Merge "Upgrade rust/crates/smallvec to 1.4.2" am: 5237030d92 am: 2b62ce0785 am: c0ccb926b3 am: f575d2366e am: c35cd24066

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/smallvec/+/1397507

Change-Id: I06b663d4ec2324bc9de4efe7391f629e498dd7c8

3 files changed, 2876 insertions, 0 deletions

diff --git a/src/lib.rs b/src/lib.rs
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+++ b/src/lib.rs
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+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Small vectors in various sizes. These store a certain number of elements inline, and fall back
+//! to the heap for larger allocations.  This can be a useful optimization for improving cache
+//! locality and reducing allocator traffic for workloads that fit within the inline buffer.
+//!
+//! ## `no_std` support
+//!
+//! By default, `smallvec` does not depend on `std`.  However, the optional
+//! `write` feature implements the `std::io::Write` trait for vectors of `u8`.
+//! When this feature is enabled, `smallvec` depends on `std`.
+//!
+//! ## Optional features
+//!
+//! ### `serde`
+//!
+//! When this optional dependency is enabled, `SmallVec` implements the `serde::Serialize` and
+//! `serde::Deserialize` traits.
+//!
+//! ### `write`
+//!
+//! When this feature is enabled, `SmallVec<[u8; _]>` implements the `std::io::Write` trait.
+//! This feature is not compatible with `#![no_std]` programs.
+//!
+//! ### `union`
+//!
+//! **This feature is unstable and requires a nightly build of the Rust toolchain.**
+//!
+//! When the `union` feature is enabled `smallvec` will track its state (inline or spilled)
+//! without the use of an enum tag, reducing the size of the `smallvec` by one machine word.
+//! This means that there is potentially no space overhead compared to `Vec`.
+//! Note that `smallvec` can still be larger than `Vec` if the inline buffer is larger than two
+//! machine words.
+//!
+//! To use this feature add `features = ["union"]` in the `smallvec` section of Cargo.toml.
+//! Note that this feature requires a nightly compiler (for now).
+//!
+//! Tracking issue: [rust-lang/rust#55149](https://github.com/rust-lang/rust/issues/55149)
+//!
+//! ### `const_generics`
+//!
+//! **This feature is unstable and requires a nightly build of the Rust toolchain.**
+//!
+//! When this feature is enabled, `SmallVec` works with any arrays of any size, not just a fixed
+//! list of sizes.
+//!
+//! Tracking issue: [rust-lang/rust#44580](https://github.com/rust-lang/rust/issues/44580)
+//!
+//! ### `specialization`
+//!
+//! **This feature is unstable and requires a nightly build of the Rust toolchain.**
+//!
+//! When this feature is enabled, `SmallVec::from(slice)` has improved performance for slices
+//! of `Copy` types.  (Without this feature, you can use `SmallVec::from_slice` to get optimal
+//! performance for `Copy` types.)
+//!
+//! Tracking issue: [rust-lang/rust#31844](https://github.com/rust-lang/rust/issues/31844)
+//!
+//! ### `may_dangle`
+//!
+//! **This feature is unstable and requires a nightly build of the Rust toolchain.**
+//!
+//! This feature makes the Rust compiler less strict about use of vectors that contain borrowed
+//! references. For details, see the
+//! [Rustonomicon](https://doc.rust-lang.org/1.42.0/nomicon/dropck.html#an-escape-hatch).
+//!
+//! Tracking issue: [rust-lang/rust#34761](https://github.com/rust-lang/rust/issues/34761)
+
+#![no_std]
+#![cfg_attr(feature = "union", feature(untagged_unions))]
+#![cfg_attr(feature = "specialization", allow(incomplete_features))]
+#![cfg_attr(feature = "specialization", feature(specialization))]
+#![cfg_attr(feature = "may_dangle", feature(dropck_eyepatch))]
+#![cfg_attr(feature = "const_generics", feature(min_const_generics))]
+#![deny(missing_docs)]
+
+#[doc(hidden)]
+pub extern crate alloc;
+
+#[cfg(any(test, feature = "write"))]
+extern crate std;
+
+#[cfg(test)]
+mod tests;
+
+use alloc::alloc::{Layout, LayoutErr};
+use alloc::boxed::Box;
+use alloc::{vec, vec::Vec};
+use core::borrow::{Borrow, BorrowMut};
+use core::cmp;
+use core::fmt;
+use core::hash::{Hash, Hasher};
+use core::hint::unreachable_unchecked;
+use core::iter::{repeat, FromIterator, FusedIterator, IntoIterator};
+use core::mem;
+use core::mem::MaybeUninit;
+use core::ops::{self, Range, RangeBounds};
+use core::ptr::{self, NonNull};
+use core::slice::{self, SliceIndex};
+
+#[cfg(feature = "serde")]
+use serde::{
+    de::{Deserialize, Deserializer, SeqAccess, Visitor},
+    ser::{Serialize, SerializeSeq, Serializer},
+};
+
+#[cfg(feature = "serde")]
+use core::marker::PhantomData;
+
+#[cfg(feature = "write")]
+use std::io;
+
+/// Creates a [`SmallVec`] containing the arguments.
+///
+/// `smallvec!` allows `SmallVec`s to be defined with the same syntax as array expressions.
+/// There are two forms of this macro:
+///
+/// - Create a [`SmallVec`] containing a given list of elements:
+///
+/// ```
+/// # #[macro_use] extern crate smallvec;
+/// # use smallvec::SmallVec;
+/// # fn main() {
+/// let v: SmallVec<[_; 128]> = smallvec![1, 2, 3];
+/// assert_eq!(v[0], 1);
+/// assert_eq!(v[1], 2);
+/// assert_eq!(v[2], 3);
+/// # }
+/// ```
+///
+/// - Create a [`SmallVec`] from a given element and size:
+///
+/// ```
+/// # #[macro_use] extern crate smallvec;
+/// # use smallvec::SmallVec;
+/// # fn main() {
+/// let v: SmallVec<[_; 0x8000]> = smallvec![1; 3];
+/// assert_eq!(v, SmallVec::from_buf([1, 1, 1]));
+/// # }
+/// ```
+///
+/// Note that unlike array expressions this syntax supports all elements
+/// which implement [`Clone`] and the number of elements doesn't have to be
+/// a constant.
+///
+/// This will use `clone` to duplicate an expression, so one should be careful
+/// using this with types having a nonstandard `Clone` implementation. For
+/// example, `smallvec![Rc::new(1); 5]` will create a vector of five references
+/// to the same boxed integer value, not five references pointing to independently
+/// boxed integers.
+
+#[macro_export]
+macro_rules! smallvec {
+    // count helper: transform any expression into 1
+    (@one $x:expr) => (1usize);
+    ($elem:expr; $n:expr) => ({
+        $crate::SmallVec::from_elem($elem, $n)
+    });
+    ($($x:expr),*$(,)*) => ({
+        let count = 0usize $(+ $crate::smallvec!(@one $x))*;
+        #[allow(unused_mut)]
+        let mut vec = $crate::SmallVec::new();
+        if count <= vec.inline_size() {
+            $(vec.push($x);)*
+            vec
+        } else {
+            $crate::SmallVec::from_vec($crate::alloc::vec![$($x,)*])
+        }
+    });
+}
+
+/// `panic!()` in debug builds, optimization hint in release.
+#[cfg(not(feature = "union"))]
+macro_rules! debug_unreachable {
+    () => {
+        debug_unreachable!("entered unreachable code")
+    };
+    ($e:expr) => {
+        if cfg!(not(debug_assertions)) {
+            unreachable_unchecked();
+        } else {
+            panic!($e);
+        }
+    };
+}
+
+/// Trait to be implemented by a collection that can be extended from a slice
+///
+/// ## Example
+///
+/// ```rust
+/// use smallvec::{ExtendFromSlice, SmallVec};
+///
+/// fn initialize<V: ExtendFromSlice<u8>>(v: &mut V) {
+///     v.extend_from_slice(b"Test!");
+/// }
+///
+/// let mut vec = Vec::new();
+/// initialize(&mut vec);
+/// assert_eq!(&vec, b"Test!");
+///
+/// let mut small_vec = SmallVec::<[u8; 8]>::new();
+/// initialize(&mut small_vec);
+/// assert_eq!(&small_vec as &[_], b"Test!");
+/// ```
+#[doc(hidden)]
+#[deprecated]
+pub trait ExtendFromSlice<T> {
+    /// Extends a collection from a slice of its element type
+    fn extend_from_slice(&mut self, other: &[T]);
+}
+
+#[allow(deprecated)]
+impl<T: Clone> ExtendFromSlice<T> for Vec<T> {
+    fn extend_from_slice(&mut self, other: &[T]) {
+        Vec::extend_from_slice(self, other)
+    }
+}
+
+/// Error type for APIs with fallible heap allocation
+#[derive(Debug)]
+pub enum CollectionAllocErr {
+    /// Overflow `usize::MAX` or other error during size computation
+    CapacityOverflow,
+    /// The allocator return an error
+    AllocErr {
+        /// The layout that was passed to the allocator
+        layout: Layout,
+    },
+}
+
+impl From<LayoutErr> for CollectionAllocErr {
+    fn from(_: LayoutErr) -> Self {
+        CollectionAllocErr::CapacityOverflow
+    }
+}
+
+fn infallible<T>(result: Result<T, CollectionAllocErr>) -> T {
+    match result {
+        Ok(x) => x,
+        Err(CollectionAllocErr::CapacityOverflow) => panic!("capacity overflow"),
+        Err(CollectionAllocErr::AllocErr { layout }) => alloc::alloc::handle_alloc_error(layout),
+    }
+}
+
+/// FIXME: use `Layout::array` when we require a Rust version where it’s stable
+/// https://github.com/rust-lang/rust/issues/55724
+fn layout_array<T>(n: usize) -> Result<Layout, CollectionAllocErr> {
+    let size = mem::size_of::<T>()
+        .checked_mul(n)
+        .ok_or(CollectionAllocErr::CapacityOverflow)?;
+    let align = mem::align_of::<T>();
+    Layout::from_size_align(size, align).map_err(|_| CollectionAllocErr::CapacityOverflow)
+}
+
+unsafe fn deallocate<T>(ptr: *mut T, capacity: usize) {
+    // This unwrap should succeed since the same did when allocating.
+    let layout = layout_array::<T>(capacity).unwrap();
+    alloc::alloc::dealloc(ptr as *mut u8, layout)
+}
+
+/// An iterator that removes the items from a `SmallVec` and yields them by value.
+///
+/// Returned from [`SmallVec::drain`][1].
