      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

    //! Unbounded channel implemented as a linked list.

    use std::cell::UnsafeCell;
    use std::marker::PhantomData;
    use std::mem::{self, ManuallyDrop};
    use std::ptr;
    use std::sync::atomic::{self, AtomicPtr, AtomicUsize, Ordering};
    use std::time::Instant;

    use crossbeam_utils::{Backoff, CachePadded};

    use context::Context;
    use err::{RecvTimeoutError, SendTimeoutError, TryRecvError, TrySendError};
    use select::{Operation, SelectHandle, Selected, Token};
    use waker::SyncWaker;

    // TODO(stjepang): Once we bump the minimum required Rust version to 1.28 or newer, re-apply the
    // following changes by @kleimkuhler:
    //
    // 1. https://github.com/crossbeam-rs/crossbeam-channel/pull/100
    // 2. https://github.com/crossbeam-rs/crossbeam-channel/pull/101

    // Bits indicating the state of a slot:
    // * If a message has been written into the slot, `WRITE` is set.
    // * If a message has been read from the slot, `READ` is set.
    // * If the block is being destroyed, `DESTROY` is set.
    const WRITE: usize = 1;
    const READ: usize = 2;
    const DESTROY: usize = 4;

    // Each block covers one "lap" of indices.
    const LAP: usize = 32;
    // The maximum number of messages a block can hold.
    const BLOCK_CAP: usize = LAP - 1;
    // How many lower bits are reserved for metadata.
    const SHIFT: usize = 1;
    // Has two different purposes:
    // * If set in head, indicates that the block is not the last one.
    // * If set in tail, indicates that the channel is disconnected.
    const MARK_BIT: usize = 1;

    /// A slot in a block.
    struct Slot<T> {
        /// The message.
        msg: UnsafeCell<ManuallyDrop<T>>,

        /// The state of the slot.
        state: AtomicUsize,
    }

    impl<T> Slot<T> {
        /// Waits until a message is written into the slot.
        fn wait_write(&self) {
            let backoff = Backoff::new();
            while self.state.load(Ordering::Acquire) & WRITE == 0 {
                backoff.snooze();
            }
        }
    }

    /// A block in a linked list.
    ///
    /// Each block in the list can hold up to `BLOCK_CAP` messages.
    struct Block<T> {
        /// The next block in the linked list.
        next: AtomicPtr<Block<T>>,

        /// Slots for messages.
        slots: [Slot<T>; BLOCK_CAP],
    }

    impl<T> Block<T> {
        /// Creates an empty block.
        fn new() -> Block<T> {
            unsafe { mem::zeroed() }
        }

        /// Waits until the next pointer is set.
        fn wait_next(&self) -> *mut Block<T> {
            let backoff = Backoff::new();
            loop {
                let next = self.next.load(Ordering::Acquire);
                if !next.is_null() {
                    return next;
                }
                backoff.snooze();
            }
        }

        /// Sets the `DESTROY` bit in slots starting from `start` and destroys the block.
        unsafe fn destroy(this: *mut Block<T>, start: usize) {
            // It is not necessary to set the `DESTROY bit in the last slot because that slot has begun
            // destruction of the block.
            for i in start..BLOCK_CAP - 1 {
                let slot = (*this).slots.get_unchecked(i);

                // Mark the `DESTROY` bit if a thread is still using the slot.
                if slot.state.load(Ordering::Acquire) & READ == 0
                    && slot.state.fetch_or(DESTROY, Ordering::AcqRel) & READ == 0
                {
                    // If a thread is still using the slot, it will continue destruction of the block.
                    return;
                }
            }

            // No thread is using the block, now it is safe to destroy it.
            drop(Box::from_raw(this));
        }
    }

    /// A position in a channel.
    #[derive(Debug)]
    struct Position<T> {
        /// The index in the channel.
        index: AtomicUsize,

        /// The block in the linked list.
        block: AtomicPtr<Block<T>>,
    }

    /// The token type for the list flavor.
    pub struct ListToken {
        /// The block of slots.
        block: *const u8,

