Understanding Rust Error E0492

Rust Error E0492: A Borrow of a Constant Containing Interior Mutability

The error E0492 occurs when a borrowed reference is attempted to a constant containing interior mutability. Constants are immutable by nature, but interior mutability allows values to be mutated through shared pointers. This abnormal behavior causes the E0492 error.

Problematic code scenario

```rust
use std::sync::atomic::AtomicUsize;

const A: AtomicUsize = AtomicUsize::new(0);
const B: &'static AtomicUsize = &A;
// error: cannot borrow a constant which may contain interior mutability,
//        create a static instead
```
This code example attempts to borrow a constant containing interior mutability, leading to Rust error E0492.

Solving the Error

To fix this error, use `static` instead of `const` for variables containing interior mutability. This allows for safe mutation of the value in memory.

Corrected Code Example

```rust
use std::sync::atomic::AtomicUsize;

static A: AtomicUsize = AtomicUsize::new(0);
static B: &'static AtomicUsize = &A; // ok!
```
In this example, we replaced `const` with `static`, resolving the error E0492.

Working with Cell Types

The E0492 error can also arise when using cell types, as they perform operations that are not thread-safe. Cell types do not implement the `Sync` trait, so they cannot be used directly as statics. However, this can be circumvented by using an unsafe wrapper.

Unsafe Wrapper Example

```rust
use std::cell::Cell;

struct NotThreadSafe<T> {
    value: Cell<T>,
}

unsafe impl<T> Sync for NotThreadSafe<T> {}

static A: NotThreadSafe<usize> = NotThreadSafe { value : Cell::new(1) };
static B: &'static NotThreadSafe<usize> = &A; // ok!
```
This code example uses an unsafe wrapper to make a cell type compatible with statics, although synchronization of access must be managed manually.