+///
+/// [1]: struct.SmallVec.html#method.drain
+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 + 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::Item> {
+        self.iter
+            .next()
+            .map(|reference| unsafe { ptr::read(reference) })
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
+}
+
+impl<'a, T: 'a + Array> DoubleEndedIterator for Drain<'a, T> {
+    #[inline]
+    fn next_back(&mut self) -> Option<T::Item> {
+        self.iter
+            .next_back()
+            .map(|reference| unsafe { ptr::read(reference) })
+    }
+}
+
+impl<'a, T: Array> ExactSizeIterator for Drain<'a, T> {
+    #[inline]
+    fn len(&self) -> usize {
+        self.iter.len()
+    }
+}
+
+impl<'a, T: Array> FusedIterator for Drain<'a, T> {}
+
+impl<'a, T: 'a + Array> Drop for Drain<'a, T> {
+    fn drop(&mut self) {
+        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);
+            }
+        }
+    }
+}
+
+#[cfg(feature = "union")]
+union SmallVecData<A: Array> {
+    inline: MaybeUninit<A>,
+    heap: (*mut A::Item, usize),
+}
+
+#[cfg(feature = "union")]
+impl<A: Array> SmallVecData<A> {
+    #[inline]
+    unsafe fn inline(&self) -> *const A::Item {
+        self.inline.as_ptr() as *const A::Item
+    }
+    #[inline]
+    unsafe fn inline_mut(&mut self) -> *mut A::Item {
+        self.inline.as_mut_ptr() as *mut A::Item
+    }
+    #[inline]
+    fn from_inline(inline: MaybeUninit<A>) -> SmallVecData<A> {
+        SmallVecData { inline }
+    }
+    #[inline]
+    unsafe fn into_inline(self) -> MaybeUninit<A> {
+        self.inline
+    }
+    #[inline]
+    unsafe fn heap(&self) -> (*mut A::Item, usize) {
+        self.heap
+    }
+    #[inline]
+    unsafe fn heap_mut(&mut self) -> &mut (*mut A::Item, usize) {
+        &mut self.heap
+    }
+    #[inline]
+    fn from_heap(ptr: *mut A::Item, len: usize) -> SmallVecData<A> {
+        SmallVecData { heap: (ptr, len) }
+    }
+}
+
+#[cfg(not(feature = "union"))]
+enum SmallVecData<A: Array> {
+    Inline(MaybeUninit<A>),
+    Heap((*mut A::Item, usize)),
+}
+
+#[cfg(not(feature = "union"))]
+impl<A: Array> SmallVecData<A> {
+    #[inline]
+    unsafe fn inline(&self) -> *const A::Item {
+        match self {
+            SmallVecData::Inline(a) => a.as_ptr() as *const A::Item,
+            _ => debug_unreachable!(),
+        }
+    }
+    #[inline]
+    unsafe fn inline_mut(&mut self) -> *mut A::Item {
+        match self {
+            SmallVecData::Inline(a) => a.as_mut_ptr() as *mut A::Item,
+            _ => debug_unreachable!(),
+        }
+    }
+    #[inline]
+    fn from_inline(inline: MaybeUninit<A>) -> SmallVecData<A> {
+        SmallVecData::Inline(inline)
+    }
+    #[inline]
+    unsafe fn into_inline(self) -> MaybeUninit<A> {
+        match self {
+            SmallVecData::Inline(a) => a,
+            _ => debug_unreachable!(),
+        }
+    }
+    #[inline]
+    unsafe fn heap(&self) -> (*mut A::Item, usize) {
+        match self {
+            SmallVecData::Heap(data) => *data,
+            _ => debug_unreachable!(),
+        }
+    }
+    #[inline]
+    unsafe fn heap_mut(&mut self) -> &mut (*mut A::Item, usize) {
+        match self {
+            SmallVecData::Heap(data) => data,
+            _ => debug_unreachable!(),
+        }
+    }
+    #[inline]
+    fn from_heap(ptr: *mut A::Item, len: usize) -> SmallVecData<A> {
+        SmallVecData::Heap((ptr, len))
+    }
+}
+
+unsafe impl<A: Array + Send> Send for SmallVecData<A> {}
+unsafe impl<A: Array + Sync> Sync for SmallVecData<A> {}
+
+/// A `Vec`-like container that can store a small number of elements inline.
+///
+/// `SmallVec` acts like a vector, but can store a limited amount of data inline within the
+/// `SmallVec` struct rather than in a separate allocation.  If the data exceeds this limit, the
+/// `SmallVec` will "spill" its data onto the heap, allocating a new buffer to hold it.
+///
+/// The amount of data that a `SmallVec` can store inline depends on its backing store. The backing
+/// store can be any type that implements the `Array` trait; usually it is a small fixed-sized
+/// array.  For example a `SmallVec<[u64; 8]>` can hold up to eight 64-bit integers inline.
+///
+/// ## Example
+///
+/// ```rust
+/// use smallvec::SmallVec;
+/// let mut v = SmallVec::<[u8; 4]>::new(); // initialize an empty vector
+///
+/// // The vector can hold up to 4 items without spilling onto the heap.
+/// v.extend(0..4);
+/// assert_eq!(v.len(), 4);
+/// assert!(!v.spilled());
+///
+/// // Pushing another element will force the buffer to spill:
+/// v.push(4);
+/// assert_eq!(v.len(), 5);
+/// assert!(v.spilled());
+/// ```
+pub struct SmallVec<A: Array> {
+    // The capacity field is used to determine which of the storage variants is active:
+    // If capacity <= Self::inline_capacity() then the inline variant is used and capacity holds the current length of the vector (number of elements actually in use).
+    // If capacity > Self::inline_capacity() then the heap variant is used and capacity holds the size of the memory allocation.
+    capacity: usize,
+    data: SmallVecData<A>,
+}
+
+impl<A: Array> SmallVec<A> {
+    /// Construct an empty vector
+    #[inline]
+    pub fn new() -> SmallVec<A> {
+        // Try to detect invalid custom implementations of `Array`. Hopefuly,
+        // this check should be optimized away entirely for valid ones.
+        assert!(
+            mem::size_of::<A>() == A::size() * mem::size_of::<A::Item>()
+                && mem::align_of::<A>() >= mem::align_of::<A::Item>()
+        );
+        SmallVec {
+            capacity: 0,
+            data: SmallVecData::from_inline(MaybeUninit::uninit()),
+        }
+    }
+
+    /// Construct an empty vector with enough capacity pre-allocated to store at least `n`
+    /// elements.
+    ///
+    /// Will create a heap allocation only if `n` is larger than the inline capacity.
+    ///
+    /// ```
+    /// # use smallvec::SmallVec;
+    ///
+    /// let v: SmallVec<[u8; 3]> = SmallVec::with_capacity(100);
+    ///
+    /// assert!(v.is_empty());
+    /// assert!(v.capacity() >= 100);
+    /// ```
+    #[inline]
+    pub fn with_capacity(n: usize) -> Self {
+        let mut v = SmallVec::new();
+        v.reserve_exact(n);
+        v
+    }
+
+    /// Construct a new `SmallVec` from a `Vec<A::Item>`.
+    ///
+    /// Elements will be copied to the inline buffer if vec.capacity() <= Self::inline_capacity().
+    ///
+    /// ```rust
+    /// use smallvec::SmallVec;
+    ///
+    /// let vec = vec![1, 2, 3, 4, 5];
+    /// let small_vec: SmallVec<[_; 3]> = SmallVec::from_vec(vec);
+    ///
+    /// assert_eq!(&*small_vec, &[1, 2, 3, 4, 5]);
+    /// ```
+    #[inline]
+    pub fn from_vec(mut vec: Vec<A::Item>) -> SmallVec<A> {
+        if vec.capacity() <= Self::inline_capacity() {
+            unsafe {
+                let mut data = SmallVecData::<A>::from_inline(MaybeUninit::uninit());
+                let len = vec.len();
+                vec.set_len(0);
+                ptr::copy_nonoverlapping(vec.as_ptr(), data.inline_mut(), len);
+
+                SmallVec {
+                    capacity: len,
+                    data,
+                }
+            }
+        } else {
+            let (ptr, cap, len) = (vec.as_mut_ptr(), vec.capacity(), vec.len());
+            mem::forget(vec);
+
+            SmallVec {
+                capacity: cap,
+                data: SmallVecData::from_heap(ptr, len),
+            }
+        }
+    }
+
+    /// Constructs a new `SmallVec` on the stack from an `A` without
+    /// copying elements.
+    ///
+    /// ```rust
+    /// use smallvec::SmallVec;
+    ///
+    /// let buf = [1, 2, 3, 4, 5];
+    /// let small_vec: SmallVec<_> = SmallVec::from_buf(buf);
+    ///
+    /// assert_eq!(&*small_vec, &[1, 2, 3, 4, 5]);
+    /// ```
+    #[inline]
+    pub fn from_buf(buf: A) -> SmallVec<A> {
+        SmallVec {
+            capacity: A::size(),
+            data: SmallVecData::from_inline(MaybeUninit::new(buf)),
+        }
+    }
+
+    /// Constructs a new `SmallVec` on the stack from an `A` without
+    /// copying elements. Also sets the length, which must be less or
+    /// equal to the size of `buf`.
+    ///
+    /// ```rust
+    /// use smallvec::SmallVec;
+    ///
+    /// let buf = [1, 2, 3, 4, 5, 0, 0, 0];
+    /// let small_vec: SmallVec<_> = SmallVec::from_buf_and_len(buf, 5);
+    ///
+    /// assert_eq!(&*small_vec, &[1, 2, 3, 4, 5]);
+    /// ```
+    #[inline]
+    pub fn from_buf_and_len(buf: A, len: usize) -> SmallVec<A> {
+        assert!(len <= A::size());
+        unsafe { SmallVec::from_buf_and_len_unchecked(MaybeUninit::new(buf), len) }
+    }
+
+    /// Constructs a new `SmallVec` on the stack from an `A` without
+    /// copying elements. Also sets the length. The user is responsible
+    /// for ensuring that `len <= A::size()`.
+    ///
+    /// ```rust
+    /// use smallvec::SmallVec;
+    /// use std::mem::MaybeUninit;
+    ///
+    /// let buf = [1, 2, 3, 4, 5, 0, 0, 0];
+    /// let small_vec: SmallVec<_> = unsafe {
+    ///     SmallVec::from_buf_and_len_unchecked(MaybeUninit::new(buf), 5)
+    /// };
+    ///
+    /// assert_eq!(&*small_vec, &[1, 2, 3, 4, 5]);
+    /// ```
+    #[inline]
+    pub unsafe fn from_buf_and_len_unchecked(buf: MaybeUninit<A>, len: usize) -> SmallVec<A> {
+        SmallVec {
+            capacity: len,
+            data: SmallVecData::from_inline(buf),
+        }
+    }
+
+    /// Sets the length of a vector.
+    ///
+    /// This will explicitly set the size of the vector, without actually
+    /// modifying its buffers, so it is up to the caller to ensure that the
+    /// vector is actually the specified size.
+    pub unsafe fn set_len(&mut self, new_len: usize) {
+        let (_, len_ptr, _) = self.triple_mut();
+        *len_ptr = new_len;
+    }
+
+    /// The maximum number of elements this vector can hold inline
+    #[inline]
+    fn inline_capacity() -> usize {
+        if mem::size_of::<A::Item>() > 0 {
+            A::size()
+        } else {
+            // For zero-size items code like `ptr.add(offset)` always returns the same pointer.
+            // Therefore all items are at the same address,
+            // and any array size has capacity for infinitely many items.
+            // The capacity is limited by the bit width of the length field.
+            //
+            // `Vec` also does this:
+            // https://github.com/rust-lang/rust/blob/1.44.0/src/liballoc/raw_vec.rs#L186
+            //
+            // In our case, this also ensures that a smallvec of zero-size items never spills,
+            // and we never try to allocate zero bytes which `std::alloc::alloc` disallows.
+            core::usize::MAX
+        }
+    }
+
+    /// The maximum number of elements this vector can hold inline
+    #[inline]
+    pub fn inline_size(&self) -> usize {
+        Self::inline_capacity()
+    }
+
+    /// The number of elements stored in the vector
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.triple().1
+    }
+
+    /// Returns `true` if the vector is empty
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// The number of items the vector can hold without reallocating
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.triple().2
+    }
+
+    /// Returns a tuple with (data ptr, len, capacity)
+    /// Useful to get all SmallVec properties with a single check of the current storage variant.
+    #[inline]
+    fn triple(&self) -> (*const A::Item, usize, usize) {
+        unsafe {
+            if self.spilled() {
+                let (ptr, len) = self.data.heap();
+                (ptr, len, self.capacity)
+            } else {
+                (self.data.inline(), self.capacity, Self::inline_capacity())
+            }
+        }
+    }
+
+    /// Returns a tuple with (data ptr, len ptr, capacity)
+    #[inline]
+    fn triple_mut(&mut self) -> (*mut A::Item, &mut usize, usize) {
+        unsafe {
+            if self.spilled() {
+                let &mut (ptr, ref mut len_ptr) = self.data.heap_mut();
+                (ptr, len_ptr, self.capacity)
+            } else {
+                (
+                    self.data.inline_mut(),
+                    &mut self.capacity,
+                    Self::inline_capacity(),
+                )
+            }
+        }
+    }
+
+    /// Returns `true` if the data has spilled into a separate heap-allocated buffer.
