Add `array::IntoIter` as a consuming/by-value array iterator

The iterator is implemented using const generics. It implements the
traits `Iterator`, `DoubleEndedIterator`, `ExactSizeIterator`,
`FusedIterator` and `TrustedLen`. It also contains a public method
`new` to create it from an array.

`IntoIterator` was not implemented for arrays yet, as there are still
some open questions regarding backwards compatibility. This commit
only adds the iterator impl and does not yet offer a convenient way
to obtain that iterator.

2 files changed, 698 insertions, 0 deletions

diff --git a/src/libcore/array/iter.rs b/src/libcore/array/iter.rs
new file mode 100644
index 00000000000..850a599c659
--- /dev/null
+++ b/src/libcore/array/iter.rs
@@ -0,0 +1,266 @@
+//! Defines the `IntoIter` owned iterator for arrays.
+
+use crate::{
+    fmt,
+    iter::{ExactSizeIterator, FusedIterator, TrustedLen},
+    mem::{self, MaybeUninit},
+    ops::Range,
+    ptr,
+};
+use super::LengthAtMost32;
+
+
+/// A by-value [array] iterator.
+///
+/// [array]: ../../std/primitive.array.html
+#[unstable(feature = "array_value_iter", issue = "0")]
+pub struct IntoIter<T, const N: usize>
+where
+    [T; N]: LengthAtMost32,
+{
+    /// This is the array we are iterating over.
+    ///
+    /// Elements with index `i` where `alive.start <= i < alive.end` have not
+    /// been yielded yet and are valid array entries. Elements with indices `i
+    /// < alive.start` or `i >= alive.end` have been yielded already and must
+    /// not be accessed anymore! Those dead elements might even be in a
+    /// completely uninitialized state!
+    ///
+    /// So the invariants are:
+    /// - `data[alive]` is alive (i.e. contains valid elements)
+    /// - `data[..alive.start]` and `data[alive.end..]` are dead (i.e. the
+    ///   elements were already read and must not be touched anymore!)
+    data: [MaybeUninit<T>; N],
+
+    /// The elements in `data` that have not been yielded yet.
+    ///
+    /// Invariants:
+    /// - `alive.start <= alive.end`
+    /// - `alive.end <= N`
+    alive: Range<usize>,
+}
+
+impl<T, const N: usize> IntoIter<T, {N}>
+where
+    [T; N]: LengthAtMost32,
+{
+    /// Creates a new iterator over the given `array`.
+    ///
+    /// *Note*: this method might never get stabilized and/or removed in the
+    /// future as there will likely be another, preferred way of obtaining this
+    /// iterator (either via `IntoIterator` for arrays or via another way).
+    #[unstable(feature = "array_value_iter", issue = "0")]
+    pub fn new(array: [T; N]) -> Self {
+        // The transmute here is actually safe. The docs of `MaybeUninit`
+        // promise:
+        //
+        // > `MaybeUninit<T>` is guaranteed to have the same size and alignment
+        // > as `T`.
+        //
+        // The docs even show a transmute from an array of `MaybeUninit<T>` to
+        // an array of `T`.
+        //
+        // With that, this initialization satisfies the invariants.
+
+        // FIXME(LukasKalbertodt): actually use `mem::transmute` here, once it
+        // works with const generics:
+        //     `mem::transmute::<[T; {N}], [MaybeUninit<T>; {N}]>(array)`
+        //
+        // Until then, we do it manually here. We first create a bitwise copy
+        // but cast the pointer so that it is treated as a different type. Then
+        // we forget `array` so that it is not dropped.
+        let data = unsafe {
+            let data = ptr::read(&array as *const [T; N] as *const [MaybeUninit<T>; N]);
+            mem::forget(array);
+            data
+        };
+
+        Self {
+            data,
+            alive: 0..N,
+        }
+    }
+
+    /// Returns an immutable slice of all elements that have not been yielded
+    /// yet.
+    fn as_slice(&self) -> &[T] {
+        // This transmute is safe. As mentioned in `new`, `MaybeUninit` retains
+        // the size and alignment of `T`. Furthermore, we know that all
+        // elements within `alive` are properly initialized.
+        let slice = &self.data[self.alive.clone()];
+        unsafe {
+            mem::transmute::<&[MaybeUninit<T>], &[T]>(slice)
+        }
+    }
+}
+
+
+#[stable(feature = "array_value_iter_impls", since = "1.38.0")]
+impl<T, const N: usize> Iterator for IntoIter<T, {N}>
+where
+    [T; N]: LengthAtMost32,
+{
+    type Item = T;
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.alive.start == self.alive.end {
+            return None;
+        }
+
+        // Bump start index.
