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
use std::error;
use std::io;
use std::path::PathBuf;
use std::rc::Rc;
use std::str::Utf8Error;
use std::string::FromUtf8Error;
use std::sync::Arc;
use std::{fmt::Debug, fmt::Display, num::ParseIntError};

use codemap::{File, Span};
use codemap_diagnostic::{ColorConfig, Diagnostic, Emitter, Level, SpanLabel, SpanStyle};
use smol_str::SmolStr;
use xml::writer::Error as XmlError;

use crate::spans::ToSpan;
use crate::value::{CoercionKind, NixString};
use crate::{SourceCode, Value};

/// "CatchableErrorKind" errors -- those which can be detected by
/// `builtins.tryEval`.
///
/// Note: this type is deliberately *not* incorporated as a variant
/// of ErrorKind, because then Result<Value,ErrorKind> would have
/// redundant representations for catchable errors, which would make
/// it too easy to handle errors incorrectly:
///
///   - Ok(Value::Catchable(cek))
///   - Err(ErrorKind::ThisVariantDoesNotExist(cek))
///
/// Because CatchableErrorKind is not a variant of ErrorKind, you
/// will often see functions which return a type like:
///
///   Result<Result<T,CatchableErrorKind>,ErrorKind>
///
/// ... where T is any type other than Value.  This is unfortunate,
/// because Rust's magic `?`-syntax does not work on nested Result
/// values like this.
// TODO(amjoseph): investigate result<T,Either<CatchableErrorKind,ErrorKind>>
#[derive(Clone, Debug)]
pub enum CatchableErrorKind {
    Throw(Box<str>),
    AssertionFailed,
    UnimplementedFeature(Box<str>),
    /// Resolving a user-supplied angle brackets path literal failed in some way.
    NixPathResolution(Box<str>),
}

impl Display for CatchableErrorKind {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            CatchableErrorKind::Throw(s) => write!(f, "error thrown: {}", s),
            CatchableErrorKind::AssertionFailed => write!(f, "assertion failed"),
            CatchableErrorKind::UnimplementedFeature(s) => {
                write!(f, "feature {} is not implemented yet", s)
            }
            CatchableErrorKind::NixPathResolution(s) => {
                write!(f, "Nix path entry could not be resolved: {}", s)
            }
        }
    }
}

#[derive(Clone, Debug)]
pub enum ErrorKind {
    /// These are user-generated errors through builtins.
    Abort(String),

    DivisionByZero,

    DuplicateAttrsKey {
        key: String,
    },

    /// Attempted to specify an invalid key type (e.g. integer) in a
    /// dynamic attribute name.
    InvalidAttributeName(Value),

    AttributeNotFound {
        name: String,
    },

    /// Attempted to index into a list beyond its boundaries.
    IndexOutOfBounds {
        index: i64,
    },

    /// Attempted to call `builtins.tail` on an empty list.
    TailEmptyList,

    TypeError {
        expected: &'static str,
        actual: &'static str,
    },

    Incomparable {
        lhs: &'static str,
        rhs: &'static str,
    },

    /// Resolving a user-supplied relative or home-relative path literal failed in some way.
    RelativePathResolution(String),

    /// Dynamic keys are not allowed in some scopes.
    DynamicKeyInScope(&'static str),

    /// Unknown variable in statically known scope.
    UnknownStaticVariable,

    /// Unknown variable in dynamic scope (with, rec, ...).
    UnknownDynamicVariable(String),

    /// User is defining the same variable twice at the same depth.
    VariableAlreadyDefined(Span),

    /// Attempt to call something that is not callable.
    NotCallable(&'static str),

    /// Infinite recursion encountered while forcing thunks.
    InfiniteRecursion {
        first_force: Span,
        suspended_at: Option<Span>,
        content_span: Option<Span>,
    },

    ParseErrors(Vec<rnix::parser::ParseError>),

    /// An error occured while executing some native code (e.g. a
    /// builtin), and needs to be chained up.
    NativeError {
        gen_type: &'static str,
        err: Box<Error>,
    },

    /// An error occured while executing Tvix bytecode, but needs to
    /// be chained up.
    BytecodeError(Box<Error>),

    /// Given type can't be coerced to a string in the respective context
    NotCoercibleToString {
        from: &'static str,
        kind: CoercionKind,
    },

    /// The given string doesn't represent an absolute path
    NotAnAbsolutePath(PathBuf),

    /// An error occurred when parsing an integer
    ParseIntError(ParseIntError),

    // Errors specific to nested attribute sets and merges thereof.
    /// Nested attributes can not be merged with an inherited value.
    UnmergeableInherit {
        name: SmolStr,
    },

