lru/

lib.rs

// MIT License

// Copyright (c) 2016 Jerome Froelich

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

//! An implementation of a LRU cache. The cache supports `get`, `get_mut`, `put`,
//! and `pop` operations, all of which are O(1). This crate was heavily influenced
//! by the [LRU Cache implementation in an earlier version of Rust's std::collections crate](https://doc.rust-lang.org/0.12.0/std/collections/lru_cache/struct.LruCache.html).
//!
//! ## Example
//!
//! ```rust
//! extern crate lru;
//!
//! use lru::LruCache;
//! use std::num::NonZeroUsize;
//!
//! fn main() {
//!         let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
//!         cache.put("apple", 3);
//!         cache.put("banana", 2);
//!
//!         assert_eq!(*cache.get(&"apple").unwrap(), 3);
//!         assert_eq!(*cache.get(&"banana").unwrap(), 2);
//!         assert!(cache.get(&"pear").is_none());
//!
//!         assert_eq!(cache.put("banana", 4), Some(2));
//!         assert_eq!(cache.put("pear", 5), None);
//!
//!         assert_eq!(*cache.get(&"pear").unwrap(), 5);
//!         assert_eq!(*cache.get(&"banana").unwrap(), 4);
//!         assert!(cache.get(&"apple").is_none());
//!
//!         {
//!             let v = cache.get_mut(&"banana").unwrap();
//!             *v = 6;
//!         }
//!
//!         assert_eq!(*cache.get(&"banana").unwrap(), 6);
//! }
//! ```

#![no_std]

#[cfg(feature = "hashbrown")]
extern crate hashbrown;

#[cfg(test)]
extern crate scoped_threadpool;

use alloc::borrow::Borrow;
use alloc::boxed::Box;
use core::fmt;
use core::hash::{BuildHasher, Hash, Hasher};
use core::iter::FusedIterator;
use core::marker::PhantomData;
use core::mem;
use core::num::NonZeroUsize;
use core::ptr::{self, NonNull};

#[cfg(any(test, not(feature = "hashbrown")))]
extern crate std;

#[cfg(feature = "hashbrown")]
use hashbrown::HashMap;
#[cfg(not(feature = "hashbrown"))]
use std::collections::HashMap;

extern crate alloc;

// Struct used to hold a reference to a key
struct KeyRef<K> {
    k: *const K,
}

impl<K: Hash> Hash for KeyRef<K> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        unsafe { (*self.k).hash(state) }
    }
}

impl<K: PartialEq> PartialEq for KeyRef<K> {
    // NB: The unconditional_recursion lint was added in 1.76.0 and can be removed
    // once the current stable version of Rust is 1.76.0 or higher.
    #![allow(unknown_lints)]
    #[allow(clippy::unconditional_recursion)]
    fn eq(&self, other: &KeyRef<K>) -> bool {
        unsafe { (*self.k).eq(&*other.k) }
    }
}

impl<K: Eq> Eq for KeyRef<K> {}

// This type exists to allow a "blanket" Borrow impl for KeyRef without conflicting with the
//  stdlib blanket impl
#[repr(transparent)]
struct KeyWrapper<K: ?Sized>(K);

impl<K: ?Sized> KeyWrapper<K> {
    fn from_ref(key: &K) -> &Self {
        // safety: KeyWrapper is transparent, so casting the ref like this is allowable
        unsafe { &*(key as *const K as *const KeyWrapper<K>) }
    }
}

impl<K: ?Sized + Hash> Hash for KeyWrapper<K> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.0.hash(state)
    }
}

impl<K: ?Sized + PartialEq> PartialEq for KeyWrapper<K> {
    // NB: The unconditional_recursion lint was added in 1.76.0 and can be removed
    // once the current stable version of Rust is 1.76.0 or higher.
    #![allow(unknown_lints)]
    #[allow(clippy::unconditional_recursion)]
    fn eq(&self, other: &Self) -> bool {
        self.0.eq(&other.0)
    }
}

impl<K: ?Sized + Eq> Eq for KeyWrapper<K> {}

impl<K, Q> Borrow<KeyWrapper<Q>> for KeyRef<K>
where
    K: Borrow<Q>,
    Q: ?Sized,
{
    fn borrow(&self) -> &KeyWrapper<Q> {
        let key = unsafe { &*self.k }.borrow();
        KeyWrapper::from_ref(key)
    }
}

// Struct used to hold a key value pair. Also contains references to previous and next entries
// so we can maintain the entries in a linked list ordered by their use.
struct LruEntry<K, V> {
    key: mem::MaybeUninit<K>,
    val: mem::MaybeUninit<V>,
    prev: *mut LruEntry<K, V>,
    next: *mut LruEntry<K, V>,
}

impl<K, V> LruEntry<K, V> {
    fn new(key: K, val: V) -> Self {
        LruEntry {
            key: mem::MaybeUninit::new(key),
            val: mem::MaybeUninit::new(val),
            prev: ptr::null_mut(),
            next: ptr::null_mut(),
        }
    }

    fn new_sigil() -> Self {
        LruEntry {
            key: mem::MaybeUninit::uninit(),
            val: mem::MaybeUninit::uninit(),
            prev: ptr::null_mut(),
            next: ptr::null_mut(),
        }
    }
}

#[cfg(feature = "hashbrown")]
pub type DefaultHasher = hashbrown::DefaultHashBuilder;
#[cfg(not(feature = "hashbrown"))]
pub type DefaultHasher = std::collections::hash_map::RandomState;

/// An LRU Cache
pub struct LruCache<K, V, S = DefaultHasher> {
    map: HashMap<KeyRef<K>, NonNull<LruEntry<K, V>>, S>,
    cap: NonZeroUsize,

    // head and tail are sigil nodes to facilitate inserting entries
    head: *mut LruEntry<K, V>,
    tail: *mut LruEntry<K, V>,
}

impl<K, V> Clone for LruCache<K, V>
where
    K: Hash + PartialEq + Eq + Clone,
    V: Clone,
{
    fn clone(&self) -> Self {
        let mut new_lru = LruCache::new(self.cap());

        for (key, value) in self.iter().rev() {
            new_lru.push(key.clone(), value.clone());
        }

        new_lru
    }
}

impl<K: Hash + Eq, V> LruCache<K, V> {
    /// Creates a new LRU Cache that holds at most `cap` items.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache: LruCache<isize, &str> = LruCache::new(NonZeroUsize::new(10).unwrap());
    /// ```
    pub fn new(cap: NonZeroUsize) -> LruCache<K, V> {
        LruCache::construct(cap, HashMap::with_capacity(cap.get()))
    }

    /// Creates a new LRU Cache that never automatically evicts items.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache: LruCache<isize, &str> = LruCache::unbounded();
    /// ```
    pub fn unbounded() -> LruCache<K, V> {
        LruCache::construct(NonZeroUsize::MAX, HashMap::default())
    }
}

impl<K: Hash + Eq, V, S: BuildHasher> LruCache<K, V, S> {
    /// Creates a new LRU Cache that holds at most `cap` items and
    /// uses the provided hash builder to hash keys.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::{LruCache, DefaultHasher};
    /// use std::num::NonZeroUsize;
    ///
    /// let s = DefaultHasher::default();
    /// let mut cache: LruCache<isize, &str> = LruCache::with_hasher(NonZeroUsize::new(10).unwrap(), s);
    /// ```
    pub fn with_hasher(cap: NonZeroUsize, hash_builder: S) -> LruCache<K, V, S> {
        LruCache::construct(
            cap,
            HashMap::with_capacity_and_hasher(cap.into(), hash_builder),
        )
    }

    /// Creates a new LRU Cache that never automatically evicts items and
    /// uses the provided hash builder to hash keys.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::{LruCache, DefaultHasher};
    ///
    /// let s = DefaultHasher::default();
    /// let mut cache: LruCache<isize, &str> = LruCache::unbounded_with_hasher(s);
    /// ```
    pub fn unbounded_with_hasher(hash_builder: S) -> LruCache<K, V, S> {
        LruCache::construct(NonZeroUsize::MAX, HashMap::with_hasher(hash_builder))
    }

    /// Creates a new LRU Cache with the given capacity.
    fn construct(
        cap: NonZeroUsize,
        map: HashMap<KeyRef<K>, NonNull<LruEntry<K, V>>, S>,
    ) -> LruCache<K, V, S> {
        // NB: The compiler warns that cache does not need to be marked as mutable if we
        // declare it as such since we only mutate it inside the unsafe block.
        let cache = LruCache {
            map,
            cap,
            head: Box::into_raw(Box::new(LruEntry::new_sigil())),
            tail: Box::into_raw(Box::new(LruEntry::new_sigil())),
        };

        unsafe {
            (*cache.head).next = cache.tail;
            (*cache.tail).prev = cache.head;
        }

        cache
    }

    /// Puts a key-value pair into cache. If the key already exists in the cache, then it updates
    /// the key's value and returns the old value. Otherwise, `None` is returned.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// assert_eq!(None, cache.put(1, "a"));
    /// assert_eq!(None, cache.put(2, "b"));
    /// assert_eq!(Some("b"), cache.put(2, "beta"));
    ///
    /// assert_eq!(cache.get(&1), Some(&"a"));
    /// assert_eq!(cache.get(&2), Some(&"beta"));
    /// ```
    pub fn put(&mut self, k: K, v: V) -> Option<V> {
        self.capturing_put(k, v, false).map(|(_, v)| v)
    }

    /// Pushes a key-value pair into the cache. If an entry with key `k` already exists in
    /// the cache or another cache entry is removed (due to the lru's capacity),
    /// then it returns the old entry's key-value pair. Otherwise, returns `None`.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// assert_eq!(None, cache.push(1, "a"));
    /// assert_eq!(None, cache.push(2, "b"));
    ///
    /// // This push call returns (2, "b") because that was previously 2's entry in the cache.
    /// assert_eq!(Some((2, "b")), cache.push(2, "beta"));
    ///
    /// // This push call returns (1, "a") because the cache is at capacity and 1's entry was the lru entry.
    /// assert_eq!(Some((1, "a")), cache.push(3, "alpha"));
    ///
    /// assert_eq!(cache.get(&1), None);
    /// assert_eq!(cache.get(&2), Some(&"beta"));
    /// assert_eq!(cache.get(&3), Some(&"alpha"));
    /// ```
    pub fn push(&mut self, k: K, v: V) -> Option<(K, V)> {
        self.capturing_put(k, v, true)
    }

    // Used internally by `put` and `push` to add a new entry to the lru.
    // Takes ownership of and returns entries replaced due to the cache's capacity
    // when `capture` is true.
    fn capturing_put(&mut self, k: K, mut v: V, capture: bool) -> Option<(K, V)> {
        let node_ref = self.map.get_mut(&KeyRef { k: &k });

        match node_ref {
            Some(node_ref) => {
                // if the key is already in the cache just update its value and move it to the
                // front of the list
                let node_ptr: *mut LruEntry<K, V> = node_ref.as_ptr();