+    #[inline]
+    pub fn spilled(&self) -> bool {
+        self.capacity > Self::inline_capacity()
+    }
+
+    /// 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 core::ops::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 {
+            self.set_len(start);
+
+            let range_slice = slice::from_raw_parts_mut(self.as_mut_ptr().add(start), end - start);
+
+            Drain {
+                tail_start: end,
+                tail_len: len - end,
+                iter: range_slice.iter(),
+                vec: NonNull::from(self),
+            }
+        }
+    }
+
+    /// Append an item to the vector.
+    #[inline]
+    pub fn push(&mut self, value: A::Item) {
+        unsafe {
+            let (_, &mut len, cap) = self.triple_mut();
+            if len == cap {
+                self.reserve(1);
+            }
+            let (ptr, len_ptr, _) = self.triple_mut();
+            *len_ptr = len + 1;
+            ptr::write(ptr.add(len), value);
+        }
+    }
+
+    /// Remove an item from the end of the vector and return it, or None if empty.
+    #[inline]
+    pub fn pop(&mut self) -> Option<A::Item> {
+        unsafe {
+            let (ptr, len_ptr, _) = self.triple_mut();
+            if *len_ptr == 0 {
+                return None;
+            }
+            let last_index = *len_ptr - 1;
+            *len_ptr = last_index;
+            Some(ptr::read(ptr.add(last_index)))
+        }
+    }
+
+    /// Re-allocate to set the capacity to `max(new_cap, inline_size())`.
+    ///
+    /// Panics if `new_cap` is less than the vector's length
+    /// or if the capacity computation overflows `usize`.
+    pub fn grow(&mut self, new_cap: usize) {
+        infallible(self.try_grow(new_cap))
+    }
+
+    /// Re-allocate to set the capacity to `max(new_cap, inline_size())`.
+    ///
+    /// Panics if `new_cap` is less than the vector's length
+    pub fn try_grow(&mut self, new_cap: usize) -> Result<(), CollectionAllocErr> {
+        unsafe {
+            let (ptr, &mut len, cap) = self.triple_mut();
+            let unspilled = !self.spilled();
+            assert!(new_cap >= len);
+            if new_cap <= self.inline_size() {
+                if unspilled {
+                    return Ok(());
+                }
+                self.data = SmallVecData::from_inline(MaybeUninit::uninit());
+                ptr::copy_nonoverlapping(ptr, self.data.inline_mut(), len);
+                self.capacity = len;
+                deallocate(ptr, cap);
+            } else if new_cap != cap {
+                let layout = layout_array::<A::Item>(new_cap)?;
+                debug_assert!(layout.size() > 0);
+                let new_alloc;
+                if unspilled {
+                    new_alloc = NonNull::new(alloc::alloc::alloc(layout))
+                        .ok_or(CollectionAllocErr::AllocErr { layout })?
+                        .cast()
+                        .as_ptr();
+                    ptr::copy_nonoverlapping(ptr, new_alloc, len);
+                } else {
+                    // This should never fail since the same succeeded
+                    // when previously allocating `ptr`.
+                    let old_layout = layout_array::<A::Item>(cap)?;
+
+                    let new_ptr = alloc::alloc::realloc(ptr as *mut u8, old_layout, layout.size());
+                    new_alloc = NonNull::new(new_ptr)
+                        .ok_or(CollectionAllocErr::AllocErr { layout })?
+                        .cast()
+                        .as_ptr();
+                }
+                self.data = SmallVecData::from_heap(new_alloc, len);
+                self.capacity = new_cap;
+            }
+            Ok(())
+        }
+    }
+
+    /// Reserve capacity for `additional` more elements to be inserted.
+    ///
+    /// May reserve more space to avoid frequent reallocations.
+    ///
+    /// Panics if the capacity computation overflows `usize`.
+    #[inline]
+    pub fn reserve(&mut self, additional: usize) {
+        infallible(self.try_reserve(additional))
+    }
+
+    /// Reserve capacity for `additional` more elements to be inserted.
+    ///
+    /// May reserve more space to avoid frequent reallocations.
+    pub fn try_reserve(&mut self, additional: usize) -> Result<(), CollectionAllocErr> {
+        // prefer triple_mut() even if triple() would work
+        // so that the optimizer removes duplicated calls to it
+        // from callers like insert()
+        let (_, &mut len, cap) = self.triple_mut();
+        if cap - len >= additional {
+            return Ok(());
+        }
+        let new_cap = len
+            .checked_add(additional)
+            .and_then(usize::checked_next_power_of_two)
+            .ok_or(CollectionAllocErr::CapacityOverflow)?;
+        self.try_grow(new_cap)
+    }
+
+    /// Reserve the minimum capacity for `additional` more elements to be inserted.
+    ///
+    /// Panics if the new capacity overflows `usize`.
+    pub fn reserve_exact(&mut self, additional: usize) {
+        infallible(self.try_reserve_exact(additional))
+    }
+
+    /// Reserve the minimum capacity for `additional` more elements to be inserted.
+    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), CollectionAllocErr> {
+        let (_, &mut len, cap) = self.triple_mut();
+        if cap - len >= additional {
+            return Ok(());
+        }
+        let new_cap = len
+            .checked_add(additional)
+            .ok_or(CollectionAllocErr::CapacityOverflow)?;
+        self.try_grow(new_cap)
+    }
+
+    /// Shrink the capacity of the vector as much as possible.
+    ///
+    /// When possible, this will move data from an external heap buffer to the vector's inline
+    /// storage.
+    pub fn shrink_to_fit(&mut self) {
+        if !self.spilled() {
+            return;
+        }
+        let len = self.len();
+        if self.inline_size() >= len {
+            unsafe {
+                let (ptr, len) = self.data.heap();
+                self.data = SmallVecData::from_inline(MaybeUninit::uninit());
+                ptr::copy_nonoverlapping(ptr, self.data.inline_mut(), len);
+                deallocate(ptr, self.capacity);
+                self.capacity = len;
+            }
+        } else if self.capacity() > len {
+            self.grow(len);
+        }
+    }
+
+    /// Shorten the vector, keeping the first `len` elements and dropping the rest.
+    ///
+    /// If `len` is greater than or equal to the vector's current length, this has no
+    /// effect.
+    ///
+    /// This does not re-allocate.  If you want the vector's capacity to shrink, call
+    /// `shrink_to_fit` after truncating.
+    pub fn truncate(&mut self, len: usize) {
+        unsafe {
+            let (ptr, len_ptr, _) = self.triple_mut();
+            while len < *len_ptr {
+                let last_index = *len_ptr - 1;
+                *len_ptr = last_index;
+                ptr::drop_in_place(ptr.add(last_index));
+            }
+        }
+    }
+
+    /// Extracts a slice containing the entire vector.
+    ///
+    /// Equivalent to `&s[..]`.
+    pub fn as_slice(&self) -> &[A::Item] {
+        self
+    }
+
+    /// Extracts a mutable slice of the entire vector.
+    ///
+    /// Equivalent to `&mut s[..]`.
+    pub fn as_mut_slice(&mut self) -> &mut [A::Item] {
+        self
+    }
+
+    /// Remove the element at position `index`, replacing it with the last element.
+    ///
+    /// This does not preserve ordering, but is O(1).
+    ///
+    /// Panics if `index` is out of bounds.
+    #[inline]
+    pub fn swap_remove(&mut self, index: usize) -> A::Item {
+        let len = self.len();
+        self.swap(len - 1, index);
+        self.pop()
+            .unwrap_or_else(|| unsafe { unreachable_unchecked() })
+    }
+
+    /// Remove all elements from the vector.
+    #[inline]
+    pub fn clear(&mut self) {
+        self.truncate(0);
+    }
+
+    /// Remove and return the element at position `index`, shifting all elements after it to the
+    /// left.
+    ///
+    /// Panics if `index` is out of bounds.
+    pub fn remove(&mut self, index: usize) -> A::Item {
+        unsafe {
+            let (mut ptr, len_ptr, _) = self.triple_mut();
+            let len = *len_ptr;
+            assert!(index < len);
+            *len_ptr = len - 1;
+            ptr = ptr.add(index);
+            let item = ptr::read(ptr);
+            ptr::copy(ptr.add(1), ptr, len - index - 1);
+            item
+        }
+    }
+
+    /// Insert an element at position `index`, shifting all elements after it to the right.
+    ///
+    /// Panics if `index` is out of bounds.
+    pub fn insert(&mut self, index: usize, element: A::Item) {
+        self.reserve(1);
+
+        unsafe {
+            let (mut ptr, len_ptr, _) = self.triple_mut();
+            let len = *len_ptr;
+            assert!(index <= len);
+            *len_ptr = len + 1;
+            ptr = ptr.add(index);
+            ptr::copy(ptr, ptr.add(1), len - index);
+            ptr::write(ptr, element);
+        }
+    }
+
+    /// Insert multiple elements at position `index`, shifting all following elements toward the
+    /// back.
+    pub fn insert_many<I: IntoIterator<Item = A::Item>>(&mut self, index: usize, iterable: I) {
+        let iter = iterable.into_iter();
+        if index == self.len() {
+            return self.extend(iter);
+        }
+
+        let (lower_size_bound, _) = iter.size_hint();
+        assert!(lower_size_bound <= core::isize::MAX as usize); // Ensure offset is indexable
+        assert!(index + lower_size_bound >= index); // Protect against overflow
+        self.reserve(lower_size_bound);
+
+        unsafe {
+            let old_len = self.len();
+            assert!(index <= old_len);
+            let start = self.as_mut_ptr();
+            let mut ptr = start.add(index);
+
+            // Move the trailing elements.
+            ptr::copy(ptr, ptr.add(lower_size_bound), old_len - index);
+
+            // In case the iterator panics, don't double-drop the items we just copied above.
+            self.set_len(0);
+            let mut guard = DropOnPanic {
+                start,
+                skip: index..(index + lower_size_bound),
+                len: old_len + lower_size_bound,
+            };
+
+            let mut num_added = 0;
+            for element in iter {
+                let mut cur = ptr.add(num_added);
+                if num_added >= lower_size_bound {
+                    // Iterator provided more elements than the hint.  Move trailing items again.
+                    self.reserve(1);
+                    let start = self.as_mut_ptr();
+                    ptr = start.add(index);
+                    cur = ptr.add(num_added);
+                    ptr::copy(cur, cur.add(1), old_len - index);
+
+                    guard.start = start;
+                    guard.len += 1;
+                    guard.skip.end += 1;
+                }
+                ptr::write(cur, element);
+                guard.skip.start += 1;
+                num_added += 1;
+            }
+            mem::forget(guard);
+
+            if num_added < lower_size_bound {
+                // Iterator provided fewer elements than the hint
+                ptr::copy(
+                    ptr.add(lower_size_bound),
+                    ptr.add(num_added),
+                    old_len - index,
+                );
+            }
+
+            self.set_len(old_len + num_added);
+        }
+
+        struct DropOnPanic<T> {
+            start: *mut T,
+            skip: Range<usize>,
+            len: usize,
+        }
+
+        impl<T> Drop for DropOnPanic<T> {
+            fn drop(&mut self) {
+                for i in 0..self.len {
+                    if !self.skip.contains(&i) {
+                        unsafe {
+                            ptr::drop_in_place(self.start.add(i));
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    /// Convert a SmallVec to a Vec, without reallocating if the SmallVec has already spilled onto
+    /// the heap.
