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    515

    use crate::convert::TryFrom;
    use crate::mem;
    use crate::ops::{self, Add, Sub, Try};
    use crate::usize;

    use super::{FusedIterator, TrustedLen};

    /// Objects that can be stepped over in both directions.
    ///
    /// The `steps_between` function provides a way to efficiently compare
    /// two `Step` objects.
    #[unstable(feature = "step_trait",
               reason = "likely to be replaced by finer-grained traits",
               issue = "42168")]
    pub trait Step: Clone + PartialOrd + Sized {
        /// Returns the number of steps between two step objects. The count is
        /// inclusive of `start` and exclusive of `end`.
        ///
        /// Returns `None` if it is not possible to calculate `steps_between`
        /// without overflow.
        fn steps_between(start: &Self, end: &Self) -> Option<usize>;

        /// Replaces this step with `1`, returning a clone of itself.
        ///
        /// The output of this method should always be greater than the output of replace_zero.
        fn replace_one(&mut self) -> Self;

        /// Replaces this step with `0`, returning a clone of itself.
        ///
        /// The output of this method should always be less than the output of replace_one.
        fn replace_zero(&mut self) -> Self;

        /// Adds one to this step, returning the result.
        fn add_one(&self) -> Self;

        /// Subtracts one to this step, returning the result.
        fn sub_one(&self) -> Self;

        /// Adds a `usize`, returning `None` on overflow.
        fn add_usize(&self, n: usize) -> Option<Self>;

        /// Subtracts a `usize`, returning `None` on underflow.
        fn sub_usize(&self, n: usize) -> Option<Self> {
            // this default implementation makes the addition of `sub_usize` a non-breaking change
            let _ = n;
            unimplemented!()
        }
    }

    // These are still macro-generated because the integer literals resolve to different types.
    macro_rules! step_identical_methods {
        () => {
            #[inline]
            fn replace_one(&mut self) -> Self {
                mem::replace(self, 1)
            }

            #[inline]
            fn replace_zero(&mut self) -> Self {
                mem::replace(self, 0)
            }

            #[inline]
            fn add_one(&self) -> Self {
                Add::add(*self, 1)
            }

            #[inline]
            fn sub_one(&self) -> Self {
                Sub::sub(*self, 1)
            }
        }
    }

    macro_rules! step_impl_unsigned {
        ($($t:ty)*) => ($(
            #[unstable(feature = "step_trait",
                       reason = "likely to be replaced by finer-grained traits",
                       issue = "42168")]
            impl Step for $t {
                #[inline]
                fn steps_between(start: &$t, end: &$t) -> Option<usize> {
                    if *start < *end {
                        usize::try_from(*end - *start).ok()
                    } else {
                        Some(0)
                    }
                }

                #[inline]
                #[allow(unreachable_patterns)]
                fn add_usize(&self, n: usize) -> Option<Self> {
                    match <$t>::try_from(n) {
                        Ok(n_as_t) => self.checked_add(n_as_t),
                        Err(_) => None,
                    }
                }

                #[inline]
                #[allow(unreachable_patterns)]
                fn sub_usize(&self, n: usize) -> Option<Self> {
                    match <$t>::try_from(n) {
                        Ok(n_as_t) => self.checked_sub(n_as_t),
                        Err(_) => None,
                    }
                }

                step_identical_methods!();
            }
        )*)
    }
    macro_rules! step_impl_signed {
        ($( [$t:ty : $unsigned:ty] )*) => ($(
            #[unstable(feature = "step_trait",
                       reason = "likely to be replaced by finer-grained traits",
                       issue = "42168")]
            impl Step for $t {
                #[inline]
                fn steps_between(start: &$t, end: &$t) -> Option<usize> {
                    if *start < *end {
                        // Use .wrapping_sub and cast to unsigned to compute the
                        // difference that may not fit inside the range of $t.
                        usize::try_from(end.wrapping_sub(*start) as $unsigned).ok()
                    } else {
                        Some(0)
                    }
                }

                #[inline]
                #[allow(unreachable_patterns)]
                fn add_usize(&self, n: usize) -> Option<Self> {
                    match <$unsigned>::try_from(n) {
                        Ok(n_as_unsigned) => {
                            // Wrapping in unsigned space handles cases like
                            // `-120_i8.add_usize(200) == Some(80_i8)`,
                            // even though 200_usize is out of range for i8.
                            let wrapped = (*self as $unsigned).wrapping_add(n_as_unsigned) as $t;
                            if wrapped >= *self {
                                Some(wrapped)
                            } else {
                                None  // Addition overflowed
                            }
                        }
                        Err(_) => None,
                    }
                }

