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    //! Macros used by iterators of slice.

    // Inlining is_empty and len makes a huge performance difference
    macro_rules! is_empty {
        // The way we encode the length of a ZST iterator, this works both for ZST
        // and non-ZST.
        ($self: ident) => {
            $self.ptr.as_ptr() as *const T == $self.end
        };
    }

    // To get rid of some bounds checks (see `position`), we compute the length in a somewhat
    // unexpected way. (Tested by `codegen/slice-position-bounds-check`.)
    macro_rules! len {
        ($self: ident) => {{
            #![allow(unused_unsafe)] // we're sometimes used within an unsafe block

            let start = $self.ptr;
            let size = size_from_ptr(start.as_ptr());
            if size == 0 {
                // This _cannot_ use `unchecked_sub` because we depend on wrapping
                // to represent the length of long ZST slice iterators.
                ($self.end as usize).wrapping_sub(start.as_ptr() as usize)
            } else {
                // We know that `start <= end`, so can do better than `offset_from`,
                // which needs to deal in signed.  By setting appropriate flags here
                // we can tell LLVM this, which helps it remove bounds checks.
                // SAFETY: By the type invariant, `start <= end`
                let diff = unsafe { unchecked_sub($self.end as usize, start.as_ptr() as usize) };
                // By also telling LLVM that the pointers are apart by an exact
                // multiple of the type size, it can optimize `len() == 0` down to
                // `start == end` instead of `(end - start) < size`.
                // SAFETY: By the type invariant, the pointers are aligned so the
                //         distance between them must be a multiple of pointee size
                unsafe { exact_div(diff, size) }
            }
        }};
    }

    // The shared definition of the `Iter` and `IterMut` iterators
    macro_rules! iterator {
        (
            struct $name:ident -> $ptr:ty,
            $elem:ty,
            $raw_mut:tt,
            {$( $mut_:tt )?},
            {$($extra:tt)*}
        ) => {
            // Returns the first element and moves the start of the iterator forwards by 1.
            // Greatly improves performance compared to an inlined function. The iterator
            // must not be empty.
            macro_rules! next_unchecked {
                ($self: ident) => {& $( $mut_ )? *$self.post_inc_start(1)}
            }

            // Returns the last element and moves the end of the iterator backwards by 1.
            // Greatly improves performance compared to an inlined function. The iterator
            // must not be empty.
            macro_rules! next_back_unchecked {
                ($self: ident) => {& $( $mut_ )? *$self.pre_dec_end(1)}
            }

            // Shrinks the iterator when T is a ZST, by moving the end of the iterator
            // backwards by `n`. `n` must not exceed `self.len()`.
            macro_rules! zst_shrink {
                ($self: ident, $n: ident) => {
                    $self.end = ($self.end as * $raw_mut u8).wrapping_offset(-$n) as * $raw_mut T;
                }
            }

            impl<'a, T> $name<'a, T> {
                // Helper function for creating a slice from the iterator.
                #[inline(always)]
                fn make_slice(&self) -> &'a [T] {
                    // SAFETY: the iterator was created from a slice with pointer
                    // `self.ptr` and length `len!(self)`. This guarantees that all
                    // the prerequisites for `from_raw_parts` are fulfilled.
                    unsafe { from_raw_parts(self.ptr.as_ptr(), len!(self)) }
                }

                // Helper function for moving the start of the iterator forwards by `offset` elements,
                // returning the old start.
                // Unsafe because the offset must not exceed `self.len()`.
                #[inline(always)]
                unsafe fn post_inc_start(&mut self, offset: isize) -> * $raw_mut T {
                    if mem::size_of::<T>() == 0 {
                        zst_shrink!(self, offset);
                        self.ptr.as_ptr()
                    } else {
                        let old = self.ptr.as_ptr();
                        // SAFETY: the caller guarantees that `offset` doesn't exceed `self.len()`,
                        // so this new pointer is inside `self` and thus guaranteed to be non-null.
                        self.ptr = unsafe { NonNull::new_unchecked(self.ptr.as_ptr().offset(offset)) };
                        old
                    }
                }

                // Helper function for moving the end of the iterator backwards by `offset` elements,
                // returning the new end.
                // Unsafe because the offset must not exceed `self.len()`.
                #[inline(always)]
                unsafe fn pre_dec_end(&mut self, offset: isize) -> * $raw_mut T {
                    if mem::size_of::<T>() == 0 {
                        zst_shrink!(self, offset);
                        self.ptr.as_ptr()
                    } else {
                        // SAFETY: the caller guarantees that `offset` doesn't exceed `self.len()`,
                        // which is guaranteed to not overflow an `isize`. Also, the resulting pointer
                        // is in bounds of `slice`, which fulfills the other requirements for `offset`.
                        self.end = unsafe { self.end.offset(-offset) };
                        self.end
                    }
                }
            }

