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    use crate::alloc::alloc::{alloc, dealloc, handle_alloc_error};
    use crate::scopeguard::guard;
    use crate::TryReserveError;
    use core::alloc::Layout;
    use core::hint;
    use core::iter::FusedIterator;
    use core::marker::PhantomData;
    use core::mem;
    use core::mem::ManuallyDrop;
    use core::ptr::NonNull;

    cfg_if! {
        // Use the SSE2 implementation if possible: it allows us to scan 16 buckets
        // at once instead of 8. We don't bother with AVX since it would require
        // runtime dispatch and wouldn't gain us much anyways: the probability of
        // finding a match drops off drastically after the first few buckets.
        //
        // I attempted an implementation on ARM using NEON instructions, but it
        // turns out that most NEON instructions have multi-cycle latency, which in
        // the end outweighs any gains over the generic implementation.
        if #[cfg(all(
            target_feature = "sse2",
            any(target_arch = "x86", target_arch = "x86_64"),
            not(miri)
        ))] {
            mod sse2;
            use sse2 as imp;
        } else {
            #[path = "generic.rs"]
            mod generic;
            use generic as imp;
        }
    }

    mod bitmask;

    use self::bitmask::{BitMask, BitMaskIter};
    use self::imp::Group;

    // Branch prediction hint. This is currently only available on nightly but it
    // consistently improves performance by 10-15%.
    #[cfg(feature = "nightly")]
    use core::intrinsics::{likely, unlikely};
    #[cfg(not(feature = "nightly"))]
    #[inline]
    fn likely(b: bool) -> bool {
        b
    }
    #[cfg(not(feature = "nightly"))]
    #[inline]
    fn unlikely(b: bool) -> bool {
        b
    }

    #[cfg(feature = "nightly")]
    #[cfg_attr(feature = "inline-more", inline)]
    unsafe fn offset_from<T>(to: *const T, from: *const T) -> usize {
        to.offset_from(from) as usize
    }
    #[cfg(not(feature = "nightly"))]
    #[cfg_attr(feature = "inline-more", inline)]
    unsafe fn offset_from<T>(to: *const T, from: *const T) -> usize {
        (to as usize - from as usize) / mem::size_of::<T>()
    }

    /// Whether memory allocation errors should return an error or abort.
    #[derive(Copy, Clone)]
    enum Fallibility {
        Fallible,
        Infallible,
    }

    impl Fallibility {
        /// Error to return on capacity overflow.
        #[cfg_attr(feature = "inline-more", inline)]
        fn capacity_overflow(self) -> TryReserveError {
            match self {
                Fallibility::Fallible => TryReserveError::CapacityOverflow,
                Fallibility::Infallible => panic!("Hash table capacity overflow"),
            }
        }

        /// Error to return on allocation error.
        #[cfg_attr(feature = "inline-more", inline)]
        fn alloc_err(self, layout: Layout) -> TryReserveError {
            match self {
                Fallibility::Fallible => TryReserveError::AllocError { layout },
                Fallibility::Infallible => handle_alloc_error(layout),
            }
        }
    }

    /// Control byte value for an empty bucket.
    const EMPTY: u8 = 0b1111_1111;

    /// Control byte value for a deleted bucket.
    const DELETED: u8 = 0b1000_0000;

    /// Checks whether a control byte represents a full bucket (top bit is clear).
    #[inline]
    fn is_full(ctrl: u8) -> bool {
        ctrl & 0x80 == 0
    }

    /// Checks whether a control byte represents a special value (top bit is set).
    #[inline]
    fn is_special(ctrl: u8) -> bool {
        ctrl & 0x80 != 0
    }

    /// Checks whether a special control value is EMPTY (just check 1 bit).
    #[inline]
    fn special_is_empty(ctrl: u8) -> bool {
        debug_assert!(is_special(ctrl));
        ctrl & 0x01 != 0
    }

    /// Primary hash function, used to select the initial bucket to probe from.
    #[inline]
    #[allow(clippy::cast_possible_truncation)]
    fn h1(hash: u64) -> usize {
        // On 32-bit platforms we simply ignore the higher hash bits.
        hash as usize
    }

    /// Secondary hash function, saved in the low 7 bits of the control byte.
    #[inline]
    #[allow(clippy::cast_possible_truncation)]
    fn h2(hash: u64) -> u8 {
        // Grab the top 7 bits of the hash. While the hash is normally a full 64-bit
        // value, some hash functions (such as FxHash) produce a usize result
        // instead, which means that the top 32 bits are 0 on 32-bit platforms.
        let hash_len = usize::min(mem::size_of::<usize>(), mem::size_of::<u64>());
        let top7 = hash >> (hash_len * 8 - 7);
        (top7 & 0x7f) as u8 // truncation
    }

    /// Probe sequence based on triangular numbers, which is guaranteed (since our
    /// table size is a power of two) to visit every group of elements exactly once.
    ///
    /// A triangular probe has us jump by 1 more group every time. So first we
    /// jump by 1 group (meaning we just continue our linear scan), then 2 groups
    /// (skipping over 1 group), then 3 groups (skipping over 2 groups), and so on.
    ///
    /// Proof that the probe will visit every group in the table:
    /// <https://fgiesen.wordpress.com/2015/02/22/triangular-numbers-mod-2n/>
    struct ProbeSeq {
        bucket_mask: usize,
        pos: usize,
        stride: usize,
    }

    impl Iterator for ProbeSeq {
        type Item = usize;

        #[inline]
        fn next(&mut self) -> Option<usize> {
            // We should have found an empty bucket by now and ended the probe.
            debug_assert!(
                self.stride <= self.bucket_mask,
                "Went past end of probe sequence"
            );

            let result = self.pos;
            self.stride += Group::WIDTH;
            self.pos += self.stride;
            self.pos &= self.bucket_mask;
            Some(result)
        }
    }

    /// Returns the number of buckets needed to hold the given number of items,
    /// taking the maximum load factor into account.
    ///
    /// Returns `None` if an overflow occurs.
    // Workaround for emscripten bug emscripten-core/emscripten-fastcomp#258
    #[cfg_attr(target_os = "emscripten", inline(never))]
    #[cfg_attr(not(target_os = "emscripten"), inline)]
    fn capacity_to_buckets(cap: usize) -> Option<usize> {
        debug_assert_ne!(cap, 0);

        // For small tables we require at least 1 empty bucket so that lookups are
        // guaranteed to terminate if an element doesn't exist in the table.
        if cap < 8 {
            // We don't bother with a table size of 2 buckets since that can only
            // hold a single element. Instead we skip directly to a 4 bucket table
            // which can hold 3 elements.
            return Some(if cap < 4 { 4 } else { 8 });
        }

        // Otherwise require 1/8 buckets to be empty (87.5% load)
        //
        // Be careful when modifying this, calculate_layout relies on the
        // overflow check here.
        let adjusted_cap = cap.checked_mul(8)? / 7;

        // Any overflows will have been caught by the checked_mul. Also, any
        // rounding errors from the division above will be cleaned up by
        // next_power_of_two (which can't overflow because of the previous divison).
        Some(adjusted_cap.next_power_of_two())
    }

    /// Returns the maximum effective capacity for the given bucket mask, taking
    /// the maximum load factor into account.
    #[inline]
    fn bucket_mask_to_capacity(bucket_mask: usize) -> usize {
        if bucket_mask < 8 {
            // For tables with 1/2/4/8 buckets, we always reserve one empty slot.
            // Keep in mind that the bucket mask is one less than the bucket count.
            bucket_mask
        } else {
            // For larger tables we reserve 12.5% of the slots as empty.
            ((bucket_mask + 1) / 8) * 7
        }
    }

    /// Returns a Layout which describes the allocation required for a hash table,
    /// and the offset of the control bytes in the allocation.
    /// (the offset is also one past last element of buckets)
    ///
    /// Returns `None` if an overflow occurs.
    #[cfg_attr(feature = "inline-more", inline)]
    #[cfg(feature = "nightly")]
    fn calculate_layout<T>(buckets: usize) -> Option<(Layout, usize)> {
        debug_assert!(buckets.is_power_of_two());

