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
//! Miscellaneous type-system utilities that are too small to deserve their own modules.

use crate::ich::NodeIdHashingMode;
use crate::middle::codegen_fn_attrs::CodegenFnAttrFlags;
use crate::ty::fold::TypeFolder;
use crate::ty::layout::IntegerExt;
use crate::ty::query::TyCtxtAt;
use crate::ty::subst::{GenericArgKind, Subst, SubstsRef};
use crate::ty::TyKind::*;
use crate::ty::{self, DebruijnIndex, DefIdTree, List, Ty, TyCtxt, TypeFoldable};
use rustc_apfloat::Float as _;
use rustc_ast as ast;
use rustc_attr::{self as attr, SignedInt, UnsignedInt};
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
use rustc_errors::ErrorReported;
use rustc_hir as hir;
use rustc_hir::def::DefKind;
use rustc_hir::def_id::DefId;
use rustc_macros::HashStable;
use rustc_span::DUMMY_SP;
use rustc_target::abi::{Integer, Size, TargetDataLayout};
use smallvec::SmallVec;
use std::{fmt, iter};

#[derive(Copy, Clone, Debug)]
pub struct Discr<'tcx> {
    /// Bit representation of the discriminant (e.g., `-128i8` is `0xFF_u128`).
    pub val: u128,
    pub ty: Ty<'tcx>,
}

impl<'tcx> fmt::Display for Discr<'tcx> {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self.ty.kind() {
            ty::Int(ity) => {
                let size = ty::tls::with(|tcx| Integer::from_int_ty(&tcx, ity).size());
                let x = self.val;
                // sign extend the raw representation to be an i128
                let x = size.sign_extend(x) as i128;
                write!(fmt, "{}", x)
            }
            _ => write!(fmt, "{}", self.val),
        }
    }
}

fn int_size_and_signed<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> (Size, bool) {
    let (int, signed) = match *ty.kind() {
        Int(ity) => (Integer::from_int_ty(&tcx, ity), true),
        Uint(uty) => (Integer::from_uint_ty(&tcx, uty), false),
        _ => bug!("non integer discriminant"),
    };
    (int.size(), signed)
}

impl<'tcx> Discr<'tcx> {
    /// Adds `1` to the value and wraps around if the maximum for the type is reached.
    pub fn wrap_incr(self, tcx: TyCtxt<'tcx>) -> Self {
        self.checked_add(tcx, 1).0
    }
    pub fn checked_add(self, tcx: TyCtxt<'tcx>, n: u128) -> (Self, bool) {
        let (size, signed) = int_size_and_signed(tcx, self.ty);
        let (val, oflo) = if signed {
            let min = size.signed_int_min();
            let max = size.signed_int_max();
            let val = size.sign_extend(self.val) as i128;
            assert!(n < (i128::MAX as u128));
            let n = n as i128;
            let oflo = val > max - n;
            let val = if oflo { min + (n - (max - val) - 1) } else { val + n };
            // zero the upper bits
            let val = val as u128;
            let val = size.truncate(val);
            (val, oflo)
        } else {
            let max = size.unsigned_int_max();
            let val = self.val;
            let oflo = val > max - n;
            let val = if oflo { n - (max - val) - 1 } else { val + n };
            (val, oflo)
        };
        (Self { val, ty: self.ty }, oflo)
    }
}

pub trait IntTypeExt {
    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx>;
    fn disr_incr<'tcx>(&self, tcx: TyCtxt<'tcx>, val: Option<Discr<'tcx>>) -> Option<Discr<'tcx>>;
    fn initial_discriminant<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Discr<'tcx>;
}

impl IntTypeExt for attr::IntType {
    fn to_ty<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Ty<'tcx> {
        match *self {
            SignedInt(ast::IntTy::I8) => tcx.types.i8,
            SignedInt(ast::IntTy::I16) => tcx.types.i16,
            SignedInt(ast::IntTy::I32) => tcx.types.i32,
            SignedInt(ast::IntTy::I64) => tcx.types.i64,
            SignedInt(ast::IntTy::I128) => tcx.types.i128,
            SignedInt(ast::IntTy::Isize) => tcx.types.isize,
            UnsignedInt(ast::UintTy::U8) => tcx.types.u8,
            UnsignedInt(ast::UintTy::U16) => tcx.types.u16,
            UnsignedInt(ast::UintTy::U32) => tcx.types.u32,
            UnsignedInt(ast::UintTy::U64) => tcx.types.u64,
            UnsignedInt(ast::UintTy::U128) => tcx.types.u128,
            UnsignedInt(ast::UintTy::Usize) => tcx.types.usize,
        }
    }

    fn initial_discriminant<'tcx>(&self, tcx: TyCtxt<'tcx>) -> Discr<'tcx> {
        Discr { val: 0, ty: self.to_ty(tcx) }
    }

    fn disr_incr<'tcx>(&self, tcx: TyCtxt<'tcx>, val: Option<Discr<'tcx>>) -> Option<Discr<'tcx>> {
        if let Some(val) = val {
            assert_eq!(self.to_ty(tcx), val.ty);
            let (new, oflo) = val.checked_add(tcx, 1);
            if oflo { None } else { Some(new) }
        } else {
            Some(self.initial_discriminant(tcx))
        }
    }
}

impl<'tcx> TyCtxt<'tcx> {
    /// Creates a hash of the type `Ty` which will be the same no matter what crate
    /// context it's calculated within. This is used by the `type_id` intrinsic.
    pub fn type_id_hash(self, ty: Ty<'tcx>) -> u64 {
        let mut hasher = StableHasher::new();
        let mut hcx = self.create_stable_hashing_context();

