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.

1 files changed, 432 insertions, 0 deletions

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}