        // Array of buckets
        let data = Layout::array::<T>(buckets).ok()?;

        // Array of control bytes. This must be aligned to the group size.
        //
        // We add `Group::WIDTH` control bytes at the end of the array which
        // replicate the bytes at the start of the array and thus avoids the need to
        // perform bounds-checking while probing.
        //
        // There is no possible overflow here since buckets is a power of two and
        // Group::WIDTH is a small number.
        let ctrl = unsafe { Layout::from_size_align_unchecked(buckets + Group::WIDTH, Group::WIDTH) };

        data.extend(ctrl).ok()
    }

    /// Returns a Layout which describes the allocation required for a hash table,
    /// and the offset of the control bytes in the allocation.
    /// (the offset is also one past last element of buckets)
    ///
    /// Returns `None` if an overflow occurs.
    #[cfg_attr(feature = "inline-more", inline)]
    #[cfg(not(feature = "nightly"))]
    fn calculate_layout<T>(buckets: usize) -> Option<(Layout, usize)> {
        debug_assert!(buckets.is_power_of_two());

        // Manual layout calculation since Layout methods are not yet stable.
        let ctrl_align = usize::max(mem::align_of::<T>(), Group::WIDTH);
        let ctrl_offset = mem::size_of::<T>()
            .checked_mul(buckets)?
            .checked_add(ctrl_align - 1)?
            & !(ctrl_align - 1);
        let len = ctrl_offset.checked_add(buckets + Group::WIDTH)?;

        Some((
            unsafe { Layout::from_size_align_unchecked(len, ctrl_align) },
            ctrl_offset,
        ))
    }

    /// A reference to a hash table bucket containing a `T`.
    ///
    /// This is usually just a pointer to the element itself. However if the element
    /// is a ZST, then we instead track the index of the element in the table so
    /// that `erase` works properly.
    pub struct Bucket<T> {
        // Actually it is pointer to next element than element itself
        // this is needed to maintain pointer arithmetic invariants
        // keeping direct pointer to element introduces difficulty.
        // Using `NonNull` for variance and niche layout
        ptr: NonNull<T>,
    }

    // This Send impl is needed for rayon support. This is safe since Bucket is
    // never exposed in a public API.
    unsafe impl<T> Send for Bucket<T> {}

    impl<T> Clone for Bucket<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn clone(&self) -> Self {
            Self { ptr: self.ptr }
        }
    }

    impl<T> Bucket<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        unsafe fn from_base_index(base: NonNull<T>, index: usize) -> Self {
            let ptr = if mem::size_of::<T>() == 0 {
                // won't overflow because index must be less than length
                (index + 1) as *mut T
            } else {
                base.as_ptr().sub(index)
            };
            Self {
                ptr: NonNull::new_unchecked(ptr),
            }
        }
        #[cfg_attr(feature = "inline-more", inline)]
        unsafe fn to_base_index(&self, base: NonNull<T>) -> usize {
            if mem::size_of::<T>() == 0 {
                self.ptr.as_ptr() as usize - 1
            } else {
                offset_from(base.as_ptr(), self.ptr.as_ptr())
            }
        }
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn as_ptr(&self) -> *mut T {
            if mem::size_of::<T>() == 0 {
                // Just return an arbitrary ZST pointer which is properly aligned
                mem::align_of::<T>() as *mut T
            } else {
                self.ptr.as_ptr().sub(1)
            }
        }
        #[cfg_attr(feature = "inline-more", inline)]
        unsafe fn next_n(&self, offset: usize) -> Self {
            let ptr = if mem::size_of::<T>() == 0 {
                (self.ptr.as_ptr() as usize + offset) as *mut T
            } else {
                self.ptr.as_ptr().sub(offset)
            };
            Self {
                ptr: NonNull::new_unchecked(ptr),
            }
        }
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn drop(&self) {
            self.as_ptr().drop_in_place();
        }
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn read(&self) -> T {
            self.as_ptr().read()
        }
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn write(&self, val: T) {
            self.as_ptr().write(val);
        }
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn as_ref<'a>(&self) -> &'a T {
            &*self.as_ptr()
        }
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn as_mut<'a>(&self) -> &'a mut T {
            &mut *self.as_ptr()
        }
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn copy_from_nonoverlapping(&self, other: &Self) {
            self.as_ptr().copy_from_nonoverlapping(other.as_ptr(), 1);
        }
    }

    /// A raw hash table with an unsafe API.
    pub struct RawTable<T> {
        // Mask to get an index from a hash value. The value is one less than the
        // number of buckets in the table.
        bucket_mask: usize,

        // [Padding], T1, T2, ..., Tlast, C1, C2, ...
        //                                ^ points here
        ctrl: NonNull<u8>,

        // Number of elements that can be inserted before we need to grow the table
        growth_left: usize,

        // Number of elements in the table, only really used by len()
        items: usize,

        // Tell dropck that we own instances of T.
        marker: PhantomData<T>,
    }

    impl<T> RawTable<T> {
        /// Creates a new empty hash table without allocating any memory.
        ///
        /// In effect this returns a table with exactly 1 bucket. However we can
        /// leave the data pointer dangling since that bucket is never written to
        /// due to our load factor forcing us to always have at least 1 free bucket.
        #[cfg_attr(feature = "inline-more", inline)]
        pub const fn new() -> Self {
            Self {
                // Be careful to cast the entire slice to a raw pointer.
                ctrl: unsafe { NonNull::new_unchecked(Group::static_empty() as *const _ as *mut u8) },
                bucket_mask: 0,
                items: 0,
                growth_left: 0,
                marker: PhantomData,
            }
        }

        /// Allocates a new hash table with the given number of buckets.
        ///
        /// The control bytes are left uninitialized.
        #[cfg_attr(feature = "inline-more", inline)]
        unsafe fn new_uninitialized(
            buckets: usize,
            fallability: Fallibility,
        ) -> Result<Self, TryReserveError> {
            debug_assert!(buckets.is_power_of_two());

            // Avoid `Option::ok_or_else` because it bloats LLVM IR.
            let (layout, ctrl_offset) = match calculate_layout::<T>(buckets) {
                Some(lco) => lco,
                None => return Err(fallability.capacity_overflow()),
            };
            let ptr = match NonNull::new(alloc(layout)) {
                Some(ptr) => ptr,
                None => return Err(fallability.alloc_err(layout)),
            };
            let ctrl = NonNull::new_unchecked(ptr.as_ptr().add(ctrl_offset));
            Ok(Self {
                ctrl,
                bucket_mask: buckets - 1,
                items: 0,
                growth_left: bucket_mask_to_capacity(buckets - 1),
                marker: PhantomData,
            })
        }

        /// Attempts to allocate a new hash table with at least enough capacity
        /// for inserting the given number of elements without reallocating.
        fn fallible_with_capacity(
            capacity: usize,
            fallability: Fallibility,
        ) -> Result<Self, TryReserveError> {
            if capacity == 0 {
                Ok(Self::new())
            } else {
                unsafe {
                    // Avoid `Option::ok_or_else` because it bloats LLVM IR.
                    let buckets = match capacity_to_buckets(capacity) {
                        Some(buckets) => buckets,
                        None => return Err(fallability.capacity_overflow()),
                    };
                    let result = Self::new_uninitialized(buckets, fallability)?;
                    result.ctrl(0).write_bytes(EMPTY, result.num_ctrl_bytes());

                    Ok(result)
                }
            }
        }

        /// Attempts to allocate a new hash table with at least enough capacity
        /// for inserting the given number of elements without reallocating.
        #[cfg(feature = "raw")]
        pub fn try_with_capacity(capacity: usize) -> Result<Self, TryReserveError> {
            Self::fallible_with_capacity(capacity, Fallibility::Fallible)
        }

        /// Allocates a new hash table with at least enough capacity for inserting
        /// the given number of elements without reallocating.
        pub fn with_capacity(capacity: usize) -> Self {
            // Avoid `Result::unwrap_or_else` because it bloats LLVM IR.
            match Self::fallible_with_capacity(capacity, Fallibility::Infallible) {
                Ok(capacity) => capacity,
                Err(_) => unsafe { hint::unreachable_unchecked() },
            }
        }

