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    //! Comparison traits for `[T]`.

    use crate::cmp;
    use crate::cmp::Ordering::{self, Greater, Less};
    use crate::mem;

    use super::from_raw_parts;
    use super::memchr;

    extern "C" {
        /// Calls implementation provided memcmp.
        ///
        /// Interprets the data as u8.
        ///
        /// Returns 0 for equal, < 0 for less than and > 0 for greater
        /// than.
        // FIXME(#32610): Return type should be c_int
        fn memcmp(s1: *const u8, s2: *const u8, n: usize) -> i32;
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A, B> PartialEq<[B]> for [A]
    where
        A: PartialEq<B>,
    {
        fn eq(&self, other: &[B]) -> bool {
            SlicePartialEq::equal(self, other)
        }

        fn ne(&self, other: &[B]) -> bool {
            SlicePartialEq::not_equal(self, other)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Eq> Eq for [T] {}

    /// Implements comparison of vectors [lexicographically](Ord#lexicographical-comparison).
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Ord> Ord for [T] {
        fn cmp(&self, other: &[T]) -> Ordering {
            SliceOrd::compare(self, other)
        }
    }

    /// Implements comparison of vectors [lexicographically](Ord#lexicographical-comparison).
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: PartialOrd> PartialOrd for [T] {
        fn partial_cmp(&self, other: &[T]) -> Option<Ordering> {
            SlicePartialOrd::partial_compare(self, other)
        }
    }

    #[doc(hidden)]
    // intermediate trait for specialization of slice's PartialEq
    trait SlicePartialEq<B> {
        fn equal(&self, other: &[B]) -> bool;

        fn not_equal(&self, other: &[B]) -> bool {
            !self.equal(other)
        }
    }

    // Generic slice equality
    impl<A, B> SlicePartialEq<B> for [A]
    where
        A: PartialEq<B>,
    {
        default fn equal(&self, other: &[B]) -> bool {
            if self.len() != other.len() {
                return false;
            }

            self.iter().zip(other.iter()).all(|(x, y)| x == y)
        }
    }

    // Use an equal-pointer optimization when types are `Eq`
    // We can't make `A` and `B` the same type because `min_specialization` won't
    // allow it.
    impl<A, B> SlicePartialEq<B> for [A]
    where
        A: MarkerEq<B>,
    {
        default fn equal(&self, other: &[B]) -> bool {
            if self.len() != other.len() {
                return false;
            }

            // While performance would suffer if `guaranteed_eq` just returned `false`
            // for all arguments, correctness and return value of this function are not affected.
            if self.as_ptr().guaranteed_eq(other.as_ptr() as *const A) {
                return true;
            }

            self.iter().zip(other.iter()).all(|(x, y)| x == y)
        }
    }

    // Use memcmp for bytewise equality when the types allow
    impl<A, B> SlicePartialEq<B> for [A]
    where
        A: BytewiseEquality<B>,
    {
        fn equal(&self, other: &[B]) -> bool {
            if self.len() != other.len() {
                return false;
            }

            // While performance would suffer if `guaranteed_eq` just returned `false`
            // for all arguments, correctness and return value of this function are not affected.
            if self.as_ptr().guaranteed_eq(other.as_ptr() as *const A) {
                return true;
            }
            // SAFETY: `self` and `other` are references and are thus guaranteed to be valid.
            // The two slices have been checked to have the same size above.
            unsafe {
                let size = mem::size_of_val(self);
                memcmp(self.as_ptr() as *const u8, other.as_ptr() as *const u8, size) == 0
            }
        }
    }

    #[doc(hidden)]
    // intermediate trait for specialization of slice's PartialOrd
    trait SlicePartialOrd: Sized {
        fn partial_compare(left: &[Self], right: &[Self]) -> Option<Ordering>;
    }

    impl<A: PartialOrd> SlicePartialOrd for A {
        default fn partial_compare(left: &[A], right: &[A]) -> Option<Ordering> {
            let l = cmp::min(left.len(), right.len());

            // Slice to the loop iteration range to enable bound check
            // elimination in the compiler
            let lhs = &left[..l];
            let rhs = &right[..l];

            for i in 0..l {
                match lhs[i].partial_cmp(&rhs[i]) {
                    Some(Ordering::Equal) => (),
                    non_eq => return non_eq,
                }
            }

            left.len().partial_cmp(&right.len())
        }
    }

    // This is the impl that we would like to have. Unfortunately it's not sound.
    // See `partial_ord_slice.rs`.
    /*
    impl<A> SlicePartialOrd for A
    where
        A: Ord,
    {
        default fn partial_compare(left: &[A], right: &[A]) -> Option<Ordering> {
            Some(SliceOrd::compare(left, right))
        }
    }
    */

    impl<A: AlwaysApplicableOrd> SlicePartialOrd for A {
        fn partial_compare(left: &[A], right: &[A]) -> Option<Ordering> {
            Some(SliceOrd::compare(left, right))
        }
    }