+    pub fn into_vec(self) -> Vec<A::Item> {
+        if self.spilled() {
+            unsafe {
+                let (ptr, len) = self.data.heap();
+                let v = Vec::from_raw_parts(ptr, len, self.capacity);
+                mem::forget(self);
+                v
+            }
+        } else {
+            self.into_iter().collect()
+        }
+    }
+
+    /// Converts a `SmallVec` into a `Box<[T]>` without reallocating if the `SmallVec` has already spilled
+    /// onto the heap.
+    ///
+    /// Note that this will drop any excess capacity.
+    pub fn into_boxed_slice(self) -> Box<[A::Item]> {
+        self.into_vec().into_boxed_slice()
+    }
+
+    /// Convert the SmallVec into an `A` if possible. Otherwise return `Err(Self)`.
+    ///
+    /// This method returns `Err(Self)` if the SmallVec is too short (and the `A` contains uninitialized elements),
+    /// or if the SmallVec is too long (and all the elements were spilled to the heap).
+    pub fn into_inner(self) -> Result<A, Self> {
+        if self.spilled() || self.len() != A::size() {
+            // Note: A::size, not Self::inline_capacity
+            Err(self)
+        } else {
+            unsafe {
+                let data = ptr::read(&self.data);
+                mem::forget(self);
+                Ok(data.into_inline().assume_init())
+            }
+        }
+    }
+
+    /// Retains only the elements specified by the predicate.
+    ///
+    /// In other words, remove all elements `e` such that `f(&e)` returns `false`.
+    /// This method operates in place and preserves the order of the retained
+    /// elements.
+    pub fn retain<F: FnMut(&mut A::Item) -> bool>(&mut self, mut f: F) {
+        let mut del = 0;
+        let len = self.len();
+        for i in 0..len {
+            if !f(&mut self[i]) {
+                del += 1;
+            } else if del > 0 {
+                self.swap(i - del, i);
+            }
+        }
+        self.truncate(len - del);
+    }
+
+    /// Removes consecutive duplicate elements.
+    pub fn dedup(&mut self)
+    where
+        A::Item: PartialEq<A::Item>,
+    {
+        self.dedup_by(|a, b| a == b);
+    }
+
+    /// Removes consecutive duplicate elements using the given equality relation.
+    pub fn dedup_by<F>(&mut self, mut same_bucket: F)
+    where
+        F: FnMut(&mut A::Item, &mut A::Item) -> bool,
+    {
+        // See the implementation of Vec::dedup_by in the
+        // standard library for an explanation of this algorithm.
+        let len = self.len();
+        if len <= 1 {
+            return;
+        }
+
+        let ptr = self.as_mut_ptr();
+        let mut w: usize = 1;
+
+        unsafe {
+            for r in 1..len {
+                let p_r = ptr.add(r);
+                let p_wm1 = ptr.add(w - 1);
+                if !same_bucket(&mut *p_r, &mut *p_wm1) {
+                    if r != w {
+                        let p_w = p_wm1.add(1);
+                        mem::swap(&mut *p_r, &mut *p_w);
+                    }
+                    w += 1;
+                }
+            }
+        }
+
+        self.truncate(w);
+    }
+
+    /// Removes consecutive elements that map to the same key.
+    pub fn dedup_by_key<F, K>(&mut self, mut key: F)
+    where
+        F: FnMut(&mut A::Item) -> K,
+        K: PartialEq<K>,
+    {
+        self.dedup_by(|a, b| key(a) == key(b));
+    }
+
+    /// Resizes the `SmallVec` in-place so that `len` is equal to `new_len`.
+    ///
+    /// If `new_len` is greater than `len`, the `SmallVec` is extended by the difference, with each
+    /// additional slot filled with the result of calling the closure `f`. The return values from `f`
+    //// will end up in the `SmallVec` in the order they have been generated.
+    ///
+    /// If `new_len` is less than `len`, the `SmallVec` is simply truncated.
+    ///
+    /// This method uses a closure to create new values on every push. If you'd rather `Clone` a given
+    /// value, use `resize`. If you want to use the `Default` trait to generate values, you can pass
+    /// `Default::default()` as the second argument.
+    ///
+    /// Added for std::vec::Vec compatibility (added in Rust 1.33.0)
+    ///
+    /// ```
+    /// # use smallvec::{smallvec, SmallVec};
+    /// let mut vec : SmallVec<[_; 4]> = smallvec![1, 2, 3];
+    /// vec.resize_with(5, Default::default);
+    /// assert_eq!(&*vec, &[1, 2, 3, 0, 0]);
+    ///
+    /// let mut vec : SmallVec<[_; 4]> = smallvec![];
+    /// let mut p = 1;
+    /// vec.resize_with(4, || { p *= 2; p });
+    /// assert_eq!(&*vec, &[2, 4, 8, 16]);
+    /// ```
+    pub fn resize_with<F>(&mut self, new_len: usize, f: F)
+    where
+        F: FnMut() -> A::Item,
+    {
+        let old_len = self.len();
+        if old_len < new_len {
+            let mut f = f;
+            let additional = new_len - old_len;
+            self.reserve(additional);
+            for _ in 0..additional {
+                self.push(f());
+            }
+        } else if old_len > new_len {
+            self.truncate(new_len);
+        }
+    }
+
+    /// Creates a `SmallVec` directly from the raw components of another
+    /// `SmallVec`.
+    ///
+    /// # Safety
+    ///
+    /// This is highly unsafe, due to the number of invariants that aren't
+    /// checked:
+    ///
+    /// * `ptr` needs to have been previously allocated via `SmallVec` for its
+    ///   spilled storage (at least, it's highly likely to be incorrect if it
+    ///   wasn't).
+    /// * `ptr`'s `A::Item` type needs to be the same size and alignment that
+    ///   it was allocated with
+    /// * `length` needs to be less than or equal to `capacity`.
+    /// * `capacity` needs to be the capacity that the pointer was allocated
+    ///   with.
+    ///
+    /// Violating these may cause problems like corrupting the allocator's
+    /// internal data structures.
+    ///
+    /// Additionally, `capacity` must be greater than the amount of inline
+    /// storage `A` has; that is, the new `SmallVec` must need to spill over
+    /// into heap allocated storage. This condition is asserted against.
+    ///
+    /// The ownership of `ptr` is effectively transferred to the
+    /// `SmallVec` which may then deallocate, reallocate or change the
+    /// contents of memory pointed to by the pointer at will. Ensure
+    /// that nothing else uses the pointer after calling this
+    /// function.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # #[macro_use] extern crate smallvec;
+    /// # use smallvec::SmallVec;
+    /// use std::mem;
+    /// use std::ptr;
+    ///
+    /// fn main() {
+    ///     let mut v: SmallVec<[_; 1]> = smallvec![1, 2, 3];
+    ///
+    ///     // Pull out the important parts of `v`.
+    ///     let p = v.as_mut_ptr();
+    ///     let len = v.len();
+    ///     let cap = v.capacity();
+    ///     let spilled = v.spilled();
+    ///
+    ///     unsafe {
+    ///         // Forget all about `v`. The heap allocation that stored the
+    ///         // three values won't be deallocated.
+    ///         mem::forget(v);
+    ///
+    ///         // Overwrite memory with [4, 5, 6].
+    ///         //
+    ///         // This is only safe if `spilled` is true! Otherwise, we are
+    ///         // writing into the old `SmallVec`'s inline storage on the
+    ///         // stack.
+    ///         assert!(spilled);
+    ///         for i in 0..len {
+    ///             ptr::write(p.add(i), 4 + i);
+    ///         }
+    ///
+    ///         // Put everything back together into a SmallVec with a different
+    ///         // amount of inline storage, but which is still less than `cap`.
+    ///         let rebuilt = SmallVec::<[_; 2]>::from_raw_parts(p, len, cap);
+    ///         assert_eq!(&*rebuilt, &[4, 5, 6]);
+    ///     }
+    /// }
+    #[inline]
+    pub unsafe fn from_raw_parts(ptr: *mut A::Item, length: usize, capacity: usize) -> SmallVec<A> {
+        assert!(capacity > Self::inline_capacity());
+        SmallVec {
+            capacity,
+            data: SmallVecData::from_heap(ptr, length),
+        }
+    }
+
+    /// Returns a raw pointer to the vector's buffer.
+    pub fn as_ptr(&self) -> *const A::Item {
+        // We shadow the slice method of the same name to avoid going through
+        // `deref`, which creates an intermediate reference that may place
+        // additional safety constraints on the contents of the slice.
+        self.triple().0
+    }
+
+    /// Returns a raw mutable pointer to the vector's buffer.
+    pub fn as_mut_ptr(&mut self) -> *mut A::Item {
+        // We shadow the slice method of the same name to avoid going through
+        // `deref_mut`, which creates an intermediate reference that may place
+        // additional safety constraints on the contents of the slice.
+        self.triple_mut().0
+    }
+}
+
+impl<A: Array> SmallVec<A>
+where
+    A::Item: Copy,
+{
+    /// Copy the elements from a slice into a new `SmallVec`.
+    ///
+    /// For slices of `Copy` types, this is more efficient than `SmallVec::from(slice)`.
+    pub fn from_slice(slice: &[A::Item]) -> Self {
+        let len = slice.len();
+        if len <= Self::inline_capacity() {
+            SmallVec {
+                capacity: len,
+                data: SmallVecData::from_inline(unsafe {
+                    let mut data: MaybeUninit<A> = MaybeUninit::uninit();
+                    ptr::copy_nonoverlapping(
+                        slice.as_ptr(),
+                        data.as_mut_ptr() as *mut A::Item,
+                        len,
+                    );
+                    data
+                }),
+            }
+        } else {
+            let mut b = slice.to_vec();
+            let (ptr, cap) = (b.as_mut_ptr(), b.capacity());
+            mem::forget(b);
+            SmallVec {
+                capacity: cap,
+                data: SmallVecData::from_heap(ptr, len),
+            }
+        }
+    }
+
+    /// Copy elements from a slice into the vector at position `index`, shifting any following
+    /// elements toward the back.
+    ///
+    /// For slices of `Copy` types, this is more efficient than `insert`.
+    pub fn insert_from_slice(&mut self, index: usize, slice: &[A::Item]) {
+        self.reserve(slice.len());
+
+        let len = self.len();
+        assert!(index <= len);
+
+        unsafe {
+            let slice_ptr = slice.as_ptr();
+            let ptr = self.as_mut_ptr().add(index);
+            ptr::copy(ptr, ptr.add(slice.len()), len - index);
+            ptr::copy_nonoverlapping(slice_ptr, ptr, slice.len());
+            self.set_len(len + slice.len());
+        }
+    }
+
+    /// Copy elements from a slice and append them to the vector.
+    ///
+    /// For slices of `Copy` types, this is more efficient than `extend`.
+    #[inline]
+    pub fn extend_from_slice(&mut self, slice: &[A::Item]) {
+        let len = self.len();
+        self.insert_from_slice(len, slice);
+    }
+}
+
+impl<A: Array> SmallVec<A>
+where
+    A::Item: Clone,
+{
+    /// Resizes the vector so that its length is equal to `len`.
+    ///
+    /// If `len` is less than the current length, the vector simply truncated.
+    ///
+    /// If `len` is greater than the current length, `value` is appended to the
+    /// vector until its length equals `len`.
+    pub fn resize(&mut self, len: usize, value: A::Item) {
+        let old_len = self.len();
+
+        if len > old_len {
+            self.extend(repeat(value).take(len - old_len));
+        } else {
+            self.truncate(len);
+        }
+    }
+
+    /// Creates a `SmallVec` with `n` copies of `elem`.