                #[inline]
                #[allow(unreachable_patterns)]
                fn sub_usize(&self, n: usize) -> Option<Self> {
                    match <$unsigned>::try_from(n) {
                        Ok(n_as_unsigned) => {
                            // Wrapping in unsigned space handles cases like
                            // `80_i8.sub_usize(200) == Some(-120_i8)`,
                            // even though 200_usize is out of range for i8.
                            let wrapped = (*self as $unsigned).wrapping_sub(n_as_unsigned) as $t;
                            if wrapped <= *self {
                                Some(wrapped)
                            } else {
                                None  // Subtraction underflowed
                            }
                        }
                        Err(_) => None,
                    }
                }

                step_identical_methods!();
            }
        )*)
    }

    step_impl_unsigned!(usize u8 u16 u32 u64 u128);
    step_impl_signed!([isize: usize] [i8: u8] [i16: u16]);
    step_impl_signed!([i32: u32] [i64: u64] [i128: u128]);

    macro_rules! range_exact_iter_impl {
        ($($t:ty)*) => ($(
            #[stable(feature = "rust1", since = "1.0.0")]
            impl ExactSizeIterator for ops::Range<$t> { }
        )*)
    }

    macro_rules! range_incl_exact_iter_impl {
        ($($t:ty)*) => ($(
            #[stable(feature = "inclusive_range", since = "1.26.0")]
            impl ExactSizeIterator for ops::RangeInclusive<$t> { }
        )*)
    }

    macro_rules! range_trusted_len_impl {
        ($($t:ty)*) => ($(
            #[unstable(feature = "trusted_len", issue = "37572")]
            unsafe impl TrustedLen for ops::Range<$t> { }
        )*)
    }

    macro_rules! range_incl_trusted_len_impl {
        ($($t:ty)*) => ($(
            #[unstable(feature = "trusted_len", issue = "37572")]
            unsafe impl TrustedLen for ops::RangeInclusive<$t> { }
        )*)
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Step> Iterator for ops::Range<A> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            if self.start < self.end {
                // We check for overflow here, even though it can't actually
                // happen. Adding this check does however help llvm vectorize loops
                // for some ranges that don't get vectorized otherwise,
                // and this won't actually result in an extra check in an optimized build.
                if let Some(mut n) = self.start.add_usize(1) {
                    mem::swap(&mut n, &mut self.start);
                    Some(n)
                } else {
                    None
                }
            } else {
                None
            }
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            match Step::steps_between(&self.start, &self.end) {
                Some(hint) => (hint, Some(hint)),
                None => (usize::MAX, None)
            }
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<A> {
            if let Some(plus_n) = self.start.add_usize(n) {
                if plus_n < self.end {
                    self.start = plus_n.add_one();
                    return Some(plus_n)
                }
            }

            self.start = self.end.clone();
            None
        }

        #[inline]
        fn last(mut self) -> Option<A> {
            self.next_back()
        }

        #[inline]
        fn min(mut self) -> Option<A> {
            self.next()
        }

        #[inline]
        fn max(mut self) -> Option<A> {
            self.next_back()
        }
    }

    // These macros generate `ExactSizeIterator` impls for various range types.
    // Range<{u,i}64> and RangeInclusive<{u,i}{32,64,size}> are excluded
    // because they cannot guarantee having a length <= usize::MAX, which is
    // required by ExactSizeIterator.
    range_exact_iter_impl!(usize u8 u16 u32 isize i8 i16 i32);
    range_incl_exact_iter_impl!(u8 u16 i8 i16);

    // These macros generate `TrustedLen` impls.
    //
    // They need to guarantee that .size_hint() is either exact, or that
    // the upper bound is None when it does not fit the type limits.
    range_trusted_len_impl!(usize isize u8 i8 u16 i16 u32 i32 u64 i64 u128 i128);
    range_incl_trusted_len_impl!(usize isize u8 i8 u16 i16 u32 i32 u64 i64 u128 i128);

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Step> DoubleEndedIterator for ops::Range<A> {
        #[inline]
        fn next_back(&mut self) -> Option<A> {
            if self.start < self.end {
                self.end = self.end.sub_one();
                Some(self.end.clone())
            } else {
                None
            }
        }

        #[inline]
        fn nth_back(&mut self, n: usize) -> Option<A> {
            if let Some(minus_n) = self.end.sub_usize(n) {
                if minus_n > self.start {
                    self.end = minus_n.sub_one();
                    return Some(self.end.clone())
                }
            }

            self.end = self.start.clone();
            None
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<A: Step> FusedIterator for ops::Range<A> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<A: Step> Iterator for ops::RangeFrom<A> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            let mut n = self.start.add_one();
            mem::swap(&mut n, &mut self.start);
            Some(n)
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            (usize::MAX, None)
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<A> {
            let plus_n = self.start.add_usize(n).expect("overflow in RangeFrom::nth");
            self.start = plus_n.add_one();
            Some(plus_n)
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<A: Step> FusedIterator for ops::RangeFrom<A> {}