                // gets a reference to the node to perform a swap and drops it right after
                let node_ref = unsafe { &mut (*(*node_ptr).val.as_mut_ptr()) };
                mem::swap(&mut v, node_ref);
                let _ = node_ref;

                self.detach(node_ptr);
                self.attach(node_ptr);
                Some((k, v))
            }
            None => {
                let (replaced, node) = self.replace_or_create_node(k, v);
                let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

                self.attach(node_ptr);

                let keyref = unsafe { (*node_ptr).key.as_ptr() };
                self.map.insert(KeyRef { k: keyref }, node);

                replaced.filter(|_| capture)
            }
        }
    }

    // Used internally to swap out a node if the cache is full or to create a new node if space
    // is available. Shared between `put`, `push`, `get_or_insert`, and `get_or_insert_mut`.
    #[allow(clippy::type_complexity)]
    fn replace_or_create_node(&mut self, k: K, v: V) -> (Option<(K, V)>, NonNull<LruEntry<K, V>>) {
        if self.len() == self.cap().get() {
            // if the cache is full, remove the last entry so we can use it for the new key
            let old_key = KeyRef {
                k: unsafe { &(*(*(*self.tail).prev).key.as_ptr()) },
            };
            let old_node = self.map.remove(&old_key).unwrap();
            let node_ptr: *mut LruEntry<K, V> = old_node.as_ptr();

            // read out the node's old key and value and then replace it
            let replaced = unsafe {
                (
                    mem::replace(&mut (*node_ptr).key, mem::MaybeUninit::new(k)).assume_init(),
                    mem::replace(&mut (*node_ptr).val, mem::MaybeUninit::new(v)).assume_init(),
                )
            };

            self.detach(node_ptr);

            (Some(replaced), old_node)
        } else {
            // if the cache is not full allocate a new LruEntry
            // Safety: We allocate, turn into raw, and get NonNull all in one step.
            (None, unsafe {
                NonNull::new_unchecked(Box::into_raw(Box::new(LruEntry::new(k, v))))
            })
        }
    }

    /// Returns a reference to the value of the key in the cache or `None` if it is not
    /// present in the cache. Moves the key to the head of the LRU list if it exists.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    /// cache.put(2, "c");
    /// cache.put(3, "d");
    ///
    /// assert_eq!(cache.get(&1), None);
    /// assert_eq!(cache.get(&2), Some(&"c"));
    /// assert_eq!(cache.get(&3), Some(&"d"));
    /// ```
    pub fn get<'a, Q>(&'a mut self, k: &Q) -> Option<&'a V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        if let Some(node) = self.map.get_mut(KeyWrapper::from_ref(k)) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            Some(unsafe { &*(*node_ptr).val.as_ptr() })
        } else {
            None
        }
    }

    /// Returns a mutable reference to the value of the key in the cache or `None` if it
    /// is not present in the cache. Moves the key to the head of the LRU list if it exists.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put("apple", 8);
    /// cache.put("banana", 4);
    /// cache.put("banana", 6);
    /// cache.put("pear", 2);
    ///
    /// assert_eq!(cache.get_mut(&"apple"), None);
    /// assert_eq!(cache.get_mut(&"banana"), Some(&mut 6));
    /// assert_eq!(cache.get_mut(&"pear"), Some(&mut 2));
    /// ```
    pub fn get_mut<'a, Q>(&'a mut self, k: &Q) -> Option<&'a mut V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        if let Some(node) = self.map.get_mut(KeyWrapper::from_ref(k)) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            Some(unsafe { &mut *(*node_ptr).val.as_mut_ptr() })
        } else {
            None
        }
    }

    /// Returns a key-value references pair of the key in the cache or `None` if it is not
    /// present in the cache. Moves the key to the head of the LRU list if it exists.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(String::from("1"), "a");
    /// cache.put(String::from("2"), "b");
    /// cache.put(String::from("2"), "c");
    /// cache.put(String::from("3"), "d");
    ///
    /// assert_eq!(cache.get_key_value("1"), None);
    /// assert_eq!(cache.get_key_value("2"), Some((&String::from("2"), &"c")));
    /// assert_eq!(cache.get_key_value("3"), Some((&String::from("3"), &"d")));
    /// ```
    pub fn get_key_value<'a, Q>(&'a mut self, k: &Q) -> Option<(&'a K, &'a V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        if let Some(node) = self.map.get_mut(KeyWrapper::from_ref(k)) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            Some(unsafe { (&*(*node_ptr).key.as_ptr(), &*(*node_ptr).val.as_ptr()) })
        } else {
            None
        }
    }

    /// Returns a key-value references pair of the key in the cache or `None` if it is not
    /// present in the cache. The reference to the value of the key is mutable. Moves the key to
    /// the head of the LRU list if it exists.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    /// let (k, v) = cache.get_key_value_mut(&1).unwrap();
    /// assert_eq!(k, &1);
    /// assert_eq!(v, &mut "a");
    /// *v = "aa";
    /// cache.put(3, "c");
    /// assert_eq!(cache.get_key_value(&2), None);
    /// assert_eq!(cache.get_key_value(&1), Some((&1, &"aa")));
    /// assert_eq!(cache.get_key_value(&3), Some((&3, &"c")));
    /// ```
    pub fn get_key_value_mut<'a, Q>(&'a mut self, k: &Q) -> Option<(&'a K, &'a mut V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        if let Some(node) = self.map.get_mut(KeyWrapper::from_ref(k)) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            Some(unsafe {
                (
                    &*(*node_ptr).key.as_ptr(),
                    &mut *(*node_ptr).val.as_mut_ptr(),
                )
            })
        } else {
            None
        }
    }

    /// Returns a reference to the value of the key in the cache if it is
    /// present in the cache and moves the key to the head of the LRU list.
    /// If the key does not exist the provided `FnOnce` is used to populate
    /// the list and a reference is returned.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    /// cache.put(2, "c");
    /// cache.put(3, "d");
    ///
    /// assert_eq!(cache.get_or_insert(2, ||"a"), &"c");
    /// assert_eq!(cache.get_or_insert(3, ||"a"), &"d");
    /// assert_eq!(cache.get_or_insert(1, ||"a"), &"a");
    /// assert_eq!(cache.get_or_insert(1, ||"b"), &"a");
    /// ```
    pub fn get_or_insert<F>(&mut self, k: K, f: F) -> &V
    where
        F: FnOnce() -> V,
    {
        if let Some(node) = self.map.get_mut(&KeyRef { k: &k }) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            unsafe { &*(*node_ptr).val.as_ptr() }
        } else {
            let v = f();
            let (_, node) = self.replace_or_create_node(k, v);
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.attach(node_ptr);

            let keyref = unsafe { (*node_ptr).key.as_ptr() };
            self.map.insert(KeyRef { k: keyref }, node);
            unsafe { &*(*node_ptr).val.as_ptr() }
        }
    }

    /// Returns a reference to the value of the key in the cache if it is
    /// present in the cache and moves the key to the head of the LRU list.
    /// If the key does not exist the provided `FnOnce` is used to populate
    /// the list and a reference is returned. The value referenced by the
    /// key is only cloned (using `to_owned()`) if it doesn't exist in the
    /// cache.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// use std::rc::Rc;
    ///
    /// let key1 = Rc::new("1".to_owned());
    /// let key2 = Rc::new("2".to_owned());
    /// let mut cache = LruCache::<Rc<String>, String>::new(NonZeroUsize::new(2).unwrap());
    /// assert_eq!(cache.get_or_insert_ref(&key1, ||"One".to_owned()), "One");
    /// assert_eq!(cache.get_or_insert_ref(&key2, ||"Two".to_owned()), "Two");
    /// assert_eq!(cache.get_or_insert_ref(&key2, ||"Not two".to_owned()), "Two");
    /// assert_eq!(cache.get_or_insert_ref(&key2, ||"Again not two".to_owned()), "Two");
    /// assert_eq!(Rc::strong_count(&key1), 2);
    /// assert_eq!(Rc::strong_count(&key2), 2); // key2 was only cloned once even though we
    ///                                         // queried it 3 times
    /// ```
    pub fn get_or_insert_ref<'a, Q, F>(&'a mut self, k: &'_ Q, f: F) -> &'a V
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized + alloc::borrow::ToOwned<Owned = K>,
        F: FnOnce() -> V,
    {
        if let Some(node) = self.map.get_mut(KeyWrapper::from_ref(k)) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            unsafe { &*(*node_ptr).val.as_ptr() }
        } else {
            let v = f();
            let (_, node) = self.replace_or_create_node(k.to_owned(), v);
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.attach(node_ptr);

            let keyref = unsafe { (*node_ptr).key.as_ptr() };
            self.map.insert(KeyRef { k: keyref }, node);
            unsafe { &*(*node_ptr).val.as_ptr() }
        }
    }

    /// Returns a reference to the value of the key in the cache if it is
    /// present in the cache and moves the key to the head of the LRU list.
    /// If the key does not exist the provided `FnOnce` is used to populate
    /// the list and a reference is returned. If `FnOnce` returns `Err`,
    /// returns the `Err`.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    /// cache.put(2, "c");
    /// cache.put(3, "d");
    ///
    /// let f = ||->Result<&str, String> {Err("failed".to_owned())};
    /// let a = ||->Result<&str, String> {Ok("a")};
    /// let b = ||->Result<&str, String> {Ok("b")};
    /// assert_eq!(cache.try_get_or_insert(2, a), Ok(&"c"));
    /// assert_eq!(cache.try_get_or_insert(3, a), Ok(&"d"));
    /// assert_eq!(cache.try_get_or_insert(4, f), Err("failed".to_owned()));
    /// assert_eq!(cache.try_get_or_insert(5, b), Ok(&"b"));
    /// assert_eq!(cache.try_get_or_insert(5, a), Ok(&"b"));
    /// ```
    pub fn try_get_or_insert<F, E>(&mut self, k: K, f: F) -> Result<&V, E>
    where
        F: FnOnce() -> Result<V, E>,
    {
        if let Some(node) = self.map.get_mut(&KeyRef { k: &k }) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            unsafe { Ok(&*(*node_ptr).val.as_ptr()) }
        } else {
            let v = f()?;
            let (_, node) = self.replace_or_create_node(k, v);
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.attach(node_ptr);

            let keyref = unsafe { (*node_ptr).key.as_ptr() };
            self.map.insert(KeyRef { k: keyref }, node);
            Ok(unsafe { &*(*node_ptr).val.as_ptr() })
        }
    }