+    /// ```
+    /// use smallvec::SmallVec;
+    ///
+    /// let v = SmallVec::<[char; 128]>::from_elem('d', 2);
+    /// assert_eq!(v, SmallVec::from_buf(['d', 'd']));
+    /// ```
+    pub fn from_elem(elem: A::Item, n: usize) -> Self {
+        if n > Self::inline_capacity() {
+            vec![elem; n].into()
+        } else {
+            let mut v = SmallVec::<A>::new();
+            unsafe {
+                let (ptr, len_ptr, _) = v.triple_mut();
+                let mut local_len = SetLenOnDrop::new(len_ptr);
+
+                for i in 0..n {
+                    ::core::ptr::write(ptr.add(i), elem.clone());
+                    local_len.increment_len(1);
+                }
+            }
+            v
+        }
+    }
+}
+
+impl<A: Array> ops::Deref for SmallVec<A> {
+    type Target = [A::Item];
+    #[inline]
+    fn deref(&self) -> &[A::Item] {
+        unsafe {
+            let (ptr, len, _) = self.triple();
+            slice::from_raw_parts(ptr, len)
+        }
+    }
+}
+
+impl<A: Array> ops::DerefMut for SmallVec<A> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut [A::Item] {
+        unsafe {
+            let (ptr, &mut len, _) = self.triple_mut();
+            slice::from_raw_parts_mut(ptr, len)
+        }
+    }
+}
+
+impl<A: Array> AsRef<[A::Item]> for SmallVec<A> {
+    #[inline]
+    fn as_ref(&self) -> &[A::Item] {
+        self
+    }
+}
+
+impl<A: Array> AsMut<[A::Item]> for SmallVec<A> {
+    #[inline]
+    fn as_mut(&mut self) -> &mut [A::Item] {
+        self
+    }
+}
+
+impl<A: Array> Borrow<[A::Item]> for SmallVec<A> {
+    #[inline]
+    fn borrow(&self) -> &[A::Item] {
+        self
+    }
+}
+
+impl<A: Array> BorrowMut<[A::Item]> for SmallVec<A> {
+    #[inline]
+    fn borrow_mut(&mut self) -> &mut [A::Item] {
+        self
+    }
+}
+
+#[cfg(feature = "write")]
+impl<A: Array<Item = u8>> io::Write for SmallVec<A> {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.extend_from_slice(buf);
+        Ok(buf.len())
+    }
+
+    #[inline]
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        self.extend_from_slice(buf);
+        Ok(())
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+#[cfg(feature = "serde")]
+impl<A: Array> Serialize for SmallVec<A>
+where
+    A::Item: Serialize,
+{
+    fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
+        let mut state = serializer.serialize_seq(Some(self.len()))?;
+        for item in self {
+            state.serialize_element(&item)?;
+        }
+        state.end()
+    }
+}
+
+#[cfg(feature = "serde")]
+impl<'de, A: Array> Deserialize<'de> for SmallVec<A>
+where
+    A::Item: Deserialize<'de>,
+{
+    fn deserialize<D: Deserializer<'de>>(deserializer: D) -> Result<Self, D::Error> {
+        deserializer.deserialize_seq(SmallVecVisitor {
+            phantom: PhantomData,
+        })
+    }
+}
+
+#[cfg(feature = "serde")]
+struct SmallVecVisitor<A> {
+    phantom: PhantomData<A>,
+}
+
+#[cfg(feature = "serde")]
+impl<'de, A: Array> Visitor<'de> for SmallVecVisitor<A>
+where
+    A::Item: Deserialize<'de>,
+{
+    type Value = SmallVec<A>;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        formatter.write_str("a sequence")
+    }
+
+    fn visit_seq<B>(self, mut seq: B) -> Result<Self::Value, B::Error>
+    where
+        B: SeqAccess<'de>,
+    {
+        let len = seq.size_hint().unwrap_or(0);
+        let mut values = SmallVec::with_capacity(len);
+
+        while let Some(value) = seq.next_element()? {
+            values.push(value);
+        }
+
+        Ok(values)
+    }
+}
+
+#[cfg(feature = "specialization")]
+trait SpecFrom<A: Array, S> {
+    fn spec_from(slice: S) -> SmallVec<A>;
+}
+
+#[cfg(feature = "specialization")]
+mod specialization;
+
+#[cfg(feature = "specialization")]
+impl<'a, A: Array> SpecFrom<A, &'a [A::Item]> for SmallVec<A>
+where
+    A::Item: Copy,
+{
+    #[inline]
+    fn spec_from(slice: &'a [A::Item]) -> SmallVec<A> {
+        SmallVec::from_slice(slice)
+    }
+}
+
+impl<'a, A: Array> From<&'a [A::Item]> for SmallVec<A>
+where
+    A::Item: Clone,
+{
+    #[cfg(not(feature = "specialization"))]
+    #[inline]
+    fn from(slice: &'a [A::Item]) -> SmallVec<A> {
+        slice.iter().cloned().collect()
+    }
+
+    #[cfg(feature = "specialization")]
+    #[inline]
+    fn from(slice: &'a [A::Item]) -> SmallVec<A> {
+        SmallVec::spec_from(slice)
+    }
+}
+
+impl<A: Array> From<Vec<A::Item>> for SmallVec<A> {
+    #[inline]
+    fn from(vec: Vec<A::Item>) -> SmallVec<A> {
+        SmallVec::from_vec(vec)
+    }
+}
+
+impl<A: Array> From<A> for SmallVec<A> {
+    #[inline]
+    fn from(array: A) -> SmallVec<A> {
+        SmallVec::from_buf(array)
+    }
+}
+
+impl<A: Array, I: SliceIndex<[A::Item]>> ops::Index<I> for SmallVec<A> {
+    type Output = I::Output;
+
+    fn index(&self, index: I) -> &I::Output {
+        &(**self)[index]
+    }
+}
+
+impl<A: Array, I: SliceIndex<[A::Item]>> ops::IndexMut<I> for SmallVec<A> {
+    fn index_mut(&mut self, index: I) -> &mut I::Output {
+        &mut (&mut **self)[index]
+    }
+}
+
+#[allow(deprecated)]
+impl<A: Array> ExtendFromSlice<A::Item> for SmallVec<A>
+where
+    A::Item: Copy,
+{
+    fn extend_from_slice(&mut self, other: &[A::Item]) {
+        SmallVec::extend_from_slice(self, other)
+    }
+}
+
+impl<A: Array> FromIterator<A::Item> for SmallVec<A> {
+    #[inline]
+    fn from_iter<I: IntoIterator<Item = A::Item>>(iterable: I) -> SmallVec<A> {
+        let mut v = SmallVec::new();
+        v.extend(iterable);
+        v
+    }
+}
+
+impl<A: Array> Extend<A::Item> for SmallVec<A> {
+    fn extend<I: IntoIterator<Item = A::Item>>(&mut self, iterable: I) {
+        let mut iter = iterable.into_iter();
+        let (lower_size_bound, _) = iter.size_hint();
+        self.reserve(lower_size_bound);
+
+        unsafe {
+            let (ptr, len_ptr, cap) = self.triple_mut();
+            let mut len = SetLenOnDrop::new(len_ptr);
+            while len.get() < cap {
+                if let Some(out) = iter.next() {
+                    ptr::write(ptr.add(len.get()), out);
+                    len.increment_len(1);
+                } else {
+                    return;
+                }
+            }
+        }
+
+        for elem in iter {
+            self.push(elem);
+        }
+    }
+}
+
+impl<A: Array> fmt::Debug for SmallVec<A>
+where
+    A::Item: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_list().entries(self.iter()).finish()
+    }
+}
+
+impl<A: Array> Default for SmallVec<A> {
+    #[inline]
+    fn default() -> SmallVec<A> {
+        SmallVec::new()
+    }
+}
+
+#[cfg(feature = "may_dangle")]
+unsafe impl<#[may_dangle] A: Array> Drop for SmallVec<A> {
+    fn drop(&mut self) {
+        unsafe {
+            if self.spilled() {
+                let (ptr, len) = self.data.heap();
+                Vec::from_raw_parts(ptr, len, self.capacity);
+            } else {
+                ptr::drop_in_place(&mut self[..]);
+            }
+        }
+    }
+}
+
+#[cfg(not(feature = "may_dangle"))]
+impl<A: Array> Drop for SmallVec<A> {
+    fn drop(&mut self) {
+        unsafe {
+            if self.spilled() {
+                let (ptr, len) = self.data.heap();
+                Vec::from_raw_parts(ptr, len, self.capacity);
+            } else {
+                ptr::drop_in_place(&mut self[..]);
+            }
+        }
+    }
+}
+
+impl<A: Array> Clone for SmallVec<A>
+where
+    A::Item: Clone,
+{
+    #[inline]
+    fn clone(&self) -> SmallVec<A> {
+        SmallVec::from(self.as_slice())
+    }
+}
+
+impl<A: Array, B: Array> PartialEq<SmallVec<B>> for SmallVec<A>
+where
+    A::Item: PartialEq<B::Item>,
+{
+    #[inline]
+    fn eq(&self, other: &SmallVec<B>) -> bool {
+        self[..] == other[..]
+    }
+}
+
+impl<A: Array> Eq for SmallVec<A> where A::Item: Eq {}
+
+impl<A: Array> PartialOrd for SmallVec<A>
+where
+    A::Item: PartialOrd,
+{
+    #[inline]
+    fn partial_cmp(&self, other: &SmallVec<A>) -> Option<cmp::Ordering> {
+        PartialOrd::partial_cmp(&**self, &**other)
+    }
+}
+
+impl<A: Array> Ord for SmallVec<A>
+where
+    A::Item: Ord,
+{
+    #[inline]
+    fn cmp(&self, other: &SmallVec<A>) -> cmp::Ordering {
+        Ord::cmp(&**self, &**other)
+    }
+}
+
+impl<A: Array> Hash for SmallVec<A>
+where
+    A::Item: Hash,
+{
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        (**self).hash(state)
+    }
+}
+
+unsafe impl<A: Array> Send for SmallVec<A> where A::Item: Send {}
+
+/// An iterator that consumes a `SmallVec` and yields its items by value.
+///
+/// Returned from [`SmallVec::into_iter`][1].
+///
+/// [1]: struct.SmallVec.html#method.into_iter
+pub struct IntoIter<A: Array> {
+    data: SmallVec<A>,
+    current: usize,
+    end: usize,
+}
+
+impl<A: Array> fmt::Debug for IntoIter<A>
+where
+    A::Item: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("IntoIter").field(&self.as_slice()).finish()
+    }
+}
+
+impl<A: Array + Clone> Clone for IntoIter<A>
+where
+    A::Item: Clone,
+{
+    fn clone(&self) -> IntoIter<A> {
+        SmallVec::from(self.as_slice()).into_iter()
+    }
+}
+
+impl<A: Array> Drop for IntoIter<A> {
+    fn drop(&mut self) {
+        for _ in self {}
+    }
+}
+
+impl<A: Array> Iterator for IntoIter<A> {
+    type Item = A::Item;
+
+    #[inline]
+    fn next(&mut self) -> Option<A::Item> {
+        if self.current == self.end {
+            None
+        } else {
+            unsafe {
+                let current = self.current;
+                self.current += 1;
+                Some(ptr::read(self.data.as_ptr().add(current)))
+            }
+        }
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let size = self.end - self.current;
+        (size, Some(size))
+    }
+}
+
+impl<A: Array> DoubleEndedIterator for IntoIter<A> {
+    #[inline]
+    fn next_back(&mut self) -> Option<A::Item> {
+        if self.current == self.end {
+            None
+        } else {
+            unsafe {
+                self.end -= 1;
+                Some(ptr::read(self.data.as_ptr().add(self.end)))
+            }
+        }
+    }
+}
+
+impl<A: Array> ExactSizeIterator for IntoIter<A> {}
+impl<A: Array> FusedIterator for IntoIter<A> {}
+
+impl<A: Array> IntoIter<A> {
+    /// Returns the remaining items of this iterator as a slice.
