1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
#![cfg_attr(not(feature = "full"), allow(dead_code))]

//! An intrusive double linked list of data.
//!
//! The data structure supports tracking pinned nodes. Most of the data
//! structure's APIs are `unsafe` as they require the caller to ensure the
//! specified node is actually contained by the list.

use core::cell::UnsafeCell;
use core::fmt;
use core::marker::{PhantomData, PhantomPinned};
use core::mem::ManuallyDrop;
use core::ptr::{self, NonNull};

/// An intrusive linked list.
///
/// Currently, the list is not emptied on drop. It is the caller's
/// responsibility to ensure the list is empty before dropping it.
pub(crate) struct LinkedList<L, T> {
    /// Linked list head
    head: Option<NonNull<T>>,

    /// Linked list tail
    tail: Option<NonNull<T>>,

    /// Node type marker.
    _marker: PhantomData<*const L>,
}

unsafe impl<L: Link> Send for LinkedList<L, L::Target> where L::Target: Send {}
unsafe impl<L: Link> Sync for LinkedList<L, L::Target> where L::Target: Sync {}

/// Defines how a type is tracked within a linked list.
///
/// In order to support storing a single type within multiple lists, accessing
/// the list pointers is decoupled from the entry type.
///
/// # Safety
///
/// Implementations must guarantee that `Target` types are pinned in memory. In
/// other words, when a node is inserted, the value will not be moved as long as
/// it is stored in the list.
pub(crate) unsafe trait Link {
    /// Handle to the list entry.
    ///
    /// This is usually a pointer-ish type.
    type Handle;

    /// Node type.
    type Target;

    /// Convert the handle to a raw pointer without consuming the handle.
    #[allow(clippy::wrong_self_convention)]
    fn as_raw(handle: &Self::Handle) -> NonNull<Self::Target>;

    /// Convert the raw pointer to a handle
    unsafe fn from_raw(ptr: NonNull<Self::Target>) -> Self::Handle;

    /// Return the pointers for a node
    ///
    /// # Safety
    ///
    /// The resulting pointer should have the same tag in the stacked-borrows
    /// stack as the argument. In particular, the method may not create an
    /// intermediate reference in the process of creating the resulting raw
    /// pointer.
    unsafe fn pointers(target: NonNull<Self::Target>) -> NonNull<Pointers<Self::Target>>;
}

/// Previous / next pointers.
pub(crate) struct Pointers<T> {
    inner: UnsafeCell<PointersInner<T>>,
}
/// We do not want the compiler to put the `noalias` attribute on mutable
/// references to this type, so the type has been made `!Unpin` with a
/// `PhantomPinned` field.
///
/// Additionally, we never access the `prev` or `next` fields directly, as any
/// such access would implicitly involve the creation of a reference to the
/// field, which we want to avoid since the fields are not `!Unpin`, and would
/// hence be given the `noalias` attribute if we were to do such an access. As
/// an alternative to accessing the fields directly, the `Pointers` type
/// provides getters and setters for the two fields, and those are implemented
/// using `ptr`-specific methods which avoids the creation of intermediate
/// references.
///
/// See this link for more information:
/// <https://github.com/rust-lang/rust/pull/82834>
struct PointersInner<T> {
    /// The previous node in the list. null if there is no previous node.
    prev: Option<NonNull<T>>,

    /// The next node in the list. null if there is no previous node.
    next: Option<NonNull<T>>,

    /// This type is !Unpin due to the heuristic from:
    /// <https://github.com/rust-lang/rust/pull/82834>
    _pin: PhantomPinned,
}

unsafe impl<T: Send> Send for Pointers<T> {}
unsafe impl<T: Sync> Sync for Pointers<T> {}

