unsafe-checker

| Use Case | Example |

|----------|---------|

| FFI | Calling C functions |

| Low-level abstractions | Implementing Vec, Arc |

| Performance | Measured bottleneck with safe alternative too slow |

NOT valid: Escaping borrow checker without understanding why.

```rust

// SAFETY:

unsafe { ... }

/// # Safety

///

pub unsafe fn dangerous() { ... }

```

| Operation | Safety Requirements |

|-----------|---------------------|

| *ptr deref | Valid, aligned, initialized |

| &*ptr | + No aliasing violations |

| transmute | Same size, valid bit pattern |

| extern "C" | Correct signature, ABI |

| static mut | Synchronization guaranteed |

| impl Send/Sync | Actually thread-safe |

| Error | Fix |

|-------|-----|

| Null pointer deref | Check for null before deref |

| Use after free | Ensure lifetime validity |

| Data race | Add proper synchronization |

| Alignment violation | Use #[repr(C)], check alignment |

| Invalid bit pattern | Use MaybeUninit |

| Missing SAFETY comment | Add // SAFETY: |

| Deprecated | Use Instead |

|------------|-------------|

| mem::uninitialized() | MaybeUninit |

| mem::zeroed() for refs | MaybeUninit |

| Raw pointer arithmetic | NonNull, ptr::add |

| CString::new().unwrap().as_ptr() | Store CString first |

| static mut | AtomicT or Mutex |

| Manual extern | bindgen |

| Direction | Crate |

|-----------|-------|

| C → Rust | bindgen |

| Rust → C | cbindgen |

| Python | PyO3 |

| Node.js | napi-rs |

Claude knows unsafe Rust. Focus on SAFETY comments and soundness.