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
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
//! Abstractions for creating [`io::Write`] instances.
//!
//! [`io::Write`]: std::io::Write
use std::{
    fmt,
    io::{self, Write},
    sync::{Arc, Mutex, MutexGuard},
};
use tracing_core::Metadata;

/// A type that can create [`io::Write`] instances.
///
/// `MakeWriter` is used by [`fmt::Layer`] or [`fmt::Subscriber`] to print
/// formatted text representations of [`Event`]s.
///
/// This trait is already implemented for function pointers and
/// immutably-borrowing closures that return an instance of [`io::Write`], such
/// as [`io::stdout`] and [`io::stderr`]. Additionally, it is implemented for
/// [`std::sync::Mutex`] when the type inside the mutex implements
/// [`io::Write`].
///
/// # Examples
///
/// The simplest usage is to pass in a named function that returns a writer. For
/// example, to log all events to stderr, we could write:
/// ```
/// let subscriber = tracing_subscriber::fmt()
///     .with_writer(std::io::stderr)
///     .finish();
/// # drop(subscriber);
/// ```
///
/// Any function that returns a writer can be used:
///
/// ```
/// fn make_my_great_writer() -> impl std::io::Write {
///     // ...
///     # std::io::stdout()
/// }
///
/// let subscriber = tracing_subscriber::fmt()
///     .with_writer(make_my_great_writer)
///     .finish();
/// # drop(subscriber);
/// ```
///
/// A closure can be used to introduce arbitrary logic into how the writer is
/// created. Consider the (admittedly rather silly) example of sending every 5th
/// event to stderr, and all other events to stdout:
///
/// ```
/// use std::io;
/// use std::sync::atomic::{AtomicUsize, Ordering::Relaxed};
///
/// let n = AtomicUsize::new(0);
/// let subscriber = tracing_subscriber::fmt()
///     .with_writer(move || -> Box<dyn io::Write> {
///         if n.fetch_add(1, Relaxed) % 5 == 0 {
///             Box::new(io::stderr())
///         } else {
///             Box::new(io::stdout())
///        }
///     })
///     .finish();
/// # drop(subscriber);
/// ```
///
/// A single instance of a type implementing [`io::Write`] may be used as a
/// `MakeWriter` by wrapping it in a [`Mutex`]. For example, we could
/// write to a file like so:
///
/// ```
/// use std::{fs::File, sync::Mutex};
///
/// # fn docs() -> Result<(), Box<dyn std::error::Error>> {
/// let log_file = File::create("my_cool_trace.log")?;
/// let subscriber = tracing_subscriber::fmt()
///     .with_writer(Mutex::new(log_file))
///     .finish();
/// # drop(subscriber);
/// # Ok(())
/// # }
/// ```
///
/// [`io::Write`]: std::io::Write
/// [`fmt::Layer`]: crate::fmt::Layer
/// [`fmt::Subscriber`]: crate::fmt::Subscriber
/// [`Event`]: tracing_core::event::Event
/// [`io::stdout`]: std::io::stdout()
/// [`io::stderr`]: std::io::stderr()
/// [`MakeWriter::make_writer_for`]: MakeWriter::make_writer_for
/// [`Metadata`]: tracing_core::Metadata
/// [levels]: tracing_core::Level
/// [targets]: tracing_core::Metadata::target
pub trait MakeWriter<'a> {
    /// The concrete [`io::Write`] implementation returned by [`make_writer`].
    ///
    /// [`io::Write`]: std::io::Write
    /// [`make_writer`]: MakeWriter::make_writer
    type Writer: io::Write;

    /// Returns an instance of [`Writer`].
    ///
    /// # Implementer notes
    ///
    /// [`fmt::Layer`] or [`fmt::Subscriber`] will call this method each time an event is recorded. Ensure any state
    /// that must be saved across writes is not lost when the [`Writer`] instance is dropped. If
    /// creating a [`io::Write`] instance is expensive, be sure to cache it when implementing
    /// [`MakeWriter`] to improve performance.
    ///
    /// [`Writer`]: MakeWriter::Writer
    /// [`fmt::Layer`]: crate::fmt::Layer
    /// [`fmt::Subscriber`]: crate::fmt::Subscriber
    /// [`io::Write`]: std::io::Write
    fn make_writer(&'a self) -> Self::Writer;

    /// Returns a [`Writer`] for writing data from the span or event described
    /// by the provided [`Metadata`].
    ///
    /// By default, this calls [`self.make_writer()`][make_writer], ignoring
    /// the provided metadata, but implementations can override this to provide
    /// metadata-specific behaviors.
    ///
    /// This method allows `MakeWriter` implementations to implement different
    /// behaviors based on the span or event being written. The `MakeWriter`
    /// type might return different writers based on the provided metadata, or
    /// might write some values to the writer before or after providing it to
    /// the caller.
    ///
    /// For example, we might want to write data from spans and events at the
    /// [`ERROR`] and [`WARN`] levels to `stderr`, and data from spans or events
    /// at lower levels to stdout:
    ///
    /// ```
    /// use std::io::{self, Stdout, Stderr, StdoutLock, StderrLock};
    /// use tracing_subscriber::fmt::writer::MakeWriter;
    /// use tracing_core::{Metadata, Level};
    ///
    /// pub struct MyMakeWriter {
    ///     stdout: Stdout,
    ///     stderr: Stderr,
    /// }
    ///
    /// /// A lock on either stdout or stderr, depending on the verbosity level
    /// /// of the event being written.
    /// pub enum StdioLock<'a> {
    ///     Stdout(StdoutLock<'a>),
    ///     Stderr(StderrLock<'a>),
    /// }
    ///
    /// impl<'a> io::Write for StdioLock<'a> {
    ///     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
    ///         match self {
    ///             StdioLock::Stdout(lock) => lock.write(buf),
    ///             StdioLock::Stderr(lock) => lock.write(buf),
    ///         }
    ///     }
    ///
    ///     fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
    ///         // ...
    ///         # match self {
    ///         #     StdioLock::Stdout(lock) => lock.write_all(buf),
    ///         #     StdioLock::Stderr(lock) => lock.write_all(buf),
    ///         # }
    ///     }
    ///
    ///     fn flush(&mut self) -> io::Result<()> {
    ///         // ...
    ///         # match self {
    ///         #     StdioLock::Stdout(lock) => lock.flush(),
    ///         #     StdioLock::Stderr(lock) => lock.flush(),
    ///         # }
    ///     }
    /// }
    ///
    /// impl<'a> MakeWriter<'a> for MyMakeWriter {
    ///     type Writer = StdioLock<'a>;
    ///
    ///     fn make_writer(&'a self) -> Self::Writer {
    ///         // We must have an implementation of `make_writer` that makes
    ///         // a "default" writer without any configuring metadata. Let's
    ///         // just return stdout in that case.
    ///         StdioLock::Stdout(self.stdout.lock())
    ///     }
    ///
    ///     fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer {
    ///         // Here's where we can implement our special behavior. We'll
    ///         // check if the metadata's verbosity level is WARN or ERROR,
    ///         // and return stderr in that case.
    ///         if meta.level() <= &Level::WARN {
    ///             return StdioLock::Stderr(self.stderr.lock());
    ///         }
    ///
    ///         // Otherwise, we'll return stdout.
    ///         StdioLock::Stdout(self.stdout.lock())
    ///     }
    /// }
    /// ```
    ///
    /// [`Writer`]: MakeWriter::Writer
    /// [`Metadata`]: tracing_core::Metadata
    /// [make_writer]: MakeWriter::make_writer
    /// [`WARN`]: tracing_core::Level::WARN
    /// [`ERROR`]: tracing_core::Level::ERROR
    fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer {
        let _ = meta;
        self.make_writer()
    }
}

