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
//! Metadata describing trace data.
use super::{callsite, field};
use crate::stdlib::{
    cmp, fmt,
    str::FromStr,
    sync::atomic::{AtomicUsize, Ordering},
};

/// Metadata describing a [span] or [event].
///
/// All spans and events have the following metadata:
/// - A [name], represented as a static string.
/// - A [target], a string that categorizes part of the system where the span
///   or event occurred. The `tracing` macros default to using the module
///   path where the span or event originated as the target, but it may be
///   overridden.
/// - A [verbosity level]. This determines how verbose a given span or event
///   is, and allows enabling or disabling more verbose diagnostics
///   situationally. See the documentation for the [`Level`] type for details.
/// - The names of the [fields] defined by the span or event.
/// - Whether the metadata corresponds to a span or event.
///
/// In addition, the following optional metadata describing the source code
/// location where the span or event originated _may_ be provided:
/// - The [file name]
/// - The [line number]
/// - The [module path]
///
/// Metadata is used by [`Subscriber`]s when filtering spans and events, and it
/// may also be used as part of their data payload.
///
/// When created by the `event!` or `span!` macro, the metadata describing a
/// particular event or span is constructed statically and exists as a single
/// static instance. Thus, the overhead of creating the metadata is
/// _significantly_ lower than that of creating the actual span. Therefore,
/// filtering is based on metadata, rather than on the constructed span.
///
/// <pre class="ignore" style="white-space:normal;font:inherit;">
///     <strong>Note</strong>: Although instances of <code>Metadata</code>
///     cannot be compared directly, they provide a method
///     <a href="struct.Metadata.html#method.id"><code>id</code></a>, returning
///     an opaque <a href="../callsite/struct.Identifier.html">callsite
///     identifier</a>which uniquely identifies the callsite where the metadata
///     originated. This can be used to determine if two <code>Metadata</code>
///     correspond to the same callsite.
/// </pre>
///
/// [span]: super::span
/// [event]: super::event
/// [name]: #method.name
/// [target]: #method.target
/// [fields]: #method.fields
/// [verbosity level]: #method.level
/// [file name]: #method.file
/// [line number]: #method.line
/// [module path]: #method.module
/// [`Subscriber`]: super::subscriber::Subscriber
/// [`id`]: Metadata::id
/// [callsite identifier]: super::callsite::Identifier
pub struct Metadata<'a> {
    /// The name of the span described by this metadata.
    name: &'static str,

    /// The part of the system that the span that this metadata describes
    /// occurred in.
    target: &'a str,

    /// The level of verbosity of the described span.
    level: Level,

    /// The name of the Rust module where the span occurred, or `None` if this
    /// could not be determined.
    module_path: Option<&'a str>,

    /// The name of the source code file where the span occurred, or `None` if
    /// this could not be determined.
    file: Option<&'a str>,

    /// The line number in the source code file where the span occurred, or
    /// `None` if this could not be determined.
    line: Option<u32>,

    /// The names of the key-value fields attached to the described span or
    /// event.
    fields: field::FieldSet,

    /// The kind of the callsite.
    kind: Kind,
}

/// Indicates whether the callsite is a span or event.
#[derive(Clone, Eq, PartialEq)]
pub struct Kind(u8);

