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# Error code E0389

#### Note: this error code is no longer emitted by the compiler.

An attempt was made to mutate data using a non-mutable reference. This commonly occurs when attempting to assign to a non-mutable reference of a mutable reference (`&(&mut T)`).

Erroneous code example:

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let mut fancy = FancyNum{ num: 5 };
        let fancy_ref = &(&mut fancy);
        fancy_ref.num = 6; // error: cannot assign to data in a `&` reference
        println!("{}", fancy_ref.num);
    }

Here, `&mut fancy` is mutable, but `&(&mut fancy)` is not. Creating an immutable reference to a value borrows it immutably. There can be multiple references of type `&(&mut T)` that point to the same value, so they must be immutable to prevent multiple mutable references to the same value.

To fix this, either remove the outer reference:

    struct FancyNum {
        num: u8,
    }

    fn main() {
        let mut fancy = FancyNum{ num: 5 };

        let fancy_ref = &mut fancy;
        // `fancy_ref` is now &mut FancyNum, rather than &(&mut FancyNum)

        fancy_ref.num = 6; // No error!

        println!("{}", fancy_ref.num);
    }

Or make the outer reference mutable:

    struct FancyNum {
        num: u8
    }

    fn main() {
        let mut fancy = FancyNum{ num: 5 };

        let fancy_ref = &mut (&mut fancy);
        // `fancy_ref` is now &mut(&mut FancyNum), rather than &(&mut FancyNum)

        fancy_ref.num = 6; // No error!

        println!("{}", fancy_ref.num);
    }