Unsafe Rust Functions

You can mark your own functions as unsafe if they require particular preconditions to avoid undefined behaviour.

/// Swaps the values pointed to by the given pointers.
///
/// # Safety
///
/// The pointers must be valid, properly aligned, and not otherwise accessed for
/// the duration of the function call.
unsafe fn swap(a: *mut u8, b: *mut u8) {
    // SAFETY: Our caller promised that the pointers are valid, properly aligned
    // and have no other access.
    unsafe {
        let temp = *a;
        *a = *b;
        *b = temp;
    }
}

fn main() {
    let mut a = 42;
    let mut b = 66;

    // SAFETY: The pointers must be valid, aligned and unique because they came
    // from references.
    unsafe {
        swap(&mut a, &mut b);
    }

    println!("a = {}, b = {}", a, b);
}