+        //
+        // From the check above we know that `alive.start != alive.end`.
+        // Combine this with the invariant `alive.start <= alive.end`, we know
+        // that `alive.start < alive.end`. Increasing `alive.start` by 1
+        // maintains the invariant regarding `alive`. However, due to this
+        // change, for a short time, the alive zone is not `data[alive]`
+        // anymore, but `data[idx..alive.end]`.
+        let idx = self.alive.start;
+        self.alive.start += 1;
+
+        // Read the element from the array. This is safe: `idx` is an index
+        // into the "alive" region of the array. Reading this element means
+        // that `data[idx]` is regarded as dead now (i.e. do not touch). As
+        // `idx` was the start of the alive-zone, the alive zone is now
+        // `data[alive]` again, restoring all invariants.
+        let out = unsafe { self.data.get_unchecked(idx).read() };
+
+        Some(out)
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.len();
+        (len, Some(len))
+    }
+
+    fn count(self) -> usize {
+        self.len()
+    }
+
+    fn last(mut self) -> Option<Self::Item> {
+        self.next_back()
+    }
+}
+
+#[stable(feature = "array_value_iter_impls", since = "1.38.0")]
+impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, {N}>
+where
+    [T; N]: LengthAtMost32,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        if self.alive.start == self.alive.end {
+            return None;
+        }
+
+        // Decrease end index.
+        //
+        // From the check above we know that `alive.start != alive.end`.
+        // Combine this with the invariant `alive.start <= alive.end`, we know
+        // that `alive.start < alive.end`. As `alive.start` cannot be negative,
+        // `alive.end` is at least 1, meaning that we can safely decrement it
+        // by one. This also maintains the invariant `alive.start <=
+        // alive.end`. However, due to this change, for a short time, the alive
+        // zone is not `data[alive]` anymore, but `data[alive.start..alive.end
+        // + 1]`.
+        self.alive.end -= 1;
+
+        // Read the element from the array. This is safe: `alive.end` is an
+        // index into the "alive" region of the array. Compare the previous
+        // comment that states that the alive region is
+        // `data[alive.start..alive.end + 1]`. Reading this element means that
+        // `data[alive.end]` is regarded as dead now (i.e. do not touch). As
+        // `alive.end` was the end of the alive-zone, the alive zone is now
+        // `data[alive]` again, restoring all invariants.
+        let out = unsafe { self.data.get_unchecked(self.alive.end).read() };
+
+        Some(out)
+    }
+}
+
+#[stable(feature = "array_value_iter_impls", since = "1.38.0")]
+impl<T, const N: usize> Drop for IntoIter<T, {N}>
+where
+    [T; N]: LengthAtMost32,
+{
+    fn drop(&mut self) {
+        // We simply drop each element via `for_each`. This should not incur
+        // any significant runtime overhead and avoids adding another `unsafe`
+        // block.
+        self.by_ref().for_each(drop);
+    }
+}
+
+#[stable(feature = "array_value_iter_impls", since = "1.38.0")]
+impl<T, const N: usize> ExactSizeIterator for IntoIter<T, {N}>
+where
+    [T; N]: LengthAtMost32,
+{
+    fn len(&self) -> usize {
+        // Will never underflow due to the invariant `alive.start <=
+        // alive.end`.
+        self.alive.end - self.alive.start
+    }
+    fn is_empty(&self) -> bool {
+        self.alive.is_empty()
+    }
+}
+
+#[stable(feature = "array_value_iter_impls", since = "1.38.0")]
+impl<T, const N: usize> FusedIterator for IntoIter<T, {N}>
+where
+    [T; N]: LengthAtMost32,
+{}
+
+// The iterator indeed reports the correct length. The number of "alive"
+// elements (that will still be yielded) is the length of the range `alive`.
+// This range is decremented in length in either `next` or `next_back`. It is
+// always decremented by 1 in those methods, but only if `Some(_)` is returned.
+#[stable(feature = "array_value_iter_impls", since = "1.38.0")]
+unsafe impl<T, const N: usize> TrustedLen for IntoIter<T, {N}>
+where
+    [T; N]: LengthAtMost32,
+{}
+
+#[stable(feature = "array_value_iter_impls", since = "1.38.0")]
+impl<T: Clone, const N: usize> Clone for IntoIter<T, {N}>
+where
+    [T; N]: LengthAtMost32,
+{
+    fn clone(&self) -> Self {
+        unsafe {
+            // This creates a new uninitialized array. Note that the `assume_init`
+            // refers to the array, not the individual elements. And it is Ok if
+            // the array is in an uninitialized state as all elements may be
+            // uninitialized (all bit patterns are valid). Compare the
+            // `MaybeUninit` docs for more information.