    /// Returns a reference to the value of the key in the cache if it is
    /// present in the cache and moves the key to the head of the LRU list.
    /// If the key does not exist the provided `FnOnce` is used to populate
    /// the list and a reference is returned. If `FnOnce` returns `Err`,
    /// returns the `Err`. The value referenced by the key is only cloned
    /// (using `to_owned()`) if it doesn't exist in the cache and `FnOnce`
    /// succeeds.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// use std::rc::Rc;
    ///
    /// let key1 = Rc::new("1".to_owned());
    /// let key2 = Rc::new("2".to_owned());
    /// let mut cache = LruCache::<Rc<String>, String>::new(NonZeroUsize::new(2).unwrap());
    /// let f = ||->Result<String, ()> {Err(())};
    /// let a = ||->Result<String, ()> {Ok("One".to_owned())};
    /// let b = ||->Result<String, ()> {Ok("Two".to_owned())};
    /// assert_eq!(cache.try_get_or_insert_ref(&key1, a), Ok(&"One".to_owned()));
    /// assert_eq!(cache.try_get_or_insert_ref(&key2, f), Err(()));
    /// assert_eq!(cache.try_get_or_insert_ref(&key2, b), Ok(&"Two".to_owned()));
    /// assert_eq!(cache.try_get_or_insert_ref(&key2, a), Ok(&"Two".to_owned()));
    /// assert_eq!(Rc::strong_count(&key1), 2);
    /// assert_eq!(Rc::strong_count(&key2), 2); // key2 was only cloned once even though we
    ///                                         // queried it 3 times
    /// ```
    pub fn try_get_or_insert_ref<'a, Q, F, E>(&'a mut self, k: &'_ Q, f: F) -> Result<&'a V, E>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized + alloc::borrow::ToOwned<Owned = K>,
        F: FnOnce() -> Result<V, E>,
    {
        if let Some(node) = self.map.get_mut(KeyWrapper::from_ref(k)) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            unsafe { Ok(&*(*node_ptr).val.as_ptr()) }
        } else {
            let v = f()?;
            let (_, node) = self.replace_or_create_node(k.to_owned(), v);
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.attach(node_ptr);

            let keyref = unsafe { (*node_ptr).key.as_ptr() };
            self.map.insert(KeyRef { k: keyref }, node);
            Ok(unsafe { &*(*node_ptr).val.as_ptr() })
        }
    }

    /// Returns a mutable reference to the value of the key in the cache if it is
    /// present in the cache and moves the key to the head of the LRU list.
    /// If the key does not exist the provided `FnOnce` is used to populate
    /// the list and a mutable reference is returned.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    ///
    /// let v = cache.get_or_insert_mut(2, ||"c");
    /// assert_eq!(v, &"b");
    /// *v = "d";
    /// assert_eq!(cache.get_or_insert_mut(2, ||"e"), &mut "d");
    /// assert_eq!(cache.get_or_insert_mut(3, ||"f"), &mut "f");
    /// assert_eq!(cache.get_or_insert_mut(3, ||"e"), &mut "f");
    /// ```
    pub fn get_or_insert_mut<F>(&mut self, k: K, f: F) -> &mut V
    where
        F: FnOnce() -> V,
    {
        if let Some(node) = self.map.get_mut(&KeyRef { k: &k }) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            unsafe { &mut *(*node_ptr).val.as_mut_ptr() }
        } else {
            let v = f();
            let (_, node) = self.replace_or_create_node(k, v);
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.attach(node_ptr);

            let keyref = unsafe { (*node_ptr).key.as_ptr() };
            self.map.insert(KeyRef { k: keyref }, node);
            unsafe { &mut *(*node_ptr).val.as_mut_ptr() }
        }
    }

    /// Returns a mutable reference to the value of the key in the cache if it is
    /// present in the cache and moves the key to the head of the LRU list.
    /// If the key does not exist the provided `FnOnce` is used to populate
    /// the list and a mutable reference is returned. The value referenced by the
    /// key is only cloned (using `to_owned()`) if it doesn't exist in the cache.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// use std::rc::Rc;
    ///
    /// let key1 = Rc::new("1".to_owned());
    /// let key2 = Rc::new("2".to_owned());
    /// let mut cache = LruCache::<Rc<String>, &'static str>::new(NonZeroUsize::new(2).unwrap());
    /// cache.get_or_insert_mut_ref(&key1, ||"One");
    /// let v = cache.get_or_insert_mut_ref(&key2, ||"Two");
    /// *v = "New two";
    /// assert_eq!(cache.get_or_insert_mut_ref(&key2, ||"Two"), &mut "New two");
    /// assert_eq!(Rc::strong_count(&key1), 2);
    /// assert_eq!(Rc::strong_count(&key2), 2); // key2 was only cloned once even though we
    ///                                         // queried it 2 times
    /// ```
    pub fn get_or_insert_mut_ref<'a, Q, F>(&mut self, k: &'_ Q, f: F) -> &'a mut V
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized + alloc::borrow::ToOwned<Owned = K>,
        F: FnOnce() -> V,
    {
        if let Some(node) = self.map.get_mut(KeyWrapper::from_ref(k)) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            unsafe { &mut *(*node_ptr).val.as_mut_ptr() }
        } else {
            let v = f();
            let (_, node) = self.replace_or_create_node(k.to_owned(), v);
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.attach(node_ptr);

            let keyref = unsafe { (*node_ptr).key.as_ptr() };
            self.map.insert(KeyRef { k: keyref }, node);
            unsafe { &mut *(*node_ptr).val.as_mut_ptr() }
        }
    }

    /// Returns a mutable reference to the value of the key in the cache if it is
    /// present in the cache and moves the key to the head of the LRU list.
    /// If the key does not exist the provided `FnOnce` is used to populate
    /// the list and a mutable reference is returned. If `FnOnce` returns `Err`,
    /// returns the `Err`.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    /// cache.put(2, "c");
    ///
    /// let f = ||->Result<&str, String> {Err("failed".to_owned())};
    /// let a = ||->Result<&str, String> {Ok("a")};
    /// let b = ||->Result<&str, String> {Ok("b")};
    /// if let Ok(v) = cache.try_get_or_insert_mut(2, a) {
    ///     *v = "d";
    /// }
    /// assert_eq!(cache.try_get_or_insert_mut(2, a), Ok(&mut "d"));
    /// assert_eq!(cache.try_get_or_insert_mut(3, f), Err("failed".to_owned()));
    /// assert_eq!(cache.try_get_or_insert_mut(4, b), Ok(&mut "b"));
    /// assert_eq!(cache.try_get_or_insert_mut(4, a), Ok(&mut "b"));
    /// ```
    pub fn try_get_or_insert_mut<F, E>(&mut self, k: K, f: F) -> Result<&mut V, E>
    where
        F: FnOnce() -> Result<V, E>,
    {
        if let Some(node) = self.map.get_mut(&KeyRef { k: &k }) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            unsafe { Ok(&mut *(*node_ptr).val.as_mut_ptr()) }
        } else {
            let v = f()?;
            let (_, node) = self.replace_or_create_node(k, v);
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.attach(node_ptr);

            let keyref = unsafe { (*node_ptr).key.as_ptr() };
            self.map.insert(KeyRef { k: keyref }, node);
            unsafe { Ok(&mut *(*node_ptr).val.as_mut_ptr()) }
        }
    }

    /// Returns a mutable reference to the value of the key in the cache if it is
    /// present in the cache and moves the key to the head of the LRU list.
    /// If the key does not exist the provided `FnOnce` is used to populate
    /// the list and a mutable reference is returned. If `FnOnce` returns `Err`,
    /// returns the `Err`. The value referenced by the key is only cloned
    /// (using `to_owned()`) if it doesn't exist in the cache and `FnOnce`
    /// succeeds.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// use std::rc::Rc;
    ///
    /// let key1 = Rc::new("1".to_owned());
    /// let key2 = Rc::new("2".to_owned());
    /// let mut cache = LruCache::<Rc<String>, String>::new(NonZeroUsize::new(2).unwrap());
    /// let f = ||->Result<String, ()> {Err(())};
    /// let a = ||->Result<String, ()> {Ok("One".to_owned())};
    /// let b = ||->Result<String, ()> {Ok("Two".to_owned())};
    /// assert_eq!(cache.try_get_or_insert_mut_ref(&key1, a), Ok(&mut "One".to_owned()));
    /// assert_eq!(cache.try_get_or_insert_mut_ref(&key2, f), Err(()));
    /// if let Ok(v) = cache.try_get_or_insert_mut_ref(&key2, b) {
    ///     *v = "New two".to_owned();
    /// }
    /// assert_eq!(cache.try_get_or_insert_mut_ref(&key2, a), Ok(&mut "New two".to_owned()));
    /// assert_eq!(Rc::strong_count(&key1), 2);
    /// assert_eq!(Rc::strong_count(&key2), 2); // key2 was only cloned once even though we
    ///                                         // queried it 3 times
    /// ```
    pub fn try_get_or_insert_mut_ref<'a, Q, F, E>(
        &'a mut self,
        k: &'_ Q,
        f: F,
    ) -> Result<&'a mut V, E>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized + alloc::borrow::ToOwned<Owned = K>,
        F: FnOnce() -> Result<V, E>,
    {
        if let Some(node) = self.map.get_mut(KeyWrapper::from_ref(k)) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.detach(node_ptr);
            self.attach(node_ptr);

            unsafe { Ok(&mut *(*node_ptr).val.as_mut_ptr()) }
        } else {
            let v = f()?;
            let (_, node) = self.replace_or_create_node(k.to_owned(), v);
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();

            self.attach(node_ptr);

            let keyref = unsafe { (*node_ptr).key.as_ptr() };
            self.map.insert(KeyRef { k: keyref }, node);
            unsafe { Ok(&mut *(*node_ptr).val.as_mut_ptr()) }
        }
    }

    /// Returns a reference to the value corresponding to the key in the cache or `None` if it is
    /// not present in the cache. Unlike `get`, `peek` does not update the LRU list so the key's
    /// position will be unchanged.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    ///
    /// assert_eq!(cache.peek(&1), Some(&"a"));
    /// assert_eq!(cache.peek(&2), Some(&"b"));
    /// ```
    pub fn peek<'a, Q>(&'a self, k: &Q) -> Option<&'a V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        self.map
            .get(KeyWrapper::from_ref(k))
            .map(|node| unsafe { &*node.as_ref().val.as_ptr() })
    }

