# Error code E0387

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

This error occurs when an attempt is made to mutate or mutably reference data that a closure has captured immutably.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    // Accepts a function or a closure that captures its environment immutably.
    // Closures passed to foo will not be able to mutate their closed-over state.
    fn foo<F: Fn()>(f: F) { }

    // Attempts to mutate closed-over data. Error message reads:
    // `cannot assign to data in a captured outer variable...`
    fn mutable() {
        let mut x = 0u32;
        foo(|| x = 2);
    }

    // Attempts to take a mutable reference to closed-over data. Error message
    // reads: `cannot borrow data mutably in a captured outer variable...`
    fn mut_addr() {
        let mut x = 0u32;
        foo(|| { let y = &mut x; });
    }
    }

The problem here is that foo is defined as accepting a parameter of type `Fn`. Closures passed into foo will thus be inferred to be of type `Fn`, meaning that they capture their context immutably.

If the definition of `foo` is under your control, the simplest solution is to capture the data mutably. This can be done by defining `foo` to take FnMut rather than Fn:

    #![allow(unused)]
    fn main() {
    fn foo<F: FnMut()>(f: F) { }
    }

Alternatively, we can consider using the `Cell` and `RefCell` types to achieve interior mutability through a shared reference. Our example's `mutable` function could be redefined as below:

    #![allow(unused)]
    fn main() {
    use std::cell::Cell;

    fn foo<F: Fn()>(f: F) { }

    fn mutable() {
        let x = Cell::new(0u32);
        foo(|| x.set(2));
    }
    }

You can read more in the API documentation for Cell.