Cannot borrow immutable borrowed content as mutable

Question

I'm trying to develop a message routing app. I've read the official Rust docs and some articles and thought that I got how pointers, owning, and borrowing stuff works but realized that I didn't.

use std::collections::HashMap; use std::vec::Vec;  struct Component {     address: &'static str,     available_workers: i32,     lang: i32 }  struct Components {     data: HashMap<i32, Vec<Component>> }  impl Components {     fn new() -> Components {         Components {data: HashMap::new() }     }      fn addOrUpdate(&mut self, component: Component) -> &Components {         if !self.data.contains_key(&component.lang) {              self.data.insert(component.lang, vec![component]);         } else {             let mut q = self.data.get(&component.lang); // this extra line is required because of the error: borrowed value does not live long enough             let mut queue = q.as_mut().unwrap();             queue.remove(0);             queue.push(component);         }         self     }  } 

(Also available on the playground)

Produces the error:

error: cannot borrow immutable borrowed content `**queue` as mutable   --> src/main.rs:26:13    | 26 |             queue.remove(0);    |             ^^^^^ cannot borrow as mutable  error: cannot borrow immutable borrowed content `**queue` as mutable   --> src/main.rs:27:13    | 27 |             queue.push(component);    |             ^^^^^ cannot borrow as mutable 

Could you please explain the error and it would be great if you can give me the right implementation.

Answer 1

Here is an MCVE of your problem:

use std::collections::HashMap;  struct Components {     data: HashMap<u8, Vec<u8>>, }  impl Components {     fn add_or_update(&mut self, component: u8) {         let mut q = self.data.get(&component);         let mut queue = q.as_mut().unwrap();         queue.remove(0);     } } 

Before NLL

error[E0596]: cannot borrow immutable borrowed content `**queue` as mutable   --> src/lib.rs:11:9    | 11 |         queue.remove(0);    |         ^^^^^ cannot borrow as mutable 

After NLL

error[E0596]: cannot borrow `**queue` as mutable, as it is behind a `&` reference   --> src/lib.rs:11:9    | 11 |         queue.remove(0);    |         ^^^^^ cannot borrow as mutable 

Many times, when something seems surprising like this, it's useful to print out the types involved. Let's print out the type of queue:

let mut queue: () = q.as_mut().unwrap(); 

error[E0308]: mismatched types   --> src/lib.rs:10:29    | 10 |         let mut queue: () = q.as_mut().unwrap();    |                             ^^^^^^^^^^^^^^^^^^^ expected (), found mutable reference    |    = note: expected type `()`               found type `&mut &std::vec::Vec<u8>` 

We have a mutable reference to an immutable reference to a Vec<u8>. Because we have an immutable reference to the Vec, we cannot modify it! Changing self.data.get to self.data.get_mut changes the type to &mut &mut collections::vec::Vec<u8> and the code compiles.

---

If you want to implement the concept of "insert or update", you should check into the entry API, which is more efficient and concise.

Beyond that, Rust uses snake_case for method naming, not camelCase.