nomicon/vec-zsts.md

% Handling Zero-Sized Types

It's time. We're going to fight the spectre that is zero-sized types. Safe Rust
*never* needs to care about this, but Vec is very intensive on raw pointers and
raw allocations, which are exactly the *only* two things that care about
zero-sized types. We need to be careful of two things:

* The raw allocator API has undefined behaviour if you pass in 0 for an
  allocation size.
* raw pointer offsets are no-ops for zero-sized types, which will break our
  C-style pointer iterator.

Thankfully we abstracted out pointer-iterators and allocating handling into
RawValIter and RawVec respectively. How mysteriously convenient.




## Allocating Zero-Sized Types

So if the allocator API doesn't support zero-sized allocations, what on earth
do we store as our allocation? Why, `heap::EMPTY` of course! Almost every operation
with a ZST is a no-op since ZSTs have exactly one value, and therefore no state needs
to be considered to store or load them. This actually extends to `ptr::read` and
`ptr::write`: they won't actually look at the pointer at all. As such we *never* need
to change the pointer.

Note however that our previous reliance on running out of memory before overflow is
no longer valid with zero-sized types. We must explicitly guard against capacity
overflow for zero-sized types.

Due to our current architecture, all this means is writing 3 guards, one in each
method of RawVec.

```rust,ignore
impl<T> RawVec<T> {
    fn new() -> Self {
        unsafe {
            // !0 is usize::MAX. This branch should be stripped at compile time.
            let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };

            // heap::EMPTY doubles as "unallocated" and "zero-sized allocation"
            RawVec { ptr: Unique::new(heap::EMPTY as *mut T), cap: cap }
        }
    }

    fn grow(&mut self) {
        unsafe {
            let elem_size = mem::size_of::<T>();

            // since we set the capacity to usize::MAX when elem_size is
            // 0, getting to here necessarily means the Vec is overfull.
            assert!(elem_size != 0, "capacity overflow");

            let align = mem::align_of::<T>();

            let (new_cap, ptr) = if self.cap == 0 {
                let ptr = heap::allocate(elem_size, align);
                (1, ptr)
            } else {
                let new_cap = 2 * self.cap;
                let ptr = heap::reallocate(*self.ptr as *mut _,
                                            self.cap * elem_size,
                                            new_cap * elem_size,
                                            align);
                (new_cap, ptr)
            };

            // If allocate or reallocate fail, we'll get `null` back
            if ptr.is_null() { oom() }

            self.ptr = Unique::new(ptr as *mut _);
            self.cap = new_cap;
        }
    }
}

impl<T> Drop for RawVec<T> {
    fn drop(&mut self) {
        let elem_size = mem::size_of::<T>();

        // don't free zero-sized allocations, as they were never allocated.
        if self.cap != 0 && elem_size != 0 {
            let align = mem::align_of::<T>();

            let num_bytes = elem_size * self.cap;
            unsafe {
                heap::deallocate(*self.ptr as *mut _, num_bytes, align);
            }
        }
    }
}
```

That's it. We support pushing and popping zero-sized types now. Our iterators
(that aren't provided by slice Deref) are still busted, though.




## Iterating Zero-Sized Types

Zero-sized offsets are no-ops. This means that our current design will always
initialize `start` and `end` as the same value, and our iterators will yield
nothing. The current solution to this is to cast the pointers to integers,
increment, and then cast them back:

```rust,ignore
impl<T> RawValIter<T> {
    unsafe fn new(slice: &[T]) -> Self {
        RawValIter {
            start: slice.as_ptr(),
            end: if mem::size_of::<T>() == 0 {
                ((slice.as_ptr() as usize) + slice.len()) as *const _
            } else if slice.len() == 0 {
                slice.as_ptr()
            } else {
                slice.as_ptr().offset(slice.len() as isize)
            }
        }
    }
}
```

Now we have a different bug. Instead of our iterators not running at all, our
iterators now run *forever*. We need to do the same trick in our iterator impls.
Also, our size_hint computation code will divide by 0 for ZSTs. Since we'll
basically be treating the two pointers as if they point to bytes, we'll just
map size 0 to divide by 1.

```rust,ignore
impl<T> Iterator for RawValIter<T> {
    type Item = T;
    fn next(&mut self) -> Option<T> {
        if self.start == self.end {
            None
        } else {
            unsafe {
                let result = ptr::read(self.start);
                self.start = if mem::size_of::<T>() == 0 {
                    (self.start as usize + 1) as *const _
                } else {
                    self.start.offset(1);
                }
                Some(result)
            }
        }
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let elem_size = mem::size_of::<T>();
        let len = (self.end as usize - self.start as usize)
                  / if elem_size == 0 { 1 } else { elem_size };
        (len, Some(len))
    }
}

impl<T> DoubleEndedIterator for RawValIter<T> {
    fn next_back(&mut self) -> Option<T> {
        if self.start == self.end {
            None
        } else {
            unsafe {
                self.end = if mem::size_of::<T>() == 0 {
                    (self.end as usize - 1) as *const _
                } else {
                    self.end.offset(-1);
                }
                Some(ptr::read(self.end))
            }
        }
    }
}
```

And that's it. Iteration works!