/// Extension trait adding combinators for working with types implementing
/// [`MakeWriter`].
///
/// This is not intended to be implemented directly for user-defined
/// [`MakeWriter`]s; instead, it should be imported when the desired methods are
/// used.
pub trait MakeWriterExt<'a>: MakeWriter<'a> {
    /// Wraps `self` and returns a [`MakeWriter`] that will only write output
    /// for events at or below the provided verbosity [`Level`]. For instance,
    /// `Level::TRACE` is considered to be _more verbose` than `Level::INFO`.
    ///
    /// Events whose level is more verbose than `level` will be ignored, and no
    /// output will be written.
    ///
    /// # Examples
    ///
    /// ```
    /// use tracing::Level;
    /// use tracing_subscriber::fmt::writer::MakeWriterExt;
    ///
    /// // Construct a writer that outputs events to `stderr` only if the span or
    /// // event's level is >= WARN (WARN and ERROR).
    /// let mk_writer = std::io::stderr.with_max_level(Level::WARN);
    ///
    /// tracing_subscriber::fmt().with_writer(mk_writer).init();
    /// ```
    ///
    /// Writing the `ERROR` and `WARN` levels to `stderr`, and everything else
    /// to `stdout`:
    ///
    /// ```
    /// # use tracing::Level;
    /// # use tracing_subscriber::fmt::writer::MakeWriterExt;
    ///
    /// let mk_writer = std::io::stderr
    ///     .with_max_level(Level::WARN)
    ///     .or_else(std::io::stdout);
    ///
    /// tracing_subscriber::fmt().with_writer(mk_writer).init();
    /// ```
    ///
    /// Writing the `ERROR` level to `stderr`, the `INFO` and `WARN` levels to
    /// `stdout`, and the `INFO` and DEBUG` levels to a file:
    ///
    /// ```
    /// # use tracing::Level;
    /// # use tracing_subscriber::fmt::writer::MakeWriterExt;
    /// use std::{sync::Arc, fs::File};
    /// # // don't actually create the file when running the tests.
    /// # fn docs() -> std::io::Result<()> {
    /// let debug_log = Arc::new(File::create("debug.log")?);
    ///
    /// let mk_writer = std::io::stderr
    ///     .with_max_level(Level::ERROR)
    ///     .or_else(std::io::stdout
    ///         .with_max_level(Level::INFO)
    ///         .and(debug_log.with_max_level(Level::DEBUG))
    ///     );
    ///
    /// tracing_subscriber::fmt().with_writer(mk_writer).init();
    /// # Ok(()) }
    /// ```
    ///
    /// [`Level`]: tracing_core::Level
    /// [`io::Write`]: std::io::Write
    fn with_max_level(self, level: tracing_core::Level) -> WithMaxLevel<Self>
    where
        Self: Sized,
    {
        WithMaxLevel::new(self, level)
    }

    /// Wraps `self` and returns a [`MakeWriter`] that will only write output
    /// for events at or above the provided verbosity [`Level`].
    ///
    /// Events whose level is less verbose than `level` will be ignored, and no
    /// output will be written.
    ///
    /// # Examples
    ///
    /// ```
    /// use tracing::Level;
    /// use tracing_subscriber::fmt::writer::MakeWriterExt;
    ///
    /// // Construct a writer that outputs events to `stdout` only if the span or
    /// // event's level is <= DEBUG (DEBUG and TRACE).
    /// let mk_writer = std::io::stdout.with_min_level(Level::DEBUG);
    ///
    /// tracing_subscriber::fmt().with_writer(mk_writer).init();
    /// ```
    /// This can be combined with [`MakeWriterExt::with_max_level`] to write
    /// only within a range of levels:
    ///
    /// ```
    /// # use tracing::Level;
    /// # use tracing_subscriber::fmt::writer::MakeWriterExt;
    /// // Only write the `DEBUG` and `INFO` levels to stdout.
    /// let mk_writer = std::io::stdout
    ///     .with_max_level(Level::DEBUG)
    ///     .with_min_level(Level::INFO)
    ///     // Write the `WARN` and `ERROR` levels to stderr.
    ///     .and(std::io::stderr.with_min_level(Level::WARN));
    ///
    /// tracing_subscriber::fmt().with_writer(mk_writer).init();
    /// ```
    /// [`Level`]: tracing_core::Level
    /// [`io::Write`]: std::io::Write
    fn with_min_level(self, level: tracing_core::Level) -> WithMinLevel<Self>
    where
        Self: Sized,
    {
        WithMinLevel::new(self, level)
    }