    /// Returns a mutable reference to the value corresponding to the key in the cache or `None`
    /// if it is not present in the cache. Unlike `get_mut`, `peek_mut` does not update the LRU
    /// list so the key's position will be unchanged.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    ///
    /// assert_eq!(cache.peek_mut(&1), Some(&mut "a"));
    /// assert_eq!(cache.peek_mut(&2), Some(&mut "b"));
    /// ```
    pub fn peek_mut<'a, Q>(&'a mut self, k: &Q) -> Option<&'a mut V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        match self.map.get_mut(KeyWrapper::from_ref(k)) {
            None => None,
            Some(node) => Some(unsafe { &mut *(*node.as_ptr()).val.as_mut_ptr() }),
        }
    }

    /// Returns the value corresponding to the least recently used item or `None` if the
    /// cache is empty. Like `peek`, `peek_lru` does not update the LRU list so the item's
    /// position will be unchanged.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    ///
    /// assert_eq!(cache.peek_lru(), Some((&1, &"a")));
    /// ```
    pub fn peek_lru(&self) -> Option<(&K, &V)> {
        if self.is_empty() {
            return None;
        }

        let (key, val);
        unsafe {
            let node = (*self.tail).prev;
            key = &(*(*node).key.as_ptr()) as &K;
            val = &(*(*node).val.as_ptr()) as &V;
        }

        Some((key, val))
    }

    /// Returns the value corresponding to the most recently used item or `None` if the
    /// cache is empty. Like `peek`, `peek_mru` does not update the LRU list so the item's
    /// position will be unchanged.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    ///
    /// assert_eq!(cache.peek_mru(), Some((&2, &"b")));
    /// ```
    pub fn peek_mru(&self) -> Option<(&K, &V)> {
        if self.is_empty() {
            return None;
        }

        let (key, val);
        unsafe {
            let node: *mut LruEntry<K, V> = (*self.head).next;
            key = &(*(*node).key.as_ptr()) as &K;
            val = &(*(*node).val.as_ptr()) as &V;
        }

        Some((key, val))
    }

    /// Returns a bool indicating whether the given key is in the cache. Does not update the
    /// LRU list.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    /// cache.put(3, "c");
    ///
    /// assert!(!cache.contains(&1));
    /// assert!(cache.contains(&2));
    /// assert!(cache.contains(&3));
    /// ```
    pub fn contains<Q>(&self, k: &Q) -> bool
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        self.map.contains_key(KeyWrapper::from_ref(k))
    }

    /// Removes and returns the value corresponding to the key from the cache or
    /// `None` if it does not exist.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(2, "a");
    ///
    /// assert_eq!(cache.pop(&1), None);
    /// assert_eq!(cache.pop(&2), Some("a"));
    /// assert_eq!(cache.pop(&2), None);
    /// assert_eq!(cache.len(), 0);
    /// ```
    pub fn pop<Q>(&mut self, k: &Q) -> Option<V>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        match self.map.remove(KeyWrapper::from_ref(k)) {
            None => None,
            Some(old_node) => {
                let mut old_node = unsafe {
                    let mut old_node = *Box::from_raw(old_node.as_ptr());
                    ptr::drop_in_place(old_node.key.as_mut_ptr());

                    old_node
                };

                self.detach(&mut old_node);

                let LruEntry { key: _, val, .. } = old_node;
                unsafe { Some(val.assume_init()) }
            }
        }
    }

    /// Removes and returns the key and the value corresponding to the key from the cache or
    /// `None` if it does not exist.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "a");
    ///
    /// assert_eq!(cache.pop(&1), Some("a"));
    /// assert_eq!(cache.pop_entry(&2), Some((2, "a")));
    /// assert_eq!(cache.pop(&1), None);
    /// assert_eq!(cache.pop_entry(&2), None);
    /// assert_eq!(cache.len(), 0);
    /// ```
    pub fn pop_entry<Q>(&mut self, k: &Q) -> Option<(K, V)>
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        match self.map.remove(KeyWrapper::from_ref(k)) {
            None => None,
            Some(old_node) => {
                let mut old_node = unsafe { *Box::from_raw(old_node.as_ptr()) };

                self.detach(&mut old_node);

                let LruEntry { key, val, .. } = old_node;
                unsafe { Some((key.assume_init(), val.assume_init())) }
            }
        }
    }

    /// Removes and returns the key and value corresponding to the least recently
    /// used item or `None` if the cache is empty.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(2, "a");
    /// cache.put(3, "b");
    /// cache.put(4, "c");
    /// cache.get(&3);
    ///
    /// assert_eq!(cache.pop_lru(), Some((4, "c")));
    /// assert_eq!(cache.pop_lru(), Some((3, "b")));
    /// assert_eq!(cache.pop_lru(), None);
    /// assert_eq!(cache.len(), 0);
    /// ```
    pub fn pop_lru(&mut self) -> Option<(K, V)> {
        let node = self.remove_last()?;
        // N.B.: Can't destructure directly because of https://github.com/rust-lang/rust/issues/28536
        let node = *node;
        let LruEntry { key, val, .. } = node;
        unsafe { Some((key.assume_init(), val.assume_init())) }
    }

    /// Removes and returns the key and value corresponding to the most recently
    /// used item or `None` if the cache is empty.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(2, "a");
    /// cache.put(3, "b");
    /// cache.put(4, "c");
    /// cache.get(&3);
    ///
    /// assert_eq!(cache.pop_mru(), Some((3, "b")));
    /// assert_eq!(cache.pop_mru(), Some((4, "c")));
    /// assert_eq!(cache.pop_mru(), None);
    /// assert_eq!(cache.len(), 0);
    /// ```
    pub fn pop_mru(&mut self) -> Option<(K, V)> {
        let node = self.remove_first()?;
        // N.B.: Can't destructure directly because of https://github.com/rust-lang/rust/issues/28536
        let node = *node;
        let LruEntry { key, val, .. } = node;
        unsafe { Some((key.assume_init(), val.assume_init())) }
    }

    /// Marks the key as the most recently used one.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    /// cache.put(3, "c");
    /// cache.get(&1);
    /// cache.get(&2);
    ///
    /// // If we do `pop_lru` now, we would pop 3.
    /// // assert_eq!(cache.pop_lru(), Some((3, "c")));
    ///
    /// // By promoting 3, we make sure it isn't popped.
    /// cache.promote(&3);
    /// assert_eq!(cache.pop_lru(), Some((1, "a")));
    /// ```
    pub fn promote<Q>(&mut self, k: &Q)
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        if let Some(node) = self.map.get_mut(KeyWrapper::from_ref(k)) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();
            self.detach(node_ptr);
            self.attach(node_ptr);
        }
    }

    /// Marks the key as the least recently used one.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    /// cache.put(3, "c");
    /// cache.get(&1);
    /// cache.get(&2);
    ///
    /// // If we do `pop_lru` now, we would pop 3.
    /// // assert_eq!(cache.pop_lru(), Some((3, "c")));
    ///
    /// // By demoting 1 and 2, we make sure those are popped first.
    /// cache.demote(&2);
    /// cache.demote(&1);
    /// assert_eq!(cache.pop_lru(), Some((1, "a")));
    /// assert_eq!(cache.pop_lru(), Some((2, "b")));
    /// ```
    pub fn demote<Q>(&mut self, k: &Q)
    where
        K: Borrow<Q>,
        Q: Hash + Eq + ?Sized,
    {
        if let Some(node) = self.map.get_mut(KeyWrapper::from_ref(k)) {
            let node_ptr: *mut LruEntry<K, V> = node.as_ptr();
            self.detach(node_ptr);
            self.attach_last(node_ptr);
        }
    }

    /// Returns the number of key-value pairs that are currently in the the cache.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    /// assert_eq!(cache.len(), 0);
    ///
    /// cache.put(1, "a");
    /// assert_eq!(cache.len(), 1);
    ///
    /// cache.put(2, "b");
    /// assert_eq!(cache.len(), 2);
    ///
    /// cache.put(3, "c");
    /// assert_eq!(cache.len(), 2);
    /// ```
    pub fn len(&self) -> usize {
        self.map.len()
    }

    /// Returns a bool indicating whether the cache is empty or not.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
    /// assert!(cache.is_empty());
    ///
    /// cache.put(1, "a");
    /// assert!(!cache.is_empty());
    /// ```
    pub fn is_empty(&self) -> bool {
        self.map.len() == 0
    }

    /// Returns the maximum number of key-value pairs the cache can hold.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache: LruCache<isize, &str> = LruCache::new(NonZeroUsize::new(2).unwrap());
    /// assert_eq!(cache.cap().get(), 2);
    /// ```
    pub fn cap(&self) -> NonZeroUsize {
        self.cap
    }

    /// Resizes the cache. If the new capacity is smaller than the size of the current
    /// cache any entries past the new capacity are discarded.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache: LruCache<isize, &str> = LruCache::new(NonZeroUsize::new(2).unwrap());
    ///
    /// cache.put(1, "a");
    /// cache.put(2, "b");
    /// cache.resize(NonZeroUsize::new(4).unwrap());
    /// cache.put(3, "c");
    /// cache.put(4, "d");
    ///
    /// assert_eq!(cache.len(), 4);
    /// assert_eq!(cache.get(&1), Some(&"a"));
    /// assert_eq!(cache.get(&2), Some(&"b"));
    /// assert_eq!(cache.get(&3), Some(&"c"));
    /// assert_eq!(cache.get(&4), Some(&"d"));
    /// ```
    pub fn resize(&mut self, cap: NonZeroUsize) {
        // return early if capacity doesn't change
        if cap == self.cap {
            return;
        }

        while self.map.len() > cap.get() {
            self.pop_lru();
        }
        self.map.shrink_to_fit();

        self.cap = cap;
    }

    /// Clears the contents of the cache.
    ///
    /// # Example
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    /// let mut cache: LruCache<isize, &str> = LruCache::new(NonZeroUsize::new(2).unwrap());
    /// assert_eq!(cache.len(), 0);
    ///
    /// cache.put(1, "a");
    /// assert_eq!(cache.len(), 1);
    ///
    /// cache.put(2, "b");
    /// assert_eq!(cache.len(), 2);
    ///
    /// cache.clear();
    /// assert_eq!(cache.len(), 0);
    /// ```
    pub fn clear(&mut self) {
        while self.pop_lru().is_some() {}
    }

    /// An iterator visiting all entries in most-recently used order. The iterator element type is
    /// `(&K, &V)`.
    ///
    /// # Examples
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    ///
    /// let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
    /// cache.put("a", 1);
    /// cache.put("b", 2);
    /// cache.put("c", 3);
    ///
    /// for (key, val) in cache.iter() {
    ///     println!("key: {} val: {}", key, val);
    /// }
    /// ```
    pub fn iter(&self) -> Iter<'_, K, V> {
        Iter {
            len: self.len(),
            ptr: unsafe { (*self.head).next },
            end: unsafe { (*self.tail).prev },
            phantom: PhantomData,
        }
    }