        // We want the type_id be independent of the types free regions, so we
        // erase them. The erase_regions() call will also anonymize bound
        // regions, which is desirable too.
        let ty = self.erase_regions(ty);

        hcx.while_hashing_spans(false, |hcx| {
            hcx.with_node_id_hashing_mode(NodeIdHashingMode::HashDefPath, |hcx| {
                ty.hash_stable(hcx, &mut hasher);
            });
        });
        hasher.finish()
    }

    pub fn has_error_field(self, ty: Ty<'tcx>) -> bool {
        if let ty::Adt(def, substs) = *ty.kind() {
            for field in def.all_fields() {
                let field_ty = field.ty(self, substs);
                if let Error(_) = field_ty.kind() {
                    return true;
                }
            }
        }
        false
    }

    /// Attempts to returns the deeply last field of nested structures, but
    /// does not apply any normalization in its search. Returns the same type
    /// if input `ty` is not a structure at all.
    pub fn struct_tail_without_normalization(self, ty: Ty<'tcx>) -> Ty<'tcx> {
        let tcx = self;
        tcx.struct_tail_with_normalize(ty, |ty| ty)
    }

    /// Returns the deeply last field of nested structures, or the same type if
    /// not a structure at all. Corresponds to the only possible unsized field,
    /// and its type can be used to determine unsizing strategy.
    ///
    /// Should only be called if `ty` has no inference variables and does not
    /// need its lifetimes preserved (e.g. as part of codegen); otherwise
    /// normalization attempt may cause compiler bugs.
    pub fn struct_tail_erasing_lifetimes(
        self,
        ty: Ty<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
    ) -> Ty<'tcx> {
        let tcx = self;
        tcx.struct_tail_with_normalize(ty, |ty| tcx.normalize_erasing_regions(param_env, ty))
    }

    /// Returns the deeply last field of nested structures, or the same type if
    /// not a structure at all. Corresponds to the only possible unsized field,
    /// and its type can be used to determine unsizing strategy.
    ///
    /// This is parameterized over the normalization strategy (i.e. how to
    /// handle `<T as Trait>::Assoc` and `impl Trait`); pass the identity
    /// function to indicate no normalization should take place.
    ///
    /// See also `struct_tail_erasing_lifetimes`, which is suitable for use
    /// during codegen.
    pub fn struct_tail_with_normalize(
        self,
        mut ty: Ty<'tcx>,
        normalize: impl Fn(Ty<'tcx>) -> Ty<'tcx>,
    ) -> Ty<'tcx> {
        let recursion_limit = self.recursion_limit();
        for iteration in 0.. {
            if !recursion_limit.value_within_limit(iteration) {
                return self.ty_error_with_message(
                    DUMMY_SP,
                    &format!("reached the recursion limit finding the struct tail for {}", ty),
                );
            }
            match *ty.kind() {
                ty::Adt(def, substs) => {
                    if !def.is_struct() {
                        break;
                    }
                    match def.non_enum_variant().fields.last() {
                        Some(f) => ty = f.ty(self, substs),
                        None => break,
                    }
                }

                ty::Tuple(tys) if let Some((&last_ty, _)) = tys.split_last() => {
                    ty = last_ty.expect_ty();
                }

                ty::Tuple(_) => break,

                ty::Projection(_) | ty::Opaque(..) => {
                    let normalized = normalize(ty);
                    if ty == normalized {
                        return ty;
                    } else {
                        ty = normalized;
                    }
                }

                _ => {
                    break;
                }
            }
        }
        ty
    }

    /// Same as applying `struct_tail` on `source` and `target`, but only
    /// keeps going as long as the two types are instances of the same
    /// structure definitions.
    /// For `(Foo<Foo<T>>, Foo<dyn Trait>)`, the result will be `(Foo<T>, Trait)`,
    /// whereas struct_tail produces `T`, and `Trait`, respectively.
    ///
    /// Should only be called if the types have no inference variables and do
    /// not need their lifetimes preserved (e.g., as part of codegen); otherwise,
    /// normalization attempt may cause compiler bugs.
    pub fn struct_lockstep_tails_erasing_lifetimes(
        self,
        source: Ty<'tcx>,
        target: Ty<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
    ) -> (Ty<'tcx>, Ty<'tcx>) {
        let tcx = self;
        tcx.struct_lockstep_tails_with_normalize(source, target, |ty| {
            tcx.normalize_erasing_regions(param_env, ty)
        })
    }