+    pub fn as_slice(&self) -> &[A::Item] {
+        let len = self.end - self.current;
+        unsafe { core::slice::from_raw_parts(self.data.as_ptr().add(self.current), len) }
+    }
+
+    /// Returns the remaining items of this iterator as a mutable slice.
+    pub fn as_mut_slice(&mut self) -> &mut [A::Item] {
+        let len = self.end - self.current;
+        unsafe { core::slice::from_raw_parts_mut(self.data.as_mut_ptr().add(self.current), len) }
+    }
+}
+
+impl<A: Array> IntoIterator for SmallVec<A> {
+    type IntoIter = IntoIter<A>;
+    type Item = A::Item;
+    fn into_iter(mut self) -> Self::IntoIter {
+        unsafe {
+            // Set SmallVec len to zero as `IntoIter` drop handles dropping of the elements
+            let len = self.len();
+            self.set_len(0);
+            IntoIter {
+                data: self,
+                current: 0,
+                end: len,
+            }
+        }
+    }
+}
+
+impl<'a, A: Array> IntoIterator for &'a SmallVec<A> {
+    type IntoIter = slice::Iter<'a, A::Item>;
+    type Item = &'a A::Item;
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+impl<'a, A: Array> IntoIterator for &'a mut SmallVec<A> {
+    type IntoIter = slice::IterMut<'a, A::Item>;
+    type Item = &'a mut A::Item;
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter_mut()
+    }
+}
+
+/// Types that can be used as the backing store for a SmallVec
+pub unsafe trait Array {
+    /// The type of the array's elements.
+    type Item;
+    /// Returns the number of items the array can hold.
+    fn size() -> usize;
+}
+
+/// Set the length of the vec when the `SetLenOnDrop` value goes out of scope.
+///
+/// Copied from https://github.com/rust-lang/rust/pull/36355
+struct SetLenOnDrop<'a> {
+    len: &'a mut usize,
+    local_len: usize,
+}
+
+impl<'a> SetLenOnDrop<'a> {
+    #[inline]
+    fn new(len: &'a mut usize) -> Self {
+        SetLenOnDrop {
+            local_len: *len,
+            len,
+        }
+    }
+
+    #[inline]
+    fn get(&self) -> usize {
+        self.local_len
+    }
+
+    #[inline]
+    fn increment_len(&mut self, increment: usize) {
+        self.local_len += increment;
+    }
+}
+
+impl<'a> Drop for SetLenOnDrop<'a> {
+    #[inline]
+    fn drop(&mut self) {
+        *self.len = self.local_len;
+    }
+}
+
+#[cfg(feature = "const_generics")]
+unsafe impl<T, const N: usize> Array for [T; N] {
+    type Item = T;
+    fn size() -> usize {
+        N
+    }
+}
+
+#[cfg(not(feature = "const_generics"))]
+macro_rules! impl_array(
+    ($($size:expr),+) => {
+        $(
+            unsafe impl<T> Array for [T; $size] {
+                type Item = T;
+                fn size() -> usize { $size }
+            }
+        )+
+    }
+);
+
+#[cfg(not(feature = "const_generics"))]
+impl_array!(
+    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 20, 24, 32, 36, 0x40, 0x60, 0x80,
+    0x100, 0x200, 0x400, 0x600, 0x800, 0x1000, 0x2000, 0x4000, 0x6000, 0x8000, 0x10000, 0x20000,
+    0x40000, 0x60000, 0x80000, 0x10_0000
+);
+
+/// Convenience trait for constructing a `SmallVec`
+pub trait ToSmallVec<A: Array> {
+    /// Construct a new `SmallVec` from a slice.
+    fn to_smallvec(&self) -> SmallVec<A>;
+}
+
+impl<A: Array> ToSmallVec<A> for [A::Item]
+where
+    A::Item: Copy,
+{
+    #[inline]
+    fn to_smallvec(&self) -> SmallVec<A> {
+        SmallVec::from_slice(self)
+    }
+}
diff --git a/src/specialization.rs b/src/specialization.rs
new file mode 100644
index 0000000..658fa77
--- /dev/null
+++ b/src/specialization.rs
@@ -0,0 +1,19 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementations that require `default fn`.
+
+use super::{Array, SmallVec, SpecFrom};
+
+impl<'a, A: Array> SpecFrom<A, &'a [A::Item]> for SmallVec<A>
+where
+    A::Item: Clone,
+{
+    #[inline]
+    default fn spec_from(slice: &'a [A::Item]) -> SmallVec<A> {
+        slice.into_iter().cloned().collect()
+    }
+}
diff --git a/src/tests.rs b/src/tests.rs
new file mode 100644
index 0000000..0452ae8
--- /dev/null
+++ b/src/tests.rs
@@ -0,0 +1,907 @@
+use crate::{smallvec, SmallVec};
+
+use std::iter::FromIterator;
+
+use alloc::borrow::ToOwned;
+use alloc::boxed::Box;
+use alloc::rc::Rc;
+use alloc::{vec, vec::Vec};
+
+#[test]
+pub fn test_zero() {
+    let mut v = SmallVec::<[_; 0]>::new();
+    assert!(!v.spilled());
+    v.push(0usize);
+    assert!(v.spilled());
+    assert_eq!(&*v, &[0]);
+}
+
+// We heap allocate all these strings so that double frees will show up under valgrind.
+
+#[test]
+pub fn test_inline() {
+    let mut v = SmallVec::<[_; 16]>::new();
+    v.push("hello".to_owned());
+    v.push("there".to_owned());
+    assert_eq!(&*v, &["hello".to_owned(), "there".to_owned(),][..]);
+}
+
+#[test]
+pub fn test_spill() {
+    let mut v = SmallVec::<[_; 2]>::new();
+    v.push("hello".to_owned());
+    assert_eq!(v[0], "hello");
+    v.push("there".to_owned());
+    v.push("burma".to_owned());
+    assert_eq!(v[0], "hello");
+    v.push("shave".to_owned());
+    assert_eq!(
+        &*v,
+        &[
+            "hello".to_owned(),
+            "there".to_owned(),
+            "burma".to_owned(),
+            "shave".to_owned(),
+        ][..]
+    );
+}
+
+#[test]
+pub fn test_double_spill() {
+    let mut v = SmallVec::<[_; 2]>::new();
+    v.push("hello".to_owned());
+    v.push("there".to_owned());
+    v.push("burma".to_owned());
+    v.push("shave".to_owned());
+    v.push("hello".to_owned());
+    v.push("there".to_owned());
+    v.push("burma".to_owned());
+    v.push("shave".to_owned());
+    assert_eq!(
+        &*v,
+        &[
+            "hello".to_owned(),
+            "there".to_owned(),
+            "burma".to_owned(),
+            "shave".to_owned(),
+            "hello".to_owned(),
+            "there".to_owned(),
+            "burma".to_owned(),
+            "shave".to_owned(),
+        ][..]
+    );
+}
+
+/// https://github.com/servo/rust-smallvec/issues/4
+#[test]
+fn issue_4() {
+    SmallVec::<[Box<u32>; 2]>::new();
+}
+
+/// https://github.com/servo/rust-smallvec/issues/5
+#[test]
+fn issue_5() {
+    assert!(Some(SmallVec::<[&u32; 2]>::new()).is_some());
+}
+
+#[test]
+fn test_with_capacity() {
+    let v: SmallVec<[u8; 3]> = SmallVec::with_capacity(1);
+    assert!(v.is_empty());
+    assert!(!v.spilled());
+    assert_eq!(v.capacity(), 3);
+
+    let v: SmallVec<[u8; 3]> = SmallVec::with_capacity(10);
+    assert!(v.is_empty());
+    assert!(v.spilled());
+    assert_eq!(v.capacity(), 10);
+}
+
+#[test]
+fn drain() {
+    let mut v: SmallVec<[u8; 2]> = SmallVec::new();
+    v.push(3);
+    assert_eq!(v.drain(..).collect::<Vec<_>>(), &[3]);
+
+    // spilling the vec
+    v.push(3);
+    v.push(4);
+    v.push(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]);
+
+    // spilling the vec
+    v.push(3);
+    v.push(4);
+    v.push(5);
+    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]
+fn into_iter() {
+    let mut v: SmallVec<[u8; 2]> = SmallVec::new();
+    v.push(3);
+    assert_eq!(v.into_iter().collect::<Vec<_>>(), &[3]);
+
+    // spilling the vec
+    let mut v: SmallVec<[u8; 2]> = SmallVec::new();
+    v.push(3);
+    v.push(4);
+    v.push(5);
+    assert_eq!(v.into_iter().collect::<Vec<_>>(), &[3, 4, 5]);
+}
+
+#[test]
+fn into_iter_rev() {
+    let mut v: SmallVec<[u8; 2]> = SmallVec::new();
+    v.push(3);
+    assert_eq!(v.into_iter().rev().collect::<Vec<_>>(), &[3]);
+
+    // spilling the vec
+    let mut v: SmallVec<[u8; 2]> = SmallVec::new();
+    v.push(3);
+    v.push(4);
+    v.push(5);
+    assert_eq!(v.into_iter().rev().collect::<Vec<_>>(), &[5, 4, 3]);
+}
+
+#[test]
+fn into_iter_drop() {
+    use std::cell::Cell;
+
+    struct DropCounter<'a>(&'a Cell<i32>);
+
+    impl<'a> Drop for DropCounter<'a> {
+        fn drop(&mut self) {
+            self.0.set(self.0.get() + 1);
+        }
+    }
+
+    {
+        let cell = Cell::new(0);
+        let mut v: SmallVec<[DropCounter<'_>; 2]> = SmallVec::new();
+        v.push(DropCounter(&cell));
+        v.into_iter();
+        assert_eq!(cell.get(), 1);
+    }
+
+    {
+        let cell = Cell::new(0);
+        let mut v: SmallVec<[DropCounter<'_>; 2]> = SmallVec::new();
+        v.push(DropCounter(&cell));
+        v.push(DropCounter(&cell));
+        assert!(v.into_iter().next().is_some());
+        assert_eq!(cell.get(), 2);
+    }
+
+    {
+        let cell = Cell::new(0);
+        let mut v: SmallVec<[DropCounter<'_>; 2]> = SmallVec::new();
+        v.push(DropCounter(&cell));
+        v.push(DropCounter(&cell));
+        v.push(DropCounter(&cell));
+        assert!(v.into_iter().next().is_some());
+        assert_eq!(cell.get(), 3);
+    }
+    {
+        let cell = Cell::new(0);
+        let mut v: SmallVec<[DropCounter<'_>; 2]> = SmallVec::new();
+        v.push(DropCounter(&cell));
+        v.push(DropCounter(&cell));
+        v.push(DropCounter(&cell));
+        {
+            let mut it = v.into_iter();
+            assert!(it.next().is_some());
+            assert!(it.next_back().is_some());
+        }
+        assert_eq!(cell.get(), 3);
+    }
+}
+
+#[test]
+fn test_capacity() {
+    let mut v: SmallVec<[u8; 2]> = SmallVec::new();
+    v.reserve(1);
+    assert_eq!(v.capacity(), 2);
+    assert!(!v.spilled());
+
+    v.reserve_exact(0x100);
+    assert!(v.capacity() >= 0x100);
+
+    v.push(0);
+    v.push(1);
+    v.push(2);
+    v.push(3);
+
+    v.shrink_to_fit();
+    assert!(v.capacity() < 0x100);
+}
+
+#[test]
+fn test_truncate() {
+    let mut v: SmallVec<[Box<u8>; 8]> = SmallVec::new();
+
+    for x in 0..8 {
+        v.push(Box::new(x));
+    }
+    v.truncate(4);
+
+    assert_eq!(v.len(), 4);
+    assert!(!v.spilled());
+
+    assert_eq!(*v.swap_remove(1), 1);
+    assert_eq!(*v.remove(1), 3);
+    v.insert(1, Box::new(3));
+
+    assert_eq!(&v.iter().map(|v| **v).collect::<Vec<_>>(), &[0, 3, 2]);
+}
+
+#[test]
+fn test_insert_many() {
+    let mut v: SmallVec<[u8; 8]> = SmallVec::new();
+    for x in 0..4 {
+        v.push(x);
+    }
+    assert_eq!(v.len(), 4);
+    v.insert_many(1, [5, 6].iter().cloned());
+    assert_eq!(
+        &v.iter().map(|v| *v).collect::<Vec<_>>(),
+        &[0, 5, 6, 1, 2, 3]
+    );
+}
+
+struct MockHintIter<T: Iterator> {
+    x: T,
+    hint: usize,
+}
+impl<T: Iterator> Iterator for MockHintIter<T> {
+    type Item = T::Item;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.x.next()
+    }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.hint, None)
+    }
+}
+
+#[test]
+fn test_insert_many_short_hint() {
+    let mut v: SmallVec<[u8; 8]> = SmallVec::new();
+    for x in 0..4 {
+        v.push(x);
+    }
+    assert_eq!(v.len(), 4);
+    v.insert_many(
+        1,
+        MockHintIter {
+            x: [5, 6].iter().cloned(),
+            hint: 5,
+        },
+    );
+    assert_eq!(
+        &v.iter().map(|v| *v).collect::<Vec<_>>(),
+        &[0, 5, 6, 1, 2, 3]
+    );
+}
+
+#[test]
+fn test_insert_many_long_hint() {
+    let mut v: SmallVec<[u8; 8]> = SmallVec::new();
+    for x in 0..4 {
+        v.push(x);
+    }
+    assert_eq!(v.len(), 4);
+    v.insert_many(
+        1,
+        MockHintIter {
+            x: [5, 6].iter().cloned(),
+            hint: 1,
+        },
+    );
+    assert_eq!(
+        &v.iter().map(|v| *v).collect::<Vec<_>>(),
+        &[0, 5, 6, 1, 2, 3]
+    );
+}
+
+// https://github.com/servo/rust-smallvec/issues/96
+mod insert_many_panic {
+    use crate::{smallvec, SmallVec};
+    use alloc::boxed::Box;
+
+    struct PanicOnDoubleDrop {
+        dropped: Box<bool>,
+    }
+
+    impl PanicOnDoubleDrop {
+        fn new() -> Self {
+            Self {
+                dropped: Box::new(false),
+            }
+        }
+    }
+
+    impl Drop for PanicOnDoubleDrop {
+        fn drop(&mut self) {
+            assert!(!*self.dropped, "already dropped");
+            *self.dropped = true;
+        }
+    }
+
+    /// Claims to yield `hint` items, but actually yields `count`, then panics.