        /// Deallocates the table without dropping any entries.
        #[cfg_attr(feature = "inline-more", inline)]
        unsafe fn free_buckets(&mut self) {
            // Avoid `Option::unwrap_or_else` because it bloats LLVM IR.
            let (layout, ctrl_offset) = match calculate_layout::<T>(self.buckets()) {
                Some(lco) => lco,
                None => hint::unreachable_unchecked(),
            };
            dealloc(self.ctrl.as_ptr().sub(ctrl_offset), layout);
        }

        /// Returns pointer to one past last element of data table.
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn data_end(&self) -> NonNull<T> {
            NonNull::new_unchecked(self.ctrl.as_ptr() as *mut T)
        }

        /// Returns pointer to start of data table.
        #[cfg_attr(feature = "inline-more", inline)]
        #[cfg(feature = "nightly")]
        pub unsafe fn data_start(&self) -> *mut T {
            self.data_end().as_ptr().wrapping_sub(self.buckets())
        }

        /// Returns the index of a bucket from a `Bucket`.
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn bucket_index(&self, bucket: &Bucket<T>) -> usize {
            bucket.to_base_index(self.data_end())
        }

        /// Returns a pointer to a control byte.
        #[cfg_attr(feature = "inline-more", inline)]
        unsafe fn ctrl(&self, index: usize) -> *mut u8 {
            debug_assert!(index < self.num_ctrl_bytes());
            self.ctrl.as_ptr().add(index)
        }

        /// Returns a pointer to an element in the table.
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn bucket(&self, index: usize) -> Bucket<T> {
            debug_assert_ne!(self.bucket_mask, 0);
            debug_assert!(index < self.buckets());
            Bucket::from_base_index(self.data_end(), index)
        }

        /// Erases an element from the table without dropping it.
        #[cfg_attr(feature = "inline-more", inline)]
        #[deprecated(since = "0.8.1", note = "use erase or remove instead")]
        pub unsafe fn erase_no_drop(&mut self, item: &Bucket<T>) {
            let index = self.bucket_index(item);
            debug_assert!(is_full(*self.ctrl(index)));
            let index_before = index.wrapping_sub(Group::WIDTH) & self.bucket_mask;
            let empty_before = Group::load(self.ctrl(index_before)).match_empty();
            let empty_after = Group::load(self.ctrl(index)).match_empty();

            // If we are inside a continuous block of Group::WIDTH full or deleted
            // cells then a probe window may have seen a full block when trying to
            // insert. We therefore need to keep that block non-empty so that
            // lookups will continue searching to the next probe window.
            //
            // Note that in this context `leading_zeros` refers to the bytes at the
            // end of a group, while `trailing_zeros` refers to the bytes at the
            // begining of a group.
            let ctrl = if empty_before.leading_zeros() + empty_after.trailing_zeros() >= Group::WIDTH {
                DELETED
            } else {
                self.growth_left += 1;
                EMPTY
            };
            self.set_ctrl(index, ctrl);
            self.items -= 1;
        }

        /// Erases an element from the table, dropping it in place.
        #[cfg_attr(feature = "inline-more", inline)]
        #[allow(clippy::needless_pass_by_value)]
        #[allow(deprecated)]
        pub unsafe fn erase(&mut self, item: Bucket<T>) {
            // Erase the element from the table first since drop might panic.
            self.erase_no_drop(&item);
            item.drop();
        }

        /// Finds and erases an element from the table, dropping it in place.
        /// Returns true if an element was found.
        #[cfg(feature = "raw")]
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn erase_entry(&mut self, hash: u64, eq: impl FnMut(&T) -> bool) -> bool {
            // Avoid `Option::map` because it bloats LLVM IR.
            if let Some(bucket) = self.find(hash, eq) {
                unsafe { self.erase(bucket) };
                true
            } else {
                false
            }
        }

        /// Removes an element from the table, returning it.
        #[cfg_attr(feature = "inline-more", inline)]
        #[allow(clippy::needless_pass_by_value)]
        #[allow(deprecated)]
        pub unsafe fn remove(&mut self, item: Bucket<T>) -> T {
            self.erase_no_drop(&item);
            item.read()
        }

        /// Finds and removes an element from the table, returning it.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn remove_entry(&mut self, hash: u64, eq: impl FnMut(&T) -> bool) -> Option<T> {
            // Avoid `Option::map` because it bloats LLVM IR.
            match self.find(hash, eq) {
                Some(bucket) => Some(unsafe { self.remove(bucket) }),
                None => None,
            }
        }

        /// Returns an iterator for a probe sequence on the table.
        ///
        /// This iterator never terminates, but is guaranteed to visit each bucket
        /// group exactly once. The loop using `probe_seq` must terminate upon
        /// reaching a group containing an empty bucket.
        #[cfg_attr(feature = "inline-more", inline)]
        fn probe_seq(&self, hash: u64) -> ProbeSeq {
            ProbeSeq {
                bucket_mask: self.bucket_mask,
                pos: h1(hash) & self.bucket_mask,
                stride: 0,
            }
        }

        /// Sets a control byte, and possibly also the replicated control byte at
        /// the end of the array.
        #[cfg_attr(feature = "inline-more", inline)]
        unsafe fn set_ctrl(&self, index: usize, ctrl: u8) {
            // Replicate the first Group::WIDTH control bytes at the end of
            // the array without using a branch:
            // - If index >= Group::WIDTH then index == index2.
            // - Otherwise index2 == self.bucket_mask + 1 + index.
            //
            // The very last replicated control byte is never actually read because
            // we mask the initial index for unaligned loads, but we write it
            // anyways because it makes the set_ctrl implementation simpler.
            //
            // If there are fewer buckets than Group::WIDTH then this code will
            // replicate the buckets at the end of the trailing group. For example
            // with 2 buckets and a group size of 4, the control bytes will look
            // like this:
            //
            //     Real    |             Replicated
            // ---------------------------------------------
            // | [A] | [B] | [EMPTY] | [EMPTY] | [A] | [B] |
            // ---------------------------------------------
            let index2 = ((index.wrapping_sub(Group::WIDTH)) & self.bucket_mask) + Group::WIDTH;

            *self.ctrl(index) = ctrl;
            *self.ctrl(index2) = ctrl;
        }

        /// Searches for an empty or deleted bucket which is suitable for inserting
        /// a new element.
        ///
        /// There must be at least 1 empty bucket in the table.
        #[cfg_attr(feature = "inline-more", inline)]
        fn find_insert_slot(&self, hash: u64) -> usize {
            for pos in self.probe_seq(hash) {
                unsafe {
                    let group = Group::load(self.ctrl(pos));
                    if let Some(bit) = group.match_empty_or_deleted().lowest_set_bit() {
                        let result = (pos + bit) & self.bucket_mask;

                        // In tables smaller than the group width, trailing control
                        // bytes outside the range of the table are filled with
                        // EMPTY entries. These will unfortunately trigger a
                        // match, but once masked may point to a full bucket that
                        // is already occupied. We detect this situation here and
                        // perform a second scan starting at the begining of the
                        // table. This second scan is guaranteed to find an empty
                        // slot (due to the load factor) before hitting the trailing
                        // control bytes (containing EMPTY).
                        if unlikely(is_full(*self.ctrl(result))) {
                            debug_assert!(self.bucket_mask < Group::WIDTH);
                            debug_assert_ne!(pos, 0);
                            return Group::load_aligned(self.ctrl(0))
                                .match_empty_or_deleted()
                                .lowest_set_bit_nonzero();
                        } else {
                            return result;
                        }
                    }
                }
            }

            // probe_seq never returns.
            unreachable!();
        }

        /// Marks all table buckets as empty without dropping their contents.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn clear_no_drop(&mut self) {
            if !self.is_empty_singleton() {
                unsafe {
                    self.ctrl(0).write_bytes(EMPTY, self.num_ctrl_bytes());
                }
            }
            self.items = 0;
            self.growth_left = bucket_mask_to_capacity(self.bucket_mask);
        }