    /// Nested attributes can not be merged with values that are not
    /// literal attribute sets.
    UnmergeableValue,

    /// Parse errors occured while importing a file.
    ImportParseError {
        path: PathBuf,
        file: Arc<File>,
        errors: Vec<rnix::parser::ParseError>,
    },

    /// Compilation errors occured while importing a file.
    ImportCompilerError {
        path: PathBuf,
        errors: Vec<Error>,
    },

    /// I/O errors
    IO {
        path: Option<PathBuf>,
        error: Rc<io::Error>,
    },

    /// Errors parsing JSON, or serializing as JSON.
    JsonError(String),

    /// Nix value that can not be serialised to JSON.
    NotSerialisableToJson(&'static str),

    /// Errors converting TOML to a value
    FromTomlError(String),

    /// An unexpected argument was supplied to a function that takes formal parameters
    UnexpectedArgument {
        arg: NixString,
        formals_span: Span,
    },

    /// Invalid UTF-8 was encoutered somewhere
    Utf8,

    /// Errors while serialising to XML.
    Xml(Rc<XmlError>),

    /// Variant for errors that bubble up to eval from other Tvix
    /// components.
    TvixError(Rc<dyn error::Error>),

    /// Variant for code paths that are known bugs in Tvix (usually
    /// issues with the compiler/VM interaction).
    TvixBug {
        msg: &'static str,
        metadata: Option<Rc<dyn Debug>>,
    },

    /// Tvix internal warning for features triggered by users that are
    /// not actually implemented yet, and without which eval can not
    /// proceed.
    NotImplemented(&'static str),

    /// Internal variant which should disappear during error construction.
    WithContext {
        context: String,
        underlying: Box<ErrorKind>,
    },

    /// Unexpected context string
    UnexpectedContext,

    /// Top-level evaluation result was a catchable Nix error, and
    /// should fail the evaluation.
    ///
    /// This variant **must** only be used at the top-level of
    /// tvix-eval when returning a result to the user, never inside of
    /// eval code.
    CatchableError(CatchableErrorKind),

    /// Invalid hash type specified, must be one of "md5", "sha1", "sha256"
    /// or "sha512"
    UnknownHashType(String),
}

impl error::Error for Error {
    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
        match &self.kind {
            ErrorKind::NativeError { err, .. } | ErrorKind::BytecodeError(err) => err.source(),
            ErrorKind::ParseErrors(err) => err.first().map(|e| e as &dyn error::Error),
            ErrorKind::ParseIntError(err) => Some(err),
            ErrorKind::ImportParseError { errors, .. } => {
                errors.first().map(|e| e as &dyn error::Error)
            }
            ErrorKind::ImportCompilerError { errors, .. } => {
                errors.first().map(|e| e as &dyn error::Error)
            }
            ErrorKind::IO { error, .. } => Some(error.as_ref()),
            ErrorKind::Xml(error) => Some(error.as_ref()),
            ErrorKind::TvixError(error) => Some(error.as_ref()),
            _ => None,
        }
    }
}

impl From<ParseIntError> for ErrorKind {
    fn from(e: ParseIntError) -> Self {
        Self::ParseIntError(e)
    }
}

impl From<Utf8Error> for ErrorKind {
    fn from(_: Utf8Error) -> Self {
        Self::NotImplemented("FromUtf8Error not handled: https://b.tvl.fyi/issues/189")
    }
}

impl From<FromUtf8Error> for ErrorKind {
    fn from(_: FromUtf8Error) -> Self {
        Self::NotImplemented("FromUtf8Error not handled: https://b.tvl.fyi/issues/189")
    }
}

impl From<bstr::Utf8Error> for ErrorKind {
    fn from(_: bstr::Utf8Error) -> Self {
        Self::Utf8
    }
}

impl From<bstr::FromUtf8Error> for ErrorKind {
    fn from(_value: bstr::FromUtf8Error) -> Self {
        Self::Utf8
    }
}

impl From<XmlError> for ErrorKind {
    fn from(err: XmlError) -> Self {
        Self::Xml(Rc::new(err))
    }
}

impl From<io::Error> for ErrorKind {
    fn from(e: io::Error) -> Self {
        ErrorKind::IO {
            path: None,
            error: Rc::new(e),
        }
    }
}

impl From<serde_json::Error> for ErrorKind {
    fn from(err: serde_json::Error) -> Self {
        // Can't just put the `serde_json::Error` in the ErrorKind since it doesn't impl `Clone`
        Self::JsonError(err.to_string())
    }
}

impl From<toml::de::Error> for ErrorKind {
    fn from(err: toml::de::Error) -> Self {
        Self::FromTomlError(format!("error in TOML serialization: {err}"))
    }
}