+    struct BadIter {
+        hint: usize,
+        count: usize,
+    }
+
+    impl Iterator for BadIter {
+        type Item = PanicOnDoubleDrop;
+        fn size_hint(&self) -> (usize, Option<usize>) {
+            (self.hint, None)
+        }
+        fn next(&mut self) -> Option<Self::Item> {
+            if self.count == 0 {
+                panic!()
+            }
+            self.count -= 1;
+            Some(PanicOnDoubleDrop::new())
+        }
+    }
+
+    #[test]
+    fn panic_early_at_start() {
+        let mut vec: SmallVec<[PanicOnDoubleDrop; 0]> =
+            smallvec![PanicOnDoubleDrop::new(), PanicOnDoubleDrop::new(),];
+        let result = ::std::panic::catch_unwind(move || {
+            vec.insert_many(0, BadIter { hint: 1, count: 0 });
+        });
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn panic_early_in_middle() {
+        let mut vec: SmallVec<[PanicOnDoubleDrop; 0]> =
+            smallvec![PanicOnDoubleDrop::new(), PanicOnDoubleDrop::new(),];
+        let result = ::std::panic::catch_unwind(move || {
+            vec.insert_many(1, BadIter { hint: 4, count: 2 });
+        });
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn panic_early_at_end() {
+        let mut vec: SmallVec<[PanicOnDoubleDrop; 0]> =
+            smallvec![PanicOnDoubleDrop::new(), PanicOnDoubleDrop::new(),];
+        let result = ::std::panic::catch_unwind(move || {
+            vec.insert_many(2, BadIter { hint: 3, count: 1 });
+        });
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn panic_late_at_start() {
+        let mut vec: SmallVec<[PanicOnDoubleDrop; 0]> =
+            smallvec![PanicOnDoubleDrop::new(), PanicOnDoubleDrop::new(),];
+        let result = ::std::panic::catch_unwind(move || {
+            vec.insert_many(0, BadIter { hint: 3, count: 5 });
+        });
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn panic_late_at_end() {
+        let mut vec: SmallVec<[PanicOnDoubleDrop; 0]> =
+            smallvec![PanicOnDoubleDrop::new(), PanicOnDoubleDrop::new(),];
+        let result = ::std::panic::catch_unwind(move || {
+            vec.insert_many(2, BadIter { hint: 3, count: 5 });
+        });
+        assert!(result.is_err());
+    }
+}
+
+#[test]
+#[should_panic]
+fn test_invalid_grow() {
+    let mut v: SmallVec<[u8; 8]> = SmallVec::new();
+    v.extend(0..8);
+    v.grow(5);
+}
+
+#[test]
+fn test_insert_from_slice() {
+    let mut v: SmallVec<[u8; 8]> = SmallVec::new();
+    for x in 0..4 {
+        v.push(x);
+    }
+    assert_eq!(v.len(), 4);
+    v.insert_from_slice(1, &[5, 6]);
+    assert_eq!(
+        &v.iter().map(|v| *v).collect::<Vec<_>>(),
+        &[0, 5, 6, 1, 2, 3]
+    );
+}
+
+#[test]
+fn test_extend_from_slice() {
+    let mut v: SmallVec<[u8; 8]> = SmallVec::new();
+    for x in 0..4 {
+        v.push(x);
+    }
+    assert_eq!(v.len(), 4);
+    v.extend_from_slice(&[5, 6]);
+    assert_eq!(
+        &v.iter().map(|v| *v).collect::<Vec<_>>(),
+        &[0, 1, 2, 3, 5, 6]
+    );
+}
+
+#[test]
+#[should_panic]
+fn test_drop_panic_smallvec() {
+    // This test should only panic once, and not double panic,
+    // which would mean a double drop
+    struct DropPanic;
+
+    impl Drop for DropPanic {
+        fn drop(&mut self) {
+            panic!("drop");
+        }
+    }
+
+    let mut v = SmallVec::<[_; 1]>::new();
+    v.push(DropPanic);
+}
+
+#[test]
+fn test_eq() {
+    let mut a: SmallVec<[u32; 2]> = SmallVec::new();
+    let mut b: SmallVec<[u32; 2]> = SmallVec::new();
+    let mut c: SmallVec<[u32; 2]> = SmallVec::new();
+    // a = [1, 2]
+    a.push(1);
+    a.push(2);
+    // b = [1, 2]
+    b.push(1);
+    b.push(2);
+    // c = [3, 4]
+    c.push(3);
+    c.push(4);
+
+    assert!(a == b);
+    assert!(a != c);
+}
+
+#[test]
+fn test_ord() {
+    let mut a: SmallVec<[u32; 2]> = SmallVec::new();
+    let mut b: SmallVec<[u32; 2]> = SmallVec::new();
+    let mut c: SmallVec<[u32; 2]> = SmallVec::new();
+    // a = [1]
+    a.push(1);
+    // b = [1, 1]
+    b.push(1);
+    b.push(1);
+    // c = [1, 2]
+    c.push(1);
+    c.push(2);
+
+    assert!(a < b);
+    assert!(b > a);
+    assert!(b < c);
+    assert!(c > b);
+}
+
+#[test]
+fn test_hash() {
+    use std::collections::hash_map::DefaultHasher;
+    use std::hash::Hash;
+
+    {
+        let mut a: SmallVec<[u32; 2]> = SmallVec::new();
+        let b = [1, 2];
+        a.extend(b.iter().cloned());
+        let mut hasher = DefaultHasher::new();
+        assert_eq!(a.hash(&mut hasher), b.hash(&mut hasher));
+    }
+    {
+        let mut a: SmallVec<[u32; 2]> = SmallVec::new();
+        let b = [1, 2, 11, 12];
+        a.extend(b.iter().cloned());
+        let mut hasher = DefaultHasher::new();
+        assert_eq!(a.hash(&mut hasher), b.hash(&mut hasher));
+    }
+}
+
+#[test]
+fn test_as_ref() {
+    let mut a: SmallVec<[u32; 2]> = SmallVec::new();
+    a.push(1);
+    assert_eq!(a.as_ref(), [1]);
+    a.push(2);
+    assert_eq!(a.as_ref(), [1, 2]);
+    a.push(3);
+    assert_eq!(a.as_ref(), [1, 2, 3]);
+}
+
+#[test]
+fn test_as_mut() {
+    let mut a: SmallVec<[u32; 2]> = SmallVec::new();
+    a.push(1);
+    assert_eq!(a.as_mut(), [1]);
+    a.push(2);
+    assert_eq!(a.as_mut(), [1, 2]);
+    a.push(3);
+    assert_eq!(a.as_mut(), [1, 2, 3]);
+    a.as_mut()[1] = 4;
+    assert_eq!(a.as_mut(), [1, 4, 3]);
+}
+
+#[test]
+fn test_borrow() {
+    use std::borrow::Borrow;
+
+    let mut a: SmallVec<[u32; 2]> = SmallVec::new();
+    a.push(1);
+    assert_eq!(a.borrow(), [1]);
+    a.push(2);
+    assert_eq!(a.borrow(), [1, 2]);
+    a.push(3);
+    assert_eq!(a.borrow(), [1, 2, 3]);
+}
+
+#[test]
+fn test_borrow_mut() {
+    use std::borrow::BorrowMut;
+
+    let mut a: SmallVec<[u32; 2]> = SmallVec::new();
+    a.push(1);
+    assert_eq!(a.borrow_mut(), [1]);
+    a.push(2);
+    assert_eq!(a.borrow_mut(), [1, 2]);
+    a.push(3);
+    assert_eq!(a.borrow_mut(), [1, 2, 3]);
+    BorrowMut::<[u32]>::borrow_mut(&mut a)[1] = 4;
+    assert_eq!(a.borrow_mut(), [1, 4, 3]);
+}
+
+#[test]
+fn test_from() {
+    assert_eq!(&SmallVec::<[u32; 2]>::from(&[1][..])[..], [1]);
+    assert_eq!(&SmallVec::<[u32; 2]>::from(&[1, 2, 3][..])[..], [1, 2, 3]);
+
+    let vec = vec![];
+    let small_vec: SmallVec<[u8; 3]> = SmallVec::from(vec);
+    assert_eq!(&*small_vec, &[]);
+    drop(small_vec);
+
+    let vec = vec![1, 2, 3, 4, 5];
+    let small_vec: SmallVec<[u8; 3]> = SmallVec::from(vec);
+    assert_eq!(&*small_vec, &[1, 2, 3, 4, 5]);
+    drop(small_vec);
+
+    let vec = vec![1, 2, 3, 4, 5];
+    let small_vec: SmallVec<[u8; 1]> = SmallVec::from(vec);
+    assert_eq!(&*small_vec, &[1, 2, 3, 4, 5]);
+    drop(small_vec);
+
+    let array = [1];
+    let small_vec: SmallVec<[u8; 1]> = SmallVec::from(array);
+    assert_eq!(&*small_vec, &[1]);
+    drop(small_vec);
+
+    let array = [99; 128];
+    let small_vec: SmallVec<[u8; 128]> = SmallVec::from(array);
+    assert_eq!(&*small_vec, vec![99u8; 128].as_slice());
+    drop(small_vec);
+}
+
+#[test]
+fn test_from_slice() {
+    assert_eq!(&SmallVec::<[u32; 2]>::from_slice(&[1][..])[..], [1]);
+    assert_eq!(
+        &SmallVec::<[u32; 2]>::from_slice(&[1, 2, 3][..])[..],
+        [1, 2, 3]
+    );
+}
+
+#[test]
+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(..2).len(), 2);
+    assert_eq!(vec.into_iter().len(), 1);
+}
+
+#[test]
+fn test_into_iter_as_slice() {
+    let vec = SmallVec::<[u32; 2]>::from(&[1, 2, 3][..]);
+    let mut iter = vec.clone().into_iter();
+    assert_eq!(iter.as_slice(), &[1, 2, 3]);
+    assert_eq!(iter.as_mut_slice(), &[1, 2, 3]);
+    iter.next();
+    assert_eq!(iter.as_slice(), &[2, 3]);
+    assert_eq!(iter.as_mut_slice(), &[2, 3]);
+    iter.next_back();
+    assert_eq!(iter.as_slice(), &[2]);
+    assert_eq!(iter.as_mut_slice(), &[2]);
+}
+
+#[test]
+fn test_into_iter_clone() {
+    // Test that the cloned iterator yields identical elements and that it owns its own copy
+    // (i.e. no use after move errors).