    /// Wraps `self` with a predicate that takes a span or event's [`Metadata`]
    /// and returns a `bool`. The returned [`MakeWriter`]'s
    /// [`MakeWriter::make_writer_for`] method will check the predicate to
    /// determine if  a writer should be produced for a given span or event.
    ///
    /// If the predicate returns `false`, the wrapped [`MakeWriter`]'s
    /// [`make_writer_for`][mwf] will return [`OptionalWriter::none`][own].
    /// Otherwise, it calls the wrapped [`MakeWriter`]'s
    /// [`make_writer_for`][mwf] method, and returns the produced writer.
    ///
    /// This can be used to filter an output based on arbitrary [`Metadata`]
    /// parameters.
    ///
    /// # Examples
    ///
    /// Writing events with a specific target to an HTTP access log, and other
    /// events to stdout:
    ///
    /// ```
    /// use tracing_subscriber::fmt::writer::MakeWriterExt;
    /// use std::{sync::Arc, fs::File};
    /// # // don't actually create the file when running the tests.
    /// # fn docs() -> std::io::Result<()> {
    /// let access_log = Arc::new(File::create("access.log")?);
    ///
    /// let mk_writer = access_log
    ///     // Only write events with the target "http::access_log" to the
    ///     // access log file.
    ///     .with_filter(|meta| meta.target() == "http::access_log")
    ///     // Write events with all other targets to stdout.
    ///     .or_else(std::io::stdout);
    ///
    /// tracing_subscriber::fmt().with_writer(mk_writer).init();
    /// # Ok(())
    /// # }
    /// ```
    ///
    /// Conditionally enabling or disabling a log file:
    /// ```
    /// use tracing_subscriber::fmt::writer::MakeWriterExt;
    /// use std::{
    ///     sync::{Arc, atomic::{AtomicBool, Ordering}},
    ///     fs::File,
    /// };
    ///
    /// static DEBUG_LOG_ENABLED: AtomicBool = AtomicBool::new(false);
    ///
    /// # // don't actually create the file when running the tests.
    /// # fn docs() -> std::io::Result<()> {
    /// // Create the debug log file
    /// let debug_file = Arc::new(File::create("debug.log")?)
    ///     // Enable the debug log only if the flag is enabled.
    ///     .with_filter(|_| DEBUG_LOG_ENABLED.load(Ordering::Acquire));
    ///
    /// // Always write to stdout
    /// let mk_writer = std::io::stdout
    ///     // Write to the debug file if it's enabled
    ///     .and(debug_file);
    ///
    /// tracing_subscriber::fmt().with_writer(mk_writer).init();
    ///
    /// // ...
    ///
    /// // Later, we can toggle on or off the debug log file.
    /// DEBUG_LOG_ENABLED.store(true, Ordering::Release);
    /// # Ok(())
    /// # }
    /// ```
    ///
    /// [`Metadata`]: tracing_core::Metadata
    /// [mwf]: MakeWriter::make_writer_for
    /// [own]: EitherWriter::none
    fn with_filter<F>(self, filter: F) -> WithFilter<Self, F>
    where
        Self: Sized,
        F: Fn(&Metadata<'_>) -> bool,
    {
        WithFilter::new(self, filter)
    }

    /// Combines `self` with another type implementing [`MakeWriter`], returning
    /// a new [`MakeWriter`] that produces [writers] that write to *both*
    /// outputs.
    ///
    /// If writing to either writer returns an error, the returned writer will
    /// return that error. However, both writers will still be written to before
    /// the error is returned, so it is possible for one writer to fail while
    /// the other is written to successfully.
    ///
    /// # Examples
    ///
    /// ```
    /// use tracing_subscriber::fmt::writer::MakeWriterExt;
    ///
    /// // Construct a writer that outputs events to `stdout` *and* `stderr`.
    /// let mk_writer = std::io::stdout.and(std::io::stderr);
    ///
    /// tracing_subscriber::fmt().with_writer(mk_writer).init();
    /// ```
    ///
    /// `and` can be used in conjunction with filtering combinators. For
    /// example, if we want to write to a number of outputs depending on the
    /// level of an event, we could write:
    ///
    /// ```
    /// use tracing::Level;
    /// # use tracing_subscriber::fmt::writer::MakeWriterExt;
    /// use std::{sync::Arc, fs::File};
    /// # // don't actually create the file when running the tests.
    /// # fn docs() -> std::io::Result<()> {
    /// let debug_log = Arc::new(File::create("debug.log")?);
    ///
    /// // Write everything to the debug log.
    /// let mk_writer = debug_log
    ///     // Write the `ERROR` and `WARN` levels to stderr.
    ///     .and(std::io::stderr.with_max_level(Level::WARN))
    ///     // Write `INFO` to `stdout`.
    ///     .and(std::io::stdout
    ///         .with_max_level(Level::INFO)
    ///         .with_min_level(Level::INFO)
    ///     );
    ///
    /// tracing_subscriber::fmt().with_writer(mk_writer).init();
    /// # Ok(()) }
    /// ```
    ///
    /// [writers]: std::io::Write
    fn and<B>(self, other: B) -> Tee<Self, B>
    where
        Self: Sized,
        B: MakeWriter<'a> + Sized,
    {
        Tee::new(self, other)
    }