// ===== impl LinkedList =====

impl<L, T> LinkedList<L, T> {
    /// Creates an empty linked list.
    pub(crate) const fn new() -> LinkedList<L, T> {
        LinkedList {
            head: None,
            tail: None,
            _marker: PhantomData,
        }
    }
}

impl<L: Link> LinkedList<L, L::Target> {
    /// Adds an element first in the list.
    pub(crate) fn push_front(&mut self, val: L::Handle) {
        // The value should not be dropped, it is being inserted into the list
        let val = ManuallyDrop::new(val);
        let ptr = L::as_raw(&val);
        assert_ne!(self.head, Some(ptr));
        unsafe {
            L::pointers(ptr).as_mut().set_next(self.head);
            L::pointers(ptr).as_mut().set_prev(None);

            if let Some(head) = self.head {
                L::pointers(head).as_mut().set_prev(Some(ptr));
            }

            self.head = Some(ptr);

            if self.tail.is_none() {
                self.tail = Some(ptr);
            }
        }
    }

    /// Removes the first element from a list and returns it, or None if it is
    /// empty.
    pub(crate) fn pop_front(&mut self) -> Option<L::Handle> {
        unsafe {
            let head = self.head?;
            self.head = L::pointers(head).as_ref().get_next();

            if let Some(new_head) = L::pointers(head).as_ref().get_next() {
                L::pointers(new_head).as_mut().set_prev(None);
            } else {
                self.tail = None;
            }

            L::pointers(head).as_mut().set_prev(None);
            L::pointers(head).as_mut().set_next(None);

            Some(L::from_raw(head))
        }
    }

    /// Removes the last element from a list and returns it, or None if it is
    /// empty.
    pub(crate) fn pop_back(&mut self) -> Option<L::Handle> {
        unsafe {
            let last = self.tail?;
            self.tail = L::pointers(last).as_ref().get_prev();

            if let Some(prev) = L::pointers(last).as_ref().get_prev() {
                L::pointers(prev).as_mut().set_next(None);
            } else {
                self.head = None;
            }

            L::pointers(last).as_mut().set_prev(None);
            L::pointers(last).as_mut().set_next(None);

            Some(L::from_raw(last))
        }
    }

    /// Returns whether the linked list does not contain any node
    pub(crate) fn is_empty(&self) -> bool {
        if self.head.is_some() {
            return false;
        }

        assert!(self.tail.is_none());
        true
    }

    /// Removes the specified node from the list
    ///
    /// # Safety
    ///
    /// The caller **must** ensure that exactly one of the following is true:
    /// - `node` is currently contained by `self`,
    /// - `node` is not contained by any list,
    /// - `node` is currently contained by some other `GuardedLinkedList` **and**
    ///   the caller has an exclusive access to that list. This condition is
    ///   used by the linked list in `sync::Notify`.
    pub(crate) unsafe fn remove(&mut self, node: NonNull<L::Target>) -> Option<L::Handle> {
        if let Some(prev) = L::pointers(node).as_ref().get_prev() {
            debug_assert_eq!(L::pointers(prev).as_ref().get_next(), Some(node));
            L::pointers(prev)
                .as_mut()
                .set_next(L::pointers(node).as_ref().get_next());
        } else {
            if self.head != Some(node) {
                return None;
            }

            self.head = L::pointers(node).as_ref().get_next();
        }

        if let Some(next) = L::pointers(node).as_ref().get_next() {
            debug_assert_eq!(L::pointers(next).as_ref().get_prev(), Some(node));
            L::pointers(next)
                .as_mut()
                .set_prev(L::pointers(node).as_ref().get_prev());
        } else {
            // This might be the last item in the list
            if self.tail != Some(node) {
                return None;
            }

            self.tail = L::pointers(node).as_ref().get_prev();
        }

        L::pointers(node).as_mut().set_next(None);
        L::pointers(node).as_mut().set_prev(None);

        Some(L::from_raw(node))
    }
}

impl<L: Link> fmt::Debug for LinkedList<L, L::Target> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("LinkedList")
            .field("head", &self.head)
            .field("tail", &self.tail)
            .finish()
    }
}