        /// Removes all elements from the table without freeing the backing memory.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn clear(&mut self) {
            // Ensure that the table is reset even if one of the drops panic
            let self_ = guard(self, |self_| self_.clear_no_drop());

            if mem::needs_drop::<T>() && self_.len() != 0 {
                unsafe {
                    for item in self_.iter() {
                        item.drop();
                    }
                }
            }
        }

        /// Shrinks the table to fit `max(self.len(), min_size)` elements.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn shrink_to(&mut self, min_size: usize, hasher: impl Fn(&T) -> u64) {
            // Calculate the minimal number of elements that we need to reserve
            // space for.
            let min_size = usize::max(self.items, min_size);
            if min_size == 0 {
                *self = Self::new();
                return;
            }

            // Calculate the number of buckets that we need for this number of
            // elements. If the calculation overflows then the requested bucket
            // count must be larger than what we have right and nothing needs to be
            // done.
            let min_buckets = match capacity_to_buckets(min_size) {
                Some(buckets) => buckets,
                None => return,
            };

            // If we have more buckets than we need, shrink the table.
            if min_buckets < self.buckets() {
                // Fast path if the table is empty
                if self.items == 0 {
                    *self = Self::with_capacity(min_size)
                } else {
                    // Avoid `Result::unwrap_or_else` because it bloats LLVM IR.
                    if self
                        .resize(min_size, hasher, Fallibility::Infallible)
                        .is_err()
                    {
                        unsafe { hint::unreachable_unchecked() }
                    }
                }
            }
        }

        /// Ensures that at least `additional` items can be inserted into the table
        /// without reallocation.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn reserve(&mut self, additional: usize, hasher: impl Fn(&T) -> u64) {
            if additional > self.growth_left {
                // Avoid `Result::unwrap_or_else` because it bloats LLVM IR.
                if self
                    .reserve_rehash(additional, hasher, Fallibility::Infallible)
                    .is_err()
                {
                    unsafe { hint::unreachable_unchecked() }
                }
            }
        }

        /// Tries to ensure that at least `additional` items can be inserted into
        /// the table without reallocation.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn try_reserve(
            &mut self,
            additional: usize,
            hasher: impl Fn(&T) -> u64,
        ) -> Result<(), TryReserveError> {
            if additional > self.growth_left {
                self.reserve_rehash(additional, hasher, Fallibility::Fallible)
            } else {
                Ok(())
            }
        }

        /// Out-of-line slow path for `reserve` and `try_reserve`.
        #[cold]
        #[inline(never)]
        fn reserve_rehash(
            &mut self,
            additional: usize,
            hasher: impl Fn(&T) -> u64,
            fallability: Fallibility,
        ) -> Result<(), TryReserveError> {
            // Avoid `Option::ok_or_else` because it bloats LLVM IR.
            let new_items = match self.items.checked_add(additional) {
                Some(new_items) => new_items,
                None => return Err(fallability.capacity_overflow()),
            };
            let full_capacity = bucket_mask_to_capacity(self.bucket_mask);
            if new_items <= full_capacity / 2 {
                // Rehash in-place without re-allocating if we have plenty of spare
                // capacity that is locked up due to DELETED entries.
                self.rehash_in_place(hasher);
                Ok(())
            } else {
                // Otherwise, conservatively resize to at least the next size up
                // to avoid churning deletes into frequent rehashes.
                self.resize(
                    usize::max(new_items, full_capacity + 1),
                    hasher,
                    fallability,
                )
            }
        }

        /// Rehashes the contents of the table in place (i.e. without changing the
        /// allocation).
        ///
        /// If `hasher` panics then some the table's contents may be lost.
        fn rehash_in_place(&mut self, hasher: impl Fn(&T) -> u64) {
            unsafe {
                // Bulk convert all full control bytes to DELETED, and all DELETED
                // control bytes to EMPTY. This effectively frees up all buckets
                // containing a DELETED entry.
                for i in (0..self.buckets()).step_by(Group::WIDTH) {
                    let group = Group::load_aligned(self.ctrl(i));
                    let group = group.convert_special_to_empty_and_full_to_deleted();
                    group.store_aligned(self.ctrl(i));
                }

                // Fix up the trailing control bytes. See the comments in set_ctrl
                // for the handling of tables smaller than the group width.
                if self.buckets() < Group::WIDTH {
                    self.ctrl(0)
                        .copy_to(self.ctrl(Group::WIDTH), self.buckets());
                } else {
                    self.ctrl(0)
                        .copy_to(self.ctrl(self.buckets()), Group::WIDTH);
                }

                // If the hash function panics then properly clean up any elements
                // that we haven't rehashed yet. We unfortunately can't preserve the
                // element since we lost their hash and have no way of recovering it
                // without risking another panic.
                let mut guard = guard(self, |self_| {
                    if mem::needs_drop::<T>() {
                        for i in 0..self_.buckets() {
                            if *self_.ctrl(i) == DELETED {
                                self_.set_ctrl(i, EMPTY);
                                self_.bucket(i).drop();
                                self_.items -= 1;
                            }
                        }
                    }
                    self_.growth_left = bucket_mask_to_capacity(self_.bucket_mask) - self_.items;
                });

                // At this point, DELETED elements are elements that we haven't
                // rehashed yet. Find them and re-insert them at their ideal
                // position.
                'outer: for i in 0..guard.buckets() {
                    if *guard.ctrl(i) != DELETED {
                        continue;
                    }
                    'inner: loop {
                        // Hash the current item
                        let item = guard.bucket(i);
                        let hash = hasher(item.as_ref());

                        // Search for a suitable place to put it
                        let new_i = guard.find_insert_slot(hash);

                        // Probing works by scanning through all of the control
                        // bytes in groups, which may not be aligned to the group
                        // size. If both the new and old position fall within the
                        // same unaligned group, then there is no benefit in moving
                        // it and we can just continue to the next item.
                        let probe_index = |pos: usize| {
                            (pos.wrapping_sub(guard.probe_seq(hash).pos) & guard.bucket_mask)
                                / Group::WIDTH
                        };
                        if likely(probe_index(i) == probe_index(new_i)) {
                            guard.set_ctrl(i, h2(hash));
                            continue 'outer;
                        }

                        // We are moving the current item to a new position. Write
                        // our H2 to the control byte of the new position.
                        let prev_ctrl = *guard.ctrl(new_i);
                        guard.set_ctrl(new_i, h2(hash));

                        if prev_ctrl == EMPTY {
                            // If the target slot is empty, simply move the current
                            // element into the new slot and clear the old control
                            // byte.
                            guard.set_ctrl(i, EMPTY);
                            guard.bucket(new_i).copy_from_nonoverlapping(&item);
                            continue 'outer;
                        } else {
                            // If the target slot is occupied, swap the two elements
                            // and then continue processing the element that we just
                            // swapped into the old slot.
                            debug_assert_eq!(prev_ctrl, DELETED);
                            mem::swap(guard.bucket(new_i).as_mut(), item.as_mut());
                            continue 'inner;
                        }
                    }
                }

                guard.growth_left = bucket_mask_to_capacity(guard.bucket_mask) - guard.items;
                mem::forget(guard);
            }
        }

        /// Allocates a new table of a different size and moves the contents of the
        /// current table into it.
        fn resize(
            &mut self,
            capacity: usize,
            hasher: impl Fn(&T) -> u64,
            fallability: Fallibility,
        ) -> Result<(), TryReserveError> {
            unsafe {
                debug_assert!(self.items <= capacity);

                // Allocate and initialize the new table.
                let mut new_table = Self::fallible_with_capacity(capacity, fallability)?;
                new_table.growth_left -= self.items;
                new_table.items = self.items;

                // The hash function may panic, in which case we simply free the new
                // table without dropping any elements that may have been copied into
                // it.
                //
                // This guard is also used to free the old table on success, see
                // the comment at the bottom of this function.
                let mut new_table = guard(ManuallyDrop::new(new_table), |new_table| {
                    if !new_table.is_empty_singleton() {
                        new_table.free_buckets();
                    }
                });

