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
//! The `Visitor` responsible for actually checking a `mir::Body` for invalid operations.

use rustc_errors::{Diagnostic, ErrorGuaranteed};
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_index::bit_set::BitSet;
use rustc_infer::infer::TyCtxtInferExt;
use rustc_infer::traits::{ImplSource, Obligation, ObligationCause};
use rustc_middle::mir::visit::{MutatingUseContext, NonMutatingUseContext, PlaceContext, Visitor};
use rustc_middle::mir::*;
use rustc_middle::ty::subst::{GenericArgKind, InternalSubsts};
use rustc_middle::ty::{self, adjustment::PointerCast, Instance, InstanceDef, Ty, TyCtxt};
use rustc_middle::ty::{Binder, TraitPredicate, TraitRef, TypeVisitable};
use rustc_mir_dataflow::{self, Analysis};
use rustc_span::{sym, Span, Symbol};
use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
use rustc_trait_selection::traits::SelectionContext;

use std::mem;
use std::ops::Deref;

use super::ops::{self, NonConstOp, Status};
use super::qualifs::{self, CustomEq, HasMutInterior, NeedsDrop, NeedsNonConstDrop};
use super::resolver::FlowSensitiveAnalysis;
use super::{ConstCx, Qualif};
use crate::const_eval::is_unstable_const_fn;
use crate::errors::UnstableInStable;

type QualifResults<'mir, 'tcx, Q> =
    rustc_mir_dataflow::ResultsCursor<'mir, 'tcx, FlowSensitiveAnalysis<'mir, 'mir, 'tcx, Q>>;

#[derive(Default)]
pub struct Qualifs<'mir, 'tcx> {
    has_mut_interior: Option<QualifResults<'mir, 'tcx, HasMutInterior>>,
    needs_drop: Option<QualifResults<'mir, 'tcx, NeedsDrop>>,
    needs_non_const_drop: Option<QualifResults<'mir, 'tcx, NeedsNonConstDrop>>,
}

impl<'mir, 'tcx> Qualifs<'mir, 'tcx> {
    /// Returns `true` if `local` is `NeedsDrop` at the given `Location`.
    ///
    /// Only updates the cursor if absolutely necessary
    pub fn needs_drop(
        &mut self,
        ccx: &'mir ConstCx<'mir, 'tcx>,
        local: Local,
        location: Location,
    ) -> bool {
        let ty = ccx.body.local_decls[local].ty;
        // Peeking into opaque types causes cycles if the current function declares said opaque
        // type. Thus we avoid short circuiting on the type and instead run the more expensive
        // analysis that looks at the actual usage within this function
        if !ty.has_opaque_types() && !NeedsDrop::in_any_value_of_ty(ccx, ty) {
            return false;
        }

        let needs_drop = self.needs_drop.get_or_insert_with(|| {
            let ConstCx { tcx, body, .. } = *ccx;

            FlowSensitiveAnalysis::new(NeedsDrop, ccx)
                .into_engine(tcx, &body)
                .iterate_to_fixpoint()
                .into_results_cursor(&body)
        });

        needs_drop.seek_before_primary_effect(location);
        needs_drop.get().contains(local)
    }

    /// Returns `true` if `local` is `NeedsNonConstDrop` at the given `Location`.
    ///
    /// Only updates the cursor if absolutely necessary
    pub fn needs_non_const_drop(
        &mut self,
        ccx: &'mir ConstCx<'mir, 'tcx>,
        local: Local,
        location: Location,
    ) -> bool {
        let ty = ccx.body.local_decls[local].ty;
        if !NeedsNonConstDrop::in_any_value_of_ty(ccx, ty) {
            return false;
        }

        let needs_non_const_drop = self.needs_non_const_drop.get_or_insert_with(|| {
            let ConstCx { tcx, body, .. } = *ccx;

            FlowSensitiveAnalysis::new(NeedsNonConstDrop, ccx)
                .into_engine(tcx, &body)
                .iterate_to_fixpoint()
                .into_results_cursor(&body)
        });

        needs_non_const_drop.seek_before_primary_effect(location);
        needs_non_const_drop.get().contains(local)
    }