/// Describes the level of verbosity of a span or event.
///
/// # Comparing Levels
///
/// `Level` implements the [`PartialOrd`] and [`Ord`] traits, allowing two
/// `Level`s to be compared to determine which is considered more or less
/// verbose. Levels which are more verbose are considered "greater than" levels
/// which are less verbose, with [`Level::ERROR`] considered the lowest, and
/// [`Level::TRACE`] considered the highest.
///
/// For example:
/// ```
/// use tracing_core::Level;
///
/// assert!(Level::TRACE > Level::DEBUG);
/// assert!(Level::ERROR < Level::WARN);
/// assert!(Level::INFO <= Level::DEBUG);
/// assert_eq!(Level::TRACE, Level::TRACE);
/// ```
///
/// # Filtering
///
/// `Level`s are typically used to implement filtering that determines which
/// spans and events are enabled. Depending on the use case, more or less
/// verbose diagnostics may be desired. For example, when running in
/// development, [`DEBUG`]-level traces may be enabled by default. When running in
/// production, only [`INFO`]-level and lower traces might be enabled. Libraries
/// may include very verbose diagnostics at the [`DEBUG`] and/or [`TRACE`] levels.
/// Applications using those libraries typically chose to ignore those traces. However, when
/// debugging an issue involving said libraries, it may be useful to temporarily
/// enable the more verbose traces.
///
/// The [`LevelFilter`] type is provided to enable filtering traces by
/// verbosity. `Level`s can be compared against [`LevelFilter`]s, and
/// [`LevelFilter`] has a variant for each `Level`, which compares analogously
/// to that level. In addition, [`LevelFilter`] adds a [`LevelFilter::OFF`]
/// variant, which is considered "less verbose" than every other `Level`. This is
/// intended to allow filters to completely disable tracing in a particular context.
///
/// For example:
/// ```
/// use tracing_core::{Level, LevelFilter};
///
/// assert!(LevelFilter::OFF < Level::TRACE);
/// assert!(LevelFilter::TRACE > Level::DEBUG);
/// assert!(LevelFilter::ERROR < Level::WARN);
/// assert!(LevelFilter::INFO <= Level::DEBUG);
/// assert!(LevelFilter::INFO >= Level::INFO);
/// ```
///
/// ## Examples
///
/// Below is a simple example of how a [`Subscriber`] could implement filtering through
/// a [`LevelFilter`]. When a span or event is recorded, the [`Subscriber::enabled`] method
/// compares the span or event's `Level` against the configured [`LevelFilter`].
/// The optional [`Subscriber::max_level_hint`] method can also be implemented to allow spans
/// and events above a maximum verbosity level to be skipped more efficiently,
/// often improving performance in short-lived programs.
///
/// ```
/// use tracing_core::{span, Event, Level, LevelFilter, Subscriber, Metadata};
/// # use tracing_core::span::{Id, Record, Current};
///
/// #[derive(Debug)]
/// pub struct MySubscriber {
///     /// The most verbose level that this subscriber will enable.
///     max_level: LevelFilter,
///
///     // ...
/// }
///
/// impl MySubscriber {
///     /// Returns a new `MySubscriber` which will record spans and events up to
///     /// `max_level`.
///     pub fn with_max_level(max_level: LevelFilter) -> Self {
///         Self {
///             max_level,
///             // ...
///         }
///     }
/// }
/// impl Subscriber for MySubscriber {
///     fn enabled(&self, meta: &Metadata<'_>) -> bool {
///         // A span or event is enabled if it is at or below the configured
///         // maximum level.
///         meta.level() <= &self.max_level
///     }
///
///     // This optional method returns the most verbose level that this
///     // subscriber will enable. Although implementing this method is not
///     // *required*, it permits additional optimizations when it is provided,
///     // allowing spans and events above the max level to be skipped
///     // more efficiently.
///     fn max_level_hint(&self) -> Option<LevelFilter> {
///         Some(self.max_level)
///     }
///
///     // Implement the rest of the subscriber...
///     fn new_span(&self, span: &span::Attributes<'_>) -> span::Id {
///         // ...
///         # drop(span); Id::from_u64(1)
///     }

///     fn event(&self, event: &Event<'_>) {
///         // ...
///         # drop(event);
///     }
///
///     // ...
///     # fn enter(&self, _: &Id) {}
///     # fn exit(&self, _: &Id) {}
///     # fn record(&self, _: &Id, _: &Record<'_>) {}
///     # fn record_follows_from(&self, _: &Id, _: &Id) {}
/// }
/// ```
///
/// It is worth noting that the `tracing-subscriber` crate provides [additional
/// APIs][envfilter] for performing more sophisticated filtering, such as
/// enabling different levels based on which module or crate a span or event is
/// recorded in.
///
/// [`DEBUG`]: Level::DEBUG
/// [`INFO`]: Level::INFO
/// [`TRACE`]: Level::TRACE
/// [`Subscriber::enabled`]: crate::subscriber::Subscriber::enabled
/// [`Subscriber::max_level_hint`]: crate::subscriber::Subscriber::max_level_hint
/// [`Subscriber`]: crate::subscriber::Subscriber
/// [envfilter]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/filter/struct.EnvFilter.html
#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
pub struct Level(LevelInner);

/// A filter comparable to a verbosity [`Level`].
///
/// If a [`Level`] is considered less than a `LevelFilter`, it should be
/// considered enabled; if greater than or equal to the `LevelFilter`,
/// that level is disabled. See [`LevelFilter::current`] for more
/// details.
///
/// Note that this is essentially identical to the `Level` type, but with the
/// addition of an [`OFF`] level that completely disables all trace
/// instrumentation.
///
/// See the documentation for the [`Level`] type to see how `Level`s
/// and `LevelFilter`s interact.
///
/// [`OFF`]: LevelFilter::OFF
#[repr(transparent)]
#[derive(Copy, Clone, Eq, PartialEq, Hash)]
pub struct LevelFilter(Option<Level>);