+            let mut new_data: [MaybeUninit<T>; N] = MaybeUninit::uninit().assume_init();
+
+            // Clone all alive elements.
+            for idx in self.alive.clone() {
+                // The element at `idx` in the old array is alive, so we can
+                // safely call `get_ref()`. We then clone it, and write the
+                // clone into the new array.
+                let clone = self.data.get_unchecked(idx).get_ref().clone();
+                new_data.get_unchecked_mut(idx).write(clone);
+            }
+
+            Self {
+                data: new_data,
+                alive: self.alive.clone(),
+            }
+        }
+    }
+}
+
+#[stable(feature = "array_value_iter_impls", since = "1.38.0")]
+impl<T: fmt::Debug, const N: usize> fmt::Debug for IntoIter<T, {N}>
+where
+    [T; N]: LengthAtMost32,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        // Only print the elements that were not yielded yet: we cannot
+        // access the yielded elements anymore.
+        f.debug_tuple("IntoIter")
+            .field(&self.as_slice())
+            .finish()
+    }
+}
diff --git a/src/libcore/array/mod.rs b/src/libcore/array/mod.rs
new file mode 100644
index 00000000000..120658e9a43
--- /dev/null
+++ b/src/libcore/array/mod.rs
@@ -0,0 +1,432 @@
+//! Implementations of things like `Eq` for fixed-length arrays
+//! up to a certain length. Eventually we should able to generalize
+//! to all lengths.
+//!
+//! *[See also the array primitive type](../../std/primitive.array.html).*
+
+#![stable(feature = "core_array", since = "1.36.0")]
+
+use crate::borrow::{Borrow, BorrowMut};
+use crate::cmp::Ordering;
+use crate::convert::{Infallible, TryFrom};
+use crate::fmt;
+use crate::hash::{Hash, self};
+use crate::marker::Unsize;
+use crate::slice::{Iter, IterMut};
+
+#[cfg(not(bootstrap))]
+mod iter;
+
+#[cfg(not(bootstrap))]
+#[unstable(feature = "array_value_iter", issue = "0")]
+pub use iter::IntoIter;
+
+/// Utility trait implemented only on arrays of fixed size
+///
+/// This trait can be used to implement other traits on fixed-size arrays
+/// without causing much metadata bloat.
+///
+/// The trait is marked unsafe in order to restrict implementors to fixed-size
+/// arrays. User of this trait can assume that implementors have the exact
+/// layout in memory of a fixed size array (for example, for unsafe
+/// initialization).
+///
+/// Note that the traits [`AsRef`] and [`AsMut`] provide similar methods for types that
+/// may not be fixed-size arrays. Implementors should prefer those traits
+/// instead.
+///
+/// [`AsRef`]: ../convert/trait.AsRef.html
+/// [`AsMut`]: ../convert/trait.AsMut.html
+#[unstable(feature = "fixed_size_array", issue = "27778")]
+pub unsafe trait FixedSizeArray<T> {
+    /// Converts the array to immutable slice
+    #[unstable(feature = "fixed_size_array", issue = "27778")]
+    fn as_slice(&self) -> &[T];
+    /// Converts the array to mutable slice
+    #[unstable(feature = "fixed_size_array", issue = "27778")]
+    fn as_mut_slice(&mut self) -> &mut [T];
+}
+
+#[unstable(feature = "fixed_size_array", issue = "27778")]
+unsafe impl<T, A: Unsize<[T]>> FixedSizeArray<T> for A {
+    #[inline]
+    fn as_slice(&self) -> &[T] {
+        self
+    }
+    #[inline]
+    fn as_mut_slice(&mut self) -> &mut [T] {
+        self
+    }
+}
+
+/// The error type returned when a conversion from a slice to an array fails.
