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
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
//! Dispatches trace events to [`Collect`]s.
//!
//! The _dispatcher_ is the component of the tracing system which is responsible
//! for forwarding trace data from the instrumentation points that generate it
//! to the collector that collects it.
//!
//! # Using the Trace Dispatcher
//!
//! Every thread in a program using `tracing` has a _default collector_. When
//! events occur, or spans are created, they are dispatched to the thread's
//! current collector.
//!
//! ## Setting the Default Collector
//!
//! By default, the current collector is an empty implementation that does
//! nothing. Trace data provided to this "do nothing" implementation is
//! immediately discarded, and is not available for any purpose.
//!
//! To use another collector implementation, it must be set as the default.
//! There are two methods for doing so: [`with_default`] and
//! [`set_global_default`]. `with_default` sets the default collector for the
//! duration of a scope, while `set_global_default` sets a default collector
//! for the entire process.
//!
//! To use either of these functions, we must first wrap our collector in a
//! [`Dispatch`], a cloneable, type-erased reference to a collector. For
//! example:
//! ```rust
//! # pub struct FooCollector;
//! # use tracing_core::{
//! #   dispatch, Event, Metadata,
//! #   span::{Attributes, Current, Id, Record}
//! # };
//! # impl tracing_core::Collect for FooCollector {
//! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
//! #   fn record(&self, _: &Id, _: &Record) {}
//! #   fn event(&self, _: &Event) {}
//! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
//! #   fn enabled(&self, _: &Metadata) -> bool { false }
//! #   fn enter(&self, _: &Id) {}
//! #   fn exit(&self, _: &Id) {}
//! #   fn current_span(&self) -> Current { Current::unknown() }
//! # }
//! # impl FooCollector { fn new() -> Self { FooCollector } }
//! # #[cfg(feature = "alloc")]
//! use dispatch::Dispatch;
//!
//! # #[cfg(feature = "alloc")]
//! let my_collector = FooCollector::new();
//! # #[cfg(feature = "alloc")]
//! let my_dispatch = Dispatch::new(my_collector);
//! ```
//! Then, we can use [`with_default`] to set our `Dispatch` as the default for
//! the duration of a block:
//! ```rust
//! # pub struct FooCollector;
//! # use tracing_core::{
//! #   dispatch, Event, Metadata,
//! #   span::{Attributes, Current, Id, Record}
//! # };
//! # impl tracing_core::Collect for FooCollector {
//! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
//! #   fn record(&self, _: &Id, _: &Record) {}
//! #   fn event(&self, _: &Event) {}
//! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
//! #   fn enabled(&self, _: &Metadata) -> bool { false }
//! #   fn enter(&self, _: &Id) {}
//! #   fn exit(&self, _: &Id) {}
//! #   fn current_span(&self) -> Current { Current::unknown() }
//! # }
//! # impl FooCollector { fn new() -> Self { FooCollector } }
//! # let _my_collector = FooCollector::new();
//! # #[cfg(feature = "std")]
//! # let my_dispatch = dispatch::Dispatch::new(_my_collector);
//! // no default collector
//!
//! # #[cfg(feature = "std")]
//! dispatch::with_default(&my_dispatch, || {
//!     // my_collector is the default
//! });
//!
//! // no default collector again
//! ```
//! It's important to note that `with_default` will not propagate the current
//! thread's default collector to any threads spawned within the `with_default`
//! block. To propagate the default collector to new threads, either use
//! `with_default` from the new thread, or use `set_global_default`.
//!
//! As an alternative to `with_default`, we can use [`set_global_default`] to
//! set a `Dispatch` as the default for all threads, for the lifetime of the
//! program. For example:
//! ```rust
//! # pub struct FooCollector;
//! # use tracing_core::{
//! #   dispatch, Event, Metadata,
//! #   span::{Attributes, Current, Id, Record}
//! # };
//! # impl tracing_core::Collect for FooCollector {
//! #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
//! #   fn record(&self, _: &Id, _: &Record) {}
//! #   fn event(&self, _: &Event) {}
//! #   fn record_follows_from(&self, _: &Id, _: &Id) {}
//! #   fn enabled(&self, _: &Metadata) -> bool { false }
//! #   fn enter(&self, _: &Id) {}
//! #   fn exit(&self, _: &Id) {}
//! #   fn current_span(&self) -> Current { Current::unknown() }
//! # }
//! # impl FooCollector { fn new() -> Self { FooCollector } }
//! # #[cfg(feature = "std")]
//! # let my_collector = FooCollector::new();
//! # #[cfg(feature = "std")]
//! # let my_dispatch = dispatch::Dispatch::new(my_collector);
//! // no default collector
//!
//! # #[cfg(feature = "std")]
//! dispatch::set_global_default(my_dispatch)
//!     // `set_global_default` will return an error if the global default
//!     // collector has already been set.
//!     .expect("global default was already set!");
//!
//! // `my_collector` is now the default
//! ```
//!
//! <div class="example-wrap" style="display:inline-block">
//! <pre class="ignore" style="white-space:normal;font:inherit;">
//!
//! **Note**: the thread-local scoped dispatcher ([`with_default`]) requires the
//! Rust standard library. `no_std` users should use [`set_global_default`] instead.
//!
//! </pre></div>
//!
//! ## Accessing the Default Collector
//!
//! A thread's current default collector can be accessed using the
//! [`get_default`] function, which executes a closure with a reference to the
//! currently default `Dispatch`. This is used primarily by `tracing`
//! instrumentation.
use crate::{
    collect::{self, Collect},
    span, Event, LevelFilter, Metadata,
};