/// Indicates that a string could not be parsed to a valid level.
#[derive(Clone, Debug)]
pub struct ParseLevelFilterError(());

static MAX_LEVEL: AtomicUsize = AtomicUsize::new(LevelFilter::OFF_USIZE);

// ===== impl Metadata =====

impl<'a> Metadata<'a> {
    /// Construct new metadata for a span or event, with a name, target, level, field
    /// names, and optional source code location.
    pub const fn new(
        name: &'static str,
        target: &'a str,
        level: Level,
        file: Option<&'a str>,
        line: Option<u32>,
        module_path: Option<&'a str>,
        fields: field::FieldSet,
        kind: Kind,
    ) -> Self {
        Metadata {
            name,
            target,
            level,
            module_path,
            file,
            line,
            fields,
            kind,
        }
    }

    /// Returns the names of the fields on the described span or event.
    pub fn fields(&self) -> &field::FieldSet {
        &self.fields
    }

    /// Returns the level of verbosity of the described span or event.
    pub fn level(&self) -> &Level {
        &self.level
    }

    /// Returns the name of the span.
    pub fn name(&self) -> &'static str {
        self.name
    }

    /// Returns a string describing the part of the system where the span or
    /// event that this metadata describes occurred.
    ///
    /// Typically, this is the module path, but alternate targets may be set
    /// when spans or events are constructed.
    pub fn target(&self) -> &'a str {
        self.target
    }

    /// Returns the path to the Rust module where the span occurred, or
    /// `None` if the module path is unknown.
    pub fn module_path(&self) -> Option<&'a str> {
        self.module_path
    }

    /// Returns the name of the source code file where the span
    /// occurred, or `None` if the file is unknown
    pub fn file(&self) -> Option<&'a str> {
        self.file
    }

    /// Returns the line number in the source code file where the span
    /// occurred, or `None` if the line number is unknown.
    pub fn line(&self) -> Option<u32> {
        self.line
    }

    /// Returns an opaque `Identifier` that uniquely identifies the callsite
    /// this `Metadata` originated from.
    #[inline]
    pub fn callsite(&self) -> callsite::Identifier {
        self.fields.callsite()
    }

    /// Returns true if the callsite kind is `Event`.
    pub fn is_event(&self) -> bool {
        self.kind.is_event()
    }

    /// Return true if the callsite kind is `Span`.
    pub fn is_span(&self) -> bool {
        self.kind.is_span()
    }
}

impl<'a> fmt::Debug for Metadata<'a> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut meta = f.debug_struct("Metadata");
        meta.field("name", &self.name)
            .field("target", &self.target)
            .field("level", &self.level);

        if let Some(path) = self.module_path() {
            meta.field("module_path", &path);
        }

        match (self.file(), self.line()) {
            (Some(file), Some(line)) => {
                meta.field("location", &format_args!("{}:{}", file, line));
            }
            (Some(file), None) => {
                meta.field("file", &format_args!("{}", file));
            }

            // Note: a line num with no file is a kind of weird case that _probably_ never occurs...
            (None, Some(line)) => {
                meta.field("line", &line);
            }
            (None, None) => {}
        };

        meta.field("fields", &format_args!("{}", self.fields))
            .field("callsite", &self.callsite())
            .field("kind", &self.kind)
            .finish()
    }
}

impl Kind {
    const EVENT_BIT: u8 = 1 << 0;
    const SPAN_BIT: u8 = 1 << 1;
    const HINT_BIT: u8 = 1 << 2;

    /// `Event` callsite
    pub const EVENT: Kind = Kind(Self::EVENT_BIT);

    /// `Span` callsite
    pub const SPAN: Kind = Kind(Self::SPAN_BIT);

    /// `enabled!` callsite. [`Subscriber`][`crate::subscriber::Subscriber`]s can assume
    /// this `Kind` means they will never recieve a
    /// full event with this [`Metadata`].
    pub const HINT: Kind = Kind(Self::HINT_BIT);

    /// Return true if the callsite kind is `Span`
    pub fn is_span(&self) -> bool {
        self.0 & Self::SPAN_BIT == Self::SPAN_BIT
    }

    /// Return true if the callsite kind is `Event`
    pub fn is_event(&self) -> bool {
        self.0 & Self::EVENT_BIT == Self::EVENT_BIT
    }

    /// Return true if the callsite kind is `Hint`
    pub fn is_hint(&self) -> bool {
        self.0 & Self::HINT_BIT == Self::HINT_BIT
    }

