mut_ref.rs

// The problem arises as I have the following type representing a cache
struct Cache {
  foo_data: Vec<Foo>,
  bar_data: Vec<Bar>,
  // ...
}

// So, there’s a function called get_by_id() that basically takes an index into a one of the Vec<T> above and returns an Option<&T>.
// Though, because of resource dependency, that function takes a mutable Cache so that, in a trait implementation for each type,
// you can ask for a load of another resource in the cache:
impl Cache {
  // ...
  
  fn get_by_id(&mut self, id: u32) -> Result<…, …>
}

// The problem is that when implementing that function for a wrapper type over Cache, I try to have two &mut:

struct Zoo { … }; // it has a Cache

impl Zoo {
  fn get_by_id(&mut self, id: u32) -> Result<…, …> {
    // ...
    
    // imagine we need to resynchronize the resource here
    if let Ok(data) = self.foo_data.get_mut(id) {
      // ok, the resource is already in the cache; though here, we’ve locked &mut self line 26
      
      // ...
      
      reload(&data.something, self); // error: cannot borrow because already borrowed!
    }
  }
}