        /// The offset into the block.
        offset: usize,
    }

    impl Default for ListToken {
        #[inline]
        fn default() -> Self {
            ListToken {
                block: ptr::null(),
                offset: 0,
            }
        }
    }

    /// Unbounded channel implemented as a linked list.
    ///
    /// Each message sent into the channel is assigned a sequence number, i.e. an index. Indices are
    /// represented as numbers of type `usize` and wrap on overflow.
    ///
    /// Consecutive messages are grouped into blocks in order to put less pressure on the allocator and
    /// improve cache efficiency.
    pub struct Channel<T> {
        /// The head of the channel.
        head: CachePadded<Position<T>>,

        /// The tail of the channel.
        tail: CachePadded<Position<T>>,

        /// Receivers waiting while the channel is empty and not disconnected.
        receivers: SyncWaker,

        /// Indicates that dropping a `Channel<T>` may drop messages of type `T`.
        _marker: PhantomData<T>,
    }

    impl<T> Channel<T> {
        /// Creates a new unbounded channel.
        pub fn new() -> Self {
            Channel {
                head: CachePadded::new(Position {
                    block: AtomicPtr::new(ptr::null_mut()),
                    index: AtomicUsize::new(0),
                }),
                tail: CachePadded::new(Position {
                    block: AtomicPtr::new(ptr::null_mut()),
                    index: AtomicUsize::new(0),
                }),
                receivers: SyncWaker::new(),
                _marker: PhantomData,
            }
        }

        /// Returns a receiver handle to the channel.
        pub fn receiver(&self) -> Receiver<T> {
            Receiver(self)
        }

        /// Returns a sender handle to the channel.
        pub fn sender(&self) -> Sender<T> {
            Sender(self)
        }

        /// Attempts to reserve a slot for sending a message.
        fn start_send(&self, token: &mut Token) -> bool {
            let backoff = Backoff::new();
            let mut tail = self.tail.index.load(Ordering::Acquire);
            let mut block = self.tail.block.load(Ordering::Acquire);
            let mut next_block = None;

            loop {
                // Check if the channel is disconnected.
                if tail & MARK_BIT != 0 {
                    token.list.block = ptr::null();
                    return true;
                }

                // Calculate the offset of the index into the block.
                let offset = (tail >> SHIFT) % LAP;

                // If we reached the end of the block, wait until the next one is installed.
                if offset == BLOCK_CAP {
                    backoff.snooze();
                    tail = self.tail.index.load(Ordering::Acquire);
                    block = self.tail.block.load(Ordering::Acquire);
                    continue;
                }

                // If we're going to have to install the next block, allocate it in advance in order to
                // make the wait for other threads as short as possible.
                if offset + 1 == BLOCK_CAP && next_block.is_none() {
                    next_block = Some(Box::new(Block::<T>::new()));
                }

                // If this is the first message to be sent into the channel, we need to allocate the
                // first block and install it.
                if block.is_null() {
                    let new = Box::into_raw(Box::new(Block::<T>::new()));

                    if self.tail.block.compare_and_swap(block, new, Ordering::Release) == block {
                        self.head.block.store(new, Ordering::Release);
                        block = new;
                    } else {
                        next_block = unsafe { Some(Box::from_raw(new)) };
                        tail = self.tail.index.load(Ordering::Acquire);
                        block = self.tail.block.load(Ordering::Acquire);
                        continue;
                    }
                }

                let new_tail = tail + (1 << SHIFT);

                // Try advancing the tail forward.
                match self.tail.index
                    .compare_exchange_weak(
                        tail,
                        new_tail,
                        Ordering::SeqCst,
                        Ordering::Acquire,
                    )
                {
                    Ok(_) => unsafe {
                        // If we've reached the end of the block, install the next one.
                        if offset + 1 == BLOCK_CAP {
                            let next_block = Box::into_raw(next_block.unwrap());
                            self.tail.block.store(next_block, Ordering::Release);
                            self.tail.index.fetch_add(1 << SHIFT, Ordering::Release);
                            (*block).next.store(next_block, Ordering::Release);
                        }

