Cannot borrow data in a dereference of `RefMut<'_, &Atom>` as mutable

Hi there!

I have a type like this:

pub struct AtomWithHierarchyMut<'a> {
    /// The Chain containing this Atom
    pub chain: &'a Chain,
    /// The Residue containing this Atom
    pub residue: &'a Residue,
    /// The Conformer containing this Atom
    pub conformer: &'a Conformer,
    /// This Atom
    pub atom: RefCell<&'a Atom>,

and I want to mutate the atom field in place. The Atom struct in turn looks like this:

pub struct Atom {
    /// The occupancy of the Atom
    occupancy: f64,


And what I tried then is:

let atom_hi_mut = AtomWithHierarchyMut::new(chain, residue, conformer, RefCell::new(atom));
let mut atom_mut = atom_hi_mut.atom.borrow_mut();
// This method mutates the `occupancy` field.

But I get this error message:
cannot borrow data in a dereference of RefMut<', &Atom>as mutable traitDerefMutis required to modify through a dereference, but it is not implemented forRefMut<', &Atom>``

I'm still trying to wrap my head around how RefCell works and how I can use it to mutate stuff and it was my impression that I can use it for a case like this but I don't quite understand the error message. Do I need to implement DerefMut for Atom or does this hint at something else?

The RefCell will let you go from &AtomWithHierarchyMut to &mut &Atom, but you need an &mut Atom to modify the Atom. Unfortunately, flattening the double reference &mut &Atom results in an &Atom as one of the two references is immutable.

Note: When reading this, pay close attention to the order and number of ampersands.

Maybe you wanted &'a RefCell<Atom>?

So RefCell will not let me mutate anything inside if it's behind a reference, yes? I think that's the key of my misunderstanding here.

Indeed. A RefCell lets you go from &RefCell<T> to &mut T, but in your case the reference and cell is reversed.

Ok, thanks for clarifying! I'll have to rethink my approach.

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