use core::{
    any::Any,
    fmt,
    sync::atomic::{AtomicBool, AtomicUsize, Ordering},
};

#[cfg(feature = "std")]
use std::{
    cell::{Cell, RefCell, RefMut},
    error,
    sync::Weak,
};

#[cfg(feature = "alloc")]
use alloc::sync::Arc;

#[cfg(feature = "alloc")]
use core::ops::Deref;

/// `Dispatch` trace data to a [`Collect`].
#[derive(Clone)]
pub struct Dispatch {
    #[cfg(feature = "alloc")]
    collector: Kind<Arc<dyn Collect + Send + Sync>>,

    #[cfg(not(feature = "alloc"))]
    collector: &'static (dyn Collect + Send + Sync),
}

#[cfg(feature = "alloc")]
#[derive(Clone)]
enum Kind<T> {
    Global(&'static (dyn Collect + Send + Sync)),
    Scoped(T),
}

#[cfg(feature = "std")]
thread_local! {
    static CURRENT_STATE: State = State {
        default: RefCell::new(Dispatch::none()),
        can_enter: Cell::new(true),
    };
}

static EXISTS: AtomicBool = AtomicBool::new(false);
static GLOBAL_INIT: AtomicUsize = AtomicUsize::new(UNINITIALIZED);

#[cfg(feature = "std")]
static SCOPED_COUNT: AtomicUsize = AtomicUsize::new(0);

const UNINITIALIZED: usize = 0;
const INITIALIZING: usize = 1;
const INITIALIZED: usize = 2;

static mut GLOBAL_DISPATCH: Dispatch = Dispatch {
    #[cfg(feature = "alloc")]
    collector: Kind::Global(&NO_COLLECTOR),
    #[cfg(not(feature = "alloc"))]
    collector: &NO_COLLECTOR,
};
static NONE: Dispatch = Dispatch {
    #[cfg(feature = "alloc")]
    collector: Kind::Global(&NO_COLLECTOR),
    #[cfg(not(feature = "alloc"))]
    collector: &NO_COLLECTOR,
};
static NO_COLLECTOR: NoCollector = NoCollector;

/// The dispatch state of a thread.
#[cfg(feature = "std")]
struct State {
    /// This thread's current default dispatcher.
    default: RefCell<Dispatch>,
    /// Whether or not we can currently begin dispatching a trace event.
    ///
    /// This is set to `false` when functions such as `enter`, `exit`, `event`,
    /// and `new_span` are called on this thread's default dispatcher, to
    /// prevent further trace events triggered inside those functions from
    /// creating an infinite recursion. When we finish handling a dispatch, this
    /// is set back to `true`.
    can_enter: Cell<bool>,
}

/// While this guard is active, additional calls to collector functions on
/// the default dispatcher will not be able to access the dispatch context.
/// Dropping the guard will allow the dispatch context to be re-entered.
#[cfg(feature = "std")]
struct Entered<'a>(&'a State);

/// A guard that resets the current default dispatcher to the prior
/// default dispatcher when dropped.
#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
#[derive(Debug)]
pub struct DefaultGuard(Option<Dispatch>);

/// Sets this dispatch as the default for the duration of a closure.
///
/// The default dispatcher is used when creating a new [span] or
/// [`Event`].
///
/// <div class="example-wrap" style="display:inline-block">
/// <pre class="ignore" style="white-space:normal;font:inherit;">
/// <strong>Note</strong>: This function required the Rust standard library.
/// <!-- hack: this whitespace makes rustdoc interpret the next line as markdown again -->
///
/// `no_std` users should use [`set_global_default`] instead.
///
/// </pre></div>
///
/// [span]: super::span
/// [`Event`]: super::event::Event
#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
pub fn with_default<T>(dispatcher: &Dispatch, f: impl FnOnce() -> T) -> T {
    // When this guard is dropped, the default dispatcher will be reset to the
    // prior default. Using this (rather than simply resetting after calling
    // `f`) ensures that we always reset to the prior dispatcher even if `f`
    // panics.
    let _guard = set_default(dispatcher);
    f()
}