    /// Same as applying `struct_tail` on `source` and `target`, but only
    /// keeps going as long as the two types are instances of the same
    /// structure definitions.
    /// For `(Foo<Foo<T>>, Foo<dyn Trait>)`, the result will be `(Foo<T>, Trait)`,
    /// whereas struct_tail produces `T`, and `Trait`, respectively.
    ///
    /// See also `struct_lockstep_tails_erasing_lifetimes`, which is suitable for use
    /// during codegen.
    pub fn struct_lockstep_tails_with_normalize(
        self,
        source: Ty<'tcx>,
        target: Ty<'tcx>,
        normalize: impl Fn(Ty<'tcx>) -> Ty<'tcx>,
    ) -> (Ty<'tcx>, Ty<'tcx>) {
        let (mut a, mut b) = (source, target);
        loop {
            match (&a.kind(), &b.kind()) {
                (&Adt(a_def, a_substs), &Adt(b_def, b_substs))
                    if a_def == b_def && a_def.is_struct() =>
                {
                    if let Some(f) = a_def.non_enum_variant().fields.last() {
                        a = f.ty(self, a_substs);
                        b = f.ty(self, b_substs);
                    } else {
                        break;
                    }
                }
                (&Tuple(a_tys), &Tuple(b_tys)) if a_tys.len() == b_tys.len() => {
                    if let Some(a_last) = a_tys.last() {
                        a = a_last.expect_ty();
                        b = b_tys.last().unwrap().expect_ty();
                    } else {
                        break;
                    }
                }
                (ty::Projection(_) | ty::Opaque(..), _)
                | (_, ty::Projection(_) | ty::Opaque(..)) => {
                    // If either side is a projection, attempt to
                    // progress via normalization. (Should be safe to
                    // apply to both sides as normalization is
                    // idempotent.)
                    let a_norm = normalize(a);
                    let b_norm = normalize(b);
                    if a == a_norm && b == b_norm {
                        break;
                    } else {
                        a = a_norm;
                        b = b_norm;
                    }
                }

                _ => break,
            }
        }
        (a, b)
    }

    /// Calculate the destructor of a given type.
    pub fn calculate_dtor(
        self,
        adt_did: DefId,
        validate: impl Fn(Self, DefId) -> Result<(), ErrorReported>,
    ) -> Option<ty::Destructor> {
        let drop_trait = self.lang_items().drop_trait()?;
        self.ensure().coherent_trait(drop_trait);

        let ty = self.type_of(adt_did);
        let (did, constness) = self.find_map_relevant_impl(drop_trait, ty, |impl_did| {
            if let Some(item) = self.associated_items(impl_did).in_definition_order().next() {
                if validate(self, impl_did).is_ok() {
                    return Some((item.def_id, self.impl_constness(impl_did)));
                }
            }
            None
        })?;

        Some(ty::Destructor { did, constness })
    }

    /// Returns the set of types that are required to be alive in
    /// order to run the destructor of `def` (see RFCs 769 and
    /// 1238).
    ///
    /// Note that this returns only the constraints for the
    /// destructor of `def` itself. For the destructors of the
    /// contents, you need `adt_dtorck_constraint`.
    pub fn destructor_constraints(self, def: &'tcx ty::AdtDef) -> Vec<ty::subst::GenericArg<'tcx>> {
        let dtor = match def.destructor(self) {
            None => {
                debug!("destructor_constraints({:?}) - no dtor", def.did);
                return vec![];
            }
            Some(dtor) => dtor.did,
        };

        let impl_def_id = self.associated_item(dtor).container.id();
        let impl_generics = self.generics_of(impl_def_id);

        // We have a destructor - all the parameters that are not
        // pure_wrt_drop (i.e, don't have a #[may_dangle] attribute)
        // must be live.

        // We need to return the list of parameters from the ADTs
        // generics/substs that correspond to impure parameters on the
        // impl's generics. This is a bit ugly, but conceptually simple:
        //
        // Suppose our ADT looks like the following
        //
        //     struct S<X, Y, Z>(X, Y, Z);
        //
        // and the impl is
        //
        //     impl<#[may_dangle] P0, P1, P2> Drop for S<P1, P2, P0>
        //
        // We want to return the parameters (X, Y). For that, we match
        // up the item-substs <X, Y, Z> with the substs on the impl ADT,
        // <P1, P2, P0>, and then look up which of the impl substs refer to
        // parameters marked as pure.

        let impl_substs = match *self.type_of(impl_def_id).kind() {
            ty::Adt(def_, substs) if def_ == def => substs,
            _ => bug!(),
        };

        let item_substs = match *self.type_of(def.did).kind() {
            ty::Adt(def_, substs) if def_ == def => substs,
            _ => bug!(),
        };

        let result = iter::zip(item_substs, impl_substs)
            .filter(|&(_, k)| {
                match k.unpack() {
                    GenericArgKind::Lifetime(&ty::RegionKind::ReEarlyBound(ref ebr)) => {
                        !impl_generics.region_param(ebr, self).pure_wrt_drop
                    }
                    GenericArgKind::Type(&ty::TyS { kind: ty::Param(ref pt), .. }) => {
                        !impl_generics.type_param(pt, self).pure_wrt_drop
                    }
                    GenericArgKind::Const(&ty::Const {
                        val: ty::ConstKind::Param(ref pc), ..
                    }) => !impl_generics.const_param(pc, self).pure_wrt_drop,
                    GenericArgKind::Lifetime(_)
                    | GenericArgKind::Type(_)
                    | GenericArgKind::Const(_) => {
                        // Not a type, const or region param: this should be reported
                        // as an error.
                        false
                    }
                }
            })
            .map(|(item_param, _)| item_param)
            .collect();
        debug!("destructor_constraint({:?}) = {:?}", def.did, result);
        result
    }

