| #### Note: this error code is no longer emitted by the compiler. |
| |
| This error occurs when an attempt is made to mutate the target of a mutable |
| reference stored inside an immutable container. |
| |
| For example, this can happen when storing a `&mut` inside an immutable `Box`: |
| |
| ``` |
| let mut x: i64 = 1; |
| let y: Box<_> = Box::new(&mut x); |
| **y = 2; // error, cannot assign to data in an immutable container |
| ``` |
| |
| This error can be fixed by making the container mutable: |
| |
| ``` |
| let mut x: i64 = 1; |
| let mut y: Box<_> = Box::new(&mut x); |
| **y = 2; |
| ``` |
| |
| It can also be fixed by using a type with interior mutability, such as `Cell` |
| or `RefCell`: |
| |
| ``` |
| use std::cell::Cell; |
| |
| let x: i64 = 1; |
| let y: Box<Cell<_>> = Box::new(Cell::new(x)); |
| y.set(2); |
| ``` |