/// Sets the dispatch as the default dispatch for the duration of the lifetime
/// of the returned DefaultGuard
///
/// <div class="example-wrap" style="display:inline-block">
/// <pre class="ignore" style="white-space:normal;font:inherit;">
///
/// **Note**: This function required the Rust standard library.
/// `no_std` users should use [`set_global_default`] instead.
///
/// </pre></div>
#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
#[must_use = "Dropping the guard unregisters the dispatcher."]
pub fn set_default(dispatcher: &Dispatch) -> DefaultGuard {
    // When this guard is dropped, the default dispatcher will be reset to the
    // prior default. Using this ensures that we always reset to the prior
    // dispatcher even if the thread calling this function panics.
    State::set_default(dispatcher.clone())
}

/// Sets this dispatch as the global default for the duration of the entire program.
/// Will be used as a fallback if no thread-local dispatch has been set in a thread
/// (using `with_default`.)
///
/// Can only be set once; subsequent attempts to set the global default will fail.
/// Returns `Err` if the global default has already been set.
///
///
/// <div class="example-wrap" style="display:inline-block"><pre class="compile_fail" style="white-space:normal;font:inherit;">
/// <strong>Warning</strong>: In general, libraries should <em>not</em> call
/// <code>set_global_default()</code>! Doing so will cause conflicts when
/// executables that depend on the library try to set the default collector later.
/// </pre></div>
///
/// [span]: super::span
/// [`Event`]: super::event::Event
pub fn set_global_default(dispatcher: Dispatch) -> Result<(), SetGlobalDefaultError> {
    // if `compare_exchange` returns Result::Ok(_), then `new` has been set and
    // `current`—now the prior value—has been returned in the `Ok()` branch.
    if GLOBAL_INIT
        .compare_exchange(
            UNINITIALIZED,
            INITIALIZING,
            Ordering::SeqCst,
            Ordering::SeqCst,
        )
        .is_ok()
    {
        #[cfg(feature = "alloc")]
        let collector = {
            let collector = match dispatcher.collector {
                Kind::Global(s) => s,
                Kind::Scoped(s) => unsafe {
                    // safety: this leaks the collector onto the heap. the
                    // reference count will always be at least 1.
                    &*Arc::into_raw(s)
                },
            };
            Kind::Global(collector)
        };

        #[cfg(not(feature = "alloc"))]
        let collector = dispatcher.collector;

        unsafe {
            GLOBAL_DISPATCH = Dispatch { collector };
        }
        GLOBAL_INIT.store(INITIALIZED, Ordering::SeqCst);
        EXISTS.store(true, Ordering::Release);
        Ok(())
    } else {
        Err(SetGlobalDefaultError { _no_construct: () })
    }
}

/// Returns true if a `tracing` dispatcher has ever been set.
///
/// This may be used to completely elide trace points if tracing is not in use
/// at all or has yet to be initialized.
#[doc(hidden)]
#[inline(always)]
pub fn has_been_set() -> bool {
    EXISTS.load(Ordering::Relaxed)
}

/// Returned if setting the global dispatcher fails.
#[derive(Debug)]
pub struct SetGlobalDefaultError {
    _no_construct: (),
}

impl fmt::Display for SetGlobalDefaultError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("a global default trace dispatcher has already been set")
    }
}

#[cfg(feature = "std")]
#[cfg_attr(docsrs, doc(cfg(feature = "std")))]
impl error::Error for SetGlobalDefaultError {}

/// Executes a closure with a reference to this thread's current [dispatcher].
///
/// Note that calls to `get_default` should not be nested; if this function is
/// called while inside of another `get_default`, that closure will be provided
/// with `Dispatch::none` rather than the previously set dispatcher.
///
/// [dispatcher]: super::dispatch::Dispatch
#[cfg(feature = "std")]
pub fn get_default<T, F>(mut f: F) -> T
where
    F: FnMut(&Dispatch) -> T,
{
    if SCOPED_COUNT.load(Ordering::Acquire) == 0 {
        // fast path if no scoped dispatcher has been set; just use the global
        // default.
        return f(get_global());
    }

    // While this guard is active, additional calls to collector functions on
    // the default dispatcher will not be able to access the dispatch context.
    // Dropping the guard will allow the dispatch context to be re-entered.
    struct Entered<'a>(&'a Cell<bool>);
    impl<'a> Drop for Entered<'a> {
        #[inline]
        fn drop(&mut self) {
            self.0.set(true);
        }
    }