                // Copy all elements to the new table.
                for item in self.iter() {
                    // This may panic.
                    let hash = hasher(item.as_ref());

                    // We can use a simpler version of insert() here since:
                    // - there are no DELETED entries.
                    // - we know there is enough space in the table.
                    // - all elements are unique.
                    let index = new_table.find_insert_slot(hash);
                    new_table.set_ctrl(index, h2(hash));
                    new_table.bucket(index).copy_from_nonoverlapping(&item);
                }

                // We successfully copied all elements without panicking. Now replace
                // self with the new table. The old table will have its memory freed but
                // the items will not be dropped (since they have been moved into the
                // new table).
                mem::swap(self, &mut new_table);

                Ok(())
            }
        }

        /// Inserts a new element into the table, and returns its raw bucket.
        ///
        /// This does not check if the given element already exists in the table.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn insert(&mut self, hash: u64, value: T, hasher: impl Fn(&T) -> u64) -> Bucket<T> {
            unsafe {
                let mut index = self.find_insert_slot(hash);

                // We can avoid growing the table once we have reached our load
                // factor if we are replacing a tombstone. This works since the
                // number of EMPTY slots does not change in this case.
                let old_ctrl = *self.ctrl(index);
                if unlikely(self.growth_left == 0 && special_is_empty(old_ctrl)) {
                    self.reserve(1, hasher);
                    index = self.find_insert_slot(hash);
                }

                let bucket = self.bucket(index);
                self.growth_left -= special_is_empty(old_ctrl) as usize;
                self.set_ctrl(index, h2(hash));
                bucket.write(value);
                self.items += 1;
                bucket
            }
        }

        /// Inserts a new element into the table, and returns a mutable reference to it.
        ///
        /// This does not check if the given element already exists in the table.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn insert_entry(&mut self, hash: u64, value: T, hasher: impl Fn(&T) -> u64) -> &mut T {
            unsafe { self.insert(hash, value, hasher).as_mut() }
        }

        /// Inserts a new element into the table, without growing the table.
        ///
        /// There must be enough space in the table to insert the new element.
        ///
        /// This does not check if the given element already exists in the table.
        #[cfg_attr(feature = "inline-more", inline)]
        #[cfg(any(feature = "raw", feature = "rustc-internal-api"))]
        pub fn insert_no_grow(&mut self, hash: u64, value: T) -> Bucket<T> {
            unsafe {
                let index = self.find_insert_slot(hash);
                let bucket = self.bucket(index);

                // If we are replacing a DELETED entry then we don't need to update
                // the load counter.
                let old_ctrl = *self.ctrl(index);
                self.growth_left -= special_is_empty(old_ctrl) as usize;

                self.set_ctrl(index, h2(hash));
                bucket.write(value);
                self.items += 1;
                bucket
            }
        }

        /// Temporary removes a bucket, applying the given function to the removed
        /// element and optionally put back the returned value in the same bucket.
        ///
        /// Returns `true` if the bucket still contains an element
        ///
        /// This does not check if the given bucket is actually occupied.
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn replace_bucket_with<F>(&mut self, bucket: Bucket<T>, f: F) -> bool
        where
            F: FnOnce(T) -> Option<T>,
        {
            let index = self.bucket_index(&bucket);
            let old_ctrl = *self.ctrl(index);
            debug_assert!(is_full(old_ctrl));
            let old_growth_left = self.growth_left;
            let item = self.remove(bucket);
            if let Some(new_item) = f(item) {
                self.growth_left = old_growth_left;
                self.set_ctrl(index, old_ctrl);
                self.items += 1;
                self.bucket(index).write(new_item);
                true
            } else {
                false
            }
        }

        /// Searches for an element in the table.
        #[inline]
        pub fn find(&self, hash: u64, mut eq: impl FnMut(&T) -> bool) -> Option<Bucket<T>> {
            unsafe {
                for bucket in self.iter_hash(hash) {
                    let elm = bucket.as_ref();
                    if likely(eq(elm)) {
                        return Some(bucket);
                    }
                }
                None
            }
        }

        /// Gets a reference to an element in the table.
        #[inline]
        pub fn get(&self, hash: u64, eq: impl FnMut(&T) -> bool) -> Option<&T> {
            // Avoid `Option::map` because it bloats LLVM IR.
            match self.find(hash, eq) {
                Some(bucket) => Some(unsafe { bucket.as_ref() }),
                None => None,
            }
        }

        /// Gets a mutable reference to an element in the table.
        #[inline]
        pub fn get_mut(&mut self, hash: u64, eq: impl FnMut(&T) -> bool) -> Option<&mut T> {
            // Avoid `Option::map` because it bloats LLVM IR.
            match self.find(hash, eq) {
                Some(bucket) => Some(unsafe { bucket.as_mut() }),
                None => None,
            }
        }

        /// Returns the number of elements the map can hold without reallocating.
        ///
        /// This number is a lower bound; the table might be able to hold
        /// more, but is guaranteed to be able to hold at least this many.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn capacity(&self) -> usize {
            self.items + self.growth_left
        }

        /// Returns the number of elements in the table.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn len(&self) -> usize {
            self.items
        }

        /// Returns the number of buckets in the table.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn buckets(&self) -> usize {
            self.bucket_mask + 1
        }

        /// Returns the number of control bytes in the table.
        #[cfg_attr(feature = "inline-more", inline)]
        fn num_ctrl_bytes(&self) -> usize {
            self.bucket_mask + 1 + Group::WIDTH
        }

        /// Returns whether this table points to the empty singleton with a capacity
        /// of 0.
        #[cfg_attr(feature = "inline-more", inline)]
        fn is_empty_singleton(&self) -> bool {
            self.bucket_mask == 0
        }

        /// Returns an iterator over every element in the table. It is up to
        /// the caller to ensure that the `RawTable` outlives the `RawIter`.
        /// Because we cannot make the `next` method unsafe on the `RawIter`
        /// struct, we have to make the `iter` method unsafe.
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn iter(&self) -> RawIter<T> {
            let data = Bucket::from_base_index(self.data_end(), 0);
            RawIter {
                iter: RawIterRange::new(self.ctrl.as_ptr(), data, self.buckets()),
                items: self.items,
            }
        }

        /// Returns an iterator over occupied buckets that could match a given hash.
        ///
        /// In rare cases, the iterator may return a bucket with a different hash.
        ///
        /// It is up to the caller to ensure that the `RawTable` outlives the
        /// `RawIterHash`. Because we cannot make the `next` method unsafe on the
        /// `RawIterHash` struct, we have to make the `iter_hash` method unsafe.
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn iter_hash(&self, hash: u64) -> RawIterHash<'_, T> {
            RawIterHash::new(self, hash)
        }

        /// Returns an iterator which removes all elements from the table without
        /// freeing the memory.
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn drain(&mut self) -> RawDrain<'_, T> {
            unsafe {
                let iter = self.iter();
                self.drain_iter_from(iter)
            }
        }

        /// Returns an iterator which removes all elements from the table without
        /// freeing the memory.
        ///
        /// Iteration starts at the provided iterator's current location.
        ///
        /// It is up to the caller to ensure that the iterator is valid for this
        /// `RawTable` and covers all items that remain in the table.
        #[cfg_attr(feature = "inline-more", inline)]
        pub unsafe fn drain_iter_from(&mut self, iter: RawIter<T>) -> RawDrain<'_, T> {
            debug_assert_eq!(iter.len(), self.len());
            RawDrain {
                iter,
                table: ManuallyDrop::new(mem::replace(self, Self::new())),
                orig_table: NonNull::from(self),
                marker: PhantomData,
            }
        }

        /// Returns an iterator which consumes all elements from the table.
        ///
        /// Iteration starts at the provided iterator's current location.
        ///
        /// It is up to the caller to ensure that the iterator is valid for this
        /// `RawTable` and covers all items that remain in the table.
        pub unsafe fn into_iter_from(self, iter: RawIter<T>) -> RawIntoIter<T> {
            debug_assert_eq!(iter.len(), self.len());

            let alloc = self.into_alloc();
            RawIntoIter {
                iter,
                alloc,
                marker: PhantomData,
            }
        }