+    let mut iter = SmallVec::<[u8; 2]>::from_iter(0..3).into_iter();
+    let mut clone_iter = iter.clone();
+    while let Some(x) = iter.next() {
+        assert_eq!(x, clone_iter.next().unwrap());
+    }
+    assert_eq!(clone_iter.next(), None);
+}
+
+#[test]
+fn test_into_iter_clone_partially_consumed_iterator() {
+    // Test that the cloned iterator only contains the remaining elements of the original iterator.
+    let mut iter = SmallVec::<[u8; 2]>::from_iter(0..3).into_iter().skip(1);
+    let mut clone_iter = iter.clone();
+    while let Some(x) = iter.next() {
+        assert_eq!(x, clone_iter.next().unwrap());
+    }
+    assert_eq!(clone_iter.next(), None);
+}
+
+#[test]
+fn test_into_iter_clone_empty_smallvec() {
+    let mut iter = SmallVec::<[u8; 2]>::new().into_iter();
+    let mut clone_iter = iter.clone();
+    assert_eq!(iter.next(), None);
+    assert_eq!(clone_iter.next(), None);
+}
+
+#[test]
+fn shrink_to_fit_unspill() {
+    let mut vec = SmallVec::<[u8; 2]>::from_iter(0..3);
+    vec.pop();
+    assert!(vec.spilled());
+    vec.shrink_to_fit();
+    assert!(!vec.spilled(), "shrink_to_fit will un-spill if possible");
+}
+
+#[test]
+fn test_into_vec() {
+    let vec = SmallVec::<[u8; 2]>::from_iter(0..2);
+    assert_eq!(vec.into_vec(), vec![0, 1]);
+
+    let vec = SmallVec::<[u8; 2]>::from_iter(0..3);
+    assert_eq!(vec.into_vec(), vec![0, 1, 2]);
+}
+
+#[test]
+fn test_into_inner() {
+    let vec = SmallVec::<[u8; 2]>::from_iter(0..2);
+    assert_eq!(vec.into_inner(), Ok([0, 1]));
+
+    let vec = SmallVec::<[u8; 2]>::from_iter(0..1);
+    assert_eq!(vec.clone().into_inner(), Err(vec));
+
+    let vec = SmallVec::<[u8; 2]>::from_iter(0..3);
+    assert_eq!(vec.clone().into_inner(), Err(vec));
+}
+
+#[test]
+fn test_from_vec() {
+    let vec = vec![];
+    let small_vec: SmallVec<[u8; 3]> = SmallVec::from_vec(vec);
+    assert_eq!(&*small_vec, &[]);
+    drop(small_vec);
+
+    let vec = vec![];
+    let small_vec: SmallVec<[u8; 1]> = SmallVec::from_vec(vec);
+    assert_eq!(&*small_vec, &[]);
+    drop(small_vec);
+
+    let vec = vec![1];
+    let small_vec: SmallVec<[u8; 3]> = SmallVec::from_vec(vec);
+    assert_eq!(&*small_vec, &[1]);
+    drop(small_vec);
+
+    let vec = vec![1, 2, 3];
+    let small_vec: SmallVec<[u8; 3]> = SmallVec::from_vec(vec);
+    assert_eq!(&*small_vec, &[1, 2, 3]);
+    drop(small_vec);
+
+    let vec = vec![1, 2, 3, 4, 5];
+    let small_vec: SmallVec<[u8; 3]> = SmallVec::from_vec(vec);
+    assert_eq!(&*small_vec, &[1, 2, 3, 4, 5]);
+    drop(small_vec);
+
+    let vec = vec![1, 2, 3, 4, 5];
+    let small_vec: SmallVec<[u8; 1]> = SmallVec::from_vec(vec);
+    assert_eq!(&*small_vec, &[1, 2, 3, 4, 5]);
+    drop(small_vec);
+}
+
+#[test]
+fn test_retain() {
+    // Test inline data storate
+    let mut sv: SmallVec<[i32; 5]> = SmallVec::from_slice(&[1, 2, 3, 3, 4]);
+    sv.retain(|&mut i| i != 3);
+    assert_eq!(sv.pop(), Some(4));
+    assert_eq!(sv.pop(), Some(2));
+    assert_eq!(sv.pop(), Some(1));
+    assert_eq!(sv.pop(), None);
+
+    // Test spilled data storage
+    let mut sv: SmallVec<[i32; 3]> = SmallVec::from_slice(&[1, 2, 3, 3, 4]);
+    sv.retain(|&mut i| i != 3);
+    assert_eq!(sv.pop(), Some(4));
+    assert_eq!(sv.pop(), Some(2));
+    assert_eq!(sv.pop(), Some(1));
+    assert_eq!(sv.pop(), None);
+
+    // Test that drop implementations are called for inline.
+    let one = Rc::new(1);
+    let mut sv: SmallVec<[Rc<i32>; 3]> = SmallVec::new();
+    sv.push(Rc::clone(&one));
+    assert_eq!(Rc::strong_count(&one), 2);
+    sv.retain(|_| false);
+    assert_eq!(Rc::strong_count(&one), 1);
+
+    // Test that drop implementations are called for spilled data.
+    let mut sv: SmallVec<[Rc<i32>; 1]> = SmallVec::new();
+    sv.push(Rc::clone(&one));
+    sv.push(Rc::new(2));
+    assert_eq!(Rc::strong_count(&one), 2);
+    sv.retain(|_| false);
+    assert_eq!(Rc::strong_count(&one), 1);
+}
+
+#[test]
+fn test_dedup() {
+    let mut dupes: SmallVec<[i32; 5]> = SmallVec::from_slice(&[1, 1, 2, 3, 3]);
+    dupes.dedup();
+    assert_eq!(&*dupes, &[1, 2, 3]);
+
+    let mut empty: SmallVec<[i32; 5]> = SmallVec::new();
+    empty.dedup();
+    assert!(empty.is_empty());
+
+    let mut all_ones: SmallVec<[i32; 5]> = SmallVec::from_slice(&[1, 1, 1, 1, 1]);
+    all_ones.dedup();
+    assert_eq!(all_ones.len(), 1);
+
+    let mut no_dupes: SmallVec<[i32; 5]> = SmallVec::from_slice(&[1, 2, 3, 4, 5]);
+    no_dupes.dedup();
+    assert_eq!(no_dupes.len(), 5);
+}
+
+#[test]
+fn test_resize() {
+    let mut v: SmallVec<[i32; 8]> = SmallVec::new();
+    v.push(1);
+    v.resize(5, 0);
+    assert_eq!(v[..], [1, 0, 0, 0, 0][..]);
+
+    v.resize(2, -1);
+    assert_eq!(v[..], [1, 0][..]);
+}
+
+#[cfg(feature = "write")]
+#[test]
+fn test_write() {
+    use std::io::Write;
+
+    let data = [1, 2, 3, 4, 5];
+
+    let mut small_vec: SmallVec<[u8; 2]> = SmallVec::new();
+    let len = small_vec.write(&data[..]).unwrap();
+    assert_eq!(len, 5);
+    assert_eq!(small_vec.as_ref(), data.as_ref());
+
+    let mut small_vec: SmallVec<[u8; 2]> = SmallVec::new();
+    small_vec.write_all(&data[..]).unwrap();
+    assert_eq!(small_vec.as_ref(), data.as_ref());
+}
+
+#[cfg(feature = "serde")]
+extern crate bincode;
+
+#[cfg(feature = "serde")]
+#[test]
+fn test_serde() {
+    use self::bincode::{config, deserialize};
+    let mut small_vec: SmallVec<[i32; 2]> = SmallVec::new();
+    small_vec.push(1);
+    let encoded = config().limit(100).serialize(&small_vec).unwrap();
+    let decoded: SmallVec<[i32; 2]> = deserialize(&encoded).unwrap();
+    assert_eq!(small_vec, decoded);
+    small_vec.push(2);
+    // Spill the vec
+    small_vec.push(3);
+    small_vec.push(4);
+    // Check again after spilling.
+    let encoded = config().limit(100).serialize(&small_vec).unwrap();
+    let decoded: SmallVec<[i32; 2]> = deserialize(&encoded).unwrap();
+    assert_eq!(small_vec, decoded);
+}
+
+#[test]
+fn grow_to_shrink() {
+    let mut v: SmallVec<[u8; 2]> = SmallVec::new();
+    v.push(1);
+    v.push(2);
+    v.push(3);
+    assert!(v.spilled());
+    v.clear();
+    // Shrink to inline.
+    v.grow(2);
+    assert!(!v.spilled());
+    assert_eq!(v.capacity(), 2);
+    assert_eq!(v.len(), 0);
+    v.push(4);
+    assert_eq!(v[..], [4]);
+}
+
+#[test]
+fn resumable_extend() {
+    let s = "a b c";
+    // This iterator yields: (Some('a'), None, Some('b'), None, Some('c')), None
+    let it = s
+        .chars()
+        .scan(0, |_, ch| if ch.is_whitespace() { None } else { Some(ch) });
+    let mut v: SmallVec<[char; 4]> = SmallVec::new();
+    v.extend(it);
+    assert_eq!(v[..], ['a']);
+}
+
+// #139
+#[test]
+fn uninhabited() {
+    enum Void {}
+    let _sv = SmallVec::<[Void; 8]>::new();
+}
+
+#[test]
+fn grow_spilled_same_size() {
+    let mut v: SmallVec<[u8; 2]> = SmallVec::new();
+    v.push(0);
+    v.push(1);
+    v.push(2);
+    assert!(v.spilled());
+    assert_eq!(v.capacity(), 4);
+    // grow with the same capacity
+    v.grow(4);
+    assert_eq!(v.capacity(), 4);
+    assert_eq!(v[..], [0, 1, 2]);
+}
+
+#[cfg(feature = "const_generics")]
+#[test]
+fn const_generics() {
+    let _v = SmallVec::<[i32; 987]>::default();
+}
+
+#[test]
+fn empty_macro() {
+    let _v: SmallVec<[u8; 1]> = smallvec![];
+}
+
+#[test]
+fn zero_size_items() {
+    SmallVec::<[(); 0]>::new().push(());
+}