+#[stable(feature = "try_from", since = "1.34.0")]
+#[derive(Debug, Copy, Clone)]
+pub struct TryFromSliceError(());
+
+#[stable(feature = "core_array", since = "1.36.0")]
+impl fmt::Display for TryFromSliceError {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Display::fmt(self.__description(), f)
+    }
+}
+
+impl TryFromSliceError {
+    #[unstable(feature = "array_error_internals",
+           reason = "available through Error trait and this method should not \
+                     be exposed publicly",
+           issue = "0")]
+    #[inline]
+    #[doc(hidden)]
+    pub fn __description(&self) -> &str {
+        "could not convert slice to array"
+    }
+}
+
+#[stable(feature = "try_from_slice_error", since = "1.36.0")]
+impl From<Infallible> for TryFromSliceError {
+    fn from(x: Infallible) -> TryFromSliceError {
+        match x {}
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T, const N: usize> AsRef<[T]> for [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    #[inline]
+    fn as_ref(&self) -> &[T] {
+        &self[..]
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T, const N: usize> AsMut<[T]> for [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    #[inline]
+    fn as_mut(&mut self) -> &mut [T] {
+        &mut self[..]
+    }
+}
+
+#[stable(feature = "array_borrow", since = "1.4.0")]
+impl<T, const N: usize> Borrow<[T]> for [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    fn borrow(&self) -> &[T] {
+        self
+    }
+}
+
+#[stable(feature = "array_borrow", since = "1.4.0")]
+impl<T, const N: usize> BorrowMut<[T]> for [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    fn borrow_mut(&mut self) -> &mut [T] {
+        self
+    }
+}
+
+#[stable(feature = "try_from", since = "1.34.0")]
+impl<T, const N: usize> TryFrom<&[T]> for [T; N]
+where
+    T: Copy,
+    [T; N]: LengthAtMost32,
+{
+    type Error = TryFromSliceError;
+
+    fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
+        <&Self>::try_from(slice).map(|r| *r)
+    }
+}
+
+#[stable(feature = "try_from", since = "1.34.0")]
+impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    type Error = TryFromSliceError;
+
+    fn try_from(slice: &[T]) -> Result<&[T; N], TryFromSliceError> {
+        if slice.len() == N {
+            let ptr = slice.as_ptr() as *const [T; N];
+            unsafe { Ok(&*ptr) }
+        } else {
+            Err(TryFromSliceError(()))
+        }
+    }
+}
+
+#[stable(feature = "try_from", since = "1.34.0")]
+impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    type Error = TryFromSliceError;
+
+    fn try_from(slice: &mut [T]) -> Result<&mut [T; N], TryFromSliceError> {
+        if slice.len() == N {
+            let ptr = slice.as_mut_ptr() as *mut [T; N];
+            unsafe { Ok(&mut *ptr) }
+        } else {
+            Err(TryFromSliceError(()))
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Hash, const N: usize> Hash for [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    fn hash<H: hash::Hasher>(&self, state: &mut H) {
+        Hash::hash(&self[..], state)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&&self[..], f)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T, const N: usize> IntoIterator for &'a [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    type Item = &'a T;
+    type IntoIter = Iter<'a, T>;
+
+    fn into_iter(self) -> Iter<'a, T> {
+        self.iter()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    type Item = &'a mut T;
+    type IntoIter = IterMut<'a, T>;
+
+    fn into_iter(self) -> IterMut<'a, T> {
+        self.iter_mut()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A, B, const N: usize> PartialEq<[B; N]> for [A; N]
+where
+    A: PartialEq<B>,
+    [A; N]: LengthAtMost32,
+    [B; N]: LengthAtMost32,
+{
+    #[inline]
+    fn eq(&self, other: &[B; N]) -> bool {
+        self[..] == other[..]
+    }
+    #[inline]
+    fn ne(&self, other: &[B; N]) -> bool {
+        self[..] != other[..]
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A, B, const N: usize> PartialEq<[B]> for [A; N]
+where
+    A: PartialEq<B>,
+    [A; N]: LengthAtMost32,
+{
+    #[inline]
+    fn eq(&self, other: &[B]) -> bool {
+        self[..] == other[..]
+    }
+    #[inline]
+    fn ne(&self, other: &[B]) -> bool {
+        self[..] != other[..]
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A, B, const N: usize> PartialEq<[A; N]> for [B]
+where
+    B: PartialEq<A>,
+    [A; N]: LengthAtMost32,
+{
+    #[inline]
+    fn eq(&self, other: &[A; N]) -> bool {
+        self[..] == other[..]
+    }
+    #[inline]
+    fn ne(&self, other: &[A; N]) -> bool {
+        self[..] != other[..]
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'b, A, B, const N: usize> PartialEq<&'b [B]> for [A; N]
+where
+    A: PartialEq<B>,
+    [A; N]: LengthAtMost32,
+{
+    #[inline]
+    fn eq(&self, other: &&'b [B]) -> bool {
+        self[..] == other[..]
