id: general-01 original_id: P.UNS.01 level: P impact: CRITICAL

Do Not Abuse Unsafe to Escape Compiler Safety Checks

Summary

Unsafe Rust should not be used as an escape hatch from the borrow checker or other compiler safety mechanisms.

Rationale

The borrow checker exists to prevent memory safety bugs. Using unsafe to bypass it defeats Rust's safety guarantees and introduces potential undefined behavior.

Bad Example

// DON'T: Using unsafe to bypass borrow checker
fn bad_alias() {
    let mut data = vec![1, 2, 3];
    let ptr = data.as_mut_ptr();

    // Unsafe used to create aliasing mutable references
    unsafe {
        let ref1 = &mut *ptr;
        let ref2 = &mut *ptr;  // UB: Two mutable references!
        *ref1 = 10;
        *ref2 = 20;
    }
}

Good Example

// DO: Work with the borrow checker, not against it
fn good_sequential() {
    let mut data = vec![1, 2, 3];
    data[0] = 10;
    data[0] = 20;  // Sequential mutations are fine
}

// DO: Use interior mutability when needed
use std::cell::RefCell;

fn good_interior_mut() {
    let data = RefCell::new(vec![1, 2, 3]);
    data.borrow_mut()[0] = 10;
}

Legitimate Uses of Unsafe

FFI: Calling C functions or implementing C-compatible interfaces
Low-level abstractions: Implementing collections, synchronization primitives
Performance: Only after profiling shows measurable improvement, and with careful safety analysis