    #[rustc_specialization_trait]
    trait AlwaysApplicableOrd: SliceOrd + Ord {}

    macro_rules! always_applicable_ord {
        ($([$($p:tt)*] $t:ty,)*) => {
            $(impl<$($p)*> AlwaysApplicableOrd for $t {})*
        }
    }

    always_applicable_ord! {
        [] u8, [] u16, [] u32, [] u64, [] u128, [] usize,
        [] i8, [] i16, [] i32, [] i64, [] i128, [] isize,
        [] bool, [] char,
        [T: ?Sized] *const T, [T: ?Sized] *mut T,
        [T: AlwaysApplicableOrd] &T,
        [T: AlwaysApplicableOrd] &mut T,
        [T: AlwaysApplicableOrd] Option<T>,
    }

    #[doc(hidden)]
    // intermediate trait for specialization of slice's Ord
    trait SliceOrd: Sized {
        fn compare(left: &[Self], right: &[Self]) -> Ordering;
    }

    impl<A: Ord> SliceOrd for A {
        default fn compare(left: &[Self], right: &[Self]) -> Ordering {
            let l = cmp::min(left.len(), right.len());

            // Slice to the loop iteration range to enable bound check
            // elimination in the compiler
            let lhs = &left[..l];
            let rhs = &right[..l];

            for i in 0..l {
                match lhs[i].cmp(&rhs[i]) {
                    Ordering::Equal => (),
                    non_eq => return non_eq,
                }
            }

            left.len().cmp(&right.len())
        }
    }

    // memcmp compares a sequence of unsigned bytes lexicographically.
    // this matches the order we want for [u8], but no others (not even [i8]).
    impl SliceOrd for u8 {
        #[inline]
        fn compare(left: &[Self], right: &[Self]) -> Ordering {
            let order =
                // SAFETY: `left` and `right` are references and are thus guaranteed to be valid.
                // We use the minimum of both lengths which guarantees that both regions are
                // valid for reads in that interval.
                unsafe { memcmp(left.as_ptr(), right.as_ptr(), cmp::min(left.len(), right.len())) };
            if order == 0 {
                left.len().cmp(&right.len())
            } else if order < 0 {
                Less
            } else {
                Greater
            }
        }
    }

    // Hack to allow specializing on `Eq` even though `Eq` has a method.
    #[rustc_unsafe_specialization_marker]
    trait MarkerEq<T>: PartialEq<T> {}

    impl<T: Eq> MarkerEq<T> for T {}

    #[doc(hidden)]
    /// Trait implemented for types that can be compared for equality using
    /// their bytewise representation
    #[rustc_specialization_trait]
    trait BytewiseEquality<T>: MarkerEq<T> + Copy {}

    macro_rules! impl_marker_for {
        ($traitname:ident, $($ty:ty)*) => {
            $(
                impl $traitname<$ty> for $ty { }
            )*
        }
    }

    impl_marker_for!(BytewiseEquality,
                     u8 i8 u16 i16 u32 i32 u64 i64 u128 i128 usize isize char bool);

    pub(super) trait SliceContains: Sized {
        fn slice_contains(&self, x: &[Self]) -> bool;
    }

    impl<T> SliceContains for T
    where
        T: PartialEq,
    {
        default fn slice_contains(&self, x: &[Self]) -> bool {
            x.iter().any(|y| *y == *self)
        }
    }

    impl SliceContains for u8 {
        #[inline]
        fn slice_contains(&self, x: &[Self]) -> bool {
            memchr::memchr(*self, x).is_some()
        }
    }

    impl SliceContains for i8 {
        #[inline]
        fn slice_contains(&self, x: &[Self]) -> bool {
            let byte = *self as u8;
            // SAFETY: `i8` and `u8` have the same memory layout, thus casting `x.as_ptr()`
            // as `*const u8` is safe. The `x.as_ptr()` comes from a reference and is thus guaranteed
            // to be valid for reads for the length of the slice `x.len()`, which cannot be larger
            // than `isize::MAX`. The returned slice is never mutated.
            let bytes: &[u8] = unsafe { from_raw_parts(x.as_ptr() as *const u8, x.len()) };
            memchr::memchr(byte, bytes).is_some()
        }
    }