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)
        }
    }
}