            #[stable(feature = "rust1", since = "1.0.0")]
            impl<T> ExactSizeIterator for $name<'_, T> {
                #[inline(always)]
                fn len(&self) -> usize {
                    len!(self)
                }

                #[inline(always)]
                fn is_empty(&self) -> bool {
                    is_empty!(self)
                }
            }

            #[stable(feature = "rust1", since = "1.0.0")]
            impl<'a, T> Iterator for $name<'a, T> {
                type Item = $elem;

                #[inline]
                fn next(&mut self) -> Option<$elem> {
                    // could be implemented with slices, but this avoids bounds checks

                    // SAFETY: `assume` calls are safe since a slice's start pointer
                    // must be non-null, and slices over non-ZSTs must also have a
                    // non-null end pointer. The call to `next_unchecked!` is safe
                    // since we check if the iterator is empty first.
                    unsafe {
                        assume(!self.ptr.as_ptr().is_null());
                        if mem::size_of::<T>() != 0 {
                            assume(!self.end.is_null());
                        }
                        if is_empty!(self) {
                            None
                        } else {
                            Some(next_unchecked!(self))
                        }
                    }
                }

                #[inline]
                fn size_hint(&self) -> (usize, Option<usize>) {
                    let exact = len!(self);
                    (exact, Some(exact))
                }

                #[inline]
                fn count(self) -> usize {
                    len!(self)
                }

                #[inline]
                fn nth(&mut self, n: usize) -> Option<$elem> {
                    if n >= len!(self) {
                        // This iterator is now empty.
                        if mem::size_of::<T>() == 0 {
                            // We have to do it this way as `ptr` may never be 0, but `end`
                            // could be (due to wrapping).
                            self.end = self.ptr.as_ptr();
                        } else {
                            // SAFETY: end can't be 0 if T isn't ZST because ptr isn't 0 and end >= ptr
                            unsafe {
                                self.ptr = NonNull::new_unchecked(self.end as *mut T);
                            }
                        }
                        return None;
                    }
                    // SAFETY: We are in bounds. `post_inc_start` does the right thing even for ZSTs.
                    unsafe {
                        self.post_inc_start(n as isize);
                        Some(next_unchecked!(self))
                    }
                }

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

                // We override the default implementation, which uses `try_fold`,
                // because this simple implementation generates less LLVM IR and is
                // faster to compile.
                #[inline]
                fn for_each<F>(mut self, mut f: F)
                where
                    Self: Sized,
                    F: FnMut(Self::Item),
                {
                    while let Some(x) = self.next() {
                        f(x);
                    }
                }

                // We override the default implementation, which uses `try_fold`,
                // because this simple implementation generates less LLVM IR and is
                // faster to compile.
                #[inline]
                fn all<F>(&mut self, mut f: F) -> bool
                where
                    Self: Sized,
                    F: FnMut(Self::Item) -> bool,
                {
                    while let Some(x) = self.next() {
                        if !f(x) {
                            return false;
                        }
                    }
                    true
                }

                // We override the default implementation, which uses `try_fold`,
                // because this simple implementation generates less LLVM IR and is
                // faster to compile.
                #[inline]
                fn any<F>(&mut self, mut f: F) -> bool
                where
                    Self: Sized,
                    F: FnMut(Self::Item) -> bool,
                {
                    while let Some(x) = self.next() {
                        if f(x) {
                            return true;
                        }
                    }
                    false
                }

                // We override the default implementation, which uses `try_fold`,
                // because this simple implementation generates less LLVM IR and is
                // faster to compile.
                #[inline]
                fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item>
                where
                    Self: Sized,
                    P: FnMut(&Self::Item) -> bool,
                {
                    while let Some(x) = self.next() {
                        if predicate(&x) {
                            return Some(x);
                        }
                    }
                    None
                }

                // We override the default implementation, which uses `try_fold`,
                // because this simple implementation generates less LLVM IR and is
                // faster to compile.
                #[inline]
                fn find_map<B, F>(&mut self, mut f: F) -> Option<B>
                where
                    Self: Sized,
                    F: FnMut(Self::Item) -> Option<B>,
                {
                    while let Some(x) = self.next() {
                        if let Some(y) = f(x) {
                            return Some(y);
                        }
                    }
                    None
                }

