Auto merge of #145 - lcnr:drain, r=mbrubeck

update drain

changes drain method to take a RangeBounds argument. Requires Rust 1.28.
This simply copies the implementation of `Vec` and should hopefully not cause any kind of UB.

closes #142

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/rust-smallvec/145)
<!-- Reviewable:end -->

lib.rs

@@ -60,13 +60,13 @@ use std::hash::{Hash, Hasher};
 use std::hint::unreachable_unchecked;
 #[cfg(feature = "std")]
 use std::io;
-use std::iter::{repeat, FromIterator, IntoIterator};
+use std::iter::{repeat, FromIterator, FusedIterator, IntoIterator};
 #[cfg(feature = "serde")]
 use std::marker::PhantomData;
 use std::mem;
 use std::mem::MaybeUninit;
-use std::ops;
-use std::ptr;
+use std::ops::{self, Bound, RangeBounds};
+use std::ptr::{self, NonNull};
 use std::slice::{self, SliceIndex};
 
 /// Creates a [`SmallVec`] containing the arguments.
@@ -182,15 +182,30 @@ unsafe fn deallocate<T>(ptr: *mut T, capacity: usize) {
 /// Returned from [`SmallVec::drain`][1].
 ///
 /// [1]: struct.SmallVec.html#method.drain
-pub struct Drain<'a, T: 'a> {
-    iter: slice::IterMut<'a, T>,
+pub struct Drain<'a, T: 'a + Array> {
+    tail_start: usize,
+    tail_len: usize,
+    iter: slice::Iter<'a, T::Item>,
+    vec: NonNull<SmallVec<T>>,
 }
 
-impl<'a, T: 'a> Iterator for Drain<'a, T> {
-    type Item = T;
+impl<'a, T: 'a + Array> fmt::Debug for Drain<'a, T>
+where
+    T::Item: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_tuple("Drain").field(&self.iter.as_slice()).finish()
+    }
+}
+
+unsafe impl<'a, T: Sync + Array> Sync for Drain<'a, T> {}
+unsafe impl<'a, T: Send + Array> Send for Drain<'a, T> {}
+
+impl<'a, T: 'a + Array> Iterator for Drain<'a, T> {
+    type Item = T::Item;
 
     #[inline]
-    fn next(&mut self) -> Option<T> {
+    fn next(&mut self) -> Option<T::Item> {
         self.iter
             .next()
             .map(|reference| unsafe { ptr::read(reference) })
@@ -202,21 +217,43 @@ impl<'a, T: 'a> Iterator for Drain<'a, T> {
     }
 }
 
-impl<'a, T: 'a> DoubleEndedIterator for Drain<'a, T> {
+impl<'a, T: 'a + Array> DoubleEndedIterator for Drain<'a, T> {
     #[inline]
-    fn next_back(&mut self) -> Option<T> {
+    fn next_back(&mut self) -> Option<T::Item> {
         self.iter
             .next_back()
             .map(|reference| unsafe { ptr::read(reference) })
     }
 }
 
-impl<'a, T> ExactSizeIterator for Drain<'a, T> {}
+impl<'a, T: Array> ExactSizeIterator for Drain<'a, T> {
+    #[inline]
+    fn len(&self) -> usize {
+        self.iter.len()
+    }
+}
 
-impl<'a, T: 'a> Drop for Drain<'a, T> {
+impl<'a, T: Array> FusedIterator for Drain<'a, T> {}
+
+impl<'a, T: 'a + Array> Drop for Drain<'a, T> {
     fn drop(&mut self) {
-        // Destroy the remaining elements.
-        for _ in self.by_ref() {}
+        self.for_each(drop);
+
+        if self.tail_len > 0 {
+            unsafe {
+                let source_vec = self.vec.as_mut();
+
+                // memmove back untouched tail, update to new length
+                let start = source_vec.len();
+                let tail = self.tail_start;
+                if tail != start {
+                    let src = source_vec.as_ptr().add(tail);
+                    let dst = source_vec.as_mut_ptr().add(start);
+                    ptr::copy(src, dst, self.tail_len);
+                }
+                source_vec.set_len(start + self.tail_len);
+            }
+        }
     }
 }
 