    /// Returns `true` if `def_id` refers to a closure (e.g., `|x| x * 2`). Note
    /// that closures have a `DefId`, but the closure *expression* also
    /// has a `HirId` that is located within the context where the
    /// closure appears (and, sadly, a corresponding `NodeId`, since
    /// those are not yet phased out). The parent of the closure's
    /// `DefId` will also be the context where it appears.
    pub fn is_closure(self, def_id: DefId) -> bool {
        matches!(self.def_kind(def_id), DefKind::Closure | DefKind::Generator)
    }

    /// Returns `true` if `def_id` refers to a trait (i.e., `trait Foo { ... }`).
    pub fn is_trait(self, def_id: DefId) -> bool {
        self.def_kind(def_id) == DefKind::Trait
    }

    /// Returns `true` if `def_id` refers to a trait alias (i.e., `trait Foo = ...;`),
    /// and `false` otherwise.
    pub fn is_trait_alias(self, def_id: DefId) -> bool {
        self.def_kind(def_id) == DefKind::TraitAlias
    }

    /// Returns `true` if this `DefId` refers to the implicit constructor for
    /// a tuple struct like `struct Foo(u32)`, and `false` otherwise.
    pub fn is_constructor(self, def_id: DefId) -> bool {
        matches!(self.def_kind(def_id), DefKind::Ctor(..))
    }

    /// Given the def-ID of a fn or closure, returns the def-ID of
    /// the innermost fn item that the closure is contained within.
    /// This is a significant `DefId` because, when we do
    /// type-checking, we type-check this fn item and all of its
    /// (transitive) closures together. Therefore, when we fetch the
    /// `typeck` the closure, for example, we really wind up
    /// fetching the `typeck` the enclosing fn item.
    pub fn closure_base_def_id(self, def_id: DefId) -> DefId {
        let mut def_id = def_id;
        while self.is_closure(def_id) {
            def_id = self.parent(def_id).unwrap_or_else(|| {
                bug!("closure {:?} has no parent", def_id);
            });
        }
        def_id
    }

    /// Given the `DefId` and substs a closure, creates the type of
    /// `self` argument that the closure expects. For example, for a
    /// `Fn` closure, this would return a reference type `&T` where
    /// `T = closure_ty`.
    ///
    /// Returns `None` if this closure's kind has not yet been inferred.
    /// This should only be possible during type checking.
    ///
    /// Note that the return value is a late-bound region and hence
    /// wrapped in a binder.
    pub fn closure_env_ty(
        self,
        closure_def_id: DefId,
        closure_substs: SubstsRef<'tcx>,
        env_region: ty::RegionKind,
    ) -> Option<Ty<'tcx>> {
        let closure_ty = self.mk_closure(closure_def_id, closure_substs);
        let closure_kind_ty = closure_substs.as_closure().kind_ty();
        let closure_kind = closure_kind_ty.to_opt_closure_kind()?;
        let env_ty = match closure_kind {
            ty::ClosureKind::Fn => self.mk_imm_ref(self.mk_region(env_region), closure_ty),
            ty::ClosureKind::FnMut => self.mk_mut_ref(self.mk_region(env_region), closure_ty),
            ty::ClosureKind::FnOnce => closure_ty,
        };
        Some(env_ty)
    }

    /// Returns `true` if the node pointed to by `def_id` is a `static` item.
    pub fn is_static(self, def_id: DefId) -> bool {
        self.static_mutability(def_id).is_some()
    }

    /// Returns `true` if this is a `static` item with the `#[thread_local]` attribute.
    pub fn is_thread_local_static(self, def_id: DefId) -> bool {
        self.codegen_fn_attrs(def_id).flags.contains(CodegenFnAttrFlags::THREAD_LOCAL)
    }

    /// Returns `true` if the node pointed to by `def_id` is a mutable `static` item.
    pub fn is_mutable_static(self, def_id: DefId) -> bool {
        self.static_mutability(def_id) == Some(hir::Mutability::Mut)
    }

    /// Get the type of the pointer to the static that we use in MIR.
    pub fn static_ptr_ty(self, def_id: DefId) -> Ty<'tcx> {
        // Make sure that any constants in the static's type are evaluated.
        let static_ty = self.normalize_erasing_regions(ty::ParamEnv::empty(), self.type_of(def_id));

        // Make sure that accesses to unsafe statics end up using raw pointers.
        // For thread-locals, this needs to be kept in sync with `Rvalue::ty`.
        if self.is_mutable_static(def_id) {
            self.mk_mut_ptr(static_ty)
        } else if self.is_foreign_item(def_id) {
            self.mk_imm_ptr(static_ty)
        } else {
            self.mk_imm_ref(self.lifetimes.re_erased, static_ty)
        }
    }

    /// Expands the given impl trait type, stopping if the type is recursive.
    pub fn try_expand_impl_trait_type(
        self,
        def_id: DefId,
        substs: SubstsRef<'tcx>,
    ) -> Result<Ty<'tcx>, Ty<'tcx>> {
        let mut visitor = OpaqueTypeExpander {
            seen_opaque_tys: FxHashSet::default(),
            expanded_cache: FxHashMap::default(),
            primary_def_id: Some(def_id),
            found_recursion: false,
            found_any_recursion: false,
            check_recursion: true,
            tcx: self,
        };

        let expanded_type = visitor.expand_opaque_ty(def_id, substs).unwrap();
        if visitor.found_recursion { Err(expanded_type) } else { Ok(expanded_type) }
    }
}

