escape_borrow_checker.rs

use std::collections::HashMap;

// escaping the borrow checker

fn test(h:&mut HashMap<String, i64>) -> &mut i64 {
    {
        let matched:Option<&mut i64> = h.get_mut("B");
        if matched.is_some() {
            // assignment to raw pointer side steps the borrow checker
            let v:*mut i64  = matched.unwrap(); 
            return unsafe { &mut *v };
            // this will not compile, as borrow escapes the function
            // let v:&mut i64  = matched.unwrap(); 
            // return v
            
        }
    }
    h.insert("B".to_owned(), -1); 
    return h.get_mut("B").unwrap();

}
fn main(){
    let mut h:HashMap<String, i64> = HashMap::new();
    h.insert("A".to_owned(), 1);
    test(&mut h);
    println!("{:?}", h);
}