@@ -548,18 +585,50 @@ impl<A: Array> SmallVec<A> {
         self.capacity > A::size()
     }
 
-    /// Empty the vector and return an iterator over its former contents.
-    pub fn drain(&mut self) -> Drain<A::Item> {
+    /// Creates a draining iterator that removes the specified range in the vector
+    /// and yields the removed items.
+    ///
+    /// Note 1: The element range is removed even if the iterator is only
+    /// partially consumed or not consumed at all.
+    ///
+    /// Note 2: It is unspecified how many elements are removed from the vector
+    /// if the `Drain` value is leaked.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the starting point is greater than the end point or if
+    /// the end point is greater than the length of the vector.
+    pub fn drain<R>(&mut self, range: R) -> Drain<A>
+    where
+        R: RangeBounds<usize>,
+    {
+        use Bound::*;
+
+        let len = self.len();
+        let start = match range.start_bound() {
+            Included(&n) => n,
+            Excluded(&n) => n + 1,
+            Unbounded => 0,
+        };
+        let end = match range.end_bound() {
+            Included(&n) => n + 1,
+            Excluded(&n) => n,
+            Unbounded => len,
+        };
+
+        assert!(start <= end);
+        assert!(end <= len);
+
         unsafe {
-            let ptr = self.as_mut_ptr();
+            self.set_len(start);
 
-            let current_len = self.len();
-            self.set_len(0);
-
-            let slice = slice::from_raw_parts_mut(ptr, current_len);
+            let range_slice = slice::from_raw_parts_mut(self.as_mut_ptr().add(start), end - start);
 
             Drain {
-                iter: slice.iter_mut(),
+                tail_start: end,
+                tail_len: len - end,
+                iter: range_slice.iter(),
+                vec: NonNull::from(self),
             }
         }
     }
@@ -1697,26 +1766,36 @@ mod tests {
     fn drain() {
         let mut v: SmallVec<[u8; 2]> = SmallVec::new();
         v.push(3);
-        assert_eq!(v.drain().collect::<Vec<_>>(), &[3]);
+        assert_eq!(v.drain(..).collect::<Vec<_>>(), &[3]);
 
         // spilling the vec
         v.push(3);
         v.push(4);
         v.push(5);
-        assert_eq!(v.drain().collect::<Vec<_>>(), &[3, 4, 5]);
+        let old_capacity = v.capacity();
+        assert_eq!(v.drain(1..).collect::<Vec<_>>(), &[4, 5]);
+        // drain should not change the capacity
+        assert_eq!(v.capacity(), old_capacity);
     }
 
     #[test]
     fn drain_rev() {
         let mut v: SmallVec<[u8; 2]> = SmallVec::new();
         v.push(3);
-        assert_eq!(v.drain().rev().collect::<Vec<_>>(), &[3]);
+        assert_eq!(v.drain(..).rev().collect::<Vec<_>>(), &[3]);
 
         // spilling the vec
         v.push(3);
         v.push(4);
         v.push(5);
-        assert_eq!(v.drain().rev().collect::<Vec<_>>(), &[5, 4, 3]);
+        assert_eq!(v.drain(..).rev().collect::<Vec<_>>(), &[5, 4, 3]);
+    }
+
+    #[test]
+    fn drain_forget() {
+        let mut v: SmallVec<[u8; 1]> = smallvec![0, 1, 2, 3, 4, 5, 6, 7];
+        std::mem::forget(v.drain(2..5));
+        assert_eq!(v.len(), 2);
     }
 
     #[test]
@@ -2157,7 +2236,8 @@ mod tests {
     fn test_exact_size_iterator() {
         let mut vec = SmallVec::<[u32; 2]>::from(&[1, 2, 3][..]);
         assert_eq!(vec.clone().into_iter().len(), 3);
-        assert_eq!(vec.drain().len(), 3);
+        assert_eq!(vec.drain(..2).len(), 2);
+        assert_eq!(vec.into_iter().len(), 1);
     }
 
     #[test]