# error[E0594]: cannot assign to data in an `Arc`

- **ID:** `rust/e0594-cannot-assign-to-data-in-an-arc`
- **Domain:** rust
- **Category:** type_error
- **Error Code:** `E0594`
- **Verification:** ai_generated
- **Fix Rate:** 90%

## Root Cause

Attempting to mutate data behind an `Arc<T>` without interior mutability (e.g., `Mutex`, `RwLock`, or `Cell`).

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| 1.65.0 | active | — | — |
| 1.68.0 | active | — | — |
| 1.72.0 | active | — | — |

## Workarounds

1. **Use `Arc<Mutex<T>>` and lock before mutation: `*arc.lock().unwrap() = new_value;`** (92% success)
   ```
   Use `Arc<Mutex<T>>` and lock before mutation: `*arc.lock().unwrap() = new_value;`
   ```
2. **Use `Arc<AtomicU32>` for simple numeric types and `store` method.** (95% success)
   ```
   Use `Arc<AtomicU32>` for simple numeric types and `store` method.
   ```

## Dead Ends

- **Using `unsafe` to cast `Arc<T>` to a mutable reference via raw pointer** — Undefined behavior due to data races; violates Rust's safety guarantees. (95% fail)
- **Wrapping the entire `Arc` in a `RefCell` without thread safety** — `RefCell` is not `Sync`, so the `Arc<RefCell<T>>` cannot be shared across threads. (70% fail)