#[derive(Clone, Debug)]
pub struct Error {
    pub kind: ErrorKind,
    pub span: Span,
    pub contexts: Vec<String>,
    pub source: SourceCode,
}

impl Error {
    pub fn new(mut kind: ErrorKind, span: Span, source: SourceCode) -> Self {
        let mut contexts = vec![];
        while let ErrorKind::WithContext {
            context,
            underlying,
        } = kind
        {
            kind = *underlying;
            contexts.push(context);
        }

        Error {
            kind,
            span,
            contexts,
            source,
        }
    }
}

impl Display for ErrorKind {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match &self {
            ErrorKind::Abort(msg) => write!(f, "evaluation aborted: {}", msg),

            ErrorKind::DivisionByZero => write!(f, "division by zero"),

            ErrorKind::DuplicateAttrsKey { key } => {
                write!(f, "attribute key '{}' already defined", key)
            }

            ErrorKind::InvalidAttributeName(val) => write!(
                f,
                "found attribute name '{}' of type '{}', but attribute names must be strings",
                val,
                val.type_of()
            ),

            ErrorKind::AttributeNotFound { name } => write!(
                f,
                "attribute with name '{}' could not be found in the set",
                name
            ),

            ErrorKind::IndexOutOfBounds { index } => {
                write!(f, "list index '{}' is out of bounds", index)
            }

            ErrorKind::TailEmptyList => write!(f, "'tail' called on an empty list"),

            ErrorKind::TypeError { expected, actual } => write!(
                f,
                "expected value of type '{}', but found a '{}'",
                expected, actual
            ),

            ErrorKind::Incomparable { lhs, rhs } => {
                write!(f, "can not compare a {} with a {}", lhs, rhs)
            }

            ErrorKind::RelativePathResolution(err) => {
                write!(f, "could not resolve path: {}", err)
            }

            ErrorKind::DynamicKeyInScope(scope) => {
                write!(f, "dynamically evaluated keys are not allowed in {}", scope)
            }

            ErrorKind::UnknownStaticVariable => write!(f, "variable not found"),

            ErrorKind::UnknownDynamicVariable(name) => write!(
                f,
                r#"variable '{}' could not be found

Note that this occured within a `with`-expression. The problem may be related
to a missing value in the attribute set(s) included via `with`."#,
                name
            ),

            ErrorKind::VariableAlreadyDefined(_) => write!(f, "variable has already been defined"),

            ErrorKind::NotCallable(other_type) => {
                write!(
                    f,
                    "only functions and builtins can be called, but this is a '{}'",
                    other_type
                )
            }

            ErrorKind::InfiniteRecursion { .. } => write!(f, "infinite recursion encountered"),

            // Errors themselves ignored here & handled in Self::spans instead
            ErrorKind::ParseErrors(_) => write!(f, "failed to parse Nix code:"),

            ErrorKind::NativeError { gen_type, .. } => {
                write!(f, "while evaluating this as native code ({})", gen_type)
            }

            ErrorKind::BytecodeError(_) => write!(f, "while evaluating this Nix code"),

            ErrorKind::NotCoercibleToString { kind, from } => {
                let kindly = if kind.strong { "strongly" } else { "weakly" };

                let hint = if *from == "set" {
                    ", missing a `__toString` or `outPath` attribute"
                } else {
                    ""
                };

                write!(f, "cannot ({kindly}) coerce {from} to a string{hint}")
            }

            ErrorKind::NotAnAbsolutePath(given) => {
                write!(
                    f,
                    "string '{}' does not represent an absolute path",
                    given.to_string_lossy()
                )
            }

            ErrorKind::ParseIntError(err) => {
                write!(f, "invalid integer: {}", err)
            }

            ErrorKind::UnmergeableInherit { name } => {
                write!(
                    f,
                    "cannot merge a nested attribute set into the inherited entry '{}'",
                    name
                )
            }

            ErrorKind::UnmergeableValue => {
                write!(
                    f,
                    "nested attribute sets or keys can only be merged with literal attribute sets"
                )
            }

            // Errors themselves ignored here & handled in Self::spans instead
            ErrorKind::ImportParseError { path, .. } => {
                write!(
                    f,
                    "parse errors occured while importing '{}'",
                    path.to_string_lossy()
                )
            }

            ErrorKind::ImportCompilerError { path, .. } => {
                writeln!(
                    f,
                    "compiler errors occured while importing '{}'",
                    path.to_string_lossy()
                )
            }

            ErrorKind::IO { path, error } => {
                write!(f, "I/O error: ")?;
                if let Some(path) = path {
                    write!(f, "{}: ", path.display())?;
                }
                write!(f, "{error}")
            }

