[SOLVED] How to get mutable reference to object inside vec inside a struct in rust?

How to get mutable reference to object inside vec inside a struct in rust?

I have a very basic problem in a rust relating to mutability of objects inside a vector. I have a need to get a mutable reference to an object from within a method of the struct as follows:

struct Allocator {
    free_regions: Vec<Region>, // vec of free regions
    used_regions: Vec<Region>, // vec of used regions
}
fn alloc(&self, layout: Layout) -> ! {
    //I want to get this mutable reference in order to avoid copying large amount of data around
    let region: &mut Region = self.free_regions.get_mut(index).expect("Could not get region");
    //This fails with  `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
}

I cannot change the function to use &mut self because this requirement is forced by a trait I'm trying to implement here. What is a proper way to get around this kind of issue? I need to be able to modify the data in those regions inside the Vec in the struct.

Solution

You need to use RefCell, or Mutex if it is shared between threads

struct Allocator {
    free_regions: RefCell<Vec<Region>>, // vec of free regions
    used_regions: RefCell<Vec<Region>>, // vec of used regions
}
fn alloc(&self, layout: Layout) -> ! {
    //I want to get this mutable reference in order to avoid copying large amount of data around
    let region: &mut Region = self.free_regions.borrow_mut().get_mut(index).expect("Could not get region");
    //This fails with  `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
}

You can find more info here https://doc.rust-lang.org/book/ch15-05-interior-mutability.html