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    #![allow(unsafe_code)]
    //! This module encapsulates the `unsafe` access to `hashbrown::raw::RawTable`,
    //! mostly in dealing with its bucket "pointers".

    use super::{equivalent, Entry, HashValue, IndexMapCore, VacantEntry};
    use core::fmt;
    use core::mem::replace;
    use hashbrown::raw::RawTable;

    type RawBucket = hashbrown::raw::Bucket<usize>;

    pub(super) struct DebugIndices<'a>(pub &'a RawTable<usize>);
    impl fmt::Debug for DebugIndices<'_> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            // SAFETY: we're not letting any of the buckets escape this function
            let indices = unsafe { self.0.iter().map(|raw_bucket| raw_bucket.read()) };
            f.debug_list().entries(indices).finish()
        }
    }

    impl<K, V> IndexMapCore<K, V> {
        /// Sweep the whole table to erase indices start..end
        pub(super) fn erase_indices_sweep(&mut self, start: usize, end: usize) {
            // SAFETY: we're not letting any of the buckets escape this function
            unsafe {
                let offset = end - start;
                for bucket in self.indices.iter() {
                    let i = bucket.read();
                    if i >= end {
                        bucket.write(i - offset);
                    } else if i >= start {
                        self.indices.erase(bucket);
                    }
                }
            }
        }

        pub(crate) fn entry(&mut self, hash: HashValue, key: K) -> Entry<'_, K, V>
        where
            K: Eq,
        {
            let eq = equivalent(&key, &self.entries);
            match self.indices.find(hash.get(), eq) {
                // SAFETY: The entry is created with a live raw bucket, at the same time
                // we have a &mut reference to the map, so it can not be modified further.
                Some(raw_bucket) => Entry::Occupied(OccupiedEntry {
                    map: self,
                    raw_bucket,
                    key,
                }),
                None => Entry::Vacant(VacantEntry {
                    map: self,
                    hash,
                    key,
                }),
            }
        }

        pub(super) fn indices_mut(&mut self) -> impl Iterator<Item = &mut usize> {
            // SAFETY: we're not letting any of the buckets escape this function,
            // only the item references that are appropriately bound to `&mut self`.
            unsafe { self.indices.iter().map(|bucket| bucket.as_mut()) }
        }

        /// Return the raw bucket for the given index
        fn find_index(&self, index: usize) -> RawBucket {
            // We'll get a "nice" bounds-check from indexing `self.entries`,
            // and then we expect to find it in the table as well.
            let hash = self.entries[index].hash.get();
            self.indices
                .find(hash, move |&i| i == index)
                .expect("index not found")
        }

        pub(crate) fn swap_indices(&mut self, a: usize, b: usize) {
            // SAFETY: Can't take two `get_mut` references from one table, so we
            // must use raw buckets to do the swap. This is still safe because we
            // are locally sure they won't dangle, and we write them individually.
            unsafe {
                let raw_bucket_a = self.find_index(a);
                let raw_bucket_b = self.find_index(b);
                raw_bucket_a.write(b);
                raw_bucket_b.write(a);
            }
            self.entries.swap(a, b);
        }
    }

    /// A view into an occupied entry in a `IndexMap`.
    /// It is part of the [`Entry`] enum.
    ///
    /// [`Entry`]: enum.Entry.html
    // SAFETY: The lifetime of the map reference also constrains the raw bucket,
    // which is essentially a raw pointer into the map indices.
    pub struct OccupiedEntry<'a, K, V> {
        map: &'a mut IndexMapCore<K, V>,
        raw_bucket: RawBucket,
        key: K,
    }

    // `hashbrown::raw::Bucket` is only `Send`, not `Sync`.
    // SAFETY: `&self` only accesses the bucket to read it.
    unsafe impl<K: Sync, V: Sync> Sync for OccupiedEntry<'_, K, V> {}

    // The parent module also adds methods that don't threaten the unsafe encapsulation.
    impl<'a, K, V> OccupiedEntry<'a, K, V> {
        /// Gets a reference to the entry's key in the map.
        ///
        /// Note that this is not the key that was used to find the entry. There may be an observable
        /// difference if the key type has any distinguishing features outside of `Hash` and `Eq`, like
        /// extra fields or the memory address of an allocation.
        pub fn key(&self) -> &K {
            &self.map.entries[self.index()].key
        }

        /// Gets a reference to the entry's value in the map.
        pub fn get(&self) -> &V {
            &self.map.entries[self.index()].value
        }

        /// Gets a mutable reference to the entry's value in the map.
        ///
        /// If you need a reference which may outlive the destruction of the
        /// `Entry` value, see `into_mut`.
        pub fn get_mut(&mut self) -> &mut V {
            let index = self.index();
            &mut self.map.entries[index].value
        }

        /// Put the new key in the occupied entry's key slot
        pub(crate) fn replace_key(self) -> K {
            let index = self.index();
            let old_key = &mut self.map.entries[index].key;
            replace(old_key, self.key)
        }

        /// Return the index of the key-value pair
        #[inline]
        pub fn index(&self) -> usize {
            // SAFETY: we have &mut map keep keeping the bucket stable
            unsafe { self.raw_bucket.read() }
        }

        /// Converts into a mutable reference to the entry's value in the map,
        /// with a lifetime bound to the map itself.
        pub fn into_mut(self) -> &'a mut V {
            let index = self.index();
            &mut self.map.entries[index].value
        }

        /// Remove and return the key, value pair stored in the map for this entry
        ///
        /// Like `Vec::swap_remove`, the pair is removed by swapping it with the
        /// last element of the map and popping it off. **This perturbs
        /// the position of what used to be the last element!**
        ///
        /// Computes in **O(1)** time (average).
        pub fn swap_remove_entry(self) -> (K, V) {
            // SAFETY: This is safe because it can only happen once (self is consumed)
            // and map.indices have not been modified since entry construction
            let index = unsafe { self.map.indices.remove(self.raw_bucket) };
            self.map.swap_remove_finish(index)
        }

        /// Remove and return the key, value pair stored in the map for this entry
        ///
        /// Like `Vec::remove`, the pair is removed by shifting all of the
        /// elements that follow it, preserving their relative order.
        /// **This perturbs the index of all of those elements!**
        ///
        /// Computes in **O(n)** time (average).
        pub fn shift_remove_entry(self) -> (K, V) {
            // SAFETY: This is safe because it can only happen once (self is consumed)
            // and map.indices have not been modified since entry construction
            let index = unsafe { self.map.indices.remove(self.raw_bucket) };
            self.map.shift_remove_finish(index)
        }
    }