        /// Converts the table into a raw allocation. The contents of the table
        /// should be dropped using a `RawIter` before freeing the allocation.
        #[cfg_attr(feature = "inline-more", inline)]
        pub(crate) fn into_alloc(self) -> Option<(NonNull<u8>, Layout)> {
            let alloc = if self.is_empty_singleton() {
                None
            } else {
                // Avoid `Option::unwrap_or_else` because it bloats LLVM IR.
                let (layout, ctrl_offset) = match calculate_layout::<T>(self.buckets()) {
                    Some(lco) => lco,
                    None => unsafe { hint::unreachable_unchecked() },
                };
                Some((
                    unsafe { NonNull::new_unchecked(self.ctrl.as_ptr().sub(ctrl_offset)) },
                    layout,
                ))
            };
            mem::forget(self);
            alloc
        }
    }

    unsafe impl<T> Send for RawTable<T> where T: Send {}
    unsafe impl<T> Sync for RawTable<T> where T: Sync {}

    impl<T: Clone> Clone for RawTable<T> {
        fn clone(&self) -> Self {
            if self.is_empty_singleton() {
                Self::new()
            } else {
                unsafe {
                    let mut new_table = ManuallyDrop::new(
                        // Avoid `Result::ok_or_else` because it bloats LLVM IR.
                        match Self::new_uninitialized(self.buckets(), Fallibility::Infallible) {
                            Ok(table) => table,
                            Err(_) => hint::unreachable_unchecked(),
                        },
                    );

                    new_table.clone_from_spec(self, |new_table| {
                        // We need to free the memory allocated for the new table.
                        new_table.free_buckets();
                    });

                    // Return the newly created table.
                    ManuallyDrop::into_inner(new_table)
                }
            }
        }

        fn clone_from(&mut self, source: &Self) {
            if source.is_empty_singleton() {
                *self = Self::new();
            } else {
                unsafe {
                    // First, drop all our elements without clearing the control bytes.
                    if mem::needs_drop::<T>() && self.len() != 0 {
                        for item in self.iter() {
                            item.drop();
                        }
                    }

                    // If necessary, resize our table to match the source.
                    if self.buckets() != source.buckets() {
                        // Skip our drop by using ptr::write.
                        if !self.is_empty_singleton() {
                            self.free_buckets();
                        }
                        (self as *mut Self).write(
                            // Avoid `Result::unwrap_or_else` because it bloats LLVM IR.
                            match Self::new_uninitialized(source.buckets(), Fallibility::Infallible) {
                                Ok(table) => table,
                                Err(_) => hint::unreachable_unchecked(),
                            },
                        );
                    }

                    self.clone_from_spec(source, |self_| {
                        // We need to leave the table in an empty state.
                        self_.clear_no_drop()
                    });
                }
            }
        }
    }

    /// Specialization of `clone_from` for `Copy` types
    trait RawTableClone {
        unsafe fn clone_from_spec(&mut self, source: &Self, on_panic: impl FnMut(&mut Self));
    }
    impl<T: Clone> RawTableClone for RawTable<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        default_fn! {
            unsafe fn clone_from_spec(&mut self, source: &Self, on_panic: impl FnMut(&mut Self)) {
                self.clone_from_impl(source, on_panic);
            }
        }
    }
    #[cfg(feature = "nightly")]
    impl<T: Copy> RawTableClone for RawTable<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        unsafe fn clone_from_spec(&mut self, source: &Self, _on_panic: impl FnMut(&mut Self)) {
            source
                .ctrl(0)
                .copy_to_nonoverlapping(self.ctrl(0), self.num_ctrl_bytes());
            source
                .data_start()
                .copy_to_nonoverlapping(self.data_start(), self.buckets());

            self.items = source.items;
            self.growth_left = source.growth_left;
        }
    }

    impl<T: Clone> RawTable<T> {
        /// Common code for clone and clone_from. Assumes `self.buckets() == source.buckets()`.
        #[cfg_attr(feature = "inline-more", inline)]
        unsafe fn clone_from_impl(&mut self, source: &Self, mut on_panic: impl FnMut(&mut Self)) {
            // Copy the control bytes unchanged. We do this in a single pass
            source
                .ctrl(0)
                .copy_to_nonoverlapping(self.ctrl(0), self.num_ctrl_bytes());

            // The cloning of elements may panic, in which case we need
            // to make sure we drop only the elements that have been
            // cloned so far.
            let mut guard = guard((0, &mut *self), |(index, self_)| {
                if mem::needs_drop::<T>() && self_.len() != 0 {
                    for i in 0..=*index {
                        if is_full(*self_.ctrl(i)) {
                            self_.bucket(i).drop();
                        }
                    }
                }

                // Depending on whether we were called from clone or clone_from, we
                // either need to free the memory for the destination table or just
                // clear the control bytes.
                on_panic(self_);
            });

            for from in source.iter() {
                let index = source.bucket_index(&from);
                let to = guard.1.bucket(index);
                to.write(from.as_ref().clone());

                // Update the index in case we need to unwind.
                guard.0 = index;
            }

            // Successfully cloned all items, no need to clean up.
            mem::forget(guard);

            self.items = source.items;
            self.growth_left = source.growth_left;
        }

        /// Variant of `clone_from` to use when a hasher is available.
        #[cfg(feature = "raw")]
        pub fn clone_from_with_hasher(&mut self, source: &Self, hasher: impl Fn(&T) -> u64) {
            // If we have enough capacity in the table, just clear it and insert
            // elements one by one. We don't do this if we have the same number of
            // buckets as the source since we can just copy the contents directly
            // in that case.
            if self.buckets() != source.buckets()
                && bucket_mask_to_capacity(self.bucket_mask) >= source.len()
            {
                self.clear();

                let guard_self = guard(&mut *self, |self_| {
                    // Clear the partially copied table if a panic occurs, otherwise
                    // items and growth_left will be out of sync with the contents
                    // of the table.
                    self_.clear();
                });

                unsafe {
                    for item in source.iter() {
                        // This may panic.
                        let item = item.as_ref().clone();
                        let hash = hasher(&item);

                        // We can use a simpler version of insert() here since:
                        // - there are no DELETED entries.
                        // - we know there is enough space in the table.
                        // - all elements are unique.
                        let index = guard_self.find_insert_slot(hash);
                        guard_self.set_ctrl(index, h2(hash));
                        guard_self.bucket(index).write(item);
                    }
                }

                // Successfully cloned all items, no need to clean up.
                mem::forget(guard_self);

                self.items = source.items;
                self.growth_left -= source.items;
            } else {
                self.clone_from(source);
            }
        }
    }

    #[cfg(feature = "nightly")]
    unsafe impl<#[may_dangle] T> Drop for RawTable<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn drop(&mut self) {
            if !self.is_empty_singleton() {
                unsafe {
                    if mem::needs_drop::<T>() && self.len() != 0 {
                        for item in self.iter() {
                            item.drop();
                        }
                    }
                    self.free_buckets();
                }
            }
        }
    }
    #[cfg(not(feature = "nightly"))]
    impl<T> Drop for RawTable<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn drop(&mut self) {
            if !self.is_empty_singleton() {
                unsafe {
                    if mem::needs_drop::<T>() && self.len() != 0 {
                        for item in self.iter() {
                            item.drop();
                        }
                    }
                    self.free_buckets();
                }
            }
        }
    }

    impl<T> IntoIterator for RawTable<T> {
        type Item = T;
        type IntoIter = RawIntoIter<T>;

        #[cfg_attr(feature = "inline-more", inline)]
        fn into_iter(self) -> RawIntoIter<T> {
            unsafe {
                let iter = self.iter();
                self.into_iter_from(iter)
            }
        }
    }

    /// Iterator over a sub-range of a table. Unlike `RawIter` this iterator does
    /// not track an item count.
    pub(crate) struct RawIterRange<T> {
        // Mask of full buckets in the current group. Bits are cleared from this
        // mask as each element is processed.
        current_group: BitMask,

        // Pointer to the buckets for the current group.
        data: Bucket<T>,

        // Pointer to the next group of control bytes,
        // Must be aligned to the group size.
        next_ctrl: *const u8,

