% 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:

```rust
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 `<=`:

```rust
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`:

```rust
// 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:

```rust
    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!