    /// Sets that this `Kind` is a [hint](Self::HINT).
    ///
    /// This can be called on [`SPAN`](Self::SPAN) and [`EVENT`](Self::EVENT)
    /// kinds to construct a hint callsite that also counts as a span or event.
    pub const fn hint(self) -> Self {
        Self(self.0 | Self::HINT_BIT)
    }
}

impl fmt::Debug for Kind {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("Kind(")?;
        let mut has_bits = false;
        let mut write_bit = |name: &str| {
            if has_bits {
                f.write_str(" | ")?;
            }
            f.write_str(name)?;
            has_bits = true;
            Ok(())
        };

        if self.is_event() {
            write_bit("EVENT")?;
        }

        if self.is_span() {
            write_bit("SPAN")?;
        }

        if self.is_hint() {
            write_bit("HINT")?;
        }

        // if none of the expected bits were set, something is messed up, so
        // just print the bits for debugging purposes
        if !has_bits {
            write!(f, "{:#b}", self.0)?;
        }

        f.write_str(")")
    }
}

// ===== impl Level =====

impl Level {
    /// The "error" level.
    ///
    /// Designates very serious errors.
    pub const ERROR: Level = Level(LevelInner::Error);
    /// The "warn" level.
    ///
    /// Designates hazardous situations.
    pub const WARN: Level = Level(LevelInner::Warn);
    /// The "info" level.
    ///
    /// Designates useful information.
    pub const INFO: Level = Level(LevelInner::Info);
    /// The "debug" level.
    ///
    /// Designates lower priority information.
    pub const DEBUG: Level = Level(LevelInner::Debug);
    /// The "trace" level.
    ///
    /// Designates very low priority, often extremely verbose, information.
    pub const TRACE: Level = Level(LevelInner::Trace);

    /// Returns the string representation of the `Level`.
    ///
    /// This returns the same string as the `fmt::Display` implementation.
    pub fn as_str(&self) -> &'static str {
        match *self {
            Level::TRACE => "TRACE",
            Level::DEBUG => "DEBUG",
            Level::INFO => "INFO",
            Level::WARN => "WARN",
            Level::ERROR => "ERROR",
        }
    }
}

impl fmt::Display for Level {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            Level::TRACE => f.pad("TRACE"),
            Level::DEBUG => f.pad("DEBUG"),
            Level::INFO => f.pad("INFO"),
            Level::WARN => f.pad("WARN"),
            Level::ERROR => f.pad("ERROR"),
        }
    }
}

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

impl FromStr for Level {
    type Err = ParseLevelError;
    fn from_str(s: &str) -> Result<Self, ParseLevelError> {
        s.parse::<usize>()
            .map_err(|_| ParseLevelError { _p: () })
            .and_then(|num| match num {
                1 => Ok(Level::ERROR),
                2 => Ok(Level::WARN),
                3 => Ok(Level::INFO),
                4 => Ok(Level::DEBUG),
                5 => Ok(Level::TRACE),
                _ => Err(ParseLevelError { _p: () }),
            })
            .or_else(|_| match s {
                s if s.eq_ignore_ascii_case("error") => Ok(Level::ERROR),
                s if s.eq_ignore_ascii_case("warn") => Ok(Level::WARN),
                s if s.eq_ignore_ascii_case("info") => Ok(Level::INFO),
                s if s.eq_ignore_ascii_case("debug") => Ok(Level::DEBUG),
                s if s.eq_ignore_ascii_case("trace") => Ok(Level::TRACE),
                _ => Err(ParseLevelError { _p: () }),
            })
    }
}

#[repr(usize)]
#[derive(Copy, Clone, Debug, Hash, Eq, PartialEq)]
enum LevelInner {
    /// The "trace" level.
    ///
    /// Designates very low priority, often extremely verbose, information.
    Trace = 0,
    /// The "debug" level.
    ///
    /// Designates lower priority information.
    Debug = 1,
    /// The "info" level.
    ///
    /// Designates useful information.
    Info = 2,
    /// The "warn" level.
    ///
    /// Designates hazardous situations.
    Warn = 3,
    /// The "error" level.
    ///
    /// Designates very serious errors.
    Error = 4,
}

// === impl LevelFilter ===

impl From<Level> for LevelFilter {
    #[inline]
    fn from(level: Level) -> Self {
        Self::from_level(level)
    }
}

impl From<Option<Level>> for LevelFilter {
    #[inline]
    fn from(level: Option<Level>) -> Self {
        Self(level)
    }
}

impl From<LevelFilter> for Option<Level> {
    #[inline]
    fn from(filter: LevelFilter) -> Self {
        filter.into_level()
    }
}