                // We override the default implementation, which uses `try_fold`,
                // because this simple implementation generates less LLVM IR and is
                // faster to compile. Also, the `assume` avoids a bounds check.
                #[inline]
                #[rustc_inherit_overflow_checks]
                fn position<P>(&mut self, mut predicate: P) -> Option<usize> where
                    Self: Sized,
                    P: FnMut(Self::Item) -> bool,
                {
                    let n = len!(self);
                    let mut i = 0;
                    while let Some(x) = self.next() {
                        if predicate(x) {
                            // SAFETY: we are guaranteed to be in bounds by the loop invariant:
                            // when `i >= n`, `self.next()` returns `None` and the loop breaks.
                            unsafe { assume(i < n) };
                            return Some(i);
                        }
                        i += 1;
                    }
                    None
                }

                // We override the default implementation, which uses `try_fold`,
                // because this simple implementation generates less LLVM IR and is
                // faster to compile. Also, the `assume` avoids a bounds check.
                #[inline]
                fn rposition<P>(&mut self, mut predicate: P) -> Option<usize> where
                    P: FnMut(Self::Item) -> bool,
                    Self: Sized + ExactSizeIterator + DoubleEndedIterator
                {
                    let n = len!(self);
                    let mut i = n;
                    while let Some(x) = self.next_back() {
                        i -= 1;
                        if predicate(x) {
                            // SAFETY: `i` must be lower than `n` since it starts at `n`
                            // and is only decreasing.
                            unsafe { assume(i < n) };
                            return Some(i);
                        }
                    }
                    None
                }

                #[doc(hidden)]
                unsafe fn get_unchecked(&mut self, idx: usize) -> Self::Item {
                    // SAFETY: the caller must guarantee that `i` is in bounds of
                    // the underlying slice, so `i` cannot overflow an `isize`, and
                    // the returned references is guaranteed to refer to an element
                    // of the slice and thus guaranteed to be valid.
                    //
                    // Also note that the caller also guarantees that we're never
                    // called with the same index again, and that no other methods
                    // that will access this subslice are called, so it is valid
                    // for the returned reference to be mutable in the case of
                    // `IterMut`
                    unsafe { & $( $mut_ )? * self.ptr.as_ptr().add(idx) }
                }

                $($extra)*
            }

            #[stable(feature = "rust1", since = "1.0.0")]
            impl<'a, T> DoubleEndedIterator for $name<'a, T> {
                #[inline]
                fn next_back(&mut self) -> Option<$elem> {
                    // could be implemented with slices, but this avoids bounds checks

                    // SAFETY: `assume` calls are safe since a slice's start pointer must be non-null,
                    // and slices over non-ZSTs must also have a non-null end pointer.
                    // The call to `next_back_unchecked!` is safe since we check if the iterator is
                    // empty first.
                    unsafe {
                        assume(!self.ptr.as_ptr().is_null());
                        if mem::size_of::<T>() != 0 {
                            assume(!self.end.is_null());
                        }
                        if is_empty!(self) {
                            None
                        } else {
                            Some(next_back_unchecked!(self))
                        }
                    }
                }

                #[inline]
                fn nth_back(&mut self, n: usize) -> Option<$elem> {
                    if n >= len!(self) {
                        // This iterator is now empty.
                        self.end = self.ptr.as_ptr();
                        return None;
                    }
                    // SAFETY: We are in bounds. `pre_dec_end` does the right thing even for ZSTs.
                    unsafe {
                        self.pre_dec_end(n as isize);
                        Some(next_back_unchecked!(self))
                    }
                }
            }

            #[stable(feature = "fused", since = "1.26.0")]
            impl<T> FusedIterator for $name<'_, T> {}

            #[unstable(feature = "trusted_len", issue = "37572")]
            unsafe impl<T> TrustedLen for $name<'_, T> {}
        }
    }

    macro_rules! forward_iterator {
        ($name:ident: $elem:ident, $iter_of:ty) => {
            #[stable(feature = "rust1", since = "1.0.0")]
            impl<'a, $elem, P> Iterator for $name<'a, $elem, P>
            where
                P: FnMut(&T) -> bool,
            {
                type Item = $iter_of;

                #[inline]
                fn next(&mut self) -> Option<$iter_of> {
                    self.inner.next()
                }

                #[inline]
                fn size_hint(&self) -> (usize, Option<usize>) {
                    self.inner.size_hint()
                }
            }

            #[stable(feature = "fused", since = "1.26.0")]
            impl<'a, $elem, P> FusedIterator for $name<'a, $elem, P> where P: FnMut(&T) -> bool {}
        };
    }