rs:147     let child = self.node.get_child(&self.path[0]);
                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~
src/http_prefix_tree.rs:146:3: 153:4 help: consider using an explicit lifetime parameter as shown: fn next(&'a mut self) -> Option<Box<Node>>
src/http_prefix_tree.rs:146   fn next(&mut self) -> Option<Box<Node>> {
src/http_prefix_tree.rs:147     let child = self.node.get_child(&self.path[0]);
src/http_prefix_tree.rs:148     if child.is_some() {
src/http_prefix_tree.rs:149       self.path = self.path[1..];
src/http_prefix_tree.rs:150       self.node = child.unwrap();
src/http_prefix_tree.rs:151     }

- decoration , . , , Rawlink DList, . , ( ) - . ?

Answer 1

, , . , Rust .

impl Node {
    fn traverse_path<'p>(&mut self, mut path: &'p [Id]) -> (&mut Node, &'p [Id]) { // '
        if self.children.contains_key(&path[0]) {
            self.children[path[0]].traverse_path(path[1..])
        } else {
            (self, path)
        }
    }
}

impl Tree {
    /// Traverse the nodes along the specified path.
    /// Return the node at which traversal stops either because the path is exhausted
    /// or because there are no more nodes matching the path.
    /// Also return any remaining steps in the path that did not have matching nodes.
    fn traverse_path<'p>(&mut self, mut path: &'p [Id]) -> (&mut Node, &'p [Id]) { // '
        self.root.traverse_path(path)
    }
}

, &mut Box<Node> &mut Node; , Box . , Node::traverse_path , contains_key(), . , , , , .

P.S.: root Tree Node, Box<Node>.