struct OpaqueTypeExpander<'tcx> {
    // Contains the DefIds of the opaque types that are currently being
    // expanded. When we expand an opaque type we insert the DefId of
    // that type, and when we finish expanding that type we remove the
    // its DefId.
    seen_opaque_tys: FxHashSet<DefId>,
    // Cache of all expansions we've seen so far. This is a critical
    // optimization for some large types produced by async fn trees.
    expanded_cache: FxHashMap<(DefId, SubstsRef<'tcx>), Ty<'tcx>>,
    primary_def_id: Option<DefId>,
    found_recursion: bool,
    found_any_recursion: bool,
    /// Whether or not to check for recursive opaque types.
    /// This is `true` when we're explicitly checking for opaque type
    /// recursion, and 'false' otherwise to avoid unnecessary work.
    check_recursion: bool,
    tcx: TyCtxt<'tcx>,
}

impl<'tcx> OpaqueTypeExpander<'tcx> {
    fn expand_opaque_ty(&mut self, def_id: DefId, substs: SubstsRef<'tcx>) -> Option<Ty<'tcx>> {
        if self.found_any_recursion {
            return None;
        }
        let substs = substs.fold_with(self);
        if !self.check_recursion || self.seen_opaque_tys.insert(def_id) {
            let expanded_ty = match self.expanded_cache.get(&(def_id, substs)) {
                Some(expanded_ty) => expanded_ty,
                None => {
                    let generic_ty = self.tcx.type_of(def_id);
                    let concrete_ty = generic_ty.subst(self.tcx, substs);
                    let expanded_ty = self.fold_ty(concrete_ty);
                    self.expanded_cache.insert((def_id, substs), expanded_ty);
                    expanded_ty
                }
            };
            if self.check_recursion {
                self.seen_opaque_tys.remove(&def_id);
            }
            Some(expanded_ty)
        } else {
            // If another opaque type that we contain is recursive, then it
            // will report the error, so we don't have to.
            self.found_any_recursion = true;
            self.found_recursion = def_id == *self.primary_def_id.as_ref().unwrap();
            None
        }
    }
}

impl<'tcx> TypeFolder<'tcx> for OpaqueTypeExpander<'tcx> {
    fn tcx(&self) -> TyCtxt<'tcx> {
        self.tcx
    }

    fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
        if let ty::Opaque(def_id, substs) = t.kind {
            self.expand_opaque_ty(def_id, substs).unwrap_or(t)
        } else if t.has_opaque_types() {
            t.super_fold_with(self)
        } else {
            t
        }
    }
}

impl<'tcx> ty::TyS<'tcx> {
    /// Returns the maximum value for the given numeric type (including `char`s)
    /// or returns `None` if the type is not numeric.
    pub fn numeric_max_val(&'tcx self, tcx: TyCtxt<'tcx>) -> Option<&'tcx ty::Const<'tcx>> {
        let val = match self.kind() {
            ty::Int(_) | ty::Uint(_) => {
                let (size, signed) = int_size_and_signed(tcx, self);
                let val =
                    if signed { size.signed_int_max() as u128 } else { size.unsigned_int_max() };
                Some(val)
            }
            ty::Char => Some(std::char::MAX as u128),
            ty::Float(fty) => Some(match fty {
                ty::FloatTy::F32 => rustc_apfloat::ieee::Single::INFINITY.to_bits(),
                ty::FloatTy::F64 => rustc_apfloat::ieee::Double::INFINITY.to_bits(),
            }),
            _ => None,
        };
        val.map(|v| ty::Const::from_bits(tcx, v, ty::ParamEnv::empty().and(self)))
    }

    /// Returns the minimum value for the given numeric type (including `char`s)
    /// or returns `None` if the type is not numeric.
    pub fn numeric_min_val(&'tcx self, tcx: TyCtxt<'tcx>) -> Option<&'tcx ty::Const<'tcx>> {
        let val = match self.kind() {
            ty::Int(_) | ty::Uint(_) => {
                let (size, signed) = int_size_and_signed(tcx, self);
                let val = if signed { size.truncate(size.signed_int_min() as u128) } else { 0 };
                Some(val)
            }
            ty::Char => Some(0),
            ty::Float(fty) => Some(match fty {
                ty::FloatTy::F32 => (-::rustc_apfloat::ieee::Single::INFINITY).to_bits(),
                ty::FloatTy::F64 => (-::rustc_apfloat::ieee::Double::INFINITY).to_bits(),
            }),
            _ => None,
        };
        val.map(|v| ty::Const::from_bits(tcx, v, ty::ParamEnv::empty().and(self)))
    }

    /// Checks whether values of this type `T` are *moved* or *copied*
    /// when referenced -- this amounts to a check for whether `T:
    /// Copy`, but note that we **don't** consider lifetimes when
    /// doing this check. This means that we may generate MIR which
    /// does copies even when the type actually doesn't satisfy the
    /// full requirements for the `Copy` trait (cc #29149) -- this
    /// winds up being reported as an error during NLL borrow check.
    pub fn is_copy_modulo_regions(
        &'tcx self,
        tcx_at: TyCtxtAt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
    ) -> bool {
        tcx_at.is_copy_raw(param_env.and(self))
    }

    /// Checks whether values of this type `T` have a size known at
    /// compile time (i.e., whether `T: Sized`). Lifetimes are ignored
    /// for the purposes of this check, so it can be an
    /// over-approximation in generic contexts, where one can have
    /// strange rules like `<T as Foo<'static>>::Bar: Sized` that
    /// actually carry lifetime requirements.
    pub fn is_sized(&'tcx self, tcx_at: TyCtxtAt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
        self.is_trivially_sized(tcx_at.tcx) || tcx_at.is_sized_raw(param_env.and(self))
    }