    CURRENT_STATE
        .try_with(|state| {
            if state.can_enter.replace(false) {
                let _guard = Entered(&state.can_enter);

                let mut default = state.default.borrow_mut();

                if default.is::<NoCollector>() {
                    // don't redo this call on the next check
                    *default = get_global().clone();
                }
                return f(&*default);
            }

            f(&Dispatch::none())
        })
        .unwrap_or_else(|_| f(&Dispatch::none()))
}

/// Executes a closure with a reference to this thread's current [dispatcher].
///
/// Note that calls to `get_default` should not be nested; if this function is
/// called while inside of another `get_default`, that closure will be provided
/// with `Dispatch::none` rather than the previously set dispatcher.
///
/// [dispatcher]: super::dispatcher::Dispatch
#[cfg(feature = "std")]
#[doc(hidden)]
#[inline(never)]
pub fn get_current<T>(f: impl FnOnce(&Dispatch) -> T) -> Option<T> {
    CURRENT_STATE
        .try_with(|state| {
            let entered = state.enter()?;
            Some(f(&*entered.current()))
        })
        .ok()?
}

/// Executes a closure with a reference to the current [dispatcher].
///
/// [dispatcher]: super::dispatcher::Dispatch
#[cfg(not(feature = "std"))]
#[doc(hidden)]
pub fn get_current<T>(f: impl FnOnce(&Dispatch) -> T) -> Option<T> {
    Some(f(&get_global()))
}

/// Executes a closure with a reference to the current [dispatcher].
///
/// [dispatcher]: super::dispatcher::Dispatch
#[cfg(not(feature = "std"))]
pub fn get_default<T, F>(mut f: F) -> T
where
    F: FnMut(&Dispatch) -> T,
{
    f(get_global())
}

#[inline(always)]
pub(crate) fn get_global() -> &'static Dispatch {
    if GLOBAL_INIT.load(Ordering::Acquire) != INITIALIZED {
        return &NONE;
    }
    unsafe {
        // This is safe given the invariant that setting the global dispatcher
        // also sets `GLOBAL_INIT` to `INITIALIZED`.
        &GLOBAL_DISPATCH
    }
}

#[cfg(feature = "std")]
pub(crate) struct Registrar(Kind<Weak<dyn Collect + Send + Sync>>);

impl Dispatch {
    /// Returns a new `Dispatch` that discards events and spans.
    #[inline]
    pub fn none() -> Self {
        Dispatch {
            #[cfg(feature = "alloc")]
            collector: Kind::Global(&NO_COLLECTOR),
            #[cfg(not(feature = "alloc"))]
            collector: &NO_COLLECTOR,
        }
    }

    /// Returns a `Dispatch` that forwards to the given [`Collect`].
    ///
    /// [`Collect`]: super::collect::Collect
    #[cfg(feature = "alloc")]
    #[cfg_attr(docsrs, doc(cfg(any(feature = "std", feature = "alloc"))))]
    pub fn new<C>(collector: C) -> Self
    where
        C: Collect + Send + Sync + 'static,
    {
        let me = Dispatch {
            collector: Kind::Scoped(Arc::new(collector)),
        };
        crate::callsite::register_dispatch(&me);
        me
    }

    /// Returns a `Dispatch` that forwards to the given static [collector].
    ///
    /// Unlike [`Dispatch::new`], this function is always available on all
    /// platforms, even when the `std` or `alloc` features are disabled.
    ///
    /// In order to use `from_static`, the `Collector` itself must be stored in
    /// a static. For example:
    ///
    /// ```rust
    /// struct MyCollector {
    ///    // ...
    /// }
    ///
    /// # use tracing_core::{span::{Id, Attributes, Current, Record}, Event, Metadata};
    /// impl tracing_core::Collect for MyCollector {
    ///     // ...
    /// #   fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
    /// #   fn record(&self, _: &Id, _: &Record) {}
    /// #   fn event(&self, _: &Event) {}
    /// #   fn record_follows_from(&self, _: &Id, _: &Id) {}
    /// #   fn enabled(&self, _: &Metadata) -> bool { false }
    /// #   fn enter(&self, _: &Id) {}
    /// #   fn exit(&self, _: &Id) {}
    /// #   fn current_span(&self) -> Current { Current::unknown() }
    /// }
    ///
    /// static COLLECTOR: MyCollector = MyCollector {
    ///     // ...
    /// };
    ///
    /// fn main() {
    ///     use tracing_core::dispatch::{self, Dispatch};
    ///
    ///     let dispatch = Dispatch::from_static(&COLLECTOR);
    ///
    ///     dispatch::set_global_default(dispatch)
    ///         .expect("no global default collector should have been set previously!");
    /// }
    /// ```
    ///
    /// Constructing the collector in a static initializer may make some forms
    /// of runtime configuration more challenging. If this is the case, users
    /// with access to `liballoc` or the Rust standard library are encouraged to
    /// use [`Dispatch::new`] rather than `from_static`. `no_std` users who
    /// cannot allocate or do not have access to `liballoc` may want to consider
    /// the [`lazy_static`] crate, or another library which allows lazy
    /// initialization of statics.
    ///
    /// [collector]: super::collect::Collect
    /// [`lazy_static`]: https://crates.io/crates/lazy_static
    pub fn from_static(collector: &'static (dyn Collect + Send + Sync)) -> Self {
        #[cfg(feature = "alloc")]
        let me = Self {
            collector: Kind::Global(collector),
        };
        #[cfg(not(feature = "alloc"))]
        let me = Self { collector };
        crate::callsite::register_dispatch(&me);
        me
    }