    /// An iterator visiting all entries in most-recently-used order, giving a mutable reference on
    /// V.  The iterator element type is `(&K, &mut V)`.
    ///
    /// # Examples
    ///
    /// ```
    /// use lru::LruCache;
    /// use std::num::NonZeroUsize;
    ///
    /// struct HddBlock {
    ///     dirty: bool,
    ///     data: [u8; 512]
    /// }
    ///
    /// let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
    /// cache.put(0, HddBlock { dirty: false, data: [0x00; 512]});
    /// cache.put(1, HddBlock { dirty: true,  data: [0x55; 512]});
    /// cache.put(2, HddBlock { dirty: true,  data: [0x77; 512]});
    ///
    /// // write dirty blocks to disk.
    /// for (block_id, block) in cache.iter_mut() {
    ///     if block.dirty {
    ///         // write block to disk
    ///         block.dirty = false
    ///     }
    /// }
    /// ```
    pub fn iter_mut(&mut self) -> IterMut<'_, K, V> {
        IterMut {
            len: self.len(),
            ptr: unsafe { (*self.head).next },
            end: unsafe { (*self.tail).prev },
            phantom: PhantomData,
        }
    }

    fn remove_first(&mut self) -> Option<Box<LruEntry<K, V>>> {
        let next;
        unsafe { next = (*self.head).next }
        if !core::ptr::eq(next, self.tail) {
            let old_key = KeyRef {
                k: unsafe { &(*(*(*self.head).next).key.as_ptr()) },
            };
            let old_node = self.map.remove(&old_key).unwrap();
            let node_ptr: *mut LruEntry<K, V> = old_node.as_ptr();
            self.detach(node_ptr);
            unsafe { Some(Box::from_raw(node_ptr)) }
        } else {
            None
        }
    }

    fn remove_last(&mut self) -> Option<Box<LruEntry<K, V>>> {
        let prev;
        unsafe { prev = (*self.tail).prev }
        if !core::ptr::eq(prev, self.head) {
            let old_key = KeyRef {
                k: unsafe { &(*(*(*self.tail).prev).key.as_ptr()) },
            };
            let old_node = self.map.remove(&old_key).unwrap();
            let node_ptr: *mut LruEntry<K, V> = old_node.as_ptr();
            self.detach(node_ptr);
            unsafe { Some(Box::from_raw(node_ptr)) }
        } else {
            None
        }
    }

    fn detach(&mut self, node: *mut LruEntry<K, V>) {
        unsafe {
            (*(*node).prev).next = (*node).next;
            (*(*node).next).prev = (*node).prev;
        }
    }

    // Attaches `node` after the sigil `self.head` node.
    fn attach(&mut self, node: *mut LruEntry<K, V>) {
        unsafe {
            (*node).next = (*self.head).next;
            (*node).prev = self.head;
            (*self.head).next = node;
            (*(*node).next).prev = node;
        }
    }

    // Attaches `node` before the sigil `self.tail` node.
    fn attach_last(&mut self, node: *mut LruEntry<K, V>) {
        unsafe {
            (*node).next = self.tail;
            (*node).prev = (*self.tail).prev;
            (*self.tail).prev = node;
            (*(*node).prev).next = node;
        }
    }
}

impl<K, V, S> Drop for LruCache<K, V, S> {
    fn drop(&mut self) {
        self.map.drain().for_each(|(_, node)| unsafe {
            let mut node = *Box::from_raw(node.as_ptr());
            ptr::drop_in_place((node).key.as_mut_ptr());
            ptr::drop_in_place((node).val.as_mut_ptr());
        });
        // We rebox the head/tail, and because these are maybe-uninit
        // they do not have the absent k/v dropped.

        let _head = unsafe { *Box::from_raw(self.head) };
        let _tail = unsafe { *Box::from_raw(self.tail) };
    }
}

impl<'a, K: Hash + Eq, V, S: BuildHasher> IntoIterator for &'a LruCache<K, V, S> {
    type Item = (&'a K, &'a V);
    type IntoIter = Iter<'a, K, V>;

    fn into_iter(self) -> Iter<'a, K, V> {
        self.iter()
    }
}

impl<'a, K: Hash + Eq, V, S: BuildHasher> IntoIterator for &'a mut LruCache<K, V, S> {
    type Item = (&'a K, &'a mut V);
    type IntoIter = IterMut<'a, K, V>;

    fn into_iter(self) -> IterMut<'a, K, V> {
        self.iter_mut()
    }
}

// The compiler does not automatically derive Send and Sync for LruCache because it contains
// raw pointers. The raw pointers are safely encapsulated by LruCache though so we can
// implement Send and Sync for it below.
unsafe impl<K: Send, V: Send, S: Send> Send for LruCache<K, V, S> {}
unsafe impl<K: Sync, V: Sync, S: Sync> Sync for LruCache<K, V, S> {}

impl<K: Hash + Eq, V, S: BuildHasher> fmt::Debug for LruCache<K, V, S> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("LruCache")
            .field("len", &self.len())
            .field("cap", &self.cap())
            .finish()
    }
}

/// An iterator over the entries of a `LruCache`.
///
/// This `struct` is created by the [`iter`] method on [`LruCache`][`LruCache`]. See its
/// documentation for more.
///
/// [`iter`]: struct.LruCache.html#method.iter
/// [`LruCache`]: struct.LruCache.html
pub struct Iter<'a, K: 'a, V: 'a> {
    len: usize,

    ptr: *const LruEntry<K, V>,
    end: *const LruEntry<K, V>,

    phantom: PhantomData<&'a K>,
}

impl<'a, K, V> Iterator for Iter<'a, K, V> {
    type Item = (&'a K, &'a V);

    fn next(&mut self) -> Option<(&'a K, &'a V)> {
        if self.len == 0 {
            return None;
        }

        let key = unsafe { &(*(*self.ptr).key.as_ptr()) as &K };
        let val = unsafe { &(*(*self.ptr).val.as_ptr()) as &V };

        self.len -= 1;
        self.ptr = unsafe { (*self.ptr).next };

        Some((key, val))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.len, Some(self.len))
    }

    fn count(self) -> usize {
        self.len
    }
}

impl<'a, K, V> DoubleEndedIterator for Iter<'a, K, V> {
    fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
        if self.len == 0 {
            return None;
        }

        let key = unsafe { &(*(*self.end).key.as_ptr()) as &K };
        let val = unsafe { &(*(*self.end).val.as_ptr()) as &V };

        self.len -= 1;
        self.end = unsafe { (*self.end).prev };

        Some((key, val))
    }
}

impl<'a, K, V> ExactSizeIterator for Iter<'a, K, V> {}
impl<'a, K, V> FusedIterator for Iter<'a, K, V> {}

impl<'a, K, V> Clone for Iter<'a, K, V> {
    fn clone(&self) -> Iter<'a, K, V> {
        Iter {
            len: self.len,
            ptr: self.ptr,
            end: self.end,
            phantom: PhantomData,
        }
    }
}

// The compiler does not automatically derive Send and Sync for Iter because it contains
// raw pointers.
unsafe impl<'a, K: Send, V: Send> Send for Iter<'a, K, V> {}
unsafe impl<'a, K: Sync, V: Sync> Sync for Iter<'a, K, V> {}

/// An iterator over mutables entries of a `LruCache`.
///
/// This `struct` is created by the [`iter_mut`] method on [`LruCache`][`LruCache`]. See its
/// documentation for more.
///
/// [`iter_mut`]: struct.LruCache.html#method.iter_mut
/// [`LruCache`]: struct.LruCache.html
pub struct IterMut<'a, K: 'a, V: 'a> {
    len: usize,

    ptr: *mut LruEntry<K, V>,
    end: *mut LruEntry<K, V>,

    phantom: PhantomData<&'a K>,
}

impl<'a, K, V> Iterator for IterMut<'a, K, V> {
    type Item = (&'a K, &'a mut V);

    fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
        if self.len == 0 {
            return None;
        }

        let key = unsafe { &mut (*(*self.ptr).key.as_mut_ptr()) as &mut K };
        let val = unsafe { &mut (*(*self.ptr).val.as_mut_ptr()) as &mut V };

        self.len -= 1;
        self.ptr = unsafe { (*self.ptr).next };

        Some((key, val))
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        (self.len, Some(self.len))
    }

    fn count(self) -> usize {
        self.len
    }
}

impl<'a, K, V> DoubleEndedIterator for IterMut<'a, K, V> {
    fn next_back(&mut self) -> Option<(&'a K, &'a mut V)> {
        if self.len == 0 {
            return None;
        }

        let key = unsafe { &mut (*(*self.end).key.as_mut_ptr()) as &mut K };
        let val = unsafe { &mut (*(*self.end).val.as_mut_ptr()) as &mut V };

        self.len -= 1;
        self.end = unsafe { (*self.end).prev };

        Some((key, val))
    }
}

impl<'a, K, V> ExactSizeIterator for IterMut<'a, K, V> {}
impl<'a, K, V> FusedIterator for IterMut<'a, K, V> {}

// The compiler does not automatically derive Send and Sync for Iter because it contains
// raw pointers.
unsafe impl<'a, K: Send, V: Send> Send for IterMut<'a, K, V> {}
unsafe impl<'a, K: Sync, V: Sync> Sync for IterMut<'a, K, V> {}

/// An iterator that moves out of a `LruCache`.
///
/// This `struct` is created by the [`into_iter`] method on [`LruCache`][`LruCache`]. See its
/// documentation for more.
///
/// [`into_iter`]: struct.LruCache.html#method.into_iter
/// [`LruCache`]: struct.LruCache.html
pub struct IntoIter<K, V>
where
    K: Hash + Eq,
{
    cache: LruCache<K, V>,
}

impl<K, V> Iterator for IntoIter<K, V>
where
    K: Hash + Eq,
{
    type Item = (K, V);

    fn next(&mut self) -> Option<(K, V)> {
        self.cache.pop_lru()
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let len = self.cache.len();
        (len, Some(len))
    }

    fn count(self) -> usize {
        self.cache.len()
    }
}

impl<K, V> ExactSizeIterator for IntoIter<K, V> where K: Hash + Eq {}
impl<K, V> FusedIterator for IntoIter<K, V> where K: Hash + Eq {}

impl<K: Hash + Eq, V> IntoIterator for LruCache<K, V> {
    type Item = (K, V);
    type IntoIter = IntoIter<K, V>;

    fn into_iter(self) -> IntoIter<K, V> {
        IntoIter { cache: self }
    }
}

#[cfg(test)]
mod tests {
    use super::LruCache;
    use core::{fmt::Debug, num::NonZeroUsize};
    use scoped_threadpool::Pool;
    use std::rc::Rc;
    use std::sync::atomic::{AtomicUsize, Ordering};

    fn assert_opt_eq<V: PartialEq + Debug>(opt: Option<&V>, v: V) {
        assert!(opt.is_some());
        assert_eq!(opt.unwrap(), &v);
    }

    fn assert_opt_eq_mut<V: PartialEq + Debug>(opt: Option<&mut V>, v: V) {
        assert!(opt.is_some());
        assert_eq!(opt.unwrap(), &v);
    }

    fn assert_opt_eq_tuple<K: PartialEq + Debug, V: PartialEq + Debug>(
        opt: Option<(&K, &V)>,
        kv: (K, V),
    ) {
        assert!(opt.is_some());
        let res = opt.unwrap();
        assert_eq!(res.0, &kv.0);
        assert_eq!(res.1, &kv.1);
    }

    fn assert_opt_eq_mut_tuple<K: PartialEq + Debug, V: PartialEq + Debug>(
        opt: Option<(&K, &mut V)>,
        kv: (K, V),
    ) {
        assert!(opt.is_some());
        let res = opt.unwrap();
        assert_eq!(res.0, &kv.0);
        assert_eq!(res.1, &kv.1);
    }

