HashMap borrow issue when trying to implement find or insert

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There are a handful of methods on HashMap to achieve these sorts of complex cases. Most notably, for your case, HashMap::entry and Entry::or_insert_with:

pub fn get_type<'a>(&'a mut self, k: i32) -> &'a i32 {
    self.types.entry(k).or_insert_with(|| {
        let value = self.next;
        self.next += 1;
        value
    })
}

In your case, however, there’s the borrow of self inside, so that won’t do.

We thus shift the borrow of self.next outside of the closure so the compiler can reason about it as disjoint from self.types. Problem solved with only one lookup, as it should be.

pub fn get_type<'a>(&'a mut self, k: i32) -> &'a i32 {
    let next = &mut self.next;

    self.types.entry(k).or_insert_with(|| {
        let value = *next;
        *next += 1;
        value
    })
}

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