I'm trying to implement a get_or_insert() style method for a binary search tree. I can't seem to find a trick to getting this to work. The following fails.
I've posted a playground example which duplicates the issue in a similar manner.
struct Foo {
data: Vec<i32>,
}
impl Foo {
fn new() -> Self {
Foo { data: vec![42; 5] }
}
fn get_mut(&mut self, index: usize) -> Option<&mut i32> {
if index < self.data.len() {
Some(&mut self.data[index])
} else {
None
}
}
fn get_or_insert(&mut self, index: usize, value: i32) -> &mut i32 { todo!() }
}
fn main() {
let mut foo = Foo::new();
let mut val = foo.get_or_insert(100, 88);
*val = 42;
}
pub fn get_or_insert(&mut self, key: &K, value: V) -> &mut V {
match self.get_mut(key) {
Some(value) => value,
None => {
self.insert(key.clone(), value);
self.get_mut(key).unwrap()
}
}
}
error[E0499]: cannot borrow `self.data` as mutable more than once at a time
--> src/main.rs:35:9
|
29 | fn get_or_insert(&mut self, index: usize, value: i32) -> &mut i32 {
| - let's call the lifetime of this reference `'1`
30 | {
31 | if let Some(value) = self.get_mut(index) {
| ---- first mutable borrow occurs here
32 | return value;
| ----- returning this value requires that `*self` is borrowed for `'1`
...
35 | self.data.extend_from_slice(vec![value; index].as_slice());
| ^^^^^^^^^ second mutable borrow occurs here
error[E0499]: cannot borrow `self.data` as mutable more than once at a time
--> src/main.rs:36:14
|
29 | fn get_or_insert(&mut self, index: usize, value: i32) -> &mut i32 {
| - let's call the lifetime of this reference `'1`
30 | {
31 | if let Some(value) = self.get_mut(index) {
| ---- first mutable borrow occurs here
32 | return value;
| ----- returning this value requires that `*self` is borrowed for `'1`
...
36 | &mut self.data[index]
| ^^^^^^^^^ second mutable borrow occurs here
Scoping the first attempt at acquiring the value also fails.
pub fn get_or_insert(&mut self, key: &K, value: V) -> &mut V {
{
let ret = self.get_mut(key);
if ret.is_some() {
return ret.unwrap();
}
}
self.insert(key.clone(), value);
self.get_mut(key).unwrap()
}
I've tried several other ways, but can't find something that works. I'm guessing this is impossible at this point. Is there some trick to getting a feature like this to work?