            ErrorKind::JsonError(msg) => {
                write!(f, "Error converting JSON to a Nix value or back: {msg}")
            }

            ErrorKind::NotSerialisableToJson(_type) => {
                write!(f, "a {} cannot be converted to JSON", _type)
            }

            ErrorKind::FromTomlError(msg) => {
                write!(f, "Error converting TOML to a Nix value: {msg}")
            }

            ErrorKind::UnexpectedArgument { arg, .. } => {
                write!(f, "Unexpected argument `{arg}` supplied to function",)
            }

            ErrorKind::Utf8 => {
                write!(f, "Invalid UTF-8 in string")
            }

            ErrorKind::Xml(error) => write!(f, "failed to serialise to XML: {error}"),

            ErrorKind::TvixError(inner_error) => {
                write!(f, "{inner_error}")
            }

            ErrorKind::TvixBug { msg, metadata } => {
                write!(f, "Tvix bug: {}", msg)?;

                if let Some(metadata) = metadata {
                    write!(f, "; metadata: {:?}", metadata)?;
                }

                Ok(())
            }

            ErrorKind::NotImplemented(feature) => {
                write!(f, "feature not yet implemented in Tvix: {}", feature)
            }

            ErrorKind::WithContext { .. } => {
                panic!("internal ErrorKind::WithContext variant leaked")
            }

            ErrorKind::UnexpectedContext => {
                write!(f, "unexpected context string")
            }

            ErrorKind::CatchableError(inner) => {
                write!(f, "{}", inner)
            }

            ErrorKind::UnknownHashType(hash_type) => {
                write!(f, "unknown hash type '{}'", hash_type)
            }
        }
    }
}

impl Display for Error {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.kind)
    }
}

pub type EvalResult<T> = Result<T, Error>;

/// Human-readable names for rnix syntaxes.
fn name_for_syntax(syntax: &rnix::SyntaxKind) -> &'static str {
    match syntax {
        rnix::SyntaxKind::TOKEN_COMMENT => "a comment",
        rnix::SyntaxKind::TOKEN_WHITESPACE => "whitespace",
        rnix::SyntaxKind::TOKEN_ASSERT => "`assert`-keyword",
        rnix::SyntaxKind::TOKEN_ELSE => "`else`-keyword",
        rnix::SyntaxKind::TOKEN_IN => "`in`-keyword",
        rnix::SyntaxKind::TOKEN_IF => "`if`-keyword",
        rnix::SyntaxKind::TOKEN_INHERIT => "`inherit`-keyword",
        rnix::SyntaxKind::TOKEN_LET => "`let`-keyword",
        rnix::SyntaxKind::TOKEN_OR => "`or`-keyword",
        rnix::SyntaxKind::TOKEN_REC => "`rec`-keyword",
        rnix::SyntaxKind::TOKEN_THEN => "`then`-keyword",
        rnix::SyntaxKind::TOKEN_WITH => "`with`-keyword",
        rnix::SyntaxKind::TOKEN_L_BRACE => "{",
        rnix::SyntaxKind::TOKEN_R_BRACE => "}",
        rnix::SyntaxKind::TOKEN_L_BRACK => "[",
        rnix::SyntaxKind::TOKEN_R_BRACK => "]",
        rnix::SyntaxKind::TOKEN_ASSIGN => "=",
        rnix::SyntaxKind::TOKEN_AT => "@",
        rnix::SyntaxKind::TOKEN_COLON => ":",
        rnix::SyntaxKind::TOKEN_COMMA => "`,`",
        rnix::SyntaxKind::TOKEN_DOT => ".",
        rnix::SyntaxKind::TOKEN_ELLIPSIS => "...",
        rnix::SyntaxKind::TOKEN_QUESTION => "?",
        rnix::SyntaxKind::TOKEN_SEMICOLON => ";",
        rnix::SyntaxKind::TOKEN_L_PAREN => "(",
        rnix::SyntaxKind::TOKEN_R_PAREN => ")",
        rnix::SyntaxKind::TOKEN_CONCAT => "++",
        rnix::SyntaxKind::TOKEN_INVERT => "!",
        rnix::SyntaxKind::TOKEN_UPDATE => "//",
        rnix::SyntaxKind::TOKEN_ADD => "+",
        rnix::SyntaxKind::TOKEN_SUB => "-",
        rnix::SyntaxKind::TOKEN_MUL => "*",
        rnix::SyntaxKind::TOKEN_DIV => "/",
        rnix::SyntaxKind::TOKEN_AND_AND => "&&",
        rnix::SyntaxKind::TOKEN_EQUAL => "==",
        rnix::SyntaxKind::TOKEN_IMPLICATION => "->",
        rnix::SyntaxKind::TOKEN_LESS => "<",
        rnix::SyntaxKind::TOKEN_LESS_OR_EQ => "<=",
        rnix::SyntaxKind::TOKEN_MORE => ">",
        rnix::SyntaxKind::TOKEN_MORE_OR_EQ => ">=",
        rnix::SyntaxKind::TOKEN_NOT_EQUAL => "!=",
        rnix::SyntaxKind::TOKEN_OR_OR => "||",
        rnix::SyntaxKind::TOKEN_FLOAT => "a float",
        rnix::SyntaxKind::TOKEN_IDENT => "an identifier",
        rnix::SyntaxKind::TOKEN_INTEGER => "an integer",
        rnix::SyntaxKind::TOKEN_INTERPOL_END => "}",
        rnix::SyntaxKind::TOKEN_INTERPOL_START => "${",
        rnix::SyntaxKind::TOKEN_PATH => "a path",
        rnix::SyntaxKind::TOKEN_URI => "a literal URI",
        rnix::SyntaxKind::TOKEN_STRING_CONTENT => "content of a string",
        rnix::SyntaxKind::TOKEN_STRING_END => "\"",
        rnix::SyntaxKind::TOKEN_STRING_START => "\"",