        // Pointer one past the last control byte of this range.
        end: *const u8,
    }

    impl<T> RawIterRange<T> {
        /// Returns a `RawIterRange` covering a subset of a table.
        ///
        /// The control byte address must be aligned to the group size.
        #[cfg_attr(feature = "inline-more", inline)]
        unsafe fn new(ctrl: *const u8, data: Bucket<T>, len: usize) -> Self {
            debug_assert_ne!(len, 0);
            debug_assert_eq!(ctrl as usize % Group::WIDTH, 0);
            let end = ctrl.add(len);

            // Load the first group and advance ctrl to point to the next group
            let current_group = Group::load_aligned(ctrl).match_full();
            let next_ctrl = ctrl.add(Group::WIDTH);

            Self {
                current_group,
                data,
                next_ctrl,
                end,
            }
        }

        /// Splits a `RawIterRange` into two halves.
        ///
        /// Returns `None` if the remaining range is smaller than or equal to the
        /// group width.
        #[cfg_attr(feature = "inline-more", inline)]
        #[cfg(feature = "rayon")]
        pub(crate) fn split(mut self) -> (Self, Option<RawIterRange<T>>) {
            unsafe {
                if self.end <= self.next_ctrl {
                    // Nothing to split if the group that we are current processing
                    // is the last one.
                    (self, None)
                } else {
                    // len is the remaining number of elements after the group that
                    // we are currently processing. It must be a multiple of the
                    // group size (small tables are caught by the check above).
                    let len = offset_from(self.end, self.next_ctrl);
                    debug_assert_eq!(len % Group::WIDTH, 0);

                    // Split the remaining elements into two halves, but round the
                    // midpoint down in case there is an odd number of groups
                    // remaining. This ensures that:
                    // - The tail is at least 1 group long.
                    // - The split is roughly even considering we still have the
                    //   current group to process.
                    let mid = (len / 2) & !(Group::WIDTH - 1);

                    let tail = Self::new(
                        self.next_ctrl.add(mid),
                        self.data.next_n(Group::WIDTH).next_n(mid),
                        len - mid,
                    );
                    debug_assert_eq!(
                        self.data.next_n(Group::WIDTH).next_n(mid).ptr,
                        tail.data.ptr
                    );
                    debug_assert_eq!(self.end, tail.end);
                    self.end = self.next_ctrl.add(mid);
                    debug_assert_eq!(self.end.add(Group::WIDTH), tail.next_ctrl);
                    (self, Some(tail))
                }
            }
        }
    }

    // We make raw iterators unconditionally Send and Sync, and let the PhantomData
    // in the actual iterator implementations determine the real Send/Sync bounds.
    unsafe impl<T> Send for RawIterRange<T> {}
    unsafe impl<T> Sync for RawIterRange<T> {}

    impl<T> Clone for RawIterRange<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn clone(&self) -> Self {
            Self {
                data: self.data.clone(),
                next_ctrl: self.next_ctrl,
                current_group: self.current_group,
                end: self.end,
            }
        }
    }

    impl<T> Iterator for RawIterRange<T> {
        type Item = Bucket<T>;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<Bucket<T>> {
            unsafe {
                loop {
                    if let Some(index) = self.current_group.lowest_set_bit() {
                        self.current_group = self.current_group.remove_lowest_bit();
                        return Some(self.data.next_n(index));
                    }

                    if self.next_ctrl >= self.end {
                        return None;
                    }

                    // We might read past self.end up to the next group boundary,
                    // but this is fine because it only occurs on tables smaller
                    // than the group size where the trailing control bytes are all
                    // EMPTY. On larger tables self.end is guaranteed to be aligned
                    // to the group size (since tables are power-of-two sized).
                    self.current_group = Group::load_aligned(self.next_ctrl).match_full();
                    self.data = self.data.next_n(Group::WIDTH);
                    self.next_ctrl = self.next_ctrl.add(Group::WIDTH);
                }
            }
        }

        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            // We don't have an item count, so just guess based on the range size.
            (
                0,
                Some(unsafe { offset_from(self.end, self.next_ctrl) + Group::WIDTH }),
            )
        }
    }

    impl<T> FusedIterator for RawIterRange<T> {}

    /// Iterator which returns a raw pointer to every full bucket in the table.
    ///
    /// For maximum flexibility this iterator is not bound by a lifetime, but you
    /// must observe several rules when using it:
    /// - You must not free the hash table while iterating (including via growing/shrinking).
    /// - It is fine to erase a bucket that has been yielded by the iterator.
    /// - Erasing a bucket that has not yet been yielded by the iterator may still
    ///   result in the iterator yielding that bucket (unless `reflect_remove` is called).
    /// - It is unspecified whether an element inserted after the iterator was
    ///   created will be yielded by that iterator (unless `reflect_insert` is called).
    /// - The order in which the iterator yields bucket is unspecified and may
    ///   change in the future.
    pub struct RawIter<T> {
        pub(crate) iter: RawIterRange<T>,
        items: usize,
    }

    impl<T> RawIter<T> {
        /// Refresh the iterator so that it reflects a removal from the given bucket.
        ///
        /// For the iterator to remain valid, this method must be called once
        /// for each removed bucket before `next` is called again.
        ///
        /// This method should be called _before_ the removal is made. It is not necessary to call this
        /// method if you are removing an item that this iterator yielded in the past.
        #[cfg(feature = "raw")]
        pub fn reflect_remove(&mut self, b: &Bucket<T>) {
            self.reflect_toggle_full(b, false);
        }

        /// Refresh the iterator so that it reflects an insertion into the given bucket.
        ///
        /// For the iterator to remain valid, this method must be called once
        /// for each insert before `next` is called again.
        ///
        /// This method does not guarantee that an insertion of a bucket witha greater
        /// index than the last one yielded will be reflected in the iterator.
        ///
        /// This method should be called _after_ the given insert is made.
        #[cfg(feature = "raw")]
        pub fn reflect_insert(&mut self, b: &Bucket<T>) {
            self.reflect_toggle_full(b, true);
        }

        /// Refresh the iterator so that it reflects a change to the state of the given bucket.
        #[cfg(feature = "raw")]
        fn reflect_toggle_full(&mut self, b: &Bucket<T>, is_insert: bool) {
            unsafe {
                if b.as_ptr() > self.iter.data.as_ptr() {
                    // The iterator has already passed the bucket's group.
                    // So the toggle isn't relevant to this iterator.
                    return;
                }

                if self.iter.next_ctrl < self.iter.end
                    && b.as_ptr() <= self.iter.data.next_n(Group::WIDTH).as_ptr()
                {
                    // The iterator has not yet reached the bucket's group.
                    // We don't need to reload anything, but we do need to adjust the item count.

                    if cfg!(debug_assertions) {
                        // Double-check that the user isn't lying to us by checking the bucket state.
                        // To do that, we need to find its control byte. We know that self.iter.data is
                        // at self.iter.next_ctrl - Group::WIDTH, so we work from there:
                        let offset = offset_from(self.iter.data.as_ptr(), b.as_ptr());
                        let ctrl = self.iter.next_ctrl.sub(Group::WIDTH).add(offset);
                        // This method should be called _before_ a removal, or _after_ an insert,
                        // so in both cases the ctrl byte should indicate that the bucket is full.
                        assert!(is_full(*ctrl));
                    }

                    if is_insert {
                        self.items += 1;
                    } else {
                        self.items -= 1;
                    }

                    return;
                }

                // The iterator is at the bucket group that the toggled bucket is in.
                // We need to do two things:
                //
                //  - Determine if the iterator already yielded the toggled bucket.
                //    If it did, we're done.
                //  - Otherwise, update the iterator cached group so that it won't
                //    yield a to-be-removed bucket, or _will_ yield a to-be-added bucket.
                //    We'll also need ot update the item count accordingly.
                if let Some(index) = self.iter.current_group.lowest_set_bit() {
                    let next_bucket = self.iter.data.next_n(index);
                    if b.as_ptr() > next_bucket.as_ptr() {
                        // The toggled bucket is "before" the bucket the iterator would yield next. We
                        // therefore don't need to do anything --- the iterator has already passed the
                        // bucket in question.
                        //
                        // The item count must already be correct, since a removal or insert "prior" to
                        // the iterator's position wouldn't affect the item count.
                    } else {
                        // The removed bucket is an upcoming bucket. We need to make sure it does _not_
                        // get yielded, and also that it's no longer included in the item count.
                        //
                        // NOTE: We can't just reload the group here, both since that might reflect
                        // inserts we've already passed, and because that might inadvertently unset the
                        // bits for _other_ removals. If we do that, we'd have to also decrement the
                        // item count for those other bits that we unset. But the presumably subsequent
                        // call to reflect for those buckets might _also_ decrement the item count.
                        // Instead, we _just_ flip the bit for the particular bucket the caller asked
                        // us to reflect.
                        let our_bit = offset_from(self.iter.data.as_ptr(), b.as_ptr());
                        let was_full = self.iter.current_group.flip(our_bit);
                        debug_assert_ne!(was_full, is_insert);