    #[cfg(feature = "std")]
    pub(crate) fn registrar(&self) -> Registrar {
        Registrar(match self.collector {
            Kind::Scoped(ref s) => Kind::Scoped(Arc::downgrade(s)),
            Kind::Global(s) => Kind::Global(s),
        })
    }

    #[inline(always)]
    #[cfg(feature = "alloc")]
    fn collector(&self) -> &(dyn Collect + Send + Sync) {
        match self.collector {
            Kind::Scoped(ref s) => Arc::deref(s),
            Kind::Global(s) => s,
        }
    }

    #[inline(always)]
    #[cfg(not(feature = "alloc"))]
    fn collector(&self) -> &(dyn Collect + Send + Sync) {
        self.collector
    }

    /// Registers a new callsite with this collector, returning whether or not
    /// the collector is interested in being notified about the callsite.
    ///
    /// This calls the [`register_callsite`] function on the [`Collect`]
    /// that this `Dispatch` forwards to.
    ///
    /// [`Collect`]: super::collect::Collect
    /// [`register_callsite`]: super::collect::Collect::register_callsite
    #[inline]
    pub fn register_callsite(&self, metadata: &'static Metadata<'static>) -> collect::Interest {
        self.collector().register_callsite(metadata)
    }

    /// Returns the highest [verbosity level][level] that this [collector] will
    /// enable, or `None`, if the collector does not implement level-based
    /// filtering or chooses not to implement this method.
    ///
    /// This calls the [`max_level_hint`] function on the [`Collect`]
    /// that this `Dispatch` forwards to.
    ///
    /// [level]: super::Level
    /// [collector]: super::collect::Collect
    /// [`Collect`]: super::collect::Collect
    /// [`register_callsite`]: super::collect::Collect::max_level_hint
    // TODO(eliza): consider making this a public API?
    #[inline]
    pub(crate) fn max_level_hint(&self) -> Option<LevelFilter> {
        self.collector().max_level_hint()
    }

    /// Record the construction of a new span, returning a new [ID] for the
    /// span being constructed.
    ///
    /// This calls the [`new_span`] function on the [`Collect`] that this
    /// `Dispatch` forwards to.
    ///
    /// [ID]: super::span::Id
    /// [`Collect`]: super::collect::Collect
    /// [`new_span`]: super::collect::Collect::new_span
    #[inline]
    pub fn new_span(&self, span: &span::Attributes<'_>) -> span::Id {
        self.collector().new_span(span)
    }

    /// Record a set of values on a span.
    ///
    /// This calls the [`record`] function on the [`Collect`] that this
    /// `Dispatch` forwards to.
    ///
    /// [`Collect`]: super::collect::Collect
    /// [`record`]: super::collect::Collect::record
    #[inline]
    pub fn record(&self, span: &span::Id, values: &span::Record<'_>) {
        self.collector().record(span, values)
    }

    /// Adds an indication that `span` follows from the span with the id
    /// `follows`.
    ///
    /// This calls the [`record_follows_from`] function on the [`Collect`]
    /// that this `Dispatch` forwards to.
    ///
    /// [`Collect`]: super::collect::Collect
    /// [`record_follows_from`]: super::collect::Collect::record_follows_from
    #[inline]
    pub fn record_follows_from(&self, span: &span::Id, follows: &span::Id) {
        self.collector().record_follows_from(span, follows)
    }