+    }
+    #[inline]
+    fn ne(&self, other: &&'b [B]) -> bool {
+        self[..] != other[..]
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'b, A, B, const N: usize> PartialEq<[A; N]> for &'b [B]
+where
+    B: PartialEq<A>,
+    [A; N]: LengthAtMost32,
+{
+    #[inline]
+    fn eq(&self, other: &[A; N]) -> bool {
+        self[..] == other[..]
+    }
+    #[inline]
+    fn ne(&self, other: &[A; N]) -> bool {
+        self[..] != other[..]
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'b, A, B, const N: usize> PartialEq<&'b mut [B]> for [A; N]
+where
+    A: PartialEq<B>,
+    [A; N]: LengthAtMost32,
+{
+    #[inline]
+    fn eq(&self, other: &&'b mut [B]) -> bool {
+        self[..] == other[..]
+    }
+    #[inline]
+    fn ne(&self, other: &&'b mut [B]) -> bool {
+        self[..] != other[..]
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<'b, A, B, const N: usize> PartialEq<[A; N]> for &'b mut [B]
+where
+    B: PartialEq<A>,
+    [A; N]: LengthAtMost32,
+{
+    #[inline]
+    fn eq(&self, other: &[A; N]) -> bool {
+        self[..] == other[..]
+    }
+    #[inline]
+    fn ne(&self, other: &[A; N]) -> bool {
+        self[..] != other[..]
+    }
+}
+
+// NOTE: some less important impls are omitted to reduce code bloat
+// __impl_slice_eq2! { [A; $N], &'b [B; $N] }
+// __impl_slice_eq2! { [A; $N], &'b mut [B; $N] }
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Eq, const N: usize> Eq for [T; N] where [T; N]: LengthAtMost32 {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: PartialOrd, const N: usize> PartialOrd for [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    #[inline]
+    fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
+        PartialOrd::partial_cmp(&&self[..], &&other[..])
+    }
+    #[inline]
+    fn lt(&self, other: &[T; N]) -> bool {
+        PartialOrd::lt(&&self[..], &&other[..])
+    }
+    #[inline]
+    fn le(&self, other: &[T; N]) -> bool {
+        PartialOrd::le(&&self[..], &&other[..])
+    }
+    #[inline]
+    fn ge(&self, other: &[T; N]) -> bool {
+        PartialOrd::ge(&&self[..], &&other[..])
+    }
+    #[inline]
+    fn gt(&self, other: &[T; N]) -> bool {
+        PartialOrd::gt(&&self[..], &&other[..])
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Ord, const N: usize> Ord for [T; N]
+where
+    [T; N]: LengthAtMost32,
+{
+    #[inline]
+    fn cmp(&self, other: &[T; N]) -> Ordering {
+        Ord::cmp(&&self[..], &&other[..])
+    }
+}
+
+/// Implemented for lengths where trait impls are allowed on arrays in core/std
+#[rustc_on_unimplemented(
+    message="arrays only have std trait implementations for lengths 0..=32",
+)]
+#[unstable(feature = "const_generic_impls_guard", issue = "0",
+    reason = "will never be stable, just a temporary step until const generics are stable")]
+pub trait LengthAtMost32 {}
+
+macro_rules! array_impls {
+    ($($N:literal)+) => {
+        $(
+            #[unstable(feature = "const_generic_impls_guard", issue = "0")]
+            impl<T> LengthAtMost32 for [T; $N] {}
+        )+
+    }
+}
+
+array_impls! {
+     0  1  2  3  4  5  6  7  8  9
+    10 11 12 13 14 15 16 17 18 19
+    20 21 22 23 24 25 26 27 28 29
+    30 31 32
+}
+
+// The Default impls cannot be generated using the array_impls! macro because
+// they require array literals.
+
+macro_rules! array_impl_default {
+    {$n:expr, $t:ident $($ts:ident)*} => {
+        #[stable(since = "1.4.0", feature = "array_default")]
+        impl<T> Default for [T; $n] where T: Default {
+            fn default() -> [T; $n] {
+                [$t::default(), $($ts::default()),*]
+            }
+        }
+        array_impl_default!{($n - 1), $($ts)*}
+    };
+    {$n:expr,} => {
+        #[stable(since = "1.4.0", feature = "array_default")]
+        impl<T> Default for [T; $n] {
+            fn default() -> [T; $n] { [] }
+        }
+    };
+}
+
+array_impl_default!{32, T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T}