#[cfg(any(
    feature = "fs",
    feature = "rt",
    all(unix, feature = "process"),
    feature = "signal",
    feature = "sync",
))]
impl<L: Link> LinkedList<L, L::Target> {
    pub(crate) fn last(&self) -> Option<&L::Target> {
        let tail = self.tail.as_ref()?;
        unsafe { Some(&*tail.as_ptr()) }
    }
}

impl<L: Link> Default for LinkedList<L, L::Target> {
    fn default() -> Self {
        Self::new()
    }
}

// ===== impl DrainFilter =====

cfg_io_driver_impl! {
    pub(crate) struct DrainFilter<'a, T: Link, F> {
        list: &'a mut LinkedList<T, T::Target>,
        filter: F,
        curr: Option<NonNull<T::Target>>,
    }

    impl<T: Link> LinkedList<T, T::Target> {
        pub(crate) fn drain_filter<F>(&mut self, filter: F) -> DrainFilter<'_, T, F>
        where
            F: FnMut(&T::Target) -> bool,
        {
            let curr = self.head;
            DrainFilter {
                curr,
                filter,
                list: self,
            }
        }
    }

    impl<'a, T, F> Iterator for DrainFilter<'a, T, F>
    where
        T: Link,
        F: FnMut(&T::Target) -> bool,
    {
        type Item = T::Handle;

        fn next(&mut self) -> Option<Self::Item> {
            while let Some(curr) = self.curr {
                // safety: the pointer references data contained by the list
                self.curr = unsafe { T::pointers(curr).as_ref() }.get_next();

                // safety: the value is still owned by the linked list.
                if (self.filter)(unsafe { &mut *curr.as_ptr() }) {
                    return unsafe { self.list.remove(curr) };
                }
            }

            None
        }
    }
}

cfg_taskdump! {
    impl<T: Link> LinkedList<T, T::Target> {
        pub(crate) fn for_each<F>(&mut self, mut f: F)
        where
            F: FnMut(&T::Handle),
        {
            let mut next = self.head;

            while let Some(curr) = next {
                unsafe {
                    let handle = ManuallyDrop::new(T::from_raw(curr));
                    f(&handle);
                    next = T::pointers(curr).as_ref().get_next();
                }
            }
        }
    }
}

// ===== impl GuardedLinkedList =====

feature! {
    #![any(
        feature = "process",
        feature = "sync",
        feature = "rt",
        feature = "signal",
    )]

    /// An intrusive linked list, but instead of keeping pointers to the head
    /// and tail nodes, it uses a special guard node linked with those nodes.
    /// It means that the list is circular and every pointer of a node from
    /// the list is not `None`, including pointers from the guard node.
    ///
    /// If a list is empty, then both pointers of the guard node are pointing
    /// at the guard node itself.
    pub(crate) struct GuardedLinkedList<L, T> {
        /// Pointer to the guard node.
        guard: NonNull<T>,

        /// Node type marker.
        _marker: PhantomData<*const L>,
    }

    impl<L: Link> LinkedList<L, L::Target> {
        /// Turns a linked list into the guarded version by linking the guard node
        /// with the head and tail nodes. Like with other nodes, you should guarantee
        /// that the guard node is pinned in memory.
        pub(crate) fn into_guarded(self, guard_handle: L::Handle) -> GuardedLinkedList<L, L::Target> {
            // `guard_handle` is a NonNull pointer, we don't have to care about dropping it.
            let guard = L::as_raw(&guard_handle);

            unsafe {
                if let Some(head) = self.head {
                    debug_assert!(L::pointers(head).as_ref().get_prev().is_none());
                    L::pointers(head).as_mut().set_prev(Some(guard));
                    L::pointers(guard).as_mut().set_next(Some(head));

                    // The list is not empty, so the tail cannot be `None`.
                    let tail = self.tail.unwrap();
                    debug_assert!(L::pointers(tail).as_ref().get_next().is_none());
                    L::pointers(tail).as_mut().set_next(Some(guard));
                    L::pointers(guard).as_mut().set_prev(Some(tail));
                } else {
                    // The list is empty.
                    L::pointers(guard).as_mut().set_prev(Some(guard));
                    L::pointers(guard).as_mut().set_next(Some(guard));
                }
            }