    /// Checks whether values of this type `T` implement the `Freeze`
    /// trait -- frozen types are those that do not contain an
    /// `UnsafeCell` anywhere. This is a language concept used to
    /// distinguish "true immutability", which is relevant to
    /// optimization as well as the rules around static values. Note
    /// that the `Freeze` trait is not exposed to end users and is
    /// effectively an implementation detail.
    pub fn is_freeze(&'tcx self, tcx_at: TyCtxtAt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
        self.is_trivially_freeze() || tcx_at.is_freeze_raw(param_env.and(self))
    }

    /// Fast path helper for testing if a type is `Freeze`.
    ///
    /// Returning true means the type is known to be `Freeze`. Returning
    /// `false` means nothing -- could be `Freeze`, might not be.
    fn is_trivially_freeze(&self) -> bool {
        match self.kind() {
            ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Bool
            | ty::Char
            | ty::Str
            | ty::Never
            | ty::Ref(..)
            | ty::RawPtr(_)
            | ty::FnDef(..)
            | ty::Error(_)
            | ty::FnPtr(_) => true,
            ty::Tuple(_) => self.tuple_fields().all(Self::is_trivially_freeze),
            ty::Slice(elem_ty) | ty::Array(elem_ty, _) => elem_ty.is_trivially_freeze(),
            ty::Adt(..)
            | ty::Bound(..)
            | ty::Closure(..)
            | ty::Dynamic(..)
            | ty::Foreign(_)
            | ty::Generator(..)
            | ty::GeneratorWitness(_)
            | ty::Infer(_)
            | ty::Opaque(..)
            | ty::Param(_)
            | ty::Placeholder(_)
            | ty::Projection(_) => false,
        }
    }

    /// Checks whether values of this type `T` implement the `Unpin` trait.
    pub fn is_unpin(&'tcx self, tcx_at: TyCtxtAt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
        self.is_trivially_unpin() || tcx_at.is_unpin_raw(param_env.and(self))
    }

    /// Fast path helper for testing if a type is `Unpin`.
    ///
    /// Returning true means the type is known to be `Unpin`. Returning
    /// `false` means nothing -- could be `Unpin`, might not be.
    fn is_trivially_unpin(&self) -> bool {
        match self.kind() {
            ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Bool
            | ty::Char
            | ty::Str
            | ty::Never
            | ty::Ref(..)
            | ty::RawPtr(_)
            | ty::FnDef(..)
            | ty::Error(_)
            | ty::FnPtr(_) => true,
            ty::Tuple(_) => self.tuple_fields().all(Self::is_trivially_unpin),
            ty::Slice(elem_ty) | ty::Array(elem_ty, _) => elem_ty.is_trivially_unpin(),
            ty::Adt(..)
            | ty::Bound(..)
            | ty::Closure(..)
            | ty::Dynamic(..)
            | ty::Foreign(_)
            | ty::Generator(..)
            | ty::GeneratorWitness(_)
            | ty::Infer(_)
            | ty::Opaque(..)
            | ty::Param(_)
            | ty::Placeholder(_)
            | ty::Projection(_) => false,
        }
    }

    /// If `ty.needs_drop(...)` returns `true`, then `ty` is definitely
    /// non-copy and *might* have a destructor attached; if it returns
    /// `false`, then `ty` definitely has no destructor (i.e., no drop glue).
    ///
    /// (Note that this implies that if `ty` has a destructor attached,
    /// then `needs_drop` will definitely return `true` for `ty`.)
    ///
    /// Note that this method is used to check eligible types in unions.
    #[inline]
    pub fn needs_drop(&'tcx self, tcx: TyCtxt<'tcx>, param_env: ty::ParamEnv<'tcx>) -> bool {
        // Avoid querying in simple cases.
        match needs_drop_components(self, &tcx.data_layout) {
            Err(AlwaysRequiresDrop) => true,
            Ok(components) => {
                let query_ty = match *components {
                    [] => return false,
                    // If we've got a single component, call the query with that
                    // to increase the chance that we hit the query cache.
                    [component_ty] => component_ty,
                    _ => self,
                };
                // This doesn't depend on regions, so try to minimize distinct
                // query keys used.
                let erased = tcx.normalize_erasing_regions(param_env, query_ty);
                tcx.needs_drop_raw(param_env.and(erased))
            }
        }
    }

    /// Checks if `ty` has has a significant drop.
    ///
    /// Note that this method can return false even if `ty` has a destructor
    /// attached; even if that is the case then the adt has been marked with
    /// the attribute `rustc_insignificant_dtor`.
    ///
    /// Note that this method is used to check for change in drop order for
    /// 2229 drop reorder migration analysis.
    #[inline]
    pub fn has_significant_drop(
        &'tcx self,
        tcx: TyCtxt<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
    ) -> bool {
        // Avoid querying in simple cases.
        match needs_drop_components(self, &tcx.data_layout) {
            Err(AlwaysRequiresDrop) => true,
            Ok(components) => {
                let query_ty = match *components {
                    [] => return false,
                    // If we've got a single component, call the query with that
                    // to increase the chance that we hit the query cache.
                    [component_ty] => component_ty,
                    _ => self,
                };

