How to work around self borrowing with map .or_insert_with? Rust (1.11) [duplicate]

Answer 1

The problem is that the closure captures self, whereas it only needs to capture a mutable reference to the last_id field.

Rust allows us to take independent mutable borrows on distinct fields, so we can use that to our advantage and pass a mutable reference to the last_id field to the closure.

use std::collections::HashMap;

struct Foo {
    local_ids: HashMap<i32, i32>,
    last_id: i32,
}

impl Foo {
    fn foo(&mut self, external_id: i32) {
        let last_id = &mut self.last_id;
        let id = self.local_ids
                     .entry(external_id)
                     .or_insert_with(|| { *last_id += 1; *last_id });
    }
}

When we use the expression self.last_id in the closure, the closure captures self directly, but Rust doesn't realize that the borrows are independent, so we need to be more explicit.