    #[test]
    fn test_unbounded() {
        let mut cache = LruCache::unbounded();
        for i in 0..13370 {
            cache.put(i, ());
        }
        assert_eq!(cache.len(), 13370);
    }

    #[test]
    #[cfg(feature = "hashbrown")]
    fn test_with_hasher() {
        use core::num::NonZeroUsize;

        use hashbrown::DefaultHashBuilder;

        let s = DefaultHashBuilder::default();
        let mut cache = LruCache::with_hasher(NonZeroUsize::new(16).unwrap(), s);

        for i in 0..13370 {
            cache.put(i, ());
        }
        assert_eq!(cache.len(), 16);
    }

    #[test]
    fn test_put_and_get() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
        assert!(cache.is_empty());

        assert_eq!(cache.put("apple", "red"), None);
        assert_eq!(cache.put("banana", "yellow"), None);

        assert_eq!(cache.cap().get(), 2);
        assert_eq!(cache.len(), 2);
        assert!(!cache.is_empty());
        assert_opt_eq(cache.get(&"apple"), "red");
        assert_opt_eq(cache.get(&"banana"), "yellow");
    }

    #[test]
    fn test_put_and_get_or_insert() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
        assert!(cache.is_empty());

        assert_eq!(cache.put("apple", "red"), None);
        assert_eq!(cache.put("banana", "yellow"), None);

        assert_eq!(cache.cap().get(), 2);
        assert_eq!(cache.len(), 2);
        assert!(!cache.is_empty());
        assert_eq!(cache.get_or_insert("apple", || "orange"), &"red");
        assert_eq!(cache.get_or_insert("banana", || "orange"), &"yellow");
        assert_eq!(cache.get_or_insert("lemon", || "orange"), &"orange");
        assert_eq!(cache.get_or_insert("lemon", || "red"), &"orange");
    }

    #[test]
    fn test_get_or_insert_ref() {
        use alloc::borrow::ToOwned;
        use alloc::string::String;

        let key1 = Rc::new("1".to_owned());
        let key2 = Rc::new("2".to_owned());
        let mut cache = LruCache::<Rc<String>, String>::new(NonZeroUsize::new(2).unwrap());
        assert!(cache.is_empty());
        assert_eq!(cache.get_or_insert_ref(&key1, || "One".to_owned()), "One");
        assert_eq!(cache.get_or_insert_ref(&key2, || "Two".to_owned()), "Two");
        assert_eq!(cache.len(), 2);
        assert!(!cache.is_empty());
        assert_eq!(
            cache.get_or_insert_ref(&key2, || "Not two".to_owned()),
            "Two"
        );
        assert_eq!(
            cache.get_or_insert_ref(&key2, || "Again not two".to_owned()),
            "Two"
        );
        assert_eq!(Rc::strong_count(&key1), 2);
        assert_eq!(Rc::strong_count(&key2), 2);
    }

    #[test]
    fn test_try_get_or_insert() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        assert_eq!(
            cache.try_get_or_insert::<_, &str>("apple", || Ok("red")),
            Ok(&"red")
        );
        assert_eq!(
            cache.try_get_or_insert::<_, &str>("apple", || Err("failed")),
            Ok(&"red")
        );
        assert_eq!(
            cache.try_get_or_insert::<_, &str>("banana", || Ok("orange")),
            Ok(&"orange")
        );
        assert_eq!(
            cache.try_get_or_insert::<_, &str>("lemon", || Err("failed")),
            Err("failed")
        );
        assert_eq!(
            cache.try_get_or_insert::<_, &str>("banana", || Err("failed")),
            Ok(&"orange")
        );
    }

    #[test]
    fn test_try_get_or_insert_ref() {
        use alloc::borrow::ToOwned;
        use alloc::string::String;

        let key1 = Rc::new("1".to_owned());
        let key2 = Rc::new("2".to_owned());
        let mut cache = LruCache::<Rc<String>, String>::new(NonZeroUsize::new(2).unwrap());
        let f = || -> Result<String, ()> { Err(()) };
        let a = || -> Result<String, ()> { Ok("One".to_owned()) };
        let b = || -> Result<String, ()> { Ok("Two".to_owned()) };
        assert_eq!(cache.try_get_or_insert_ref(&key1, a), Ok(&"One".to_owned()));
        assert_eq!(cache.try_get_or_insert_ref(&key2, f), Err(()));
        assert_eq!(cache.try_get_or_insert_ref(&key2, b), Ok(&"Two".to_owned()));
        assert_eq!(cache.try_get_or_insert_ref(&key2, a), Ok(&"Two".to_owned()));
        assert_eq!(cache.len(), 2);
        assert_eq!(Rc::strong_count(&key1), 2);
        assert_eq!(Rc::strong_count(&key2), 2);
    }

    #[test]
    fn test_put_and_get_or_insert_mut() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
        assert!(cache.is_empty());

        assert_eq!(cache.put("apple", "red"), None);
        assert_eq!(cache.put("banana", "yellow"), None);

        assert_eq!(cache.cap().get(), 2);
        assert_eq!(cache.len(), 2);

        let v = cache.get_or_insert_mut("apple", || "orange");
        assert_eq!(v, &"red");
        *v = "blue";

        assert_eq!(cache.get_or_insert_mut("apple", || "orange"), &"blue");
        assert_eq!(cache.get_or_insert_mut("banana", || "orange"), &"yellow");
        assert_eq!(cache.get_or_insert_mut("lemon", || "orange"), &"orange");
        assert_eq!(cache.get_or_insert_mut("lemon", || "red"), &"orange");
    }

    #[test]
    fn test_get_or_insert_mut_ref() {
        use alloc::borrow::ToOwned;
        use alloc::string::String;

        let key1 = Rc::new("1".to_owned());
        let key2 = Rc::new("2".to_owned());
        let mut cache = LruCache::<Rc<String>, &'static str>::new(NonZeroUsize::new(2).unwrap());
        assert_eq!(cache.get_or_insert_mut_ref(&key1, || "One"), &mut "One");
        let v = cache.get_or_insert_mut_ref(&key2, || "Two");
        *v = "New two";
        assert_eq!(cache.get_or_insert_mut_ref(&key2, || "Two"), &mut "New two");
        assert_eq!(Rc::strong_count(&key1), 2);
        assert_eq!(Rc::strong_count(&key2), 2);
    }

    #[test]
    fn test_try_get_or_insert_mut() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        cache.put(1, "a");
        cache.put(2, "b");
        cache.put(2, "c");

        let f = || -> Result<&str, &str> { Err("failed") };
        let a = || -> Result<&str, &str> { Ok("a") };
        let b = || -> Result<&str, &str> { Ok("b") };
        if let Ok(v) = cache.try_get_or_insert_mut(2, a) {
            *v = "d";
        }
        assert_eq!(cache.try_get_or_insert_mut(2, a), Ok(&mut "d"));
        assert_eq!(cache.try_get_or_insert_mut(3, f), Err("failed"));
        assert_eq!(cache.try_get_or_insert_mut(4, b), Ok(&mut "b"));
        assert_eq!(cache.try_get_or_insert_mut(4, a), Ok(&mut "b"));
    }

    #[test]
    fn test_try_get_or_insert_mut_ref() {
        use alloc::borrow::ToOwned;
        use alloc::string::String;

        let key1 = Rc::new("1".to_owned());
        let key2 = Rc::new("2".to_owned());
        let mut cache = LruCache::<Rc<String>, String>::new(NonZeroUsize::new(2).unwrap());
        let f = || -> Result<String, ()> { Err(()) };
        let a = || -> Result<String, ()> { Ok("One".to_owned()) };
        let b = || -> Result<String, ()> { Ok("Two".to_owned()) };
        assert_eq!(
            cache.try_get_or_insert_mut_ref(&key1, a),
            Ok(&mut "One".to_owned())
        );
        assert_eq!(cache.try_get_or_insert_mut_ref(&key2, f), Err(()));
        if let Ok(v) = cache.try_get_or_insert_mut_ref(&key2, b) {
            assert_eq!(v, &mut "Two");
            *v = "New two".to_owned();
        }
        assert_eq!(
            cache.try_get_or_insert_mut_ref(&key2, a),
            Ok(&mut "New two".to_owned())
        );
        assert_eq!(Rc::strong_count(&key1), 2);
        assert_eq!(Rc::strong_count(&key2), 2);
    }

    #[test]
    fn test_put_and_get_mut() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        cache.put("apple", "red");
        cache.put("banana", "yellow");

        assert_eq!(cache.cap().get(), 2);
        assert_eq!(cache.len(), 2);
        assert_opt_eq_mut(cache.get_mut(&"apple"), "red");
        assert_opt_eq_mut(cache.get_mut(&"banana"), "yellow");
    }

    #[test]
    fn test_get_mut_and_update() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        cache.put("apple", 1);
        cache.put("banana", 3);

        {
            let v = cache.get_mut(&"apple").unwrap();
            *v = 4;
        }

        assert_eq!(cache.cap().get(), 2);
        assert_eq!(cache.len(), 2);
        assert_opt_eq_mut(cache.get_mut(&"apple"), 4);
        assert_opt_eq_mut(cache.get_mut(&"banana"), 3);
    }

    #[test]
    fn test_put_update() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        assert_eq!(cache.put("apple", "red"), None);
        assert_eq!(cache.put("apple", "green"), Some("red"));

        assert_eq!(cache.len(), 1);
        assert_opt_eq(cache.get(&"apple"), "green");
    }