    /// Returns `true` if `local` is `HasMutInterior` at the given `Location`.
    ///
    /// Only updates the cursor if absolutely necessary.
    pub fn has_mut_interior(
        &mut self,
        ccx: &'mir ConstCx<'mir, 'tcx>,
        local: Local,
        location: Location,
    ) -> bool {
        let ty = ccx.body.local_decls[local].ty;
        // Peeking into opaque types causes cycles if the current function declares said opaque
        // type. Thus we avoid short circuiting on the type and instead run the more expensive
        // analysis that looks at the actual usage within this function
        if !ty.has_opaque_types() && !HasMutInterior::in_any_value_of_ty(ccx, ty) {
            return false;
        }

        let has_mut_interior = self.has_mut_interior.get_or_insert_with(|| {
            let ConstCx { tcx, body, .. } = *ccx;

            FlowSensitiveAnalysis::new(HasMutInterior, ccx)
                .into_engine(tcx, &body)
                .iterate_to_fixpoint()
                .into_results_cursor(&body)
        });

        has_mut_interior.seek_before_primary_effect(location);
        has_mut_interior.get().contains(local)
    }

    fn in_return_place(
        &mut self,
        ccx: &'mir ConstCx<'mir, 'tcx>,
        tainted_by_errors: Option<ErrorGuaranteed>,
    ) -> ConstQualifs {
        // Find the `Return` terminator if one exists.
        //
        // If no `Return` terminator exists, this MIR is divergent. Just return the conservative
        // qualifs for the return type.
        let return_block = ccx
            .body
            .basic_blocks
            .iter_enumerated()
            .find(|(_, block)| matches!(block.terminator().kind, TerminatorKind::Return))
            .map(|(bb, _)| bb);

        let Some(return_block) = return_block else {
            return qualifs::in_any_value_of_ty(ccx, ccx.body.return_ty(), tainted_by_errors);
        };

        let return_loc = ccx.body.terminator_loc(return_block);

        let custom_eq = match ccx.const_kind() {
            // We don't care whether a `const fn` returns a value that is not structurally
            // matchable. Functions calls are opaque and always use type-based qualification, so
            // this value should never be used.
            hir::ConstContext::ConstFn => true,

            // If we know that all values of the return type are structurally matchable, there's no
            // need to run dataflow.
            // Opaque types do not participate in const generics or pattern matching, so we can safely count them out.
            _ if ccx.body.return_ty().has_opaque_types()
                || !CustomEq::in_any_value_of_ty(ccx, ccx.body.return_ty()) =>
            {
                false
            }

            hir::ConstContext::Const | hir::ConstContext::Static(_) => {
                let mut cursor = FlowSensitiveAnalysis::new(CustomEq, ccx)
                    .into_engine(ccx.tcx, &ccx.body)
                    .iterate_to_fixpoint()
                    .into_results_cursor(&ccx.body);

                cursor.seek_after_primary_effect(return_loc);
                cursor.get().contains(RETURN_PLACE)
            }
        };

        ConstQualifs {
            needs_drop: self.needs_drop(ccx, RETURN_PLACE, return_loc),
            needs_non_const_drop: self.needs_non_const_drop(ccx, RETURN_PLACE, return_loc),
            has_mut_interior: self.has_mut_interior(ccx, RETURN_PLACE, return_loc),
            custom_eq,
            tainted_by_errors,
        }
    }
}

pub struct Checker<'mir, 'tcx> {
    ccx: &'mir ConstCx<'mir, 'tcx>,
    qualifs: Qualifs<'mir, 'tcx>,