    /// Combines `self` with another type implementing [`MakeWriter`], returning
    /// a new [`MakeWriter`] that calls `other`'s [`make_writer`] if `self`'s
    /// `make_writer` returns [`OptionalWriter::none`][own].
    ///
    /// # Examples
    ///
    /// ```
    /// use tracing::Level;
    /// use tracing_subscriber::fmt::writer::MakeWriterExt;
    ///
    /// // Produces a writer that writes to `stderr` if the level is >= WARN,
    /// // or returns `OptionalWriter::none()` otherwise.
    /// let stderr = std::io::stderr.with_max_level(Level::WARN);
    ///
    /// // If the `stderr` `MakeWriter` is disabled by the max level filter,
    /// // write to stdout instead:
    /// let mk_writer = stderr.or_else(std::io::stdout);
    ///
    /// tracing_subscriber::fmt().with_writer(mk_writer).init();
    /// ```
    ///
    /// [`make_writer`]: MakeWriter::make_writer
    /// [own]: EitherWriter::none
    fn or_else<W, B>(self, other: B) -> OrElse<Self, B>
    where
        Self: MakeWriter<'a, Writer = OptionalWriter<W>> + Sized,
        B: MakeWriter<'a> + Sized,
        W: Write,
    {
        OrElse::new(self, other)
    }
}

/// A writer intended to support [`libtest`'s output capturing][capturing] for use in unit tests.
///
/// `TestWriter` is used by [`fmt::Subscriber`] or [`fmt::Layer`] to enable capturing support.
///
/// `cargo test` can only capture output from the standard library's [`print!`] macro. See
/// [`libtest`'s output capturing][capturing] for more details about output capturing.
///
/// Writing to [`io::stdout`] and [`io::stderr`] produces the same results as using
/// [`libtest`'s `--nocapture` option][nocapture] which may make the results look unreadable.
///
/// [`fmt::Subscriber`]: super::Subscriber
/// [`fmt::Layer`]: super::Layer
/// [capturing]: https://doc.rust-lang.org/book/ch11-02-running-tests.html#showing-function-output
/// [nocapture]: https://doc.rust-lang.org/cargo/commands/cargo-test.html
/// [`io::stdout`]: std::io::stdout
/// [`io::stderr`]: std::io::stderr
/// [`print!`]: std::print!
#[derive(Default, Debug)]
pub struct TestWriter {
    _p: (),
}

/// A writer that erases the specific [`io::Write`] and [`MakeWriter`] types being used.
///
/// This is useful in cases where the concrete type of the writer cannot be known
/// until runtime.
///
/// # Examples
///
/// A function that returns a [`Subscriber`] that will write to either stdout or stderr:
///
/// ```rust
/// # use tracing::Subscriber;
/// # use tracing_subscriber::fmt::writer::BoxMakeWriter;
///
/// fn dynamic_writer(use_stderr: bool) -> impl Subscriber {
///     let writer = if use_stderr {
///         BoxMakeWriter::new(std::io::stderr)
///     } else {
///         BoxMakeWriter::new(std::io::stdout)
///     };
///
///     tracing_subscriber::fmt().with_writer(writer).finish()
/// }
/// ```
///
/// [`Subscriber`]: tracing::Subscriber
/// [`io::Write`]: std::io::Write
pub struct BoxMakeWriter {
    inner: Box<dyn for<'a> MakeWriter<'a, Writer = Box<dyn Write + 'a>> + Send + Sync>,
    name: &'static str,
}

/// A [writer] that is one of two types implementing [`io::Write`].
///
/// This may be used by [`MakeWriter`] implementations that may conditionally
/// return one of two writers.
///
/// [writer]: std::io::Write
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub enum EitherWriter<A, B> {
    /// A writer of type `A`.
    A(A),
    /// A writer of type `B`.
    B(B),
}

/// A [writer] which may or may not be enabled.
///
/// This may be used by [`MakeWriter`] implementations that wish to
/// conditionally enable or disable the returned writer based on a span or
/// event's [`Metadata`].
///
/// [writer]: std::io::Write
pub type OptionalWriter<T> = EitherWriter<T, std::io::Sink>;

/// A [`MakeWriter`] combinator that only returns an enabled [writer] for spans
/// and events with metadata at or below a specified verbosity [`Level`].
///
/// This is returned by the [`MakeWriterExt::with_max_level`] method. See the
/// method documentation for details.
///
/// [writer]: std::io::Write
/// [`Level`]: tracing_core::Level
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct WithMaxLevel<M> {
    make: M,
    level: tracing_core::Level,
}

/// A [`MakeWriter`] combinator that only returns an enabled [writer] for spans
/// and events with metadata at or above a specified verbosity [`Level`].
///
/// This is returned by the [`MakeWriterExt::with_min_level`] method. See the
/// method documentation for details.
///
/// [writer]: std::io::Write
/// [`Level`]: tracing_core::Level
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct WithMinLevel<M> {
    make: M,
    level: tracing_core::Level,
}

/// A [`MakeWriter`] combinator that wraps a [`MakeWriter`] with a predicate for
/// span and event [`Metadata`], so that the [`MakeWriter::make_writer_for`]
/// method returns [`OptionalWriter::some`][ows] when the predicate returns `true`,
/// and [`OptionalWriter::none`][own] when the predicate returns `false`.
///
/// This is returned by the [`MakeWriterExt::with_filter`] method. See the
/// method documentation for details.
///
/// [`Metadata`]: tracing_core::Metadata
/// [ows]: EitherWriter::some
/// [own]: EitherWriter::none
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct WithFilter<M, F> {
    make: M,
    filter: F,
}

/// Combines a [`MakeWriter`] that returns an [`OptionalWriter`] with another
/// [`MakeWriter`], so that the second [`MakeWriter`] is used when the first
/// [`MakeWriter`] returns [`OptionalWriter::none`][own].
///
/// This is returned by the [`MakeWriterExt::or_else] method. See the
/// method documentation for details.
///
/// [own]: EitherWriter::none
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct OrElse<A, B> {
    inner: A,
    or_else: B,
}

/// Combines two types implementing [`MakeWriter`] (or [`std::io::Write`]) to
/// produce a writer that writes to both [`MakeWriter`]'s returned writers.
///
/// This is returned by the [`MakeWriterExt::and`] method. See the method
/// documentation for details.
#[derive(Copy, Clone, Debug, Eq, PartialEq)]
pub struct Tee<A, B> {
    a: A,
    b: B,
}

