Rust error E0386: #### Note: this error code is no longer emitted by the compiler

The compiler team's explanation

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);

From rustc --explain E0386 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.