    /// Returns true if a span with the specified [metadata] would be
    /// recorded.
    ///
    /// This calls the [`enabled`] function on the [`Collect`] that this
    /// `Dispatch` forwards to.
    ///
    /// [metadata]: super::metadata::Metadata
    /// [`Collect`]: super::collect::Collect
    /// [`enabled`]: super::collect::Collect::enabled
    #[inline]
    pub fn enabled(&self, metadata: &Metadata<'_>) -> bool {
        self.collector().enabled(metadata)
    }

    /// Records that an [`Event`] has occurred.
    ///
    /// This calls the [`event`] function on the [`Collect`] that this
    /// `Dispatch` forwards to.
    ///
    /// [`Event`]: super::event::Event
    /// [`Collect`]: super::collect::Collect
    /// [`event`]: super::collect::Collect::event
    #[inline]
    pub fn event(&self, event: &Event<'_>) {
        self.collector().event(event)
    }

    /// Records that a span has been can_enter.
    ///
    /// This calls the [`enter`] function on the [`Collect`] that this
    /// `Dispatch` forwards to.
    ///
    /// [`Collect`]: super::collect::Collect
    /// [`enter`]: super::collect::Collect::enter
    #[inline]
    pub fn enter(&self, span: &span::Id) {
        self.collector().enter(span);
    }

    /// Records that a span has been exited.
    ///
    /// This calls the [`exit`] function on the [`Collect`] that this
    /// `Dispatch` forwards to.
    ///
    /// [`Collect`]: super::collect::Collect
    /// [`exit`]: super::collect::Collect::exit
    #[inline]
    pub fn exit(&self, span: &span::Id) {
        self.collector().exit(span);
    }

    /// Notifies the [collector] that a [span ID] has been cloned.
    ///
    /// This function must only be called with span IDs that were returned by
    /// this `Dispatch`'s [`new_span`] function. The `tracing` crate upholds
    /// this guarantee and any other libraries implementing instrumentation APIs
    /// must as well.
    ///
    /// This calls the [`clone_span`] function on the [`Collect`] that this
    /// `Dispatch` forwards to.
    ///
    /// [span ID]: super::span::Id
    /// [collector]: super::collect::Collect
    /// [`clone_span`]: super::collect::Collect::clone_span
    /// [`new_span`]: super::collect::Collect::new_span
    #[inline]
    pub fn clone_span(&self, id: &span::Id) -> span::Id {
        self.collector().clone_span(id)
    }

    /// Notifies the collector that a [span ID] has been dropped.
    ///
    /// This function must only be called with span IDs that were returned by
    /// this `Dispatch`'s [`new_span`] function. The `tracing` crate upholds
    /// this guarantee and any other libraries implementing instrumentation APIs
    /// must as well.
    ///
    /// This calls the [`drop_span`] function on the [`Collect`] that this
    ///  `Dispatch` forwards to.
    ///
    /// <div class="example-wrap" style="display:inline-block"><pre class="compile_fail" style="white-space:normal;font:inherit;">
    ///
    /// **Deprecated**: The [`try_close`] method is functionally identical, but returns `true` if the span is now closed.
    /// It should be used instead of this method.
    ///
    /// </pre></div>
    ///
    /// [span ID]: super::span::Id
    /// [`Collect`]: super::collect::Collect
    /// [`drop_span`]: super::collect::Collect::drop_span
    /// [`new_span`]: super::collect::Collect::new_span
    /// [`try_close`]: Self::try_close
    #[inline]
    #[deprecated(since = "0.1.2", note = "use `Dispatch::try_close` instead")]
    pub fn drop_span(&self, id: span::Id) {
        #[allow(deprecated)]
        self.collector().drop_span(id);
    }

    /// Notifies the collector that a [span ID] has been dropped, and returns
    /// `true` if there are now 0 IDs referring to that span.
    ///
    /// This function must only be called with span IDs that were returned by
    /// this `Dispatch`'s [`new_span`] function. The `tracing` crate upholds
    /// this guarantee and any other libraries implementing instrumentation APIs
    /// must as well.
    ///
    /// This calls the [`try_close`] function on the [`Collect`] trait
    /// that this `Dispatch` forwards to.
    ///
    /// [span ID]: super::span::Id
    /// [`Collect`]: super::collect::Collect
    /// [`try_close`]: super::collect::Collect::try_close
    /// [`new_span`]: super::collect::Collect::new_span
    #[inline]
    pub fn try_close(&self, id: span::Id) -> bool {
        self.collector().try_close(id)
    }

    /// Returns a type representing this collector's view of the current span.
    ///
    /// This calls the [`current`] function on the [`Collect`] that this
    /// `Dispatch` forwards to.
    ///
    /// [`Collect`]: super::collect::Collect
    /// [`current`]: super::collect::Collect::current_span
    #[inline]
    pub fn current_span(&self) -> span::Current {
        self.collector().current_span()
    }