        rnix::SyntaxKind::NODE_APPLY => "a function application",
        rnix::SyntaxKind::NODE_ASSERT => "an assertion",
        rnix::SyntaxKind::NODE_ATTRPATH => "an attribute path",
        rnix::SyntaxKind::NODE_DYNAMIC => "a dynamic identifier",

        rnix::SyntaxKind::NODE_IDENT => "an identifier",
        rnix::SyntaxKind::NODE_IF_ELSE => "an `if`-expression",
        rnix::SyntaxKind::NODE_SELECT => "a `select`-expression",
        rnix::SyntaxKind::NODE_INHERIT => "inherited values",
        rnix::SyntaxKind::NODE_INHERIT_FROM => "inherited values",
        rnix::SyntaxKind::NODE_STRING => "a string",
        rnix::SyntaxKind::NODE_INTERPOL => "an interpolation",
        rnix::SyntaxKind::NODE_LAMBDA => "a function",
        rnix::SyntaxKind::NODE_IDENT_PARAM => "a function parameter",
        rnix::SyntaxKind::NODE_LEGACY_LET => "a legacy `let`-expression",
        rnix::SyntaxKind::NODE_LET_IN => "a `let`-expression",
        rnix::SyntaxKind::NODE_LIST => "a list",
        rnix::SyntaxKind::NODE_BIN_OP => "a binary operator",
        rnix::SyntaxKind::NODE_PAREN => "a parenthesised expression",
        rnix::SyntaxKind::NODE_PATTERN => "a function argument pattern",
        rnix::SyntaxKind::NODE_PAT_BIND => "an argument pattern binding",
        rnix::SyntaxKind::NODE_PAT_ENTRY => "an argument pattern entry",
        rnix::SyntaxKind::NODE_ROOT => "a Nix expression",
        rnix::SyntaxKind::NODE_ATTR_SET => "an attribute set",
        rnix::SyntaxKind::NODE_ATTRPATH_VALUE => "an attribute set entry",
        rnix::SyntaxKind::NODE_UNARY_OP => "a unary operator",
        rnix::SyntaxKind::NODE_LITERAL => "a literal value",
        rnix::SyntaxKind::NODE_WITH => "a `with`-expression",
        rnix::SyntaxKind::NODE_PATH => "a path",
        rnix::SyntaxKind::NODE_HAS_ATTR => "`?`-operator",

        // TODO(tazjin): unsure what these variants are, lets crash!
        rnix::SyntaxKind::NODE_ERROR => todo!("NODE_ERROR found, tell tazjin!"),
        rnix::SyntaxKind::TOKEN_ERROR => todo!("TOKEN_ERROR found, tell tazjin!"),
        _ => todo!(),
    }
}

/// Construct the string representation for a list of expected parser tokens.
fn expected_syntax(one_of: &[rnix::SyntaxKind]) -> String {
    match one_of.len() {
        0 => "nothing".into(),
        1 => format!("'{}'", name_for_syntax(&one_of[0])),
        _ => {
            let mut out: String = "one of: ".into();
            let end = one_of.len() - 1;

            for (idx, item) in one_of.iter().enumerate() {
                if idx != 0 {
                    out.push_str(", ");
                } else if idx == end {
                    out.push_str(", or ");
                };

                out.push_str(name_for_syntax(item));
            }

            out
        }
    }
}

/// Process a list of parse errors into a set of span labels, annotating parse
/// errors.
fn spans_for_parse_errors(file: &File, errors: &[rnix::parser::ParseError]) -> Vec<SpanLabel> {
    // rnix has a tendency to emit some identical errors more than once, but
    // they do not enhance the user experience necessarily, so we filter them
    // out
    let mut had_eof = false;