    /// The span of the current statement.
    span: Span,

    /// A set that stores for each local whether it has a `StorageDead` for it somewhere.
    local_has_storage_dead: Option<BitSet<Local>>,

    error_emitted: Option<ErrorGuaranteed>,
    secondary_errors: Vec<Diagnostic>,
}

impl<'mir, 'tcx> Deref for Checker<'mir, 'tcx> {
    type Target = ConstCx<'mir, 'tcx>;

    fn deref(&self) -> &Self::Target {
        &self.ccx
    }
}

impl<'mir, 'tcx> Checker<'mir, 'tcx> {
    pub fn new(ccx: &'mir ConstCx<'mir, 'tcx>) -> Self {
        Checker {
            span: ccx.body.span,
            ccx,
            qualifs: Default::default(),
            local_has_storage_dead: None,
            error_emitted: None,
            secondary_errors: Vec::new(),
        }
    }

    pub fn check_body(&mut self) {
        let ConstCx { tcx, body, .. } = *self.ccx;
        let def_id = self.ccx.def_id();

        // `async` functions cannot be `const fn`. This is checked during AST lowering, so there's
        // no need to emit duplicate errors here.
        if self.ccx.is_async() || body.generator.is_some() {
            tcx.sess.delay_span_bug(body.span, "`async` functions cannot be `const fn`");
            return;
        }

        // The local type and predicate checks are not free and only relevant for `const fn`s.
        if self.const_kind() == hir::ConstContext::ConstFn {
            for (idx, local) in body.local_decls.iter_enumerated() {
                // Handle the return place below.
                if idx == RETURN_PLACE || local.internal {
                    continue;
                }

                self.span = local.source_info.span;
                self.check_local_or_return_ty(local.ty, idx);
            }

            // impl trait is gone in MIR, so check the return type of a const fn by its signature
            // instead of the type of the return place.
            self.span = body.local_decls[RETURN_PLACE].source_info.span;
            let return_ty = tcx.fn_sig(def_id).output();
            self.check_local_or_return_ty(return_ty.skip_binder(), RETURN_PLACE);
        }

        if !tcx.has_attr(def_id.to_def_id(), sym::rustc_do_not_const_check) {
            self.visit_body(&body);
        }

        // If we got through const-checking without emitting any "primary" errors, emit any
        // "secondary" errors if they occurred.
        let secondary_errors = mem::take(&mut self.secondary_errors);
        if self.error_emitted.is_none() {
            for mut error in secondary_errors {
                self.tcx.sess.diagnostic().emit_diagnostic(&mut error);
            }
        } else {
            assert!(self.tcx.sess.has_errors().is_some());
        }
    }

    fn local_has_storage_dead(&mut self, local: Local) -> bool {
        let ccx = self.ccx;
        self.local_has_storage_dead
            .get_or_insert_with(|| {
                struct StorageDeads {
                    locals: BitSet<Local>,
                }
                impl<'tcx> Visitor<'tcx> for StorageDeads {
                    fn visit_statement(&mut self, stmt: &Statement<'tcx>, _: Location) {
                        if let StatementKind::StorageDead(l) = stmt.kind {
                            self.locals.insert(l);
                        }
                    }
                }
                let mut v = StorageDeads { locals: BitSet::new_empty(ccx.body.local_decls.len()) };
                v.visit_body(ccx.body);
                v.locals
            })
            .contains(local)
    }

    pub fn qualifs_in_return_place(&mut self) -> ConstQualifs {
        self.qualifs.in_return_place(self.ccx, self.error_emitted)
    }

    /// Emits an error if an expression cannot be evaluated in the current context.
    pub fn check_op(&mut self, op: impl NonConstOp<'tcx>) {
        self.check_op_spanned(op, self.span);
    }

    /// Emits an error at the given `span` if an expression cannot be evaluated in the current
    /// context.
    pub fn check_op_spanned<O: NonConstOp<'tcx>>(&mut self, op: O, span: Span) {
        let gate = match op.status_in_item(self.ccx) {
            Status::Allowed => return,