impl LevelFilter {
    /// The "off" level.
    ///
    /// Designates that trace instrumentation should be completely disabled.
    pub const OFF: LevelFilter = LevelFilter(None);
    /// The "error" level.
    ///
    /// Designates very serious errors.
    pub const ERROR: LevelFilter = LevelFilter::from_level(Level::ERROR);
    /// The "warn" level.
    ///
    /// Designates hazardous situations.
    pub const WARN: LevelFilter = LevelFilter::from_level(Level::WARN);
    /// The "info" level.
    ///
    /// Designates useful information.
    pub const INFO: LevelFilter = LevelFilter::from_level(Level::INFO);
    /// The "debug" level.
    ///
    /// Designates lower priority information.
    pub const DEBUG: LevelFilter = LevelFilter::from_level(Level::DEBUG);
    /// The "trace" level.
    ///
    /// Designates very low priority, often extremely verbose, information.
    pub const TRACE: LevelFilter = LevelFilter(Some(Level::TRACE));

    /// Returns a `LevelFilter` that enables spans and events with verbosity up
    /// to and including `level`.
    pub const fn from_level(level: Level) -> Self {
        Self(Some(level))
    }

    /// Returns the most verbose [`Level`] that this filter accepts, or `None`
    /// if it is [`OFF`].
    ///
    /// [`Level`]: super::Level
    /// [`OFF`]: #associatedconstant.OFF
    pub const fn into_level(self) -> Option<Level> {
        self.0
    }

    // These consts are necessary because `as` casts are not allowed as
    // match patterns.
    const ERROR_USIZE: usize = LevelInner::Error as usize;
    const WARN_USIZE: usize = LevelInner::Warn as usize;
    const INFO_USIZE: usize = LevelInner::Info as usize;
    const DEBUG_USIZE: usize = LevelInner::Debug as usize;
    const TRACE_USIZE: usize = LevelInner::Trace as usize;
    // Using the value of the last variant + 1 ensures that we match the value
    // for `Option::None` as selected by the niche optimization for
    // `LevelFilter`. If this is the case, converting a `usize` value into a
    // `LevelFilter` (in `LevelFilter::current`) will be an identity conversion,
    // rather than generating a lookup table.
    const OFF_USIZE: usize = LevelInner::Error as usize + 1;

    /// Returns a `LevelFilter` that matches the most verbose [`Level`] that any
    /// currently active [`Subscriber`] will enable.
    ///
    /// User code should treat this as a *hint*. If a given span or event has a
    /// level *higher* than the returned `LevelFilter`, it will not be enabled.
    /// However, if the level is less than or equal to this value, the span or
    /// event is *not* guaranteed to be enabled; the subscriber will still
    /// filter each callsite individually.
    ///
    /// Therefore, comparing a given span or event's level to the returned
    /// `LevelFilter` **can** be used for determining if something is
    /// *disabled*, but **should not** be used for determining if something is
    /// *enabled*.
    ///
    /// [`Level`]: super::Level
    /// [`Subscriber`]: super::Subscriber
    #[inline(always)]
    pub fn current() -> Self {
        match MAX_LEVEL.load(Ordering::Relaxed) {
            Self::ERROR_USIZE => Self::ERROR,
            Self::WARN_USIZE => Self::WARN,
            Self::INFO_USIZE => Self::INFO,
            Self::DEBUG_USIZE => Self::DEBUG,
            Self::TRACE_USIZE => Self::TRACE,
            Self::OFF_USIZE => Self::OFF,
            #[cfg(debug_assertions)]
            unknown => unreachable!(
                "/!\\ `LevelFilter` representation seems to have changed! /!\\ \n\
                This is a bug (and it's pretty bad). Please contact the `tracing` \
                maintainers. Thank you and I'm sorry.\n \
                The offending repr was: {:?}",
                unknown,
            ),
            #[cfg(not(debug_assertions))]
            _ => unsafe {
                // Using `unreachable_unchecked` here (rather than
                // `unreachable!()`) is necessary to ensure that rustc generates
                // an identity conversion from integer -> discriminant, rather
                // than generating a lookup table. We want to ensure this
                // function is a single `mov` instruction (on x86) if at all
                // possible, because it is called *every* time a span/event
                // callsite is hit; and it is (potentially) the only code in the
                // hottest path for skipping a majority of callsites when level
                // filtering is in use.
                //
                // safety: This branch is only truly unreachable if we guarantee
                // that no values other than the possible enum discriminants
                // will *ever* be present. The `AtomicUsize` is initialized to
                // the `OFF` value. It is only set by the `set_max` function,
                // which takes a `LevelFilter` as a parameter. This restricts
                // the inputs to `set_max` to the set of valid discriminants.
                // Therefore, **as long as `MAX_VALUE` is only ever set by
                // `set_max`**, this is safe.
                crate::stdlib::hint::unreachable_unchecked()
            },
        }
    }

    pub(crate) fn set_max(LevelFilter(level): LevelFilter) {
        let val = match level {
            Some(Level(level)) => level as usize,
            None => Self::OFF_USIZE,
        };