    #[test]
    fn test_put_removes_oldest() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        assert_eq!(cache.put("apple", "red"), None);
        assert_eq!(cache.put("banana", "yellow"), None);
        assert_eq!(cache.put("pear", "green"), None);

        assert!(cache.get(&"apple").is_none());
        assert_opt_eq(cache.get(&"banana"), "yellow");
        assert_opt_eq(cache.get(&"pear"), "green");

        // Even though we inserted "apple" into the cache earlier it has since been removed from
        // the cache so there is no current value for `put` to return.
        assert_eq!(cache.put("apple", "green"), None);
        assert_eq!(cache.put("tomato", "red"), None);

        assert!(cache.get(&"pear").is_none());
        assert_opt_eq(cache.get(&"apple"), "green");
        assert_opt_eq(cache.get(&"tomato"), "red");
    }

    #[test]
    fn test_peek() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        cache.put("apple", "red");
        cache.put("banana", "yellow");

        assert_opt_eq(cache.peek(&"banana"), "yellow");
        assert_opt_eq(cache.peek(&"apple"), "red");

        cache.put("pear", "green");

        assert!(cache.peek(&"apple").is_none());
        assert_opt_eq(cache.peek(&"banana"), "yellow");
        assert_opt_eq(cache.peek(&"pear"), "green");
    }

    #[test]
    fn test_peek_mut() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        cache.put("apple", "red");
        cache.put("banana", "yellow");

        assert_opt_eq_mut(cache.peek_mut(&"banana"), "yellow");
        assert_opt_eq_mut(cache.peek_mut(&"apple"), "red");
        assert!(cache.peek_mut(&"pear").is_none());

        cache.put("pear", "green");

        assert!(cache.peek_mut(&"apple").is_none());
        assert_opt_eq_mut(cache.peek_mut(&"banana"), "yellow");
        assert_opt_eq_mut(cache.peek_mut(&"pear"), "green");

        {
            let v = cache.peek_mut(&"banana").unwrap();
            *v = "green";
        }

        assert_opt_eq_mut(cache.peek_mut(&"banana"), "green");
    }

    #[test]
    fn test_peek_lru() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        assert!(cache.peek_lru().is_none());

        cache.put("apple", "red");
        cache.put("banana", "yellow");
        assert_opt_eq_tuple(cache.peek_lru(), ("apple", "red"));

        cache.get(&"apple");
        assert_opt_eq_tuple(cache.peek_lru(), ("banana", "yellow"));

        cache.clear();
        assert!(cache.peek_lru().is_none());
    }

    #[test]
    fn test_peek_mru() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        assert!(cache.peek_mru().is_none());

        cache.put("apple", "red");
        cache.put("banana", "yellow");
        assert_opt_eq_tuple(cache.peek_mru(), ("banana", "yellow"));

        cache.get(&"apple");
        assert_opt_eq_tuple(cache.peek_mru(), ("apple", "red"));

        cache.clear();
        assert!(cache.peek_mru().is_none());
    }

    #[test]
    fn test_contains() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        cache.put("apple", "red");
        cache.put("banana", "yellow");
        cache.put("pear", "green");

        assert!(!cache.contains(&"apple"));
        assert!(cache.contains(&"banana"));
        assert!(cache.contains(&"pear"));
    }

    #[test]
    fn test_pop() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        cache.put("apple", "red");
        cache.put("banana", "yellow");

        assert_eq!(cache.len(), 2);
        assert_opt_eq(cache.get(&"apple"), "red");
        assert_opt_eq(cache.get(&"banana"), "yellow");

        let popped = cache.pop(&"apple");
        assert!(popped.is_some());
        assert_eq!(popped.unwrap(), "red");
        assert_eq!(cache.len(), 1);
        assert!(cache.get(&"apple").is_none());
        assert_opt_eq(cache.get(&"banana"), "yellow");
    }

    #[test]
    fn test_pop_entry() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());
        cache.put("apple", "red");
        cache.put("banana", "yellow");

        assert_eq!(cache.len(), 2);
        assert_opt_eq(cache.get(&"apple"), "red");
        assert_opt_eq(cache.get(&"banana"), "yellow");

        let popped = cache.pop_entry(&"apple");
        assert!(popped.is_some());
        assert_eq!(popped.unwrap(), ("apple", "red"));
        assert_eq!(cache.len(), 1);
        assert!(cache.get(&"apple").is_none());
        assert_opt_eq(cache.get(&"banana"), "yellow");
    }

    #[test]
    fn test_pop_lru() {
        let mut cache = LruCache::new(NonZeroUsize::new(200).unwrap());

        for i in 0..75 {
            cache.put(i, "A");
        }
        for i in 0..75 {
            cache.put(i + 100, "B");
        }
        for i in 0..75 {
            cache.put(i + 200, "C");
        }
        assert_eq!(cache.len(), 200);

        for i in 0..75 {
            assert_opt_eq(cache.get(&(74 - i + 100)), "B");
        }
        assert_opt_eq(cache.get(&25), "A");

        for i in 26..75 {
            assert_eq!(cache.pop_lru(), Some((i, "A")));
        }
        for i in 0..75 {
            assert_eq!(cache.pop_lru(), Some((i + 200, "C")));
        }
        for i in 0..75 {
            assert_eq!(cache.pop_lru(), Some((74 - i + 100, "B")));
        }
        assert_eq!(cache.pop_lru(), Some((25, "A")));
        for _ in 0..50 {
            assert_eq!(cache.pop_lru(), None);
        }
    }

    #[test]
    fn test_pop_mru() {
        let mut cache = LruCache::new(NonZeroUsize::new(200).unwrap());

        for i in 0..75 {
            cache.put(i, "A");
        }
        for i in 0..75 {
            cache.put(i + 100, "B");
        }
        for i in 0..75 {
            cache.put(i + 200, "C");
        }
        assert_eq!(cache.len(), 200);

        for i in 0..75 {
            assert_opt_eq(cache.get(&(74 - i + 100)), "B");
        }
        assert_opt_eq(cache.get(&25), "A");

        assert_eq!(cache.pop_mru(), Some((25, "A")));
        for i in 0..75 {
            assert_eq!(cache.pop_mru(), Some((i + 100, "B")));
        }
        for i in 0..75 {
            assert_eq!(cache.pop_mru(), Some((74 - i + 200, "C")));
        }
        for i in (26..75).into_iter().rev() {
            assert_eq!(cache.pop_mru(), Some((i, "A")));
        }
        for _ in 0..50 {
            assert_eq!(cache.pop_mru(), None);
        }
    }

    #[test]
    fn test_clear() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        cache.put("apple", "red");
        cache.put("banana", "yellow");

        assert_eq!(cache.len(), 2);
        assert_opt_eq(cache.get(&"apple"), "red");
        assert_opt_eq(cache.get(&"banana"), "yellow");

        cache.clear();
        assert_eq!(cache.len(), 0);
    }

    #[test]
    fn test_resize_larger() {
        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        cache.put(1, "a");
        cache.put(2, "b");
        cache.resize(NonZeroUsize::new(4).unwrap());
        cache.put(3, "c");
        cache.put(4, "d");

        assert_eq!(cache.len(), 4);
        assert_eq!(cache.get(&1), Some(&"a"));
        assert_eq!(cache.get(&2), Some(&"b"));
        assert_eq!(cache.get(&3), Some(&"c"));
        assert_eq!(cache.get(&4), Some(&"d"));
    }

    #[test]
    fn test_resize_smaller() {
        let mut cache = LruCache::new(NonZeroUsize::new(4).unwrap());

        cache.put(1, "a");
        cache.put(2, "b");
        cache.put(3, "c");
        cache.put(4, "d");

        cache.resize(NonZeroUsize::new(2).unwrap());

        assert_eq!(cache.len(), 2);
        assert!(cache.get(&1).is_none());
        assert!(cache.get(&2).is_none());
        assert_eq!(cache.get(&3), Some(&"c"));
        assert_eq!(cache.get(&4), Some(&"d"));
    }

    #[test]
    fn test_send() {
        use std::thread;

        let mut cache = LruCache::new(NonZeroUsize::new(4).unwrap());
        cache.put(1, "a");

        let handle = thread::spawn(move || {
            assert_eq!(cache.get(&1), Some(&"a"));
        });

        assert!(handle.join().is_ok());
    }

    #[test]
    fn test_multiple_threads() {
        let mut pool = Pool::new(1);
        let mut cache = LruCache::new(NonZeroUsize::new(4).unwrap());
        cache.put(1, "a");

        let cache_ref = &cache;
        pool.scoped(|scoped| {
            scoped.execute(move || {
                assert_eq!(cache_ref.peek(&1), Some(&"a"));
            });
        });

        assert_eq!((cache_ref).peek(&1), Some(&"a"));
    }

    #[test]
    fn test_iter_forwards() {
        let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
        cache.put("a", 1);
        cache.put("b", 2);
        cache.put("c", 3);

        {
            // iter const
            let mut iter = cache.iter();
            assert_eq!(iter.len(), 3);
            assert_opt_eq_tuple(iter.next(), ("c", 3));

            assert_eq!(iter.len(), 2);
            assert_opt_eq_tuple(iter.next(), ("b", 2));

            assert_eq!(iter.len(), 1);
            assert_opt_eq_tuple(iter.next(), ("a", 1));

            assert_eq!(iter.len(), 0);
            assert_eq!(iter.next(), None);
        }
        {
            // iter mut
            let mut iter = cache.iter_mut();
            assert_eq!(iter.len(), 3);
            assert_opt_eq_mut_tuple(iter.next(), ("c", 3));

            assert_eq!(iter.len(), 2);
            assert_opt_eq_mut_tuple(iter.next(), ("b", 2));

            assert_eq!(iter.len(), 1);
            assert_opt_eq_mut_tuple(iter.next(), ("a", 1));

            assert_eq!(iter.len(), 0);
            assert_eq!(iter.next(), None);
        }
    }

    #[test]
    fn test_iter_backwards() {
        let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
        cache.put("a", 1);
        cache.put("b", 2);
        cache.put("c", 3);

        {
            // iter const
            let mut iter = cache.iter();
            assert_eq!(iter.len(), 3);
            assert_opt_eq_tuple(iter.next_back(), ("a", 1));

            assert_eq!(iter.len(), 2);
            assert_opt_eq_tuple(iter.next_back(), ("b", 2));

            assert_eq!(iter.len(), 1);
            assert_opt_eq_tuple(iter.next_back(), ("c", 3));

            assert_eq!(iter.len(), 0);
            assert_eq!(iter.next_back(), None);
        }

        {
            // iter mut
            let mut iter = cache.iter_mut();
            assert_eq!(iter.len(), 3);
            assert_opt_eq_mut_tuple(iter.next_back(), ("a", 1));

            assert_eq!(iter.len(), 2);
            assert_opt_eq_mut_tuple(iter.next_back(), ("b", 2));

            assert_eq!(iter.len(), 1);
            assert_opt_eq_mut_tuple(iter.next_back(), ("c", 3));

            assert_eq!(iter.len(), 0);
            assert_eq!(iter.next_back(), None);
        }
    }