    /// Returns `true` if this `Dispatch` forwards to a collector of type
    /// `T`.
    #[inline]
    pub fn is<T: Any>(&self) -> bool {
        <dyn Collect>::is::<T>(&*self.collector())
    }

    /// Returns some reference to the [`Collect`] this `Dispatch` forwards to
    /// if it is of type `T`, or `None` if it isn't.
    ///
    /// [`Collect`]: super::collect::Collect
    #[inline]
    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
        <dyn Collect>::downcast_ref(&*self.collector())
    }
}

impl Default for Dispatch {
    /// Returns the current default dispatcher
    fn default() -> Self {
        get_default(|default| default.clone())
    }
}

impl fmt::Debug for Dispatch {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("Dispatch(...)")
    }
}

#[cfg(feature = "std")]
impl<C> From<C> for Dispatch
where
    C: Collect + Send + Sync + 'static,
{
    #[inline]
    fn from(collector: C) -> Self {
        Dispatch::new(collector)
    }
}

struct NoCollector;
impl Collect for NoCollector {
    #[inline]
    fn register_callsite(&self, _: &'static Metadata<'static>) -> collect::Interest {
        collect::Interest::never()
    }

    fn new_span(&self, _: &span::Attributes<'_>) -> span::Id {
        span::Id::from_u64(0xDEAD)
    }

    fn event(&self, _event: &Event<'_>) {}

    fn record(&self, _span: &span::Id, _values: &span::Record<'_>) {}

    fn record_follows_from(&self, _span: &span::Id, _follows: &span::Id) {}

    #[inline]
    fn enabled(&self, _metadata: &Metadata<'_>) -> bool {
        false
    }

    fn enter(&self, _span: &span::Id) {}
    fn exit(&self, _span: &span::Id) {}

    fn current_span(&self) -> span::Current {
        span::Current::none()
    }
}

#[cfg(feature = "std")]
impl Registrar {
    pub(crate) fn upgrade(&self) -> Option<Dispatch> {
        match self.0 {
            Kind::Global(s) => Some(Dispatch {
                collector: Kind::Global(s),
            }),
            Kind::Scoped(ref s) => s.upgrade().map(|s| Dispatch {
                collector: Kind::Scoped(s),
            }),
        }
    }
}

// ===== impl State =====

#[cfg(feature = "std")]
impl State {
    /// Replaces the current default dispatcher on this thread with the provided
    /// dispatcher.
    ///
    /// Dropping the returned `ResetGuard` will reset the default dispatcher to
    /// the previous value.
    #[inline]
    fn set_default(new_dispatch: Dispatch) -> DefaultGuard {
        let prior = CURRENT_STATE
            .try_with(|state| {
                state.can_enter.set(true);
                state.default.replace(new_dispatch)
            })
            .ok();
        EXISTS.store(true, Ordering::Release);
        SCOPED_COUNT.fetch_add(1, Ordering::Release);
        DefaultGuard(prior)
    }

    #[inline]
    fn enter(&self) -> Option<Entered<'_>> {
        if self.can_enter.replace(false) {
            Some(Entered(self))
        } else {
            None
        }
    }
}

// ===== impl Entered =====

#[cfg(feature = "std")]
impl<'a> Entered<'a> {
    #[inline]
    fn current(&self) -> RefMut<'a, Dispatch> {
        let mut default = self.0.default.borrow_mut();

        if default.is::<NoCollector>() {
            // don't redo this call on the next check
            *default = get_global().clone();
        }

        default
    }
}

#[cfg(feature = "std")]
impl<'a> Drop for Entered<'a> {
    #[inline]
    fn drop(&mut self) {
        self.0.can_enter.set(true);
    }
}

// ===== impl DefaultGuard =====

#[cfg(feature = "std")]
impl Drop for DefaultGuard {
    #[inline]
    fn drop(&mut self) {
        SCOPED_COUNT.fetch_sub(1, Ordering::Release);
        if let Some(dispatch) = self.0.take() {
            // Replace the dispatcher and then drop the old one outside
            // of the thread-local context. Dropping the dispatch may
            // lead to the drop of a collector which, in the process,
            // could then also attempt to access the same thread local
            // state -- causing a clash.
            let prev = CURRENT_STATE.try_with(|state| state.default.replace(dispatch));
            drop(prev)
        }
    }
}

#[cfg(test)]
mod test {

    use super::*;
    use crate::{
        callsite::Callsite,
        collect::Interest,
        metadata::{Kind, Level, Metadata},
    };

    #[test]
    fn dispatch_is() {
        let dispatcher = Dispatch::from_static(&NO_COLLECTOR);
        assert!(dispatcher.is::<NoCollector>());
    }