        // using an AcqRel swap ensures an ordered relationship of writes to the
        // max level.
        MAX_LEVEL.swap(val, Ordering::AcqRel);
    }
}

impl fmt::Display for LevelFilter {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            LevelFilter::OFF => f.pad("off"),
            LevelFilter::ERROR => f.pad("error"),
            LevelFilter::WARN => f.pad("warn"),
            LevelFilter::INFO => f.pad("info"),
            LevelFilter::DEBUG => f.pad("debug"),
            LevelFilter::TRACE => f.pad("trace"),
        }
    }
}

impl fmt::Debug for LevelFilter {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            LevelFilter::OFF => f.pad("LevelFilter::OFF"),
            LevelFilter::ERROR => f.pad("LevelFilter::ERROR"),
            LevelFilter::WARN => f.pad("LevelFilter::WARN"),
            LevelFilter::INFO => f.pad("LevelFilter::INFO"),
            LevelFilter::DEBUG => f.pad("LevelFilter::DEBUG"),
            LevelFilter::TRACE => f.pad("LevelFilter::TRACE"),
        }
    }
}

impl FromStr for LevelFilter {
    type Err = ParseLevelFilterError;
    fn from_str(from: &str) -> Result<Self, Self::Err> {
        from.parse::<usize>()
            .ok()
            .and_then(|num| match num {
                0 => Some(LevelFilter::OFF),
                1 => Some(LevelFilter::ERROR),
                2 => Some(LevelFilter::WARN),
                3 => Some(LevelFilter::INFO),
                4 => Some(LevelFilter::DEBUG),
                5 => Some(LevelFilter::TRACE),
                _ => None,
            })
            .or_else(|| match from {
                "" => Some(LevelFilter::ERROR),
                s if s.eq_ignore_ascii_case("error") => Some(LevelFilter::ERROR),
                s if s.eq_ignore_ascii_case("warn") => Some(LevelFilter::WARN),
                s if s.eq_ignore_ascii_case("info") => Some(LevelFilter::INFO),
                s if s.eq_ignore_ascii_case("debug") => Some(LevelFilter::DEBUG),
                s if s.eq_ignore_ascii_case("trace") => Some(LevelFilter::TRACE),
                s if s.eq_ignore_ascii_case("off") => Some(LevelFilter::OFF),
                _ => None,
            })
            .ok_or(ParseLevelFilterError(()))
    }
}

/// Returned if parsing a `Level` fails.
#[derive(Debug)]
pub struct ParseLevelError {
    _p: (),
}

impl fmt::Display for ParseLevelError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad(
            "error parsing level: expected one of \"error\", \"warn\", \
             \"info\", \"debug\", \"trace\", or a number 1-5",
        )
    }
}

impl fmt::Display for ParseLevelFilterError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad(
            "error parsing level filter: expected one of \"off\", \"error\", \
            \"warn\", \"info\", \"debug\", \"trace\", or a number 0-5",
        )
    }
}

