rust/vec-drain.md

% Drain

Let's move on to Drain. Drain is largely the same as IntoIter, except that
instead of consuming the Vec, it borrows the Vec and leaves its allocation
free. For now we'll only implement the "basic" full-range version.

```rust,ignore
use std::marker::PhantomData;

struct Drain<'a, T: 'a> {
    vec: PhantomData<&'a mut Vec<T>>
    start: *const T,
    end: *const T,
}

impl<'a, T> Iterator for Drain<'a, T> {
    type Item = T;
    fn next(&mut self) -> Option<T> {
        if self.start == self.end {
            None
```

-- wait, this is seeming familiar. Let's do some more compression. Both
IntoIter and Drain have the exact same structure, let's just factor it out.

```rust
struct RawValIter<T> {
    start: *const T,
    end: *const T,
}

impl<T> RawValIter<T> {
    // unsafe to construct because it has no associated lifetimes.
    // This is necessary to store a RawValIter in the same struct as
    // its actual allocation. OK since it's a private implementation
    // detail.
    unsafe fn new(slice: &[T]) -> Self {
        RawValIter {
            start: slice.as_ptr(),
            end: if slice.len() == 0 {
                slice.as_ptr()
            } else {
                slice.as_ptr().offset(slice.len() as isize)
            }
        }
    }
}

// Iterator and DoubleEndedIterator impls identical to IntoIter.
```

And IntoIter becomes the following:

```
pub struct IntoIter<T> {
    _buf: RawVec<T>, // we don't actually care about this. Just need it to live.
    iter: RawValIter<T>,
}

impl<T> Iterator for IntoIter<T> {
    type Item = T;
    fn next(&mut self) -> Option<T> { self.iter.next() }
    fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
}

impl<T> DoubleEndedIterator for IntoIter<T> {
    fn next_back(&mut self) -> Option<T> { self.iter.next_back() }
}

impl<T> Drop for IntoIter<T> {
    fn drop(&mut self) {
        for _ in &mut self.iter {}
    }
}

impl<T> Vec<T> {
    pub fn into_iter(self) -> IntoIter<T> {
        unsafe {
            let iter = RawValIter::new(&self);
            let buf = ptr::read(&self.buf);
            mem::forget(self);

            IntoIter {
                iter: iter,
                _buf: buf,
            }
        }
    }
}
```

Note that I've left a few quirks in this design to make upgrading Drain to work
with arbitrary subranges a bit easier. In particular we *could* have RawValIter
drain itself on drop, but that won't work right for a more complex Drain.
We also take a slice to simplify Drain initialization.

Alright, now Drain is really easy:

```rust
use std::marker::PhantomData;

pub struct Drain<'a, T: 'a> {
    vec: PhantomData<&'a mut Vec<T>>,
    iter: RawValIter<T>,
}

impl<'a, T> Iterator for Drain<'a, T> {
    type Item = T;
    fn next(&mut self) -> Option<T> { self.iter.next_back() }
    fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
}

impl<'a, T> DoubleEndedIterator for Drain<'a, T> {
    fn next_back(&mut self) -> Option<T> { self.iter.next_back() }
}

impl<'a, T> Drop for Drain<'a, T> {
    fn drop(&mut self) {
        for _ in &mut self.iter {}
    }
}

impl<T> Vec<T> {
    pub fn drain(&mut self) -> Drain<T> {
        // this is a mem::forget safety thing. If Drain is forgotten, we just
        // leak the whole Vec's contents. Also we need to do this *eventually*
        // anyway, so why not do it now?
        self.len = 0;

        unsafe {
            Drain {
                iter: RawValIter::new(&self),
                vec: PhantomData,
            }
        }
    }
}
```




# 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
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::min_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::min_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:

```
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.

```
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!