cannot borrow `var` as immutable because it is also borrowed as mutable

While trying to compile the following code, im getting an error:

error[E0502]: cannot borrow var as immutable because it is also borrowed as mutable

fn increment(num : &mut i32){
    *num = *num +1;
}

fn main() {
    let mut var : i32 = 123;
    let p_var: &mut i32   = &mut var;

    println!("{}", var);
    increment(p_var);
}

I have no clue what does it mean, can someone explain me exacly why am I getting this error?

>Solution :

fn increment(num: &mut i32){
    *num = *num +1;
}

fn main() {
    let mut var : i32 = 123;
    
    println!("{}", var);
    increment(&mut var);
    println!("{}", var);
}

worked for me.

I, myself, am a Rust beginner, too. So, here is what my view on this is ( I MAY be utterly wrong ):

In this code var never gives up ownership and has no mutable borrows that are still in scope when it borrows immutably. I therefore can borrow it mutably to the increment function. After it has returned that borrow is out of scope. I therefore can borrow it again to the println macro.

When you assigned to p_var that wasn’t the case any more.

This is what the incredible cargo told me:

error[E0502]: cannot borrow `var` as immutable because it is also borrowed as mutable
  --> src/main.rs:9:20
   |
7  |     let p_var : &mut i32 = &mut var;
   |                            -------- mutable borrow occurs here
8  |
9  |     println!("{}", var);
   |                    ^^^ immutable borrow occurs here
10 |     increment(p_var);
   |               ----- mutable borrow later used here
   |

It would be great if some more experienced rustacian could verify (or correct) my reasoning and assessment.