/// A type implementing [`io::Write`] for a [`MutexGuard`] where the type
/// inside the [`Mutex`] implements [`io::Write`].
///
/// This is used by the [`MakeWriter`] implementation for [`Mutex`], because
/// [`MutexGuard`] itself will not implement [`io::Write`] — instead, it
/// _dereferences_ to a type implementing [`io::Write`]. Because [`MakeWriter`]
/// requires the `Writer` type to implement [`io::Write`], it's necessary to add
/// a newtype that forwards the trait implementation.
///
/// [`io::Write`]: std::io::Write
/// [`MutexGuard`]: std::sync::MutexGuard
/// [`Mutex`]: std::sync::Mutex
#[derive(Debug)]
pub struct MutexGuardWriter<'a, W>(MutexGuard<'a, W>);

/// Implements [`std::io::Write`] for an [`Arc`]<W> where `&W: Write`.
///
/// This is an implementation detail of the [`MakeWriter`] impl for [`Arc`].
#[derive(Clone, Debug)]
pub struct ArcWriter<W>(Arc<W>);

/// A bridge between `fmt::Write` and `io::Write`.
///
/// This is used by the timestamp formatting implementation for the `time`
/// crate and by the JSON formatter. In both cases, this is needed because
/// `tracing-subscriber`'s `FormatEvent`/`FormatTime` traits expect a
/// `fmt::Write` implementation, while `serde_json::Serializer` and `time`'s
/// `format_into` methods expect an `io::Write`.
#[cfg(any(feature = "json", feature = "time"))]
pub(in crate::fmt) struct WriteAdaptor<'a> {
    fmt_write: &'a mut dyn fmt::Write,
}

impl<'a, F, W> MakeWriter<'a> for F
where
    F: Fn() -> W,
    W: io::Write,
{
    type Writer = W;

    fn make_writer(&'a self) -> Self::Writer {
        (self)()
    }
}

impl<'a, W> MakeWriter<'a> for Arc<W>
where
    &'a W: io::Write + 'a,
{
    type Writer = &'a W;
    fn make_writer(&'a self) -> Self::Writer {
        self
    }
}

impl<'a> MakeWriter<'a> for std::fs::File {
    type Writer = &'a std::fs::File;
    fn make_writer(&'a self) -> Self::Writer {
        self
    }
}

// === impl TestWriter ===

impl TestWriter {
    /// Returns a new `TestWriter` with the default configuration.
    pub fn new() -> Self {
        Self::default()
    }
}

impl io::Write for TestWriter {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        let out_str = String::from_utf8_lossy(buf);
        print!("{}", out_str);
        Ok(buf.len())
    }

    fn flush(&mut self) -> io::Result<()> {
        Ok(())
    }
}

impl<'a> MakeWriter<'a> for TestWriter {
    type Writer = Self;

    fn make_writer(&'a self) -> Self::Writer {
        Self::default()
    }
}

// === impl BoxMakeWriter ===

impl BoxMakeWriter {
    /// Constructs a `BoxMakeWriter` wrapping a type implementing [`MakeWriter`].
    ///
    pub fn new<M>(make_writer: M) -> Self
    where
        M: for<'a> MakeWriter<'a> + Send + Sync + 'static,
    {
        Self {
            inner: Box::new(Boxed(make_writer)),
            name: std::any::type_name::<M>(),
        }
    }
}

impl fmt::Debug for BoxMakeWriter {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("BoxMakeWriter")
            .field(&format_args!("<{}>", self.name))
            .finish()
    }
}

impl<'a> MakeWriter<'a> for BoxMakeWriter {
    type Writer = Box<dyn Write + 'a>;

    #[inline]
    fn make_writer(&'a self) -> Self::Writer {
        self.inner.make_writer()
    }

    #[inline]
    fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer {
        self.inner.make_writer_for(meta)
    }
}

struct Boxed<M>(M);

impl<'a, M> MakeWriter<'a> for Boxed<M>
where
    M: MakeWriter<'a>,
{
    type Writer = Box<dyn Write + 'a>;

    fn make_writer(&'a self) -> Self::Writer {
        let w = self.0.make_writer();
        Box::new(w)
    }

    fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer {
        let w = self.0.make_writer_for(meta);
        Box::new(w)
    }
}

// === impl Mutex/MutexGuardWriter ===

impl<'a, W> MakeWriter<'a> for Mutex<W>
where
    W: io::Write + 'a,
{
    type Writer = MutexGuardWriter<'a, W>;

    fn make_writer(&'a self) -> Self::Writer {
        MutexGuardWriter(self.lock().expect("lock poisoned"))
    }
}

impl<'a, W> io::Write for MutexGuardWriter<'a, W>
where
    W: io::Write,
{
    #[inline]
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        self.0.write(buf)
    }

    #[inline]
    fn flush(&mut self) -> io::Result<()> {
        self.0.flush()
    }

    #[inline]
    fn write_vectored(&mut self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize> {
        self.0.write_vectored(bufs)
    }

    #[inline]
    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
        self.0.write_all(buf)
    }

    #[inline]
    fn write_fmt(&mut self, fmt: std::fmt::Arguments<'_>) -> io::Result<()> {
        self.0.write_fmt(fmt)
    }
}

// === impl EitherWriter ===

impl<A, B> io::Write for EitherWriter<A, B>
where
    A: io::Write,
    B: io::Write,
{
    #[inline]
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        match self {
            EitherWriter::A(a) => a.write(buf),
            EitherWriter::B(b) => b.write(buf),
        }
    }

    #[inline]
    fn flush(&mut self) -> io::Result<()> {
        match self {
            EitherWriter::A(a) => a.flush(),
            EitherWriter::B(b) => b.flush(),
        }
    }

    #[inline]
    fn write_vectored(&mut self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize> {
        match self {
            EitherWriter::A(a) => a.write_vectored(bufs),
            EitherWriter::B(b) => b.write_vectored(bufs),
        }
    }

    #[inline]
    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
        match self {
            EitherWriter::A(a) => a.write_all(buf),
            EitherWriter::B(b) => b.write_all(buf),
        }
    }