                // FIXME(#86868): We should be canonicalizing, or else moving this to a method of inference
                // context, or *something* like that, but for now just avoid passing inference
                // variables to queries that can't cope with them. Instead, conservatively
                // return "true" (may change drop order).
                if query_ty.needs_infer() {
                    return true;
                }

                // This doesn't depend on regions, so try to minimize distinct
                // query keys used.
                let erased = tcx.normalize_erasing_regions(param_env, query_ty);
                tcx.has_significant_drop_raw(param_env.and(erased))
            }
        }
    }

    /// Returns `true` if equality for this type is both reflexive and structural.
    ///
    /// Reflexive equality for a type is indicated by an `Eq` impl for that type.
    ///
    /// Primitive types (`u32`, `str`) have structural equality by definition. For composite data
    /// types, equality for the type as a whole is structural when it is the same as equality
    /// between all components (fields, array elements, etc.) of that type. For ADTs, structural
    /// equality is indicated by an implementation of `PartialStructuralEq` and `StructuralEq` for
    /// that type.
    ///
    /// This function is "shallow" because it may return `true` for a composite type whose fields
    /// are not `StructuralEq`. For example, `[T; 4]` has structural equality regardless of `T`
    /// because equality for arrays is determined by the equality of each array element. If you
    /// want to know whether a given call to `PartialEq::eq` will proceed structurally all the way
    /// down, you will need to use a type visitor.
    #[inline]
    pub fn is_structural_eq_shallow(&'tcx self, tcx: TyCtxt<'tcx>) -> bool {
        match self.kind() {
            // Look for an impl of both `PartialStructuralEq` and `StructuralEq`.
            Adt(..) => tcx.has_structural_eq_impls(self),

            // Primitive types that satisfy `Eq`.
            Bool | Char | Int(_) | Uint(_) | Str | Never => true,

            // Composite types that satisfy `Eq` when all of their fields do.
            //
            // Because this function is "shallow", we return `true` for these composites regardless
            // of the type(s) contained within.
            Ref(..) | Array(..) | Slice(_) | Tuple(..) => true,

            // Raw pointers use bitwise comparison.
            RawPtr(_) | FnPtr(_) => true,

            // Floating point numbers are not `Eq`.
            Float(_) => false,

            // Conservatively return `false` for all others...

            // Anonymous function types
            FnDef(..) | Closure(..) | Dynamic(..) | Generator(..) => false,

            // Generic or inferred types
            //
            // FIXME(ecstaticmorse): Maybe we should `bug` here? This should probably only be
            // called for known, fully-monomorphized types.
            Projection(_) | Opaque(..) | Param(_) | Bound(..) | Placeholder(_) | Infer(_) => false,

            Foreign(_) | GeneratorWitness(..) | Error(_) => false,
        }
    }

    pub fn same_type(a: Ty<'tcx>, b: Ty<'tcx>) -> bool {
        match (&a.kind(), &b.kind()) {
            (&Adt(did_a, substs_a), &Adt(did_b, substs_b)) => {
                if did_a != did_b {
                    return false;
                }

                substs_a.types().zip(substs_b.types()).all(|(a, b)| Self::same_type(a, b))
            }
            _ => a == b,
        }
    }

    /// Peel off all reference types in this type until there are none left.
    ///
    /// This method is idempotent, i.e. `ty.peel_refs().peel_refs() == ty.peel_refs()`.
    ///
    /// # Examples
    ///
    /// - `u8` -> `u8`
    /// - `&'a mut u8` -> `u8`
    /// - `&'a &'b u8` -> `u8`
    /// - `&'a *const &'b u8 -> *const &'b u8`
    pub fn peel_refs(&'tcx self) -> Ty<'tcx> {
        let mut ty = self;
        while let Ref(_, inner_ty, _) = ty.kind() {
            ty = inner_ty;
        }
        ty
    }

    pub fn outer_exclusive_binder(&'tcx self) -> DebruijnIndex {
        self.outer_exclusive_binder
    }
}

pub enum ExplicitSelf<'tcx> {
    ByValue,
    ByReference(ty::Region<'tcx>, hir::Mutability),
    ByRawPointer(hir::Mutability),
    ByBox,
    Other,
}

impl<'tcx> ExplicitSelf<'tcx> {
    /// Categorizes an explicit self declaration like `self: SomeType`
    /// into either `self`, `&self`, `&mut self`, `Box<self>`, or
    /// `Other`.
    /// This is mainly used to require the arbitrary_self_types feature
    /// in the case of `Other`, to improve error messages in the common cases,
    /// and to make `Other` non-object-safe.
    ///
    /// Examples:
    ///
    /// ```
    /// impl<'a> Foo for &'a T {
    ///     // Legal declarations:
    ///     fn method1(self: &&'a T); // ExplicitSelf::ByReference
    ///     fn method2(self: &'a T); // ExplicitSelf::ByValue
    ///     fn method3(self: Box<&'a T>); // ExplicitSelf::ByBox
    ///     fn method4(self: Rc<&'a T>); // ExplicitSelf::Other
    ///
    ///     // Invalid cases will be caught by `check_method_receiver`:
    ///     fn method_err1(self: &'a mut T); // ExplicitSelf::Other
    ///     fn method_err2(self: &'static T) // ExplicitSelf::ByValue
    ///     fn method_err3(self: &&T) // ExplicitSelf::ByReference
    /// }
    /// ```
    ///
    pub fn determine<P>(self_arg_ty: Ty<'tcx>, is_self_ty: P) -> ExplicitSelf<'tcx>
    where
        P: Fn(Ty<'tcx>) -> bool,
    {
        use self::ExplicitSelf::*;

