Rust borrow checker multiple mutable reference in modifying element in linked list in a loop - rust

Question

I'm trying to build some sort of linked list, each node has a keyword and vectors of the same node. In that for loop, i will traverse the linked list to get the vector, either already exist if keyword is the same, or create a new vector. I find it impossible to *not * violate the double mut reference error since I'll need to do iter_mut(), push() on the same path. Anyone has general idea on how to approach this?
pub struct Knowledge {
pub title: String,
}
pub struct Entry {
pub keyword: String,
pub next: Vec<Box<KnowledgeEntry>>
}
pub enum KnowledgeEntry {
Normal(Knowledge),
Entry(Entry)
}
impl KnowledgeEntry {
pub fn get_entry_mut (&mut self) -> Option<&mut Vec<Box<KnowledgeEntry>>> {
match self {
KnowledgeEntry::Entry(e) => {
Some(&mut e.next)
}
KnowledgeEntry::Normal(k) => {None}
}
}
}
fn main () {
let mut entry = Entry{keyword: "".to_owned(), next: vec![]};
let mut current_container = &mut entry.next;
let new_knowledge = Knowledge{title: "title1".to_owned()};
// new knowledge to be insert in these hierarchy.
let remaining_comp: Vec<_> = vec!["hier1", "hier2", "hier3"];
// each loop will set current_container, either created from new, or found in the vector.
for comp in remaining_comp.iter() {
// true will be replace with body checking with keyword.
let mut _result = current_container.iter_mut().find(|en| {true});
if let Some(mut _res) = _result {
// found the container. Set the current container
if let KnowledgeEntry::Entry(ref mut entry) = &mut **_res {
current_container = &mut entry.next;
} else {
break;
}
} else {
// can't find the resulting container. Create a new vector, and move the container.
let new_container: Vec<Box<KnowledgeEntry>> = vec![];
let mut _temp = KnowledgeEntry::Entry(Entry {
keyword: (*comp).to_string(),
next: new_container
});
current_container.push(Box::new(_temp));
let last_var = current_container.last_mut().unwrap();
// get_entry_mut() to retrieve `_temp` that we have just moved.
current_container = (**last_var).get_entry_mut().unwrap();
}
}
// found the container correspond to "hier1" -> "hier2" -> "hier3", add the entry.
current_container.push(Box::new(KnowledgeEntry::Normal(new_knowledge)));
}
Playground
Produces the following errors:
error[E0499]: cannot borrow `*current_container` as mutable more than once at a time
--> src/main.rs:50:13
|
35 | let mut _result = current_container.iter_mut().find(|en| {true});
| ----------------- first mutable borrow occurs here
...
50 | current_container.push(Box::new(_temp));
| ^^^^^^^^^^^^^^^^^
| |
| second mutable borrow occurs here
| first borrow later used here
error[E0499]: cannot borrow `*current_container` as mutable more than once at a time
--> src/main.rs:51:28
|
35 | let mut _result = current_container.iter_mut().find(|en| {true});
| ----------------- first mutable borrow occurs here
...
51 | let last_var = current_container.last_mut().unwrap();
| ^^^^^^^^^^^^^^^^^
| |
| second mutable borrow occurs here
| first borrow later used here
error[E0499]: cannot borrow `*current_container` as mutable more than once at a time
--> src/main.rs:57:1
|
35 | let mut _result = current_container.iter_mut().find(|en| {true});
| ----------------- first mutable borrow occurs here
...
57 | current_container.push(Box::new(KnowledgeEntry::Normal(new_knowledge)));
| ^^^^^^^^^^^^^^^^^
| |
| second mutable borrow occurs here
| first borrow later used here