Multiple borrows error when fetching and setting a value in Struct

Question

Task

A simple data-structure that gets and sets a value of a field in a struct.

Issue

The following get_or_set function errors with

cannot borrow *self as mutable because it is also borrowed as immutable

Code

struct MyValue<A> {
    value: Option<A>,
}

impl<A> MyValue<A> {
    fn get(&self) -> &Option<A> {
        &self.value
    }

    fn set<F: FnOnce() -> A>(&mut self, setter: F) -> &Option<A> {
        self.value = Some(setter());
        self.get()
    }

    //this function is causing borrow error
    fn get_or_set<F: FnOnce() -> A>(&mut self, setter: F) -> &Option<A> {
        match self.get() {
            None => self.set(setter),
            some @ Some(_) => some
        }
    }
}

Why does the error occur?

Following the error the issue is that I am call self.set a second time before the first self.get goes out of scope and multiple borrows are not allowed.

Fix:

Use if & else instead. Here I'm calling self.get twice which is not nice.

fn get_or_set<F: FnOnce() -> A>(&mut self, setter: F) -> &Option<A> {
    if self.get().is_some() {
        self.get()
    } else {
        self.set(setter)
    }
}

Question

How do I get the borrow check to allow the match statement to pass as well? My attempt at setting lifetimes has not succeeded.

fn get_or_set<'a, F: FnOnce() -> A + 'a>(&'a mut self, setter: F) -> &'a Option<A>

Answer 1

You can write your function like this:

fn get_or_set1<F: FnOnce() -> A>(&mut self, setter: F) -> &Option<A> {
    match *self.get() {
        None => self.set(setter),
        _ => &self.value,
    }
}