            GuardedLinkedList { guard, _marker: PhantomData }
        }
    }

    impl<L: Link> GuardedLinkedList<L, L::Target> {
        fn tail(&self) -> Option<NonNull<L::Target>> {
            let tail_ptr = unsafe {
                L::pointers(self.guard).as_ref().get_prev().unwrap()
            };

            // Compare the tail pointer with the address of the guard node itself.
            // If the guard points at itself, then there are no other nodes and
            // the list is considered empty.
            if tail_ptr != self.guard {
                Some(tail_ptr)
            } else {
                None
            }
        }

        /// Removes the last element from a list and returns it, or None if it is
        /// empty.
        pub(crate) fn pop_back(&mut self) -> Option<L::Handle> {
            unsafe {
                let last = self.tail()?;
                let before_last = L::pointers(last).as_ref().get_prev().unwrap();

                L::pointers(self.guard).as_mut().set_prev(Some(before_last));
                L::pointers(before_last).as_mut().set_next(Some(self.guard));

                L::pointers(last).as_mut().set_prev(None);
                L::pointers(last).as_mut().set_next(None);

                Some(L::from_raw(last))
            }
        }
    }
}

// ===== impl Pointers =====

impl<T> Pointers<T> {
    /// Create a new set of empty pointers
    pub(crate) fn new() -> Pointers<T> {
        Pointers {
            inner: UnsafeCell::new(PointersInner {
                prev: None,
                next: None,
                _pin: PhantomPinned,
            }),
        }
    }

    pub(crate) fn get_prev(&self) -> Option<NonNull<T>> {
        // SAFETY: Field is accessed immutably through a reference.
        unsafe { ptr::addr_of!((*self.inner.get()).prev).read() }
    }
    pub(crate) fn get_next(&self) -> Option<NonNull<T>> {
        // SAFETY: Field is accessed immutably through a reference.
        unsafe { ptr::addr_of!((*self.inner.get()).next).read() }
    }

    fn set_prev(&mut self, value: Option<NonNull<T>>) {
        // SAFETY: Field is accessed mutably through a mutable reference.
        unsafe {
            ptr::addr_of_mut!((*self.inner.get()).prev).write(value);
        }
    }
    fn set_next(&mut self, value: Option<NonNull<T>>) {
        // SAFETY: Field is accessed mutably through a mutable reference.
        unsafe {
            ptr::addr_of_mut!((*self.inner.get()).next).write(value);
        }
    }
}

impl<T> fmt::Debug for Pointers<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let prev = self.get_prev();
        let next = self.get_next();
        f.debug_struct("Pointers")
            .field("prev", &prev)
            .field("next", &next)
            .finish()
    }
}

#[cfg(any(test, fuzzing))]
#[cfg(not(loom))]
pub(crate) mod tests {
    use super::*;

    use std::pin::Pin;

    #[derive(Debug)]
    #[repr(C)]
    struct Entry {
        pointers: Pointers<Entry>,
        val: i32,
    }

    unsafe impl<'a> Link for &'a Entry {
        type Handle = Pin<&'a Entry>;
        type Target = Entry;

        fn as_raw(handle: &Pin<&'_ Entry>) -> NonNull<Entry> {
            NonNull::from(handle.get_ref())
        }

        unsafe fn from_raw(ptr: NonNull<Entry>) -> Pin<&'a Entry> {
            Pin::new_unchecked(&*ptr.as_ptr())
        }

        unsafe fn pointers(target: NonNull<Entry>) -> NonNull<Pointers<Entry>> {
            target.cast()
        }
    }

    fn entry(val: i32) -> Pin<Box<Entry>> {
        Box::pin(Entry {
            pointers: Pointers::new(),
            val,
        })
    }

    fn ptr(r: &Pin<Box<Entry>>) -> NonNull<Entry> {
        r.as_ref().get_ref().into()
    }

    fn collect_list(list: &mut LinkedList<&'_ Entry, <&'_ Entry as Link>::Target>) -> Vec<i32> {
        let mut ret = vec![];

        while let Some(entry) = list.pop_back() {
            ret.push(entry.val);
        }

        ret
    }

    fn push_all<'a>(
        list: &mut LinkedList<&'a Entry, <&'_ Entry as Link>::Target>,
        entries: &[Pin<&'a Entry>],
    ) {
        for entry in entries.iter() {
            list.push_front(*entry);
        }
    }