            Status::Unstable(gate) if self.tcx.features().enabled(gate) => {
                let unstable_in_stable = self.ccx.is_const_stable_const_fn()
                    && !super::rustc_allow_const_fn_unstable(self.tcx, self.def_id(), gate);
                if unstable_in_stable {
                    emit_unstable_in_stable_error(self.ccx, span, gate);
                }

                return;
            }

            Status::Unstable(gate) => Some(gate),
            Status::Forbidden => None,
        };

        if self.tcx.sess.opts.unstable_opts.unleash_the_miri_inside_of_you {
            self.tcx.sess.miri_unleashed_feature(span, gate);
            return;
        }

        let mut err = op.build_error(self.ccx, span);
        assert!(err.is_error());

        match op.importance() {
            ops::DiagnosticImportance::Primary => {
                let reported = err.emit();
                self.error_emitted = Some(reported);
            }

            ops::DiagnosticImportance::Secondary => err.buffer(&mut self.secondary_errors),
        }
    }

    fn check_static(&mut self, def_id: DefId, span: Span) {
        if self.tcx.is_thread_local_static(def_id) {
            self.tcx.sess.delay_span_bug(span, "tls access is checked in `Rvalue::ThreadLocalRef");
        }
        self.check_op_spanned(ops::StaticAccess, span)
    }

    fn check_local_or_return_ty(&mut self, ty: Ty<'tcx>, local: Local) {
        let kind = self.body.local_kind(local);

        for ty in ty.walk() {
            let ty = match ty.unpack() {
                GenericArgKind::Type(ty) => ty,

                // No constraints on lifetimes or constants, except potentially
                // constants' types, but `walk` will get to them as well.
                GenericArgKind::Lifetime(_) | GenericArgKind::Const(_) => continue,
            };

            match *ty.kind() {
                ty::Ref(_, _, hir::Mutability::Mut) => self.check_op(ops::ty::MutRef(kind)),
                _ => {}
            }
        }
    }

    fn check_mut_borrow(&mut self, local: Local, kind: hir::BorrowKind) {
        match self.const_kind() {
            // In a const fn all borrows are transient or point to the places given via
            // references in the arguments (so we already checked them with
            // TransientMutBorrow/MutBorrow as appropriate).
            // The borrow checker guarantees that no new non-transient borrows are created.
            // NOTE: Once we have heap allocations during CTFE we need to figure out
            // how to prevent `const fn` to create long-lived allocations that point
            // to mutable memory.
            hir::ConstContext::ConstFn => self.check_op(ops::TransientMutBorrow(kind)),
            _ => {
                // Locals with StorageDead do not live beyond the evaluation and can
                // thus safely be borrowed without being able to be leaked to the final
                // value of the constant.
                if self.local_has_storage_dead(local) {
                    self.check_op(ops::TransientMutBorrow(kind));
                } else {
                    self.check_op(ops::MutBorrow(kind));
                }
            }
        }
    }
}

impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
    fn visit_basic_block_data(&mut self, bb: BasicBlock, block: &BasicBlockData<'tcx>) {
        trace!("visit_basic_block_data: bb={:?} is_cleanup={:?}", bb, block.is_cleanup);

        // We don't const-check basic blocks on the cleanup path since we never unwind during
        // const-eval: a panic causes an immediate compile error. In other words, cleanup blocks
        // are unreachable during const-eval.
        //
        // We can't be more conservative (e.g., by const-checking cleanup blocks anyways) because
        // locals that would never be dropped during normal execution are sometimes dropped during
        // unwinding, which means backwards-incompatible live-drop errors.
        if block.is_cleanup {
            return;
        }

        self.super_basic_block_data(bb, block);
    }

    fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
        trace!("visit_rvalue: rvalue={:?} location={:?}", rvalue, location);

        // Special-case reborrows to be more like a copy of a reference.
        match *rvalue {
            Rvalue::Ref(_, kind, place) => {
                if let Some(reborrowed_place_ref) = place_as_reborrow(self.tcx, self.body, place) {
                    let ctx = match kind {
                        BorrowKind::Shared => {
                            PlaceContext::NonMutatingUse(NonMutatingUseContext::SharedBorrow)
                        }
                        BorrowKind::Shallow => {
                            PlaceContext::NonMutatingUse(NonMutatingUseContext::ShallowBorrow)
                        }
                        BorrowKind::Unique => {
                            PlaceContext::NonMutatingUse(NonMutatingUseContext::UniqueBorrow)
                        }
                        BorrowKind::Mut { .. } => {
                            PlaceContext::MutatingUse(MutatingUseContext::Borrow)
                        }
                    };
                    self.visit_local(reborrowed_place_ref.local, ctx, location);
                    self.visit_projection(reborrowed_place_ref, ctx, location);
                    return;
                }
            }
            Rvalue::AddressOf(mutbl, place) => {
                if let Some(reborrowed_place_ref) = place_as_reborrow(self.tcx, self.body, place) {
                    let ctx = match mutbl {
                        Mutability::Not => {
                            PlaceContext::NonMutatingUse(NonMutatingUseContext::AddressOf)
                        }
                        Mutability::Mut => PlaceContext::MutatingUse(MutatingUseContext::AddressOf),
                    };
                    self.visit_local(reborrowed_place_ref.local, ctx, location);
                    self.visit_projection(reborrowed_place_ref, ctx, location);
                    return;
                }
            }
            _ => {}
        }