                        token.list.block = block as *const u8;
                        token.list.offset = offset;
                        return true;
                    }
                    Err(t) => {
                        tail = t;
                        block = self.tail.block.load(Ordering::Acquire);
                        backoff.spin();
                    }
                }
            }
        }

        /// Writes a message into the channel.
        pub unsafe fn write(&self, token: &mut Token, msg: T) -> Result<(), T> {
            // If there is no slot, the channel is disconnected.
            if token.list.block.is_null() {
                return Err(msg);
            }

            // Write the message into the slot.
            let block = token.list.block as *mut Block<T>;
            let offset = token.list.offset;
            let slot = (*block).slots.get_unchecked(offset);
            slot.msg.get().write(ManuallyDrop::new(msg));
            slot.state.fetch_or(WRITE, Ordering::Release);

            // Wake a sleeping receiver.
            self.receivers.notify();
            Ok(())
        }

        /// Attempts to reserve a slot for receiving a message.
        fn start_recv(&self, token: &mut Token) -> bool {
            let backoff = Backoff::new();
            let mut head = self.head.index.load(Ordering::Acquire);
            let mut block = self.head.block.load(Ordering::Acquire);

            loop {
                // Calculate the offset of the index into the block.
                let offset = (head >> SHIFT) % LAP;

                // If we reached the end of the block, wait until the next one is installed.
                if offset == BLOCK_CAP {
                    backoff.snooze();
                    head = self.head.index.load(Ordering::Acquire);
                    block = self.head.block.load(Ordering::Acquire);
                    continue;
                }

                let mut new_head = head + (1 << SHIFT);

                if new_head & MARK_BIT == 0 {
                    atomic::fence(Ordering::SeqCst);
                    let tail = self.tail.index.load(Ordering::Relaxed);

                    // If the tail equals the head, that means the channel is empty.
                    if head >> SHIFT == tail >> SHIFT {
                        // If the channel is disconnected...
                        if tail & MARK_BIT != 0 {
                            // ...then receive an error.
                            token.list.block = ptr::null();
                            return true;
                        } else {
                            // Otherwise, the receive operation is not ready.
                            return false;
                        }
                    }

                    // If head and tail are not in the same block, set `MARK_BIT` in head.
                    if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP {
                        new_head |= MARK_BIT;
                    }
                }

                // The block can be null here only if the first message is being sent into the channel.
                // In that case, just wait until it gets initialized.
                if block.is_null() {
                    backoff.snooze();
                    head = self.head.index.load(Ordering::Acquire);
                    block = self.head.block.load(Ordering::Acquire);
                    continue;
                }

                // Try moving the head index forward.
                match self.head.index
                    .compare_exchange_weak(
                        head,
                        new_head,
                        Ordering::SeqCst,
                        Ordering::Acquire,
                    )
                {
                    Ok(_) => unsafe {
                        // If we've reached the end of the block, move to the next one.
                        if offset + 1 == BLOCK_CAP {
                            let next = (*block).wait_next();
                            let mut next_index = (new_head & !MARK_BIT).wrapping_add(1 << SHIFT);
                            if !(*next).next.load(Ordering::Relaxed).is_null() {
                                next_index |= MARK_BIT;
                            }

                            self.head.block.store(next, Ordering::Release);
                            self.head.index.store(next_index, Ordering::Release);
                        }

                        token.list.block = block as *const u8;
                        token.list.offset = offset;
                        return true;
                    }
                    Err(h) => {
                        head = h;
                        block = self.head.block.load(Ordering::Acquire);
                        backoff.spin();
                    }
                }
            }
        }

        /// Reads a message from the channel.
        pub unsafe fn read(&self, token: &mut Token) -> Result<T, ()> {
            if token.list.block.is_null() {
                // The channel is disconnected.
                return Err(());
            }

            // Read the message.
            let block = token.list.block as *mut Block<T>;
            let offset = token.list.offset;
            let slot = (*block).slots.get_unchecked(offset);
            slot.wait_write();
            let m = slot.msg.get().read();
            let msg = ManuallyDrop::into_inner(m);

