test.rs

use std::collections::hash_map::HashMap;

struct MyStruct {
    a: u64,
    b: u64,
}

struct Test {
    counter: u64,
    map: HashMap<u64, MyStruct>,
}

impl Test {
    pub fn new() -> Test {
        Test {
            counter: 0,
            map: HashMap::new(),
        }
    }

    pub fn add_if_not_yet_exists(&mut self, key: u64, a: u64, b: u64) {
        self.map.entry(key).or_insert(self.add(a, b));
    }

    pub fn add(&mut self, a: u64, b: u64) -> MyStruct {
        self.counter += 1;
        MyStruct {a: a, b: b}
    }
}

fn main() {
    let mut test = Test::new();
    test.add_if_not_yet_exists(1, 2, 3);
}

/*
error[E0499]: cannot borrow `*self` as mutable more than once at a time
  --> src/bin/test.rs:22:39
   |
22 |         self.map.entry(key).or_insert(self.add(a, b));
   |         --------                      ^^^^          - first borrow ends here
   |         |                             |
   |         |                             second mutable borrow occurs here
   |         first mutable borrow occurs here

error: aborting due to previous error
*/