    errors
        .iter()
        .enumerate()
        .filter_map(|(idx, err)| {
            let (span, label): (Span, String) = match err {
                rnix::parser::ParseError::Unexpected(range) => (
                    range.span_for(file),
                    "found an unexpected syntax element here".into(),
                ),

                rnix::parser::ParseError::UnexpectedExtra(range) => (
                    range.span_for(file),
                    "found unexpected extra elements at the root of the expression".into(),
                ),

                rnix::parser::ParseError::UnexpectedWanted(found, range, wanted) => {
                    let span = range.span_for(file);
                    (
                        span,
                        format!(
                            "found '{}', but expected {}",
                            name_for_syntax(found),
                            expected_syntax(wanted),
                        ),
                    )
                }

                rnix::parser::ParseError::UnexpectedEOF => {
                    if had_eof {
                        return None;
                    }

                    had_eof = true;

                    (
                        file.span,
                        "code ended unexpectedly while the parser still expected more".into(),
                    )
                }

                rnix::parser::ParseError::UnexpectedEOFWanted(wanted) => {
                    had_eof = true;

                    (
                        file.span,
                        format!(
                            "code ended unexpectedly, but wanted {}",
                            expected_syntax(wanted)
                        ),
                    )
                }

                rnix::parser::ParseError::DuplicatedArgs(range, name) => (
                    range.span_for(file),
                    format!(
                        "the function argument pattern '{}' was bound more than once",
                        name
                    ),
                ),

                rnix::parser::ParseError::RecursionLimitExceeded => (
                    file.span,
                    "this code exceeds the parser's recursion limit, please report a Tvix bug"
                        .to_string(),
                ),

                // TODO: can rnix even still throw this? it's semantic!
                rnix::parser::ParseError::UnexpectedDoubleBind(range) => (
                    range.span_for(file),
                    "this pattern was bound more than once".into(),
                ),

                // The error enum is marked as `#[non_exhaustive]` in rnix,
                // which disables the compiler error for missing a variant. This
                // feature makes it possible for users to miss critical updates
                // of enum variants for a more exciting runtime experience.
                new => todo!("new parse error variant: {}", new),
            };

            Some(SpanLabel {
                span,
                label: Some(label),
                style: if idx == 0 {
                    SpanStyle::Primary
                } else {
                    SpanStyle::Secondary
                },
            })
        })
        .collect()
}

impl Error {
    pub fn fancy_format_str(&self) -> String {
        let mut out = vec![];
        Emitter::vec(&mut out, Some(&*self.source.codemap())).emit(&self.diagnostics());
        String::from_utf8_lossy(&out).to_string()
    }

    /// Render a fancy, human-readable output of this error and print
    /// it to stderr.
    pub fn fancy_format_stderr(&self) {
        Emitter::stderr(ColorConfig::Auto, Some(&*self.source.codemap())).emit(&self.diagnostics());
    }

    /// Create the optional span label displayed as an annotation on
    /// the underlined span of the error.
    fn span_label(&self) -> Option<String> {
        let label = match &self.kind {
            ErrorKind::DuplicateAttrsKey { .. } => "in this attribute set",
            ErrorKind::InvalidAttributeName(_) => "in this attribute set",
            ErrorKind::RelativePathResolution(_) => "in this path literal",
            ErrorKind::UnexpectedArgument { .. } => "in this function call",
            ErrorKind::UnexpectedContext => "in this string",

            // The spans for some errors don't have any more descriptive stuff
            // in them, or we don't utilise it yet.
            ErrorKind::Abort(_)
            | ErrorKind::AttributeNotFound { .. }
            | ErrorKind::IndexOutOfBounds { .. }
            | ErrorKind::TailEmptyList
            | ErrorKind::TypeError { .. }
            | ErrorKind::Incomparable { .. }
            | ErrorKind::DivisionByZero
            | ErrorKind::DynamicKeyInScope(_)
            | ErrorKind::UnknownStaticVariable
            | ErrorKind::UnknownDynamicVariable(_)
            | ErrorKind::VariableAlreadyDefined(_)
            | ErrorKind::NotCallable(_)
            | ErrorKind::InfiniteRecursion { .. }
            | ErrorKind::ParseErrors(_)
            | ErrorKind::NativeError { .. }
            | ErrorKind::BytecodeError(_)
            | ErrorKind::NotCoercibleToString { .. }
            | ErrorKind::NotAnAbsolutePath(_)
            | ErrorKind::ParseIntError(_)
            | ErrorKind::UnmergeableInherit { .. }
            | ErrorKind::UnmergeableValue
            | ErrorKind::ImportParseError { .. }
            | ErrorKind::ImportCompilerError { .. }
            | ErrorKind::IO { .. }
            | ErrorKind::JsonError(_)
            | ErrorKind::NotSerialisableToJson(_)
            | ErrorKind::FromTomlError(_)
            | ErrorKind::Xml(_)
            | ErrorKind::Utf8
            | ErrorKind::TvixError(_)
            | ErrorKind::TvixBug { .. }
            | ErrorKind::NotImplemented(_)
            | ErrorKind::WithContext { .. }
            | ErrorKind::UnknownHashType(_)
            | ErrorKind::CatchableError(_) => return None,
        };