            // Destroy the block if we've reached the end, or if another thread wanted to destroy but
            // couldn't because we were busy reading from the slot.
            if offset + 1 == BLOCK_CAP {
                Block::destroy(block, 0);
            } else if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 {
                Block::destroy(block, offset + 1);
            }

            Ok(msg)
        }

        /// Attempts to send a message into the channel.
        pub fn try_send(&self, msg: T) -> Result<(), TrySendError<T>> {
            self.send(msg, None).map_err(|err| match err {
                SendTimeoutError::Disconnected(msg) => TrySendError::Disconnected(msg),
                SendTimeoutError::Timeout(_) => unreachable!(),
            })
        }

        /// Sends a message into the channel.
        pub fn send(&self, msg: T, _deadline: Option<Instant>) -> Result<(), SendTimeoutError<T>> {
            let token = &mut Token::default();
            assert!(self.start_send(token));
            unsafe {
                self.write(token, msg)
                    .map_err(SendTimeoutError::Disconnected)
            }
        }

        /// Attempts to receive a message without blocking.
        pub fn try_recv(&self) -> Result<T, TryRecvError> {
            let token = &mut Token::default();

            if self.start_recv(token) {
                unsafe { self.read(token).map_err(|_| TryRecvError::Disconnected) }
            } else {
                Err(TryRecvError::Empty)
            }
        }

        /// Receives a message from the channel.
        pub fn recv(&self, deadline: Option<Instant>) -> Result<T, RecvTimeoutError> {
            let token = &mut Token::default();
            loop {
                // Try receiving a message several times.
                let backoff = Backoff::new();
                loop {
                    if self.start_recv(token) {
                        unsafe {
                            return self.read(token).map_err(|_| RecvTimeoutError::Disconnected);
                        }
                    }

                    if backoff.is_completed() {
                        break;
                    } else {
                        backoff.snooze();
                    }
                }

                // Prepare for blocking until a sender wakes us up.
                Context::with(|cx| {
                    let oper = Operation::hook(token);
                    self.receivers.register(oper, cx);

                    // Has the channel become ready just now?
                    if !self.is_empty() || self.is_disconnected() {
                        let _ = cx.try_select(Selected::Aborted);
                    }

                    // Block the current thread.
                    let sel = cx.wait_until(deadline);

                    match sel {
                        Selected::Waiting => unreachable!(),
                        Selected::Aborted | Selected::Disconnected => {
                            self.receivers.unregister(oper).unwrap();
                            // If the channel was disconnected, we still have to check for remaining
                            // messages.
                        }
                        Selected::Operation(_) => {}
                    }
                });

                if let Some(d) = deadline {
                    if Instant::now() >= d {
                        return Err(RecvTimeoutError::Timeout);
                    }
                }
            }
        }

        /// Returns the current number of messages inside the channel.
        pub fn len(&self) -> usize {
            loop {
                // Load the tail index, then load the head index.
                let mut tail = self.tail.index.load(Ordering::SeqCst);
                let mut head = self.head.index.load(Ordering::SeqCst);

                // If the tail index didn't change, we've got consistent indices to work with.
                if self.tail.index.load(Ordering::SeqCst) == tail {
                    // Erase the lower bits.
                    tail &= !((1 << SHIFT) - 1);
                    head &= !((1 << SHIFT) - 1);

                    // Rotate indices so that head falls into the first block.
                    let lap = (head >> SHIFT) / LAP;
                    tail = tail.wrapping_sub((lap * LAP) << SHIFT);
                    head = head.wrapping_sub((lap * LAP) << SHIFT);

                    // Remove the lower bits.
                    tail >>= SHIFT;
                    head >>= SHIFT;

                    // Fix up indices if they fall onto block ends.
                    if head == BLOCK_CAP {
                        head = 0;
                        tail -= LAP;
                    }
                    if tail == BLOCK_CAP {
                        tail += 1;
                    }

                    // Return the difference minus the number of blocks between tail and head.
                    return tail - head - tail / LAP;
                }
            }
        }

