How can I borrow from a HashMap to read and write at the same time?

Question

I have a function f that accepts two references, one mut and one not mut. I have values for f inside a HashMap

Answer 1

Something appears to have changed since the question was asked. In Rust 1.38.0 (possibly earlier), the following compiles and works:

use std::collections::HashMap;

fn f(a: &i32, b: &mut i32) {}

fn main() {
    let mut map = HashMap::new();

    map.insert("1", 1);
    map.insert("2", 2);

    let a: &i32 = map.get("1").unwrap();
    println!("a: {}", a);

    let b: &mut i32 = map.get_mut("2").unwrap();
    println!("b: {}", b);
    *b = 5;

    println!("Results: {:?}", map)
}

playground

There is no need for RefCell, nor is there even a need for the inner scope.