Final Code -

# The Final Code

    #![feature(ptr_internals)]
    #![feature(allocator_api)]
    #![feature(alloc_layout_extra)]

    use std::ptr::{Unique, NonNull, self};
    use std::mem;
    use std::ops::{Deref, DerefMut};
    use std::marker::PhantomData;
    use std::alloc::{Alloc, GlobalAlloc, Layout, Global, handle_alloc_error};

    struct RawVec<T> {
        ptr: Unique<T>,
        cap: usize,
    }

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

            // Unique::empty() doubles as "unallocated" and "zero-sized allocation"
            RawVec { ptr: Unique::empty(), 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 (new_cap, ptr) = if self.cap == 0 {
                    let ptr = Global.alloc(Layout::array::<T>(1).unwrap());
                    (1, ptr)
                } else {
                    let new_cap = 2 * self.cap;
                    let c: NonNull<T> = self.ptr.into();
                    let ptr = Global.realloc(c.cast(),
                                             Layout::array::<T>(self.cap).unwrap(),
                                             Layout::array::<T>(new_cap).unwrap().size());
                    (new_cap, ptr)
                };

                // If allocate or reallocate fail, oom
                if ptr.is_err() {
                    handle_alloc_error(Layout::from_size_align_unchecked(
                        new_cap * elem_size,
                        mem::align_of::<T>(),
                    ))
                }
                let ptr = ptr.unwrap();

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

    impl<T> Drop for RawVec<T> {
        fn drop(&mut self) {
            let elem_size = mem::size_of::<T>();
            if self.cap != 0 && elem_size != 0 {
                unsafe {
                    let c: NonNull<T> = self.ptr.into();
                    Global.dealloc(c.cast(),
                                   Layout::array::<T>(self.cap).unwrap());
                }
            }
        }
    }

    pub struct Vec<T> {
        buf: RawVec<T>,
        len: usize,
    }

    impl<T> Vec<T> {
        fn ptr(&self) -> *mut T { self.buf.ptr.as_ptr() }

        fn cap(&self) -> usize { self.buf.cap }

        pub fn new() -> Self {
            Vec { buf: RawVec::new(), len: 0 }
        }
        pub fn push(&mut self, elem: T) {
            if self.len == self.cap() { self.buf.grow(); }

            unsafe {
                ptr::write(self.ptr().offset(self.len as isize), elem);
            }

            // Can't fail, we'll OOM first.
            self.len += 1;
        }

        pub fn pop(&mut self) -> Option<T> {
            if self.len == 0 {
                None
            } else {
                self.len -= 1;
                unsafe {
                    Some(ptr::read(self.ptr().offset(self.len as isize)))
                }
            }
        }

        pub fn insert(&mut self, index: usize, elem: T) {
            assert!(index <= self.len, "index out of bounds");
            if self.cap() == self.len { self.buf.grow(); }

            unsafe {
                if index < self.len {
                    ptr::copy(self.ptr().offset(index as isize),
                              self.ptr().offset(index as isize + 1),
                              self.len - index);
                }
                ptr::write(self.ptr().offset(index as isize), elem);
                self.len += 1;
            }
        }

        pub fn remove(&mut self, index: usize) -> T {
            assert!(index < self.len, "index out of bounds");
            unsafe {
                self.len -= 1;
                let result = ptr::read(self.ptr().offset(index as isize));
                ptr::copy(self.ptr().offset(index as isize + 1),
                          self.ptr().offset(index as isize),
                          self.len - index);
                result
            }
        }

        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,
                }
            }
        }

        pub fn drain(&mut self) -> Drain<T> {
            unsafe {
                let iter = RawValIter::new(&self);

                // 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;

                Drain {
                    iter: iter,
                    vec: PhantomData,
                }
            }
        }
    }

    impl<T> Drop for Vec<T> {
        fn drop(&mut self) {
            while let Some(_) = self.pop() {}
            // allocation is handled by RawVec
        }
    }

    impl<T> Deref for Vec<T> {
        type Target = [T];
        fn deref(&self) -> &[T] {
            unsafe {
                ::std::slice::from_raw_parts(self.ptr(), self.len)
            }
        }
    }

    impl<T> DerefMut for Vec<T> {
        fn deref_mut(&mut self) -> &mut [T] {
            unsafe {
                ::std::slice::from_raw_parts_mut(self.ptr(), self.len)
            }
        }
    }

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

    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)
                }
            }
        }
    }

    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))
                }
            }
        }
    }

    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 {}
        }
    }

    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() }
        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) {
            // pre-drain the iter
            for _ in &mut self.iter {}
        }
    }

    fn main() {
        tests::create_push_pop();
        tests::iter_test();
        tests::test_drain();
        tests::test_zst();
        println!("All tests finished OK");
    }

    mod tests {
        use super::*;
        pub fn create_push_pop() {
            let mut v = Vec::new();
            v.push(1);
            assert_eq!(1, v.len());
            assert_eq!(1, v[0]);
            for i in v.iter_mut() {
                *i += 1;
            }
            v.insert(0, 5);
            let x = v.pop();
            assert_eq!(Some(2), x);
            assert_eq!(1, v.len());
            v.push(10);
            let x = v.remove(0);
            assert_eq!(5, x);
            assert_eq!(1, v.len());
        }

        pub fn iter_test() {
            let mut v = Vec::new();
            for i in 0..10 {
                v.push(Box::new(i))
            }
            let mut iter = v.into_iter();
            let first = iter.next().unwrap();
            let last = iter.next_back().unwrap();
            drop(iter);
            assert_eq!(0, *first);
            assert_eq!(9, *last);
        }

        pub fn test_drain() {
            let mut v = Vec::new();
            for i in 0..10 {
                v.push(Box::new(i))
            }
            {
                let mut drain = v.drain();
                let first = drain.next().unwrap();
                let last = drain.next_back().unwrap();
                assert_eq!(0, *first);
                assert_eq!(9, *last);
            }
            assert_eq!(0, v.len());
            v.push(Box::new(1));
            assert_eq!(1, *v.pop().unwrap());
        }

        pub fn test_zst() {
            let mut v = Vec::new();
            for _i in 0..10 {
                v.push(())
            }

            let mut count = 0;

            for _ in v.into_iter() {
                count += 1
            }

            assert_eq!(10, count);
        }
    }