Invalid digit found in string ParseIntError

I am solving rustlings exercises and have issue with errors2, below is my code:

pub fn total_cost(item_quantity: &str) -> Result<i32, ParseIntError> {
let processing_fee = 1;
let cost_per_item = 5;
let qty = item_quantity.parse::<i32>();

let quantity_result = match qty {
    Ok(qty) => (qty * cost_per_item + processing_fee),
    Err(err) => panic!("invalid digit found in string {:?}", err),
};

    Ok(quantity_result)
}

Ok part works but I have no idea how to implement properly Err part. Here is a test for the code:

#[test]
fn item_quantity_is_an_invalid_number() {
    assert_eq!(
        total_cost("beep boop").unwrap_err().to_string(),
        "invalid digit found in string"
    );
}

Currently I have following message:

panicked at ‘invalid digit found in string ParseIntError { kind:
InvalidDigit }’

>Solution :

The test is doing an unwrap_err(), meaning the test wants to see an Err returned from your function. You already have a ParseIntError, so return it instead of panicking; change this line

    Err(err) => panic!("invalid digit found in string {:?}", err),

to

    Err(err) => return Err(err),

That should let the test pass.

A further refinement of the code is to use the ? operator, which is specifically for returning errors when they occur. Then you would replace the entire match with:

let quantity_result = qty? * cost_per_item + processing_fee;

or, more idiomatically, put the ? on the line where the error occurred:

let qty = item_quantity.parse::<i32>()?;

so that qty itself is an i32 and not a Result<i32, ParseIntError>.