    #[unstable(feature = "trusted_len", issue = "37572")]
    unsafe impl<A: Step> TrustedLen for ops::RangeFrom<A> {}

    #[stable(feature = "inclusive_range", since = "1.26.0")]
    impl<A: Step> Iterator for ops::RangeInclusive<A> {
        type Item = A;

        #[inline]
        fn next(&mut self) -> Option<A> {
            self.compute_is_empty();
            if self.is_empty.unwrap_or_default() {
                return None;
            }
            let is_iterating = self.start < self.end;
            self.is_empty = Some(!is_iterating);
            Some(if is_iterating {
                let n = self.start.add_one();
                mem::replace(&mut self.start, n)
            } else {
                self.start.clone()
            })
        }

        #[inline]
        fn size_hint(&self) -> (usize, Option<usize>) {
            if self.is_empty() {
                return (0, Some(0));
            }

            match Step::steps_between(&self.start, &self.end) {
                Some(hint) => (hint.saturating_add(1), hint.checked_add(1)),
                None => (usize::MAX, None),
            }
        }

        #[inline]
        fn nth(&mut self, n: usize) -> Option<A> {
            self.compute_is_empty();
            if self.is_empty.unwrap_or_default() {
                return None;
            }

            if let Some(plus_n) = self.start.add_usize(n) {
                use crate::cmp::Ordering::*;

                match plus_n.partial_cmp(&self.end) {
                    Some(Less) => {
                        self.is_empty = Some(false);
                        self.start = plus_n.add_one();
                        return Some(plus_n);
                    }
                    Some(Equal) => {
                        self.is_empty = Some(true);
                        return Some(plus_n);
                    }
                    _ => {}
                }
            }

            self.is_empty = Some(true);
            None
        }

        #[inline]
        fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
        where
            Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
        {
            self.compute_is_empty();

            if self.is_empty() {
                return Try::from_ok(init);
            }

            let mut accum = init;

            while self.start < self.end {
                let n = self.start.add_one();
                let n = mem::replace(&mut self.start, n);
                accum = f(accum, n)?;
            }

            self.is_empty = Some(true);

            if self.start == self.end {
                accum = f(accum, self.start.clone())?;
            }

            Try::from_ok(accum)
        }

        #[inline]
        fn last(mut self) -> Option<A> {
            self.next_back()
        }

        #[inline]
        fn min(mut self) -> Option<A> {
            self.next()
        }

        #[inline]
        fn max(mut self) -> Option<A> {
            self.next_back()
        }
    }

    #[stable(feature = "inclusive_range", since = "1.26.0")]
    impl<A: Step> DoubleEndedIterator for ops::RangeInclusive<A> {
        #[inline]
        fn next_back(&mut self) -> Option<A> {
            self.compute_is_empty();
            if self.is_empty.unwrap_or_default() {
                return None;
            }
            let is_iterating = self.start < self.end;
            self.is_empty = Some(!is_iterating);
            Some(if is_iterating {
                let n = self.end.sub_one();
                mem::replace(&mut self.end, n)
            } else {
                self.end.clone()
            })
        }

        #[inline]
        fn nth_back(&mut self, n: usize) -> Option<A> {
            self.compute_is_empty();
            if self.is_empty.unwrap_or_default() {
                return None;
            }

            if let Some(minus_n) = self.end.sub_usize(n) {
                use crate::cmp::Ordering::*;

                match minus_n.partial_cmp(&self.start) {
                    Some(Greater) => {
                        self.is_empty = Some(false);
                        self.end = minus_n.sub_one();
                        return Some(minus_n);
                    }
                    Some(Equal) => {
                        self.is_empty = Some(true);
                        return Some(minus_n);
                    }
                    _ => {}
                }
            }

            self.is_empty = Some(true);
            None
        }

        #[inline]
        fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R where
            Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
        {
            self.compute_is_empty();

            if self.is_empty() {
                return Try::from_ok(init);
            }

            let mut accum = init;

            while self.start < self.end {
                let n = self.end.sub_one();
                let n = mem::replace(&mut self.end, n);
                accum = f(accum, n)?;
            }

            self.is_empty = Some(true);

            if self.start == self.end {
                accum = f(accum, self.start.clone())?;
            }

            Try::from_ok(accum)
        }
    }

    #[stable(feature = "fused", since = "1.26.0")]
    impl<A: Step> FusedIterator for ops::RangeInclusive<A> {}