🛈 Note: This is pre-release documentation for the upcoming tracing 0.2.0 ecosystem.

For the release documentation, please see docs.rs, instead.

logo
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
1117
//! 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, NoCollector},
    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(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::new();

/// The dispatch state of a thread.
#[cfg(feature = "std")]
struct State {
    /// This thread's current default dispatcher.
    default: RefCell<Option<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")]
#[inline(always)]
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());
    }

    get_default_slow(f)
}

#[cfg(feature = "std")]
#[inline(never)]
fn get_default_slow<T, F>(mut f: F) -> T
where
    F: FnMut(&Dispatch) -> T,
{
    // 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();
                let default = default
                    // if the local default for this thread has never been set,
                    // populate it with the global default, so we don't have to
                    // keep getting the global on every `get_default_slow` call.
                    .get_or_insert_with(|| 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 [`once_cell`] crate, or another library which allows lazy
    /// initialization of statics.
    ///
    /// [collector]: super::collect::Collect
    /// [`once_cell`]: https://crates.io/crates/once_cell
    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<'_>) {
        let collector = self.collector();
        if collector.event_enabled(event) {
            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
    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
    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
    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 {
        match &self.collector {
            #[cfg(feature = "alloc")]
            Kind::Global(collector) => f
                .debug_tuple("Dispatch::Global")
                .field(&format_args!("{:p}", collector))
                .finish(),

            #[cfg(feature = "alloc")]
            Kind::Scoped(collector) => f
                .debug_tuple("Dispatch::Scoped")
                .field(&format_args!("{:p}", collector))
                .finish(),

            #[cfg(not(feature = "alloc"))]
            collector => f
                .debug_tuple("Dispatch::Global")
                .field(&format_args!("{:p}", collector))
                .finish(),
        }
    }
}

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

#[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(Some(new_dispatch))
                    // if the scoped default was not set on this thread, set the
                    // `prior` default to the global default to populate the
                    // scoped default when unsetting *this* default
                    .unwrap_or_else(|| get_global().clone())
            })
            .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 default = self.0.default.borrow_mut();
        RefMut::map(default, |default| {
            default.get_or_insert_with(|| get_global().clone())
        })
    }
}

#[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(Some(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(&NO_COLLECTOR);
        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>());
    }
}