    #[inline]
    fn write_fmt(&mut self, fmt: std::fmt::Arguments<'_>) -> io::Result<()> {
        match self {
            EitherWriter::A(a) => a.write_fmt(fmt),
            EitherWriter::B(b) => b.write_fmt(fmt),
        }
    }
}

impl<T> OptionalWriter<T> {
    /// Returns a [disabled writer].
    ///
    /// Any bytes written to the returned writer are discarded.
    ///
    /// This is equivalent to returning [`Option::None`].
    ///
    /// [disabled writer]: std::io::sink
    #[inline]
    pub fn none() -> Self {
        EitherWriter::B(std::io::sink())
    }

    /// Returns an enabled writer of type `T`.
    ///
    /// This is equivalent to returning [`Option::Some`].
    #[inline]
    pub fn some(t: T) -> Self {
        EitherWriter::A(t)
    }
}

impl<T> From<Option<T>> for OptionalWriter<T> {
    #[inline]
    fn from(opt: Option<T>) -> Self {
        match opt {
            Some(writer) => Self::some(writer),
            None => Self::none(),
        }
    }
}

// === impl WithMaxLevel ===

impl<M> WithMaxLevel<M> {
    /// Wraps the provided [`MakeWriter`] with a maximum [`Level`], so that it
    /// returns [`OptionalWriter::none`] for spans and events whose level is
    /// more verbose than the maximum level.
    ///
    /// See [`MakeWriterExt::with_max_level`] for details.
    ///
    /// [`Level`]: tracing_core::Level
    pub fn new(make: M, level: tracing_core::Level) -> Self {
        Self { make, level }
    }
}

impl<'a, M: MakeWriter<'a>> MakeWriter<'a> for WithMaxLevel<M> {
    type Writer = OptionalWriter<M::Writer>;

    #[inline]
    fn make_writer(&'a self) -> Self::Writer {
        // If we don't know the level, assume it's disabled.
        OptionalWriter::none()
    }

    #[inline]
    fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer {
        if meta.level() <= &self.level {
            return OptionalWriter::some(self.make.make_writer_for(meta));
        }
        OptionalWriter::none()
    }
}

// === impl WithMinLevel ===

impl<M> WithMinLevel<M> {
    /// Wraps the provided [`MakeWriter`] with a minimum [`Level`], so that it
    /// returns [`OptionalWriter::none`] for spans and events whose level is
    /// less verbose than the maximum level.
    ///
    /// See [`MakeWriterExt::with_min_level`] for details.
    ///
    /// [`Level`]: tracing_core::Level
    pub fn new(make: M, level: tracing_core::Level) -> Self {
        Self { make, level }
    }
}

impl<'a, M: MakeWriter<'a>> MakeWriter<'a> for WithMinLevel<M> {
    type Writer = OptionalWriter<M::Writer>;

    #[inline]
    fn make_writer(&'a self) -> Self::Writer {
        // If we don't know the level, assume it's disabled.
        OptionalWriter::none()
    }

    #[inline]
    fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer {
        if meta.level() >= &self.level {
            return OptionalWriter::some(self.make.make_writer_for(meta));
        }
        OptionalWriter::none()
    }
}

// ==== impl WithFilter ===

impl<M, F> WithFilter<M, F> {
    /// Wraps `make` with the provided `filter`, returning a [`MakeWriter`] that
    /// will call `make.make_writer_for()` when `filter` returns `true` for a
    /// span or event's [`Metadata`], and returns a [`sink`] otherwise.
    ///
    /// See [`MakeWriterExt::with_filter`] for details.
    ///
    /// [`Metadata`]: tracing_core::Metadata
    /// [`sink`]: std::io::sink
    pub fn new(make: M, filter: F) -> Self
    where
        F: Fn(&Metadata<'_>) -> bool,
    {
        Self { make, filter }
    }
}

impl<'a, M, F> MakeWriter<'a> for WithFilter<M, F>
where
    M: MakeWriter<'a>,
    F: Fn(&Metadata<'_>) -> bool,
{
    type Writer = OptionalWriter<M::Writer>;

    #[inline]
    fn make_writer(&'a self) -> Self::Writer {
        OptionalWriter::some(self.make.make_writer())
    }

    #[inline]
    fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer {
        if (self.filter)(meta) {
            OptionalWriter::some(self.make.make_writer_for(meta))
        } else {
            OptionalWriter::none()
        }
    }
}

// === impl Tee ===

impl<A, B> Tee<A, B> {
    /// Combines two types implementing [`MakeWriter`], returning
    /// a new [`MakeWriter`] that produces [writers] that write to *both*
    /// outputs.
    ///
    /// See the documentation for [`MakeWriterExt::and`] for details.
    ///
    /// [writers]: std::io::Write
    pub fn new(a: A, b: B) -> Self {
        Self { a, b }
    }
}

impl<'a, A, B> MakeWriter<'a> for Tee<A, B>
where
    A: MakeWriter<'a>,
    B: MakeWriter<'a>,
{
    type Writer = Tee<A::Writer, B::Writer>;

    #[inline]
    fn make_writer(&'a self) -> Self::Writer {
        Tee::new(self.a.make_writer(), self.b.make_writer())
    }

    #[inline]
    fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer {
        Tee::new(self.a.make_writer_for(meta), self.b.make_writer_for(meta))
    }
}

macro_rules! impl_tee {
    ($self_:ident.$f:ident($($arg:ident),*)) => {
        {
            let res_a = $self_.a.$f($($arg),*);
            let res_b = $self_.b.$f($($arg),*);
            (res_a?, res_b?)
        }
    }
}

impl<A, B> io::Write for Tee<A, B>
where
    A: io::Write,
    B: io::Write,
{
    #[inline]
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        let (a, b) = impl_tee!(self.write(buf));
        Ok(std::cmp::max(a, b))
    }

    #[inline]
    fn flush(&mut self) -> io::Result<()> {
        impl_tee!(self.flush());
        Ok(())
    }

    #[inline]
    fn write_vectored(&mut self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize> {
        let (a, b) = impl_tee!(self.write_vectored(bufs));
        Ok(std::cmp::max(a, b))
    }

