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    //! 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};

    /// 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.
    #[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 {}
        }
    }

    macro_rules! __impl_slice_eq1 {
        ($Lhs: ty, $Rhs: ty) => {
            __impl_slice_eq1! { $Lhs, $Rhs, Sized }
        };
        ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
            #[stable(feature = "rust1", since = "1.0.0")]
            impl<'a, 'b, A: $Bound, B> PartialEq<$Rhs> for $Lhs where A: PartialEq<B> {
                #[inline]
                fn eq(&self, other: &$Rhs) -> bool { self[..] == other[..] }
                #[inline]
                fn ne(&self, other: &$Rhs) -> bool { self[..] != other[..] }
            }
        }
    }

    macro_rules! __impl_slice_eq2 {
        ($Lhs: ty, $Rhs: ty) => {
            __impl_slice_eq2! { $Lhs, $Rhs, Sized }
        };
        ($Lhs: ty, $Rhs: ty, $Bound: ident) => {
            __impl_slice_eq1!($Lhs, $Rhs, $Bound);

            #[stable(feature = "rust1", since = "1.0.0")]
            impl<'a, 'b, A: $Bound, B> PartialEq<$Lhs> for $Rhs where B: PartialEq<A> {
                #[inline]
                fn eq(&self, other: &$Lhs) -> bool { self[..] == other[..] }
                #[inline]
                fn ne(&self, other: &$Lhs) -> bool { self[..] != other[..] }
            }
        }
    }

    // macro for implementing n-element array functions and operations
    macro_rules! array_impls {
        ($($N:expr)+) => {
            $(
                #[stable(feature = "rust1", since = "1.0.0")]
                impl<T> AsRef<[T]> for [T; $N] {
                    #[inline]
                    fn as_ref(&self) -> &[T] {
                        &self[..]
                    }
                }

                #[stable(feature = "rust1", since = "1.0.0")]
                impl<T> AsMut<[T]> for [T; $N] {
                    #[inline]
                    fn as_mut(&mut self) -> &mut [T] {
                        &mut self[..]
                    }
                }

                #[stable(feature = "array_borrow", since = "1.4.0")]
                impl<T> Borrow<[T]> for [T; $N] {
                    fn borrow(&self) -> &[T] {
                        self
                    }
                }

                #[stable(feature = "array_borrow", since = "1.4.0")]
                impl<T> BorrowMut<[T]> for [T; $N] {
                    fn borrow_mut(&mut self) -> &mut [T] {
                        self
                    }
                }

                #[stable(feature = "try_from", since = "1.34.0")]
                impl<T> TryFrom<&[T]> for [T; $N] where T: Copy {
                    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> TryFrom<&'a [T]> for &'a [T; $N] {
                    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> TryFrom<&'a mut [T]> for &'a mut [T; $N] {
                    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> Hash for [T; $N] {
                    fn hash<H: hash::Hasher>(&self, state: &mut H) {
                        Hash::hash(&self[..], state)
                    }
                }

                #[stable(feature = "rust1", since = "1.0.0")]
                impl<T: fmt::Debug> fmt::Debug for [T; $N] {
                    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                        fmt::Debug::fmt(&&self[..], f)
                    }
                }

                #[stable(feature = "rust1", since = "1.0.0")]
                impl<'a, T> IntoIterator for &'a [T; $N] {
                    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> IntoIterator for &'a mut [T; $N] {
                    type Item = &'a mut T;
                    type IntoIter = IterMut<'a, T>;

                    fn into_iter(self) -> IterMut<'a, T> {
                        self.iter_mut()
                    }
                }

                // NOTE: some less important impls are omitted to reduce code bloat
                __impl_slice_eq1! { [A; $N], [B; $N] }
                __impl_slice_eq2! { [A; $N], [B] }
                __impl_slice_eq2! { [A; $N], &'b [B] }
                __impl_slice_eq2! { [A; $N], &'b mut [B] }
                // __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> Eq for [T; $N] { }

                #[stable(feature = "rust1", since = "1.0.0")]
                impl<T:PartialOrd> PartialOrd for [T; $N] {
                    #[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> Ord for [T; $N] {
                    #[inline]
                    fn cmp(&self, other: &[T; $N]) -> Ordering {
                        Ord::cmp(&&self[..], &&other[..])
                    }
                }
            )+
        }
    }

    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}

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