Understanding Rust Error E0386
Rust Error E0386 Overview
Rust Error E0386 occurs when an attempt is made to mutate the target of a mutable reference stored inside an immutable container. In other words, you are trying to change data within a mutable reference, but the container holding that reference is immutable, preventing any modifications.
Example of Rust Error E0386
Consider the following code example, which generates Rust Error E0386:
```
#![allow(unused)]
fn main() {
let mut x: i64 = 1;
let y: Box<_> = Box::new(&mut x);
**y = 2; // error, cannot assign to data in an immutable container
}
```Fixing Rust Error E0386
This error can be fixed through two possible solutions:
1. Make the container mutable
Simply change the container to a mutable type, allowing modifications to its contents. In the given example, this can be achieved by adding the `mut` keyword to the variable declaration of `y`:
```
#![allow(unused)]
fn main() {
let mut x: i64 = 1;
let mut y: Box<_> = Box::new(&mut x);
**y = 2;
}
```2. Use a type with interior mutability
Interior mutability types, such as `Cell` or `RefCell`, allow for mutations within an immutable container. Here's how to apply this solution to the example code:
```
#![allow(unused)]
fn main() {
use std::cell::Cell;
let x: i64 = 1;
let y: Box<Cell<_>> = Box::new(Cell::new(x));
y.set(2);
}
```