    #[inline]
    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
        impl_tee!(self.write_all(buf));
        Ok(())
    }

    #[inline]
    fn write_fmt(&mut self, fmt: std::fmt::Arguments<'_>) -> io::Result<()> {
        impl_tee!(self.write_fmt(fmt));
        Ok(())
    }
}

// === impl OrElse ===

impl<A, B> OrElse<A, B> {
    /// Combines
    pub fn new<'a, W>(inner: A, or_else: B) -> Self
    where
        A: MakeWriter<'a, Writer = OptionalWriter<W>>,
        B: MakeWriter<'a>,
        W: Write,
    {
        Self { inner, or_else }
    }
}

impl<'a, A, B, W> MakeWriter<'a> for OrElse<A, B>
where
    A: MakeWriter<'a, Writer = OptionalWriter<W>>,
    B: MakeWriter<'a>,
    W: io::Write,
{
    type Writer = EitherWriter<W, B::Writer>;

    #[inline]
    fn make_writer(&'a self) -> Self::Writer {
        match self.inner.make_writer() {
            EitherWriter::A(writer) => EitherWriter::A(writer),
            EitherWriter::B(_) => EitherWriter::B(self.or_else.make_writer()),
        }
    }

    #[inline]
    fn make_writer_for(&'a self, meta: &Metadata<'_>) -> Self::Writer {
        match self.inner.make_writer_for(meta) {
            EitherWriter::A(writer) => EitherWriter::A(writer),
            EitherWriter::B(_) => EitherWriter::B(self.or_else.make_writer_for(meta)),
        }
    }
}

// === impl ArcWriter ===

impl<W> io::Write for ArcWriter<W>
where
    for<'a> &'a W: io::Write,
{
    #[inline]
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        (&*self.0).write(buf)
    }

    #[inline]
    fn flush(&mut self) -> io::Result<()> {
        (&*self.0).flush()
    }

    #[inline]
    fn write_vectored(&mut self, bufs: &[io::IoSlice<'_>]) -> io::Result<usize> {
        (&*self.0).write_vectored(bufs)
    }

    #[inline]
    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
        (&*self.0).write_all(buf)
    }

    #[inline]
    fn write_fmt(&mut self, fmt: std::fmt::Arguments<'_>) -> io::Result<()> {
        (&*self.0).write_fmt(fmt)
    }
}

// === impl WriteAdaptor ===

#[cfg(any(feature = "json", feature = "time"))]
impl<'a> WriteAdaptor<'a> {
    pub(in crate::fmt) fn new(fmt_write: &'a mut dyn fmt::Write) -> Self {
        Self { fmt_write }
    }
}
#[cfg(any(feature = "json", feature = "time"))]
impl<'a> io::Write for WriteAdaptor<'a> {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        let s =
            std::str::from_utf8(buf).map_err(|e| io::Error::new(io::ErrorKind::InvalidData, e))?;

        self.fmt_write
            .write_str(s)
            .map_err(|e| io::Error::new(io::ErrorKind::Other, e))?;

        Ok(s.as_bytes().len())
    }

    fn flush(&mut self) -> io::Result<()> {
        Ok(())
    }
}

#[cfg(any(feature = "json", feature = "time"))]
impl<'a> fmt::Debug for WriteAdaptor<'a> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad("WriteAdaptor { .. }")
    }
}
// === blanket impls ===

impl<'a, M> MakeWriterExt<'a> for M where M: MakeWriter<'a> {}
#[cfg(test)]
mod test {
    use super::*;
    use crate::fmt::format::Format;
    use crate::fmt::test::{MockMakeWriter, MockWriter};
    use crate::fmt::Subscriber;
    use std::sync::atomic::{AtomicBool, Ordering};
    use std::sync::{Arc, Mutex};
    use tracing::{debug, error, info, trace, warn, Level};
    use tracing_core::dispatcher::{self, Dispatch};

    fn test_writer<T>(make_writer: T, msg: &str, buf: &Mutex<Vec<u8>>)
    where
        T: for<'writer> MakeWriter<'writer> + Send + Sync + 'static,
    {
        let subscriber = {
            #[cfg(feature = "ansi")]
            let f = Format::default().without_time().with_ansi(false);
            #[cfg(not(feature = "ansi"))]
            let f = Format::default().without_time();
            Subscriber::builder()
                .event_format(f)
                .with_writer(make_writer)
                .finish()
        };
        let dispatch = Dispatch::from(subscriber);

        dispatcher::with_default(&dispatch, || {
            error!("{}", msg);
        });

        let expected = format!("ERROR {}: {}\n", module_path!(), msg);
        let actual = String::from_utf8(buf.try_lock().unwrap().to_vec()).unwrap();
        assert!(actual.contains(expected.as_str()));
    }

    fn has_lines(buf: &Mutex<Vec<u8>>, msgs: &[(tracing::Level, &str)]) {
        let actual = String::from_utf8(buf.try_lock().unwrap().to_vec()).unwrap();
        let mut expected_lines = msgs.iter();
        for line in actual.lines() {
            let line = dbg!(line).trim();
            let (level, msg) = expected_lines
                .next()
                .unwrap_or_else(|| panic!("expected no more lines, but got: {:?}", line));
            let expected = format!("{} {}: {}", level, module_path!(), msg);
            assert_eq!(line, expected.as_str());
        }
    }

    #[test]
    fn custom_writer_closure() {
        let buf = Arc::new(Mutex::new(Vec::new()));
        let buf2 = buf.clone();
        let make_writer = move || MockWriter::new(buf2.clone());
        let msg = "my custom writer closure error";
        test_writer(make_writer, msg, &buf);
    }

    #[test]
    fn custom_writer_struct() {
        let buf = Arc::new(Mutex::new(Vec::new()));
        let make_writer = MockMakeWriter::new(buf.clone());
        let msg = "my custom writer struct error";
        test_writer(make_writer, msg, &buf);
    }

