I’m trying to create a function that updates internal data if its id has changed, and then returns a reference to it. However, I’m encountering a borrow checker error and I’m not sure why.

Below is a simplified example of what I’m attempting:

struct SomeData {
    id: i32,
    other_data: (),
}

impl SomeData {
    fn new(id: i32) -> Self {
        Self {
            id,
            other_data: (),
        }
    }
}

struct MyData {
    inner: Option<SomeData>,
}

impl MyData {
    fn update(&mut self, id: i32) -> &SomeData {
        match  self.inner.as_ref() {
            Some(inner) if inner.id == id => {
                return inner
            }
            _ => {}
        }
        // At this point ref to inner should be dropped

        
        self.inner = Some(SomeData::new(id));
        
        // For some reason "`self.inner` is assigned to here but it was already borrowed"
        self.inner.as_ref().unwrap()
    }
}

error[E0506]: cannot assign to `self.inner` because it is borrowed
  --> src/lib.rs:30:9
   |
20 |     fn update(&mut self, id: i32) -> &SomeData {
   |               - let's call the lifetime of this reference `'1`
21 |         match  self.inner.as_ref() {
   |                ---------- `self.inner` is borrowed here
22 |             Some(inner) if inner.id == id => {
23 |                 return inner
   |                        ----- returning this value requires that `self.inner` is borrowed for `'1`
...
30 |         self.inner = Some(SomeData::new(id));
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `self.inner` is assigned to here but it was already borrowed

For more information about this error, try `rustc --explain E0506`.

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