rust/working-with-unsafe.md

% Working with Unsafe

Rust generally only gives us the tools to talk about Unsafe in a scoped and binary manner. Unfortunately, reality is significantly more complicated than that. For instance, consider the following toy function:

pub fn index(idx: usize, arr: &[u8]) -> Option<u8> {
    if idx < arr.len() {
        unsafe {
            Some(*arr.get_unchecked(idx))
        }
    } else {
        None
    }
}

Clearly, this function is safe. We check that the index is in bounds, and if it is, index into the array in an unchecked manner. But even in such a trivial function, the scope of the unsafe block is questionable. Consider changing the < to a <=:

pub fn index(idx: usize, arr: &[u8]) -> Option<u8> {
    if idx <= arr.len() {
        unsafe {
            Some(*arr.get_unchecked(idx))
        }
    } else {
        None
    }
}

This program is now unsound, and yet we only modified safe code. This is the fundamental problem of safety: it's non-local. The soundness of our unsafe operations necessarily depends on the state established by "safe" operations. Although safety is modular (we still don't need to worry about about unrelated safety issues like uninitialized memory), it quickly contaminates the surrounding code.

Trickier than that is when we get into actual statefulness. Consider a simple implementation of Vec:

// Note this definition is insufficient. See the section on lifetimes.
pub struct Vec<T> {
    ptr: *mut T,
    len: usize,
    cap: usize,
}

// Note this implementation does not correctly handle zero-sized types.
// We currently live in a nice imaginary world of only positive fixed-size
// types.
impl<T> Vec<T> {
    pub fn push(&mut self, elem: T) {
        if self.len == self.cap {
            // not important for this example
            self.reallocate();
        }
        unsafe {
            ptr::write(self.ptr.offset(len as isize), elem);
            self.len += 1;
        }
    }
}

This code is simple enough to reasonably audit and verify. Now consider adding the following method:

    fn make_room(&mut self) {
        // grow the capacity
        self.cap += 1;
    }

This code is safe, but it is also completely unsound. Changing the capacity violates the invariants of Vec (that cap reflects the allocated space in the Vec). This is not something the rest of Vec can guard against. It has to trust the capacity field because there's no way to verify it.

unsafe does more than pollute a whole function: it pollutes a whole module. Generally, the only bullet-proof way to limit the scope of unsafe code is at the module boundary with privacy.

However this works perfectly. The existence of make_room is not a problem for the soundness of Vec because we didn't mark it as public. Only the module that defines this function can call it. Also, make_room directly accesses the private fields of Vec, so it can only be written in the same module as Vec.

It is therefore possible for us to write a completely safe abstraction that relies on complex invariants. This is critical to the relationship between Safe Rust and Unsafe Rust. We have already seen that Unsafe code must trust some Safe code, but can't trust arbitrary Safe code. However if Unsafe couldn't prevent client Safe code from messing with its state in arbitrary ways, safety would be a lost cause.

Safety lives!