#[cfg(feature = "std")]
impl std::error::Error for ParseLevelFilterError {}

// ==== Level and LevelFilter comparisons ====

// /!\ BIG, IMPORTANT WARNING /!\
// Do NOT mess with these implementations! They are hand-written for a reason!
//
// Since comparing `Level`s and `LevelFilter`s happens in a *very* hot path
// (potentially, every time a span or event macro is hit, regardless of whether
// or not is enabled), we *need* to ensure that these comparisons are as fast as
// possible. Therefore, we have some requirements:
//
// 1. We want to do our best to ensure that rustc will generate integer-integer
//    comparisons wherever possible.
//
//    The derived `Ord`/`PartialOrd` impls for `LevelFilter` will not do this,
//    because `LevelFilter`s are represented by `Option<Level>`, rather than as
//    a separate `#[repr(usize)]` enum. This was (unfortunately) necessary for
//    backwards-compatibility reasons, as the  `tracing` crate's original
//    version of `LevelFilter` defined `const fn` conversions between `Level`s
//    and `LevelFilter`, so we're stuck with the `Option<Level>` repr.
//    Therefore, we need hand-written `PartialOrd` impls that cast both sides of
//    the comparison to `usize`s, to force the compiler to generate integer
//    compares.
//
// 2. The hottest `Level`/`LevelFilter` comparison, the one that happens every
//    time a callsite is hit, occurs *within the `tracing` crate's macros*.
//    This means that the comparison is happening *inside* a crate that
//    *depends* on `tracing-core`, not in `tracing-core` itself. The compiler
//    will only inline function calls across crate boundaries if the called
//    function is annotated with an `#[inline]` attribute, and we *definitely*
//    want the comparison functions to be inlined: as previously mentioned, they
//    should compile down to a single integer comparison on release builds, and
//    it seems really sad to push an entire stack frame to call a function
//    consisting of one `cmp` instruction!
//
//    Therefore, we need to ensure that all the comparison methods have
//    `#[inline]` or `#[inline(always)]` attributes. It's not sufficient to just
//    add the attribute to `partial_cmp` in a manual implementation of the
//    trait, since it's the comparison operators (`lt`, `le`, `gt`, and `ge`)
//    that will actually be *used*, and the default implementation of *those*
//    methods, which calls `partial_cmp`, does not have an inline annotation.
//
// 3. We need the comparisons to be inverted. The discriminants for the
//    `LevelInner` enum are assigned in "backwards" order, with `TRACE` having
//    the *lowest* value. However, we want `TRACE` to compare greater-than all
//    other levels.
//
//    Why are the numeric values inverted? In order to ensure that `LevelFilter`
//    (which, as previously mentioned, *has* to be internally represented by an
//    `Option<Level>`) compiles down to a single integer value. This is
//    necessary for storing the global max in an `AtomicUsize`, and for ensuring
//    that we use fast integer-integer comparisons, as mentioned previously. In
//    order to ensure this, we exploit the niche optimization. The niche
//    optimization for `Option<{enum with a numeric repr}>` will choose
//    `(HIGHEST_DISCRIMINANT_VALUE + 1)` as the representation for `None`.
//    Therefore, the integer representation of `LevelFilter::OFF` (which is
//    `None`) will be the number 5. `OFF` must compare higher than every other
//    level in order for it to filter as expected. Since we want to use a single
//    `cmp` instruction, we can't special-case the integer value of `OFF` to
//    compare higher, as that will generate more code. Instead, we need it to be
//    on one end of the enum, with `ERROR` on the opposite end, so we assign the
//    value 0 to `ERROR`.
//
//    This *does* mean that when parsing `LevelFilter`s or `Level`s from
//    `String`s, the integer values are inverted, but that doesn't happen in a
//    hot path.
//
//    Note that we manually invert the comparisons by swapping the left-hand and
//    right-hand side. Using `Ordering::reverse` generates significantly worse
//    code (per Matt Godbolt's Compiler Explorer).
//
// Anyway, that's a brief history of why this code is the way it is. Don't
// change it unless you know what you're doing.

impl PartialEq<LevelFilter> for Level {
    #[inline(always)]
    fn eq(&self, other: &LevelFilter) -> bool {
        self.0 as usize == filter_as_usize(&other.0)
    }
}

impl PartialOrd for Level {
    #[inline(always)]
    fn partial_cmp(&self, other: &Level) -> Option<cmp::Ordering> {
        Some(self.cmp(other))
    }

    #[inline(always)]
    fn lt(&self, other: &Level) -> bool {
        (other.0 as usize) < (self.0 as usize)
    }

    #[inline(always)]
    fn le(&self, other: &Level) -> bool {
        (other.0 as usize) <= (self.0 as usize)
    }

    #[inline(always)]
    fn gt(&self, other: &Level) -> bool {
        (other.0 as usize) > (self.0 as usize)
    }

    #[inline(always)]
    fn ge(&self, other: &Level) -> bool {
        (other.0 as usize) >= (self.0 as usize)
    }
}

impl Ord for Level {
    #[inline(always)]
    fn cmp(&self, other: &Self) -> cmp::Ordering {
        (other.0 as usize).cmp(&(self.0 as usize))
    }
}

impl PartialOrd<LevelFilter> for Level {
    #[inline(always)]
    fn partial_cmp(&self, other: &LevelFilter) -> Option<cmp::Ordering> {
        Some(filter_as_usize(&other.0).cmp(&(self.0 as usize)))
    }

    #[inline(always)]
    fn lt(&self, other: &LevelFilter) -> bool {
        filter_as_usize(&other.0) < (self.0 as usize)
    }

    #[inline(always)]
    fn le(&self, other: &LevelFilter) -> bool {
        filter_as_usize(&other.0) <= (self.0 as usize)
    }

    #[inline(always)]
    fn gt(&self, other: &LevelFilter) -> bool {
        filter_as_usize(&other.0) > (self.0 as usize)
    }

    #[inline(always)]
    fn ge(&self, other: &LevelFilter) -> bool {
        filter_as_usize(&other.0) >= (self.0 as usize)
    }
}