    #[test]
    fn test_iter_forwards_and_backwards() {
        let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
        cache.put("a", 1);
        cache.put("b", 2);
        cache.put("c", 3);

        {
            // iter const
            let mut iter = cache.iter();
            assert_eq!(iter.len(), 3);
            assert_opt_eq_tuple(iter.next(), ("c", 3));

            assert_eq!(iter.len(), 2);
            assert_opt_eq_tuple(iter.next_back(), ("a", 1));

            assert_eq!(iter.len(), 1);
            assert_opt_eq_tuple(iter.next(), ("b", 2));

            assert_eq!(iter.len(), 0);
            assert_eq!(iter.next_back(), None);
        }
        {
            // iter mut
            let mut iter = cache.iter_mut();
            assert_eq!(iter.len(), 3);
            assert_opt_eq_mut_tuple(iter.next(), ("c", 3));

            assert_eq!(iter.len(), 2);
            assert_opt_eq_mut_tuple(iter.next_back(), ("a", 1));

            assert_eq!(iter.len(), 1);
            assert_opt_eq_mut_tuple(iter.next(), ("b", 2));

            assert_eq!(iter.len(), 0);
            assert_eq!(iter.next_back(), None);
        }
    }

    #[test]
    fn test_iter_multiple_threads() {
        let mut pool = Pool::new(1);
        let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
        cache.put("a", 1);
        cache.put("b", 2);
        cache.put("c", 3);

        let mut iter = cache.iter();
        assert_eq!(iter.len(), 3);
        assert_opt_eq_tuple(iter.next(), ("c", 3));

        {
            let iter_ref = &mut iter;
            pool.scoped(|scoped| {
                scoped.execute(move || {
                    assert_eq!(iter_ref.len(), 2);
                    assert_opt_eq_tuple(iter_ref.next(), ("b", 2));
                });
            });
        }

        assert_eq!(iter.len(), 1);
        assert_opt_eq_tuple(iter.next(), ("a", 1));

        assert_eq!(iter.len(), 0);
        assert_eq!(iter.next(), None);
    }

    #[test]
    fn test_iter_clone() {
        let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
        cache.put("a", 1);
        cache.put("b", 2);

        let mut iter = cache.iter();
        let mut iter_clone = iter.clone();

        assert_eq!(iter.len(), 2);
        assert_opt_eq_tuple(iter.next(), ("b", 2));
        assert_eq!(iter_clone.len(), 2);
        assert_opt_eq_tuple(iter_clone.next(), ("b", 2));

        assert_eq!(iter.len(), 1);
        assert_opt_eq_tuple(iter.next(), ("a", 1));
        assert_eq!(iter_clone.len(), 1);
        assert_opt_eq_tuple(iter_clone.next(), ("a", 1));

        assert_eq!(iter.len(), 0);
        assert_eq!(iter.next(), None);
        assert_eq!(iter_clone.len(), 0);
        assert_eq!(iter_clone.next(), None);
    }

    #[test]
    fn test_into_iter() {
        let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
        cache.put("a", 1);
        cache.put("b", 2);
        cache.put("c", 3);

        let mut iter = cache.into_iter();
        assert_eq!(iter.len(), 3);
        assert_eq!(iter.next(), Some(("a", 1)));

        assert_eq!(iter.len(), 2);
        assert_eq!(iter.next(), Some(("b", 2)));

        assert_eq!(iter.len(), 1);
        assert_eq!(iter.next(), Some(("c", 3)));

        assert_eq!(iter.len(), 0);
        assert_eq!(iter.next(), None);
    }

    #[test]
    fn test_that_pop_actually_detaches_node() {
        let mut cache = LruCache::new(NonZeroUsize::new(5).unwrap());

        cache.put("a", 1);
        cache.put("b", 2);
        cache.put("c", 3);
        cache.put("d", 4);
        cache.put("e", 5);

        assert_eq!(cache.pop(&"c"), Some(3));

        cache.put("f", 6);

        let mut iter = cache.iter();
        assert_opt_eq_tuple(iter.next(), ("f", 6));
        assert_opt_eq_tuple(iter.next(), ("e", 5));
        assert_opt_eq_tuple(iter.next(), ("d", 4));
        assert_opt_eq_tuple(iter.next(), ("b", 2));
        assert_opt_eq_tuple(iter.next(), ("a", 1));
        assert!(iter.next().is_none());
    }

    #[test]
    fn test_get_with_borrow() {
        use alloc::string::String;

        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        let key = String::from("apple");
        cache.put(key, "red");

        assert_opt_eq(cache.get("apple"), "red");
    }

    #[test]
    fn test_get_mut_with_borrow() {
        use alloc::string::String;

        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        let key = String::from("apple");
        cache.put(key, "red");

        assert_opt_eq_mut(cache.get_mut("apple"), "red");
    }

    #[test]
    fn test_no_memory_leaks() {
        static DROP_COUNT: AtomicUsize = AtomicUsize::new(0);

        struct DropCounter;

        impl Drop for DropCounter {
            fn drop(&mut self) {
                DROP_COUNT.fetch_add(1, Ordering::SeqCst);
            }
        }

        let n = 100;
        for _ in 0..n {
            let mut cache = LruCache::new(NonZeroUsize::new(1).unwrap());
            for i in 0..n {
                cache.put(i, DropCounter {});
            }
        }
        assert_eq!(DROP_COUNT.load(Ordering::SeqCst), n * n);
    }

    #[test]
    fn test_no_memory_leaks_with_clear() {
        static DROP_COUNT: AtomicUsize = AtomicUsize::new(0);

        struct DropCounter;

        impl Drop for DropCounter {
            fn drop(&mut self) {
                DROP_COUNT.fetch_add(1, Ordering::SeqCst);
            }
        }

        let n = 100;
        for _ in 0..n {
            let mut cache = LruCache::new(NonZeroUsize::new(1).unwrap());
            for i in 0..n {
                cache.put(i, DropCounter {});
            }
            cache.clear();
        }
        assert_eq!(DROP_COUNT.load(Ordering::SeqCst), n * n);
    }

    #[test]
    fn test_no_memory_leaks_with_resize() {
        static DROP_COUNT: AtomicUsize = AtomicUsize::new(0);

        struct DropCounter;

        impl Drop for DropCounter {
            fn drop(&mut self) {
                DROP_COUNT.fetch_add(1, Ordering::SeqCst);
            }
        }

        let n = 100;
        for _ in 0..n {
            let mut cache = LruCache::new(NonZeroUsize::new(1).unwrap());
            for i in 0..n {
                cache.put(i, DropCounter {});
            }
            cache.clear();
        }
        assert_eq!(DROP_COUNT.load(Ordering::SeqCst), n * n);
    }

    #[test]
    fn test_no_memory_leaks_with_pop() {
        static DROP_COUNT: AtomicUsize = AtomicUsize::new(0);

        #[derive(Hash, Eq)]
        struct KeyDropCounter(usize);

        impl PartialEq for KeyDropCounter {
            fn eq(&self, other: &Self) -> bool {
                self.0.eq(&other.0)
            }
        }

        impl Drop for KeyDropCounter {
            fn drop(&mut self) {
                DROP_COUNT.fetch_add(1, Ordering::SeqCst);
            }
        }

        let n = 100;
        for _ in 0..n {
            let mut cache = LruCache::new(NonZeroUsize::new(1).unwrap());

            for i in 0..100 {
                cache.put(KeyDropCounter(i), i);
                cache.pop(&KeyDropCounter(i));
            }
        }

        assert_eq!(DROP_COUNT.load(Ordering::SeqCst), n * n * 2);
    }

    #[test]
    fn test_promote_and_demote() {
        let mut cache = LruCache::new(NonZeroUsize::new(5).unwrap());
        for i in 0..5 {
            cache.push(i, i);
        }
        cache.promote(&1);
        cache.promote(&0);
        cache.demote(&3);
        cache.demote(&4);
        assert_eq!(cache.pop_lru(), Some((4, 4)));
        assert_eq!(cache.pop_lru(), Some((3, 3)));
        assert_eq!(cache.pop_lru(), Some((2, 2)));
        assert_eq!(cache.pop_lru(), Some((1, 1)));
        assert_eq!(cache.pop_lru(), Some((0, 0)));
        assert_eq!(cache.pop_lru(), None);
    }

    #[test]
    fn test_get_key_value() {
        use alloc::string::String;

        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        let key = String::from("apple");
        cache.put(key, "red");

        assert_eq!(
            cache.get_key_value("apple"),
            Some((&String::from("apple"), &"red"))
        );
        assert_eq!(cache.get_key_value("banana"), None);
    }

    #[test]
    fn test_get_key_value_mut() {
        use alloc::string::String;

        let mut cache = LruCache::new(NonZeroUsize::new(2).unwrap());

        let key = String::from("apple");
        cache.put(key, "red");

        let (k, v) = cache.get_key_value_mut("apple").unwrap();
        assert_eq!(k, &String::from("apple"));
        assert_eq!(v, &mut "red");
        *v = "green";

        assert_eq!(
            cache.get_key_value("apple"),
            Some((&String::from("apple"), &"green"))
        );
        assert_eq!(cache.get_key_value("banana"), None);
    }

    #[test]
    fn test_clone() {
        let mut cache = LruCache::new(NonZeroUsize::new(3).unwrap());
        cache.put("a", 1);
        cache.put("b", 2);
        cache.put("c", 3);

        let mut cloned = cache.clone();

        assert_eq!(cache.pop_lru(), Some(("a", 1)));
        assert_eq!(cloned.pop_lru(), Some(("a", 1)));

        assert_eq!(cache.pop_lru(), Some(("b", 2)));
        assert_eq!(cloned.pop_lru(), Some(("b", 2)));

        assert_eq!(cache.pop_lru(), Some(("c", 3)));
        assert_eq!(cloned.pop_lru(), Some(("c", 3)));

        assert_eq!(cache.pop_lru(), None);
        assert_eq!(cloned.pop_lru(), None);
    }
}

/// Doctests for what should *not* compile
///
/// ```compile_fail
/// let mut cache = lru::LruCache::<u32, u32>::unbounded();
/// let _: &'static u32 = cache.get_or_insert(0, || 92);
/// ```
///
/// ```compile_fail
/// let mut cache = lru::LruCache::<u32, u32>::unbounded();
/// let _: Option<(&'static u32, _)> = cache.peek_lru();
/// let _: Option<(_, &'static u32)> = cache.peek_lru();
/// ```
fn _test_lifetimes() {}