        Some(label.into())
    }

    /// Return the unique error code for this variant which can be
    /// used to refer users to documentation.
    fn code(&self) -> &'static str {
        match self.kind {
            ErrorKind::CatchableError(CatchableErrorKind::Throw(_)) => "E001",
            ErrorKind::Abort(_) => "E002",
            ErrorKind::CatchableError(CatchableErrorKind::AssertionFailed) => "E003",
            ErrorKind::InvalidAttributeName { .. } => "E004",
            ErrorKind::AttributeNotFound { .. } => "E005",
            ErrorKind::TypeError { .. } => "E006",
            ErrorKind::Incomparable { .. } => "E007",
            ErrorKind::CatchableError(CatchableErrorKind::NixPathResolution(_)) => "E008",
            ErrorKind::DynamicKeyInScope(_) => "E009",
            ErrorKind::UnknownStaticVariable => "E010",
            ErrorKind::UnknownDynamicVariable(_) => "E011",
            ErrorKind::VariableAlreadyDefined(_) => "E012",
            ErrorKind::NotCallable(_) => "E013",
            ErrorKind::InfiniteRecursion { .. } => "E014",
            ErrorKind::ParseErrors(_) => "E015",
            ErrorKind::DuplicateAttrsKey { .. } => "E016",
            ErrorKind::NotCoercibleToString { .. } => "E018",
            ErrorKind::IndexOutOfBounds { .. } => "E019",
            ErrorKind::NotAnAbsolutePath(_) => "E020",
            ErrorKind::ParseIntError(_) => "E021",
            ErrorKind::TailEmptyList { .. } => "E023",
            ErrorKind::UnmergeableInherit { .. } => "E024",
            ErrorKind::UnmergeableValue => "E025",
            ErrorKind::ImportParseError { .. } => "E027",
            ErrorKind::ImportCompilerError { .. } => "E028",
            ErrorKind::IO { .. } => "E029",
            ErrorKind::JsonError { .. } => "E030",
            ErrorKind::UnexpectedArgument { .. } => "E031",
            ErrorKind::RelativePathResolution(_) => "E032",
            ErrorKind::DivisionByZero => "E033",
            ErrorKind::Xml(_) => "E034",
            ErrorKind::FromTomlError(_) => "E035",
            ErrorKind::NotSerialisableToJson(_) => "E036",
            ErrorKind::UnexpectedContext => "E037",
            ErrorKind::Utf8 => "E038",
            ErrorKind::UnknownHashType(_) => "E039",

            // Special error code for errors from other Tvix
            // components. We may want to introduce a code namespacing
            // system to have these errors pass codes through.
            ErrorKind::TvixError(_) => "E997",

            // Special error code that is not part of the normal
            // ordering.
            ErrorKind::TvixBug { .. } => "E998",

            // Placeholder error while Tvix is under construction.
            ErrorKind::CatchableError(CatchableErrorKind::UnimplementedFeature(_))
            | ErrorKind::NotImplemented(_) => "E999",

            // Chained errors should yield the code of the innermost
            // error.
            ErrorKind::NativeError { ref err, .. } | ErrorKind::BytecodeError(ref err) => {
                err.code()
            }

            ErrorKind::WithContext { .. } => {
                panic!("internal ErrorKind::WithContext variant leaked")
            }
        }
    }

    fn spans(&self) -> Vec<SpanLabel> {
        let mut spans = match &self.kind {
            ErrorKind::ImportParseError { errors, file, .. } => {
                spans_for_parse_errors(file, errors)
            }

            ErrorKind::ParseErrors(errors) => {
                let file = self.source.get_file(self.span);
                spans_for_parse_errors(&file, errors)
            }

            ErrorKind::UnexpectedArgument { formals_span, .. } => {
                vec![
                    SpanLabel {
                        label: self.span_label(),
                        span: self.span,
                        style: SpanStyle::Primary,
                    },
                    SpanLabel {
                        label: Some("the accepted arguments".into()),
                        span: *formals_span,
                        style: SpanStyle::Secondary,
                    },
                ]
            }