    #[cfg(test)]
    macro_rules! assert_clean {
        ($e:ident) => {{
            assert!($e.pointers.get_next().is_none());
            assert!($e.pointers.get_prev().is_none());
        }};
    }

    #[cfg(test)]
    macro_rules! assert_ptr_eq {
        ($a:expr, $b:expr) => {{
            // Deal with mapping a Pin<&mut T> -> Option<NonNull<T>>
            assert_eq!(Some($a.as_ref().get_ref().into()), $b)
        }};
    }

    #[test]
    fn const_new() {
        const _: LinkedList<&Entry, <&Entry as Link>::Target> = LinkedList::new();
    }

    #[test]
    fn push_and_drain() {
        let a = entry(5);
        let b = entry(7);
        let c = entry(31);

        let mut list = LinkedList::new();
        assert!(list.is_empty());

        list.push_front(a.as_ref());
        assert!(!list.is_empty());
        list.push_front(b.as_ref());
        list.push_front(c.as_ref());

        let items: Vec<i32> = collect_list(&mut list);
        assert_eq!([5, 7, 31].to_vec(), items);

        assert!(list.is_empty());
    }

    #[test]
    fn push_pop_push_pop() {
        let a = entry(5);
        let b = entry(7);

        let mut list = LinkedList::<&Entry, <&Entry as Link>::Target>::new();

        list.push_front(a.as_ref());

        let entry = list.pop_back().unwrap();
        assert_eq!(5, entry.val);
        assert!(list.is_empty());

        list.push_front(b.as_ref());

        let entry = list.pop_back().unwrap();
        assert_eq!(7, entry.val);

        assert!(list.is_empty());
        assert!(list.pop_back().is_none());
    }

    #[test]
    fn remove_by_address() {
        let a = entry(5);
        let b = entry(7);
        let c = entry(31);

        unsafe {
            // Remove first
            let mut list = LinkedList::new();

            push_all(&mut list, &[c.as_ref(), b.as_ref(), a.as_ref()]);
            assert!(list.remove(ptr(&a)).is_some());
            assert_clean!(a);
            // `a` should be no longer there and can't be removed twice
            assert!(list.remove(ptr(&a)).is_none());
            assert!(!list.is_empty());

            assert!(list.remove(ptr(&b)).is_some());
            assert_clean!(b);
            // `b` should be no longer there and can't be removed twice
            assert!(list.remove(ptr(&b)).is_none());
            assert!(!list.is_empty());

            assert!(list.remove(ptr(&c)).is_some());
            assert_clean!(c);
            // `b` should be no longer there and can't be removed twice
            assert!(list.remove(ptr(&c)).is_none());
            assert!(list.is_empty());
        }

        unsafe {
            // Remove middle
            let mut list = LinkedList::new();

            push_all(&mut list, &[c.as_ref(), b.as_ref(), a.as_ref()]);

            assert!(list.remove(ptr(&a)).is_some());
            assert_clean!(a);

            assert_ptr_eq!(b, list.head);
            assert_ptr_eq!(c, b.pointers.get_next());
            assert_ptr_eq!(b, c.pointers.get_prev());

            let items = collect_list(&mut list);
            assert_eq!([31, 7].to_vec(), items);
        }

        unsafe {
            // Remove middle
            let mut list = LinkedList::new();

            push_all(&mut list, &[c.as_ref(), b.as_ref(), a.as_ref()]);

            assert!(list.remove(ptr(&b)).is_some());
            assert_clean!(b);

            assert_ptr_eq!(c, a.pointers.get_next());
            assert_ptr_eq!(a, c.pointers.get_prev());

            let items = collect_list(&mut list);
            assert_eq!([31, 5].to_vec(), items);
        }

