I am trying to lazy-initialize a simple tree on first access of a node.
My tree struct:
struct Tree {
true_branch: Option<Box<Tree>>,
false_branch: Option<Box<Tree>>,
}
To do the lazy initialization on first access I would like to return a Result<&mut Tree, &mut Tree> which is an Ok(requested_node) if the node is already initialized or an Err(last_initialized_on_the_path_to_requested_node) so that the lazy initialization can continue from that node.
Getting a node, if it initialized works fine with get_mut but I cannot get the Result version, get_last_initialized_mut, to compile.
impl Tree {
fn get_mut(&mut self, mut directions: impl Iterator<Item = bool>) -> Option<&mut Self> {
let mut current = self;
loop {
match directions.next() {
None => break,
Some(true) => {
current = current.true_branch.as_deref_mut()?;
}
Some(false) => {
current = current.false_branch.as_deref_mut()?;
}
}
}
Some(current)
}
/// This does not compile
fn get_last_initialized_mut(&mut self, mut directions: impl Iterator<Item = bool>) -> Result<&mut Self, &mut Self> {
let mut current = self;
loop {
match directions.next() {
None => break,
Some(true) => {
let next = current.true_branch.as_deref_mut();
if next.is_none() {
drop(next);
return Err(current);
} else {
current = next.unwrap();
}
}
Some(false) => {
let next = current.false_branch.as_deref_mut();
if next.is_none() {
drop(next);
return Err(current);
} else {
current = next.unwrap();
}
}
}
}
Ok(current)
}
}
The error:
error[E0499]: cannot borrow `*current` as mutable more than once at a time
--> src/lib.rs:36:36
|
27 | fn get_last_initialized_mut(&mut self, mut directions: impl Iterator<Item = bool>) -> Result<&mut Self, &mut Self> {
| - let's call the lifetime of this reference `'1`
...
33 | let next = current.true_branch.as_deref_mut();
| ---------------------------------- first mutable borrow occurs here
...
36 | return Err(current);
| ^^^^^^^ second mutable borrow occurs here
...
52 | Ok(current)
| ----------- returning this value requires that `current.true_branch` is borrowed for `'1`
What I do not get is that I drop next on line 35 and then it says current is still borrowed on line 36, by the borrow by next on line 33.
I have tried breaking with an enum and returning outside of the loop based on the break type, dropping variables. Neither of them worked.