    #[test]
    fn dispatch_downcasts() {
        let dispatcher = Dispatch::from_static(&NoCollector);
        assert!(dispatcher.downcast_ref::<NoCollector>().is_some());
    }

    struct TestCallsite;
    static TEST_CALLSITE: TestCallsite = TestCallsite;
    static TEST_META: Metadata<'static> = metadata! {
        name: "test",
        target: module_path!(),
        level: Level::DEBUG,
        fields: &[],
        callsite: &TEST_CALLSITE,
        kind: Kind::EVENT
    };

    impl Callsite for TestCallsite {
        fn set_interest(&self, _: Interest) {}
        fn metadata(&self) -> &Metadata<'_> {
            &TEST_META
        }
    }

    #[test]
    #[cfg(feature = "std")]
    fn events_dont_infinite_loop() {
        // This test ensures that an event triggered within a collector
        // won't cause an infinite loop of events.
        struct TestCollector;
        impl Collect for TestCollector {
            fn enabled(&self, _: &Metadata<'_>) -> bool {
                true
            }

            fn new_span(&self, _: &span::Attributes<'_>) -> span::Id {
                span::Id::from_u64(0xAAAA)
            }

            fn record(&self, _: &span::Id, _: &span::Record<'_>) {}

            fn record_follows_from(&self, _: &span::Id, _: &span::Id) {}

            fn event(&self, _: &Event<'_>) {
                static EVENTS: AtomicUsize = AtomicUsize::new(0);
                assert_eq!(
                    EVENTS.fetch_add(1, Ordering::Relaxed),
                    0,
                    "event method called twice!"
                );
                Event::dispatch(&TEST_META, &TEST_META.fields().value_set(&[]))
            }

            fn enter(&self, _: &span::Id) {}

            fn exit(&self, _: &span::Id) {}

            fn current_span(&self) -> span::Current {
                span::Current::unknown()
            }
        }

        with_default(&Dispatch::new(TestCollector), || {
            Event::dispatch(&TEST_META, &TEST_META.fields().value_set(&[]))
        })
    }

    #[test]
    #[cfg(feature = "std")]
    fn spans_dont_infinite_loop() {
        // This test ensures that a span created within a collector
        // won't cause an infinite loop of new spans.

        fn mk_span() {
            get_default(|current| {
                current.new_span(&span::Attributes::new(
                    &TEST_META,
                    &TEST_META.fields().value_set(&[]),
                ))
            });
        }

        struct TestCollector;
        impl Collect for TestCollector {
            fn enabled(&self, _: &Metadata<'_>) -> bool {
                true
            }

            fn new_span(&self, _: &span::Attributes<'_>) -> span::Id {
                static NEW_SPANS: AtomicUsize = AtomicUsize::new(0);
                assert_eq!(
                    NEW_SPANS.fetch_add(1, Ordering::Relaxed),
                    0,
                    "new_span method called twice!"
                );
                mk_span();
                span::Id::from_u64(0xAAAA)
            }

            fn record(&self, _: &span::Id, _: &span::Record<'_>) {}

            fn record_follows_from(&self, _: &span::Id, _: &span::Id) {}

            fn event(&self, _: &Event<'_>) {}

            fn enter(&self, _: &span::Id) {}

            fn exit(&self, _: &span::Id) {}

            fn current_span(&self) -> span::Current {
                span::Current::unknown()
            }
        }

        with_default(&Dispatch::new(TestCollector), mk_span)
    }

    #[test]
    fn default_no_collector() {
        let default_dispatcher = Dispatch::default();
        assert!(default_dispatcher.is::<NoCollector>());
    }

    #[cfg(feature = "std")]
    #[test]
    fn default_dispatch() {
        struct TestCollector;
        impl Collect for TestCollector {
            fn enabled(&self, _: &Metadata<'_>) -> bool {
                true
            }

            fn new_span(&self, _: &span::Attributes<'_>) -> span::Id {
                span::Id::from_u64(0xAAAA)
            }

            fn record(&self, _: &span::Id, _: &span::Record<'_>) {}

            fn record_follows_from(&self, _: &span::Id, _: &span::Id) {}

            fn event(&self, _: &Event<'_>) {}

            fn enter(&self, _: &span::Id) {}

            fn exit(&self, _: &span::Id) {}

            fn current_span(&self) -> span::Current {
                span::Current::unknown()
            }
        }
        let guard = set_default(&Dispatch::new(TestCollector));
        let default_dispatcher = Dispatch::default();
        assert!(default_dispatcher.is::<TestCollector>());

        drop(guard);
        let default_dispatcher = Dispatch::default();
        assert!(default_dispatcher.is::<NoCollector>());
    }
}