unsafe — When and How

// 1. Raw pointer dereference
fn main() {
    let x = 42;
    let p: *const i32 = &x;
    unsafe {
        println!("via raw ptr: {}", *p);
    }

    // 2. Calling an unsafe function (e.g. FFI)
    unsafe {
        let n = libc::abs(-7);                    // requires the `libc` crate
        println!("abs = {n}");
    }

    // 3. Safe abstraction over unsafe — the standard pattern
    let v = vec![10u32, 20, 30, 40];
    let first_two = unsafe { std::slice::from_raw_parts(v.as_ptr(), 2) };
    println!("{first_two:?}");                    // [10, 20]
}

// Don't:
//   - Use unsafe to silence the borrow checker (the compiler is usually right)
//   - Touch raw pointers when a safe wrapper exists
// Do:
//   - Confine `unsafe` blocks to the smallest scope
//   - Document INVARIANTS the caller must uphold for the unsafe API