        unsafe {
            // Remove last
            // Remove middle
            let mut list = LinkedList::new();

            push_all(&mut list, &[c.as_ref(), b.as_ref(), a.as_ref()]);

            assert!(list.remove(ptr(&c)).is_some());
            assert_clean!(c);

            assert!(b.pointers.get_next().is_none());
            assert_ptr_eq!(b, list.tail);

            let items = collect_list(&mut list);
            assert_eq!([7, 5].to_vec(), items);
        }

        unsafe {
            // Remove first of two
            let mut list = LinkedList::new();

            push_all(&mut list, &[b.as_ref(), a.as_ref()]);

            assert!(list.remove(ptr(&a)).is_some());

            assert_clean!(a);

            // a should be no longer there and can't be removed twice
            assert!(list.remove(ptr(&a)).is_none());

            assert_ptr_eq!(b, list.head);
            assert_ptr_eq!(b, list.tail);

            assert!(b.pointers.get_next().is_none());
            assert!(b.pointers.get_prev().is_none());

            let items = collect_list(&mut list);
            assert_eq!([7].to_vec(), items);
        }

        unsafe {
            // Remove last of two
            let mut list = LinkedList::new();

            push_all(&mut list, &[b.as_ref(), a.as_ref()]);

            assert!(list.remove(ptr(&b)).is_some());

            assert_clean!(b);

            assert_ptr_eq!(a, list.head);
            assert_ptr_eq!(a, list.tail);

            assert!(a.pointers.get_next().is_none());
            assert!(a.pointers.get_prev().is_none());

            let items = collect_list(&mut list);
            assert_eq!([5].to_vec(), items);
        }

        unsafe {
            // Remove last item
            let mut list = LinkedList::new();

            push_all(&mut list, &[a.as_ref()]);

            assert!(list.remove(ptr(&a)).is_some());
            assert_clean!(a);

            assert!(list.head.is_none());
            assert!(list.tail.is_none());
            let items = collect_list(&mut list);
            assert!(items.is_empty());
        }

        unsafe {
            // Remove missing
            let mut list = LinkedList::<&Entry, <&Entry as Link>::Target>::new();

            list.push_front(b.as_ref());
            list.push_front(a.as_ref());

            assert!(list.remove(ptr(&c)).is_none());
        }
    }

    /// This is a fuzz test. You run it by entering `cargo fuzz run fuzz_linked_list` in CLI in `/tokio/` module.
    #[cfg(fuzzing)]
    pub fn fuzz_linked_list(ops: &[u8]) {
        enum Op {
            Push,
            Pop,
            Remove(usize),
        }
        use std::collections::VecDeque;

        let ops = ops
            .iter()
            .map(|i| match i % 3u8 {
                0 => Op::Push,
                1 => Op::Pop,
                2 => Op::Remove((i / 3u8) as usize),
                _ => unreachable!(),
            })
            .collect::<Vec<_>>();

        let mut ll = LinkedList::<&Entry, <&Entry as Link>::Target>::new();
        let mut reference = VecDeque::new();

        let entries: Vec<_> = (0..ops.len()).map(|i| entry(i as i32)).collect();

        for (i, op) in ops.iter().enumerate() {
            match op {
                Op::Push => {
                    reference.push_front(i as i32);
                    assert_eq!(entries[i].val, i as i32);

                    ll.push_front(entries[i].as_ref());
                }
                Op::Pop => {
                    if reference.is_empty() {
                        assert!(ll.is_empty());
                        continue;
                    }

                    let v = reference.pop_back();
                    assert_eq!(v, ll.pop_back().map(|v| v.val));
                }
                Op::Remove(n) => {
                    if reference.is_empty() {
                        assert!(ll.is_empty());
                        continue;
                    }

                    let idx = n % reference.len();
                    let expect = reference.remove(idx).unwrap();

                    unsafe {
                        let entry = ll.remove(ptr(&entries[expect as usize])).unwrap();
                        assert_eq!(expect, entry.val);
                    }
                }
            }
        }
    }
}