        /// Returns the capacity of the channel.
        pub fn capacity(&self) -> Option<usize> {
            None
        }

        /// Disconnects the channel and wakes up all blocked receivers.
        pub fn disconnect(&self) {
            let tail = self.tail.index.fetch_or(MARK_BIT, Ordering::SeqCst);

            if tail & MARK_BIT == 0 {
                self.receivers.disconnect();
            }
        }

        /// Returns `true` if the channel is disconnected.
        pub fn is_disconnected(&self) -> bool {
            self.tail.index.load(Ordering::SeqCst) & MARK_BIT != 0
        }

        /// Returns `true` if the channel is empty.
        pub fn is_empty(&self) -> bool {
            let head = self.head.index.load(Ordering::SeqCst);
            let tail = self.tail.index.load(Ordering::SeqCst);
            head >> SHIFT == tail >> SHIFT
        }

        /// Returns `true` if the channel is full.
        pub fn is_full(&self) -> bool {
            false
        }
    }

    impl<T> Drop for Channel<T> {
        fn drop(&mut self) {
            let mut head = self.head.index.load(Ordering::Relaxed);
            let mut tail = self.tail.index.load(Ordering::Relaxed);
            let mut block = self.head.block.load(Ordering::Relaxed);

            // Erase the lower bits.
            head &= !((1 << SHIFT) - 1);
            tail &= !((1 << SHIFT) - 1);

            unsafe {
                // Drop all messages between head and tail and deallocate the heap-allocated blocks.
                while head != tail {
                    let offset = (head >> SHIFT) % LAP;

                    if offset < BLOCK_CAP {
                        // Drop the message in the slot.
                        let slot = (*block).slots.get_unchecked(offset);
                        ManuallyDrop::drop(&mut *(*slot).msg.get());
                    } else {
                        // Deallocate the block and move to the next one.
                        let next = (*block).next.load(Ordering::Relaxed);
                        drop(Box::from_raw(block));
                        block = next;
                    }

                    head = head.wrapping_add(1 << SHIFT);
                }

                // Deallocate the last remaining block.
                if !block.is_null() {
                    drop(Box::from_raw(block));
                }
            }
        }
    }

    /// Receiver handle to a channel.
    pub struct Receiver<'a, T: 'a>(&'a Channel<T>);

    /// Sender handle to a channel.
    pub struct Sender<'a, T: 'a>(&'a Channel<T>);

    impl<'a, T> SelectHandle for Receiver<'a, T> {
        fn try_select(&self, token: &mut Token) -> bool {
            self.0.start_recv(token)
        }

        fn deadline(&self) -> Option<Instant> {
            None
        }

        fn register(&self, oper: Operation, cx: &Context) -> bool {
            self.0.receivers.register(oper, cx);
            self.is_ready()
        }

        fn unregister(&self, oper: Operation) {
            self.0.receivers.unregister(oper);
        }

        fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
            self.try_select(token)
        }

        fn is_ready(&self) -> bool {
            !self.0.is_empty() || self.0.is_disconnected()
        }

        fn watch(&self, oper: Operation, cx: &Context) -> bool {
            self.0.receivers.watch(oper, cx);
            self.is_ready()
        }

        fn unwatch(&self, oper: Operation) {
            self.0.receivers.unwatch(oper);
        }
    }

    impl<'a, T> SelectHandle for Sender<'a, T> {
        fn try_select(&self, token: &mut Token) -> bool {
            self.0.start_send(token)
        }

        fn deadline(&self) -> Option<Instant> {
            None
        }

        fn register(&self, _oper: Operation, _cx: &Context) -> bool {
            self.is_ready()
        }

        fn unregister(&self, _oper: Operation) {}

        fn accept(&self, token: &mut Token, _cx: &Context) -> bool {
            self.try_select(token)
        }

        fn is_ready(&self) -> bool {
            true
        }

        fn watch(&self, _oper: Operation, _cx: &Context) -> bool {
            self.is_ready()
        }

        fn unwatch(&self, _oper: Operation) {}
    }

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)