    #[test]
    fn custom_writer_mutex() {
        let buf = Arc::new(Mutex::new(Vec::new()));
        let writer = MockWriter::new(buf.clone());
        let make_writer = Mutex::new(writer);
        let msg = "my mutex writer error";
        test_writer(make_writer, msg, &buf);
    }

    #[test]
    fn combinators_level_filters() {
        let info_buf = Arc::new(Mutex::new(Vec::new()));
        let info = MockMakeWriter::new(info_buf.clone());

        let debug_buf = Arc::new(Mutex::new(Vec::new()));
        let debug = MockMakeWriter::new(debug_buf.clone());

        let warn_buf = Arc::new(Mutex::new(Vec::new()));
        let warn = MockMakeWriter::new(warn_buf.clone());

        let err_buf = Arc::new(Mutex::new(Vec::new()));
        let err = MockMakeWriter::new(err_buf.clone());

        let make_writer = info
            .with_max_level(Level::INFO)
            .and(debug.with_max_level(Level::DEBUG))
            .and(warn.with_max_level(Level::WARN))
            .and(err.with_max_level(Level::ERROR));

        let c = {
            #[cfg(feature = "ansi")]
            let f = Format::default().without_time().with_ansi(false);
            #[cfg(not(feature = "ansi"))]
            let f = Format::default().without_time();
            Subscriber::builder()
                .event_format(f)
                .with_writer(make_writer)
                .with_max_level(Level::TRACE)
                .finish()
        };

        let _s = tracing::subscriber::set_default(c);

        trace!("trace");
        debug!("debug");
        info!("info");
        warn!("warn");
        error!("error");

        let all_lines = [
            (Level::TRACE, "trace"),
            (Level::DEBUG, "debug"),
            (Level::INFO, "info"),
            (Level::WARN, "warn"),
            (Level::ERROR, "error"),
        ];

        println!("max level debug");
        has_lines(&debug_buf, &all_lines[1..]);

        println!("max level info");
        has_lines(&info_buf, &all_lines[2..]);

        println!("max level warn");
        has_lines(&warn_buf, &all_lines[3..]);

        println!("max level error");
        has_lines(&err_buf, &all_lines[4..]);
    }

    #[test]
    fn combinators_or_else() {
        let some_buf = Arc::new(Mutex::new(Vec::new()));
        let some = MockMakeWriter::new(some_buf.clone());

        let or_else_buf = Arc::new(Mutex::new(Vec::new()));
        let or_else = MockMakeWriter::new(or_else_buf.clone());

        let return_some = AtomicBool::new(true);
        let make_writer = move || {
            if return_some.swap(false, Ordering::Relaxed) {
                OptionalWriter::some(some.make_writer())
            } else {
                OptionalWriter::none()
            }
        };
        let make_writer = make_writer.or_else(or_else);
        let c = {
            #[cfg(feature = "ansi")]
            let f = Format::default().without_time().with_ansi(false);
            #[cfg(not(feature = "ansi"))]
            let f = Format::default().without_time();
            Subscriber::builder()
                .event_format(f)
                .with_writer(make_writer)
                .with_max_level(Level::TRACE)
                .finish()
        };

        let _s = tracing::subscriber::set_default(c);
        info!("hello");
        info!("world");
        info!("goodbye");

        has_lines(&some_buf, &[(Level::INFO, "hello")]);
        has_lines(
            &or_else_buf,
            &[(Level::INFO, "world"), (Level::INFO, "goodbye")],
        );
    }

    #[test]
    fn combinators_or_else_chain() {
        let info_buf = Arc::new(Mutex::new(Vec::new()));
        let info = MockMakeWriter::new(info_buf.clone());

        let debug_buf = Arc::new(Mutex::new(Vec::new()));
        let debug = MockMakeWriter::new(debug_buf.clone());

        let warn_buf = Arc::new(Mutex::new(Vec::new()));
        let warn = MockMakeWriter::new(warn_buf.clone());

        let err_buf = Arc::new(Mutex::new(Vec::new()));
        let err = MockMakeWriter::new(err_buf.clone());

        let make_writer = err.with_max_level(Level::ERROR).or_else(
            warn.with_max_level(Level::WARN).or_else(
                info.with_max_level(Level::INFO)
                    .or_else(debug.with_max_level(Level::DEBUG)),
            ),
        );

        let c = {
            #[cfg(feature = "ansi")]
            let f = Format::default().without_time().with_ansi(false);
            #[cfg(not(feature = "ansi"))]
            let f = Format::default().without_time();
            Subscriber::builder()
                .event_format(f)
                .with_writer(make_writer)
                .with_max_level(Level::TRACE)
                .finish()
        };

        let _s = tracing::subscriber::set_default(c);

        trace!("trace");
        debug!("debug");
        info!("info");
        warn!("warn");
        error!("error");

        println!("max level debug");
        has_lines(&debug_buf, &[(Level::DEBUG, "debug")]);

        println!("max level info");
        has_lines(&info_buf, &[(Level::INFO, "info")]);

        println!("max level warn");
        has_lines(&warn_buf, &[(Level::WARN, "warn")]);

        println!("max level error");
        has_lines(&err_buf, &[(Level::ERROR, "error")]);
    }

    #[test]
    fn combinators_and() {
        let a_buf = Arc::new(Mutex::new(Vec::new()));
        let a = MockMakeWriter::new(a_buf.clone());

        let b_buf = Arc::new(Mutex::new(Vec::new()));
        let b = MockMakeWriter::new(b_buf.clone());

        let lines = &[(Level::INFO, "hello"), (Level::INFO, "world")];

        let make_writer = a.and(b);
        let c = {
            #[cfg(feature = "ansi")]
            let f = Format::default().without_time().with_ansi(false);
            #[cfg(not(feature = "ansi"))]
            let f = Format::default().without_time();
            Subscriber::builder()
                .event_format(f)
                .with_writer(make_writer)
                .with_max_level(Level::TRACE)
                .finish()
        };

        let _s = tracing::subscriber::set_default(c);
        info!("hello");
        info!("world");

        has_lines(&a_buf, &lines[..]);
        has_lines(&b_buf, &lines[..]);
    }
}