                        if is_insert {
                            self.items += 1;
                        } else {
                            self.items -= 1;
                        }

                        if cfg!(debug_assertions) {
                            if b.as_ptr() == next_bucket.as_ptr() {
                                // The removed bucket should no longer be next
                                debug_assert_ne!(self.iter.current_group.lowest_set_bit(), Some(index));
                            } else {
                                // We should not have changed what bucket comes next.
                                debug_assert_eq!(self.iter.current_group.lowest_set_bit(), Some(index));
                            }
                        }
                    }
                } else {
                    // We must have already iterated past the removed item.
                }
            }
        }
    }

    impl<T> Clone for RawIter<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn clone(&self) -> Self {
            Self {
                iter: self.iter.clone(),
                items: self.items,
            }
        }
    }

    impl<T> Iterator for RawIter<T> {
        type Item = Bucket<T>;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<Bucket<T>> {
            if let Some(b) = self.iter.next() {
                self.items -= 1;
                Some(b)
            } else {
                // We don't check against items == 0 here to allow the
                // compiler to optimize away the item count entirely if the
                // iterator length is never queried.
                debug_assert_eq!(self.items, 0);
                None
            }
        }

        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            (self.items, Some(self.items))
        }
    }

    impl<T> ExactSizeIterator for RawIter<T> {}
    impl<T> FusedIterator for RawIter<T> {}

    /// Iterator which consumes a table and returns elements.
    pub struct RawIntoIter<T> {
        iter: RawIter<T>,
        alloc: Option<(NonNull<u8>, Layout)>,
        marker: PhantomData<T>,
    }

    impl<T> RawIntoIter<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn iter(&self) -> RawIter<T> {
            self.iter.clone()
        }
    }

    unsafe impl<T> Send for RawIntoIter<T> where T: Send {}
    unsafe impl<T> Sync for RawIntoIter<T> where T: Sync {}

    #[cfg(feature = "nightly")]
    unsafe impl<#[may_dangle] T> Drop for RawIntoIter<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn drop(&mut self) {
            unsafe {
                // Drop all remaining elements
                if mem::needs_drop::<T>() && self.iter.len() != 0 {
                    while let Some(item) = self.iter.next() {
                        item.drop();
                    }
                }

                // Free the table
                if let Some((ptr, layout)) = self.alloc {
                    dealloc(ptr.as_ptr(), layout);
                }
            }
        }
    }
    #[cfg(not(feature = "nightly"))]
    impl<T> Drop for RawIntoIter<T> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn drop(&mut self) {
            unsafe {
                // Drop all remaining elements
                if mem::needs_drop::<T>() && self.iter.len() != 0 {
                    while let Some(item) = self.iter.next() {
                        item.drop();
                    }
                }

                // Free the table
                if let Some((ptr, layout)) = self.alloc {
                    dealloc(ptr.as_ptr(), layout);
                }
            }
        }
    }

    impl<T> Iterator for RawIntoIter<T> {
        type Item = T;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<T> {
            unsafe { Some(self.iter.next()?.read()) }
        }

        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    impl<T> ExactSizeIterator for RawIntoIter<T> {}
    impl<T> FusedIterator for RawIntoIter<T> {}

    /// Iterator which consumes elements without freeing the table storage.
    pub struct RawDrain<'a, T> {
        iter: RawIter<T>,

        // The table is moved into the iterator for the duration of the drain. This
        // ensures that an empty table is left if the drain iterator is leaked
        // without dropping.
        table: ManuallyDrop<RawTable<T>>,
        orig_table: NonNull<RawTable<T>>,

        // We don't use a &'a mut RawTable<T> because we want RawDrain to be
        // covariant over T.
        marker: PhantomData<&'a RawTable<T>>,
    }

    impl<T> RawDrain<'_, T> {
        #[cfg_attr(feature = "inline-more", inline)]
        pub fn iter(&self) -> RawIter<T> {
            self.iter.clone()
        }
    }

    unsafe impl<T> Send for RawDrain<'_, T> where T: Send {}
    unsafe impl<T> Sync for RawDrain<'_, T> where T: Sync {}

    impl<T> Drop for RawDrain<'_, T> {
        #[cfg_attr(feature = "inline-more", inline)]
        fn drop(&mut self) {
            unsafe {
                // Drop all remaining elements. Note that this may panic.
                if mem::needs_drop::<T>() && self.iter.len() != 0 {
                    while let Some(item) = self.iter.next() {
                        item.drop();
                    }
                }

                // Reset the contents of the table now that all elements have been
                // dropped.
                self.table.clear_no_drop();

                // Move the now empty table back to its original location.
                self.orig_table
                    .as_ptr()
                    .copy_from_nonoverlapping(&*self.table, 1);
            }
        }
    }

    impl<T> Iterator for RawDrain<'_, T> {
        type Item = T;

        #[cfg_attr(feature = "inline-more", inline)]
        fn next(&mut self) -> Option<T> {
            unsafe {
                let item = self.iter.next()?;
                Some(item.read())
            }
        }

        #[cfg_attr(feature = "inline-more", inline)]
        fn size_hint(&self) -> (usize, Option<usize>) {
            self.iter.size_hint()
        }
    }

    impl<T> ExactSizeIterator for RawDrain<'_, T> {}
    impl<T> FusedIterator for RawDrain<'_, T> {}

    /// Iterator over occupied buckets that could match a given hash.
    ///
    /// In rare cases, the iterator may return a bucket with a different hash.
    pub struct RawIterHash<'a, T> {
        table: &'a RawTable<T>,

        // The top 7 bits of the hash.
        h2_hash: u8,

        // The sequence of groups to probe in the search.
        probe_seq: ProbeSeq,

        // The current group and its position.
        pos: usize,
        group: Group,

        // The elements within the group with a matching h2-hash.
        bitmask: BitMaskIter,
    }

    impl<'a, T> RawIterHash<'a, T> {
        fn new(table: &'a RawTable<T>, hash: u64) -> Self {
            unsafe {
                let h2_hash = h2(hash);
                let mut probe_seq = table.probe_seq(hash);
                let pos = probe_seq.next().unwrap();
                let group = Group::load(table.ctrl(pos));
                let bitmask = group.match_byte(h2_hash).into_iter();

                RawIterHash {
                    table,
                    h2_hash,
                    probe_seq,
                    pos,
                    group,
                    bitmask,
                }
            }
        }
    }

    impl<'a, T> Iterator for RawIterHash<'a, T> {
        type Item = Bucket<T>;

        fn next(&mut self) -> Option<Bucket<T>> {
            unsafe {
                loop {
                    if let Some(bit) = self.bitmask.next() {
                        let index = (self.pos + bit) & self.table.bucket_mask;
                        let bucket = self.table.bucket(index);
                        return Some(bucket);
                    }
                    if likely(self.group.match_empty().any_bit_set()) {
                        return None;
                    }
                    self.pos = self.probe_seq.next().unwrap();
                    self.group = Group::load(self.table.ctrl(self.pos));
                    self.bitmask = self.group.match_byte(self.h2_hash).into_iter();
                }
            }
        }
    }