            ErrorKind::InfiniteRecursion {
                first_force,
                suspended_at,
                content_span,
            } => {
                let mut spans = vec![];

                if let Some(content_span) = content_span {
                    spans.push(SpanLabel {
                        label: Some("this lazily-evaluated code".into()),
                        span: *content_span,
                        style: SpanStyle::Secondary,
                    })
                }

                if let Some(suspended_at) = suspended_at {
                    spans.push(SpanLabel {
                        label: Some("which was instantiated here".into()),
                        span: *suspended_at,
                        style: SpanStyle::Secondary,
                    })
                }

                spans.push(SpanLabel {
                    label: Some("was first requested to be evaluated here".into()),
                    span: *first_force,
                    style: SpanStyle::Secondary,
                });

                spans.push(SpanLabel {
                    label: Some("but then requested again here during its own evaluation".into()),
                    span: self.span,
                    style: SpanStyle::Primary,
                });

                spans
            }

            // All other errors pretty much have the same shape.
            _ => {
                vec![SpanLabel {
                    label: self.span_label(),
                    span: self.span,
                    style: SpanStyle::Primary,
                }]
            }
        };

        for ctx in &self.contexts {
            spans.push(SpanLabel {
                label: Some(format!("while {}", ctx)),
                span: self.span,
                style: SpanStyle::Secondary,
            });
        }

        spans
    }

    /// Create the primary diagnostic for a given error.
    fn diagnostic(&self) -> Diagnostic {
        Diagnostic {
            level: Level::Error,
            message: self.to_string(),
            spans: self.spans(),
            code: Some(self.code().into()),
        }
    }

    /// Return the primary diagnostic and all further associated diagnostics (if
    /// any) of an error.
    fn diagnostics(&self) -> Vec<Diagnostic> {
        match &self.kind {
            ErrorKind::ImportCompilerError { errors, .. } => {
                let mut out = vec![self.diagnostic()];
                out.extend(errors.iter().map(|e| e.diagnostic()));
                out
            }

            // When encountering either of these error kinds, we are dealing
            // with the top of an error chain.
            //
            // An error chain creates a list of diagnostics which provide trace
            // information.
            //
            // We don't know how deep this chain is, so we avoid recursing in
            // this function while unrolling the chain.
            ErrorKind::NativeError { err: next, .. } | ErrorKind::BytecodeError(next) => {
                // Accumulated diagnostics to return.
                let mut diagnostics: Vec<Diagnostic> = vec![];

                // The next (inner) error to add to the diagnostics, after this
                // one.
                let mut next = *next.clone();

                // Diagnostic message for *this* error.
                let mut this_message = self.to_string();

                // Primary span for *this* error.
                let mut this_span = self.span;

                // Diagnostic spans for *this* error.
                let mut this_spans = self.spans();

                loop {
                    if is_new_span(
                        this_span,
                        diagnostics.last().and_then(|last| last.spans.last()),
                    ) {
                        diagnostics.push(Diagnostic {
                            level: Level::Note,
                            message: this_message,
                            spans: this_spans,
                            code: None, // only the top-level error has one
                        });
                    }

                    this_message = next.to_string();
                    this_span = next.span;
                    this_spans = next.spans();

                    match next.kind {
                        ErrorKind::NativeError { err: inner, .. }
                        | ErrorKind::BytecodeError(inner) => {
                            next = *inner;
                            continue;
                        }
                        _ => {
                            diagnostics.extend(next.diagnostics());
                            break;
                        }
                    }
                }

                diagnostics
            }

            _ => vec![self.diagnostic()],
        }
    }
}

// Check if this error is in a different span from its immediate ancestor.
fn is_new_span(this_span: Span, parent: Option<&SpanLabel>) -> bool {
    match parent {
        None => true,
        Some(parent) => parent.span != this_span,
    }
}

// Convenience methods to add context on other types.
pub trait AddContext {
    /// Add context to the error-carrying type.
    fn context<S: Into<String>>(self, ctx: S) -> Self;
}

impl AddContext for ErrorKind {
    fn context<S: Into<String>>(self, ctx: S) -> Self {
        ErrorKind::WithContext {
            context: ctx.into(),
            underlying: Box::new(self),
        }
    }
}

impl<T> AddContext for Result<T, ErrorKind> {
    fn context<S: Into<String>>(self, ctx: S) -> Self {
        self.map_err(|kind| kind.context(ctx))
    }
}

impl<T> AddContext for Result<T, Error> {
    fn context<S: Into<String>>(self, ctx: S) -> Self {
        self.map_err(|err| Error {
            kind: err.kind.context(ctx),
            ..err
        })
    }
}