#[inline(always)]
fn filter_as_usize(x: &Option<Level>) -> usize {
    match x {
        Some(Level(f)) => *f as usize,
        None => LevelFilter::OFF_USIZE,
    }
}

impl PartialEq<Level> for LevelFilter {
    #[inline(always)]
    fn eq(&self, other: &Level) -> bool {
        filter_as_usize(&self.0) == other.0 as usize
    }
}

impl PartialOrd for LevelFilter {
    #[inline(always)]
    fn partial_cmp(&self, other: &LevelFilter) -> Option<cmp::Ordering> {
        Some(self.cmp(other))
    }

    #[inline(always)]
    fn lt(&self, other: &LevelFilter) -> bool {
        filter_as_usize(&other.0) < filter_as_usize(&self.0)
    }

    #[inline(always)]
    fn le(&self, other: &LevelFilter) -> bool {
        filter_as_usize(&other.0) <= filter_as_usize(&self.0)
    }

    #[inline(always)]
    fn gt(&self, other: &LevelFilter) -> bool {
        filter_as_usize(&other.0) > filter_as_usize(&self.0)
    }

    #[inline(always)]
    fn ge(&self, other: &LevelFilter) -> bool {
        filter_as_usize(&other.0) >= filter_as_usize(&self.0)
    }
}

impl Ord for LevelFilter {
    #[inline(always)]
    fn cmp(&self, other: &Self) -> cmp::Ordering {
        filter_as_usize(&other.0).cmp(&filter_as_usize(&self.0))
    }
}

impl PartialOrd<Level> for LevelFilter {
    #[inline(always)]
    fn partial_cmp(&self, other: &Level) -> Option<cmp::Ordering> {
        Some((other.0 as usize).cmp(&filter_as_usize(&self.0)))
    }

    #[inline(always)]
    fn lt(&self, other: &Level) -> bool {
        (other.0 as usize) < filter_as_usize(&self.0)
    }

    #[inline(always)]
    fn le(&self, other: &Level) -> bool {
        (other.0 as usize) <= filter_as_usize(&self.0)
    }

    #[inline(always)]
    fn gt(&self, other: &Level) -> bool {
        (other.0 as usize) > filter_as_usize(&self.0)
    }

    #[inline(always)]
    fn ge(&self, other: &Level) -> bool {
        (other.0 as usize) >= filter_as_usize(&self.0)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::stdlib::mem;

    #[test]
    fn level_from_str() {
        assert_eq!("error".parse::<Level>().unwrap(), Level::ERROR);
        assert_eq!("4".parse::<Level>().unwrap(), Level::DEBUG);
        assert!("0".parse::<Level>().is_err())
    }

    #[test]
    fn filter_level_conversion() {
        let mapping = [
            (LevelFilter::OFF, None),
            (LevelFilter::ERROR, Some(Level::ERROR)),
            (LevelFilter::WARN, Some(Level::WARN)),
            (LevelFilter::INFO, Some(Level::INFO)),
            (LevelFilter::DEBUG, Some(Level::DEBUG)),
            (LevelFilter::TRACE, Some(Level::TRACE)),
        ];
        for (filter, level) in mapping.iter() {
            assert_eq!(filter.into_level(), *level);
            match level {
                Some(level) => {
                    let actual: LevelFilter = (*level).into();
                    assert_eq!(actual, *filter);
                }
                None => {
                    let actual: LevelFilter = None.into();
                    assert_eq!(actual, *filter);
                }
            }
        }
    }

    #[test]
    fn level_filter_is_usize_sized() {
        assert_eq!(
            mem::size_of::<LevelFilter>(),
            mem::size_of::<usize>(),
            "`LevelFilter` is no longer `usize`-sized! global MAX_LEVEL may now be invalid!"
        )
    }

    #[test]
    fn level_filter_reprs() {
        let mapping = [
            (LevelFilter::OFF, LevelInner::Error as usize + 1),
            (LevelFilter::ERROR, LevelInner::Error as usize),
            (LevelFilter::WARN, LevelInner::Warn as usize),
            (LevelFilter::INFO, LevelInner::Info as usize),
            (LevelFilter::DEBUG, LevelInner::Debug as usize),
            (LevelFilter::TRACE, LevelInner::Trace as usize),
        ];
        for &(filter, expected) in &mapping {
            let repr = unsafe {
                // safety: The entire purpose of this test is to assert that the
                // actual repr matches what we expect it to be --- we're testing
                // that *other* unsafe code is sound using the transmuted value.
                // We're not going to do anything with it that might be unsound.
                mem::transmute::<LevelFilter, usize>(filter)
            };
            assert_eq!(expected, repr, "repr changed for {:?}", filter)
        }
    }
}