        match *self_arg_ty.kind() {
            _ if is_self_ty(self_arg_ty) => ByValue,
            ty::Ref(region, ty, mutbl) if is_self_ty(ty) => ByReference(region, mutbl),
            ty::RawPtr(ty::TypeAndMut { ty, mutbl }) if is_self_ty(ty) => ByRawPointer(mutbl),
            ty::Adt(def, _) if def.is_box() && is_self_ty(self_arg_ty.boxed_ty()) => ByBox,
            _ => Other,
        }
    }
}

/// Returns a list of types such that the given type needs drop if and only if
/// *any* of the returned types need drop. Returns `Err(AlwaysRequiresDrop)` if
/// this type always needs drop.
pub fn needs_drop_components(
    ty: Ty<'tcx>,
    target_layout: &TargetDataLayout,
) -> Result<SmallVec<[Ty<'tcx>; 2]>, AlwaysRequiresDrop> {
    match ty.kind() {
        ty::Infer(ty::FreshIntTy(_))
        | ty::Infer(ty::FreshFloatTy(_))
        | ty::Bool
        | ty::Int(_)
        | ty::Uint(_)
        | ty::Float(_)
        | ty::Never
        | ty::FnDef(..)
        | ty::FnPtr(_)
        | ty::Char
        | ty::GeneratorWitness(..)
        | ty::RawPtr(_)
        | ty::Ref(..)
        | ty::Str => Ok(SmallVec::new()),

        // Foreign types can never have destructors.
        ty::Foreign(..) => Ok(SmallVec::new()),

        ty::Dynamic(..) | ty::Error(_) => Err(AlwaysRequiresDrop),

        ty::Slice(ty) => needs_drop_components(ty, target_layout),
        ty::Array(elem_ty, size) => {
            match needs_drop_components(elem_ty, target_layout) {
                Ok(v) if v.is_empty() => Ok(v),
                res => match size.val.try_to_bits(target_layout.pointer_size) {
                    // Arrays of size zero don't need drop, even if their element
                    // type does.
                    Some(0) => Ok(SmallVec::new()),
                    Some(_) => res,
                    // We don't know which of the cases above we are in, so
                    // return the whole type and let the caller decide what to
                    // do.
                    None => Ok(smallvec![ty]),
                },
            }
        }
        // If any field needs drop, then the whole tuple does.
        ty::Tuple(..) => ty.tuple_fields().try_fold(SmallVec::new(), move |mut acc, elem| {
            acc.extend(needs_drop_components(elem, target_layout)?);
            Ok(acc)
        }),

        // These require checking for `Copy` bounds or `Adt` destructors.
        ty::Adt(..)
        | ty::Projection(..)
        | ty::Param(_)
        | ty::Bound(..)
        | ty::Placeholder(..)
        | ty::Opaque(..)
        | ty::Infer(_)
        | ty::Closure(..)
        | ty::Generator(..) => Ok(smallvec![ty]),
    }
}

// Does the equivalent of
// ```
// let v = self.iter().map(|p| p.fold_with(folder)).collect::<SmallVec<[_; 8]>>();
// folder.tcx().intern_*(&v)
// ```
pub fn fold_list<'tcx, F, T>(
    list: &'tcx ty::List<T>,
    folder: &mut F,
    intern: impl FnOnce(TyCtxt<'tcx>, &[T]) -> &'tcx ty::List<T>,
) -> &'tcx ty::List<T>
where
    F: TypeFolder<'tcx>,
    T: TypeFoldable<'tcx> + PartialEq + Copy,
{
    let mut iter = list.iter();
    // Look for the first element that changed
    if let Some((i, new_t)) = iter.by_ref().enumerate().find_map(|(i, t)| {
        let new_t = t.fold_with(folder);
        if new_t == t { None } else { Some((i, new_t)) }
    }) {
        // An element changed, prepare to intern the resulting list
        let mut new_list = SmallVec::<[_; 8]>::with_capacity(list.len());
        new_list.extend_from_slice(&list[..i]);
        new_list.push(new_t);
        new_list.extend(iter.map(|t| t.fold_with(folder)));
        intern(folder.tcx(), &new_list)
    } else {
        list
    }
}

#[derive(Copy, Clone, Debug, HashStable, TyEncodable, TyDecodable)]
pub struct AlwaysRequiresDrop;

/// Normalizes all opaque types in the given value, replacing them
/// with their underlying types.
pub fn normalize_opaque_types(
    tcx: TyCtxt<'tcx>,
    val: &'tcx List<ty::Predicate<'tcx>>,
) -> &'tcx List<ty::Predicate<'tcx>> {
    let mut visitor = OpaqueTypeExpander {
        seen_opaque_tys: FxHashSet::default(),
        expanded_cache: FxHashMap::default(),
        primary_def_id: None,
        found_recursion: false,
        found_any_recursion: false,
        check_recursion: false,
        tcx,
    };
    val.fold_with(&mut visitor)
}

pub fn provide(providers: &mut ty::query::Providers) {
    *providers = ty::query::Providers { normalize_opaque_types, ..*providers }
}