        self.super_rvalue(rvalue, location);

        match *rvalue {
            Rvalue::ThreadLocalRef(_) => self.check_op(ops::ThreadLocalAccess),

            Rvalue::Use(_)
            | Rvalue::CopyForDeref(..)
            | Rvalue::Repeat(..)
            | Rvalue::Discriminant(..)
            | Rvalue::Len(_)
            | Rvalue::Aggregate(..) => {}

            Rvalue::Ref(_, kind @ BorrowKind::Mut { .. }, ref place)
            | Rvalue::Ref(_, kind @ BorrowKind::Unique, ref place) => {
                let ty = place.ty(self.body, self.tcx).ty;
                let is_allowed = match ty.kind() {
                    // Inside a `static mut`, `&mut [...]` is allowed.
                    ty::Array(..) | ty::Slice(_)
                        if self.const_kind() == hir::ConstContext::Static(hir::Mutability::Mut) =>
                    {
                        true
                    }

                    // FIXME(ecstaticmorse): We could allow `&mut []` inside a const context given
                    // that this is merely a ZST and it is already eligible for promotion.
                    // This may require an RFC?
                    /*
                    ty::Array(_, len) if len.try_eval_usize(cx.tcx, cx.param_env) == Some(0)
                        => true,
                    */
                    _ => false,
                };

                if !is_allowed {
                    if let BorrowKind::Mut { .. } = kind {
                        self.check_mut_borrow(place.local, hir::BorrowKind::Ref)
                    } else {
                        self.check_op(ops::CellBorrow);
                    }
                }
            }

            Rvalue::AddressOf(Mutability::Mut, ref place) => {
                self.check_mut_borrow(place.local, hir::BorrowKind::Raw)
            }

            Rvalue::Ref(_, BorrowKind::Shared | BorrowKind::Shallow, ref place)
            | Rvalue::AddressOf(Mutability::Not, ref place) => {
                let borrowed_place_has_mut_interior = qualifs::in_place::<HasMutInterior, _>(
                    &self.ccx,
                    &mut |local| self.qualifs.has_mut_interior(self.ccx, local, location),
                    place.as_ref(),
                );

                if borrowed_place_has_mut_interior {
                    match self.const_kind() {
                        // In a const fn all borrows are transient or point to the places given via
                        // references in the arguments (so we already checked them with
                        // TransientCellBorrow/CellBorrow as appropriate).
                        // The borrow checker guarantees that no new non-transient borrows are created.
                        // NOTE: Once we have heap allocations during CTFE we need to figure out
                        // how to prevent `const fn` to create long-lived allocations that point
                        // to (interior) mutable memory.
                        hir::ConstContext::ConstFn => self.check_op(ops::TransientCellBorrow),
                        _ => {
                            // Locals with StorageDead are definitely not part of the final constant value, and
                            // it is thus inherently safe to permit such locals to have their
                            // address taken as we can't end up with a reference to them in the
                            // final value.
                            // Note: This is only sound if every local that has a `StorageDead` has a
                            // `StorageDead` in every control flow path leading to a `return` terminator.
                            if self.local_has_storage_dead(place.local) {
                                self.check_op(ops::TransientCellBorrow);
                            } else {
                                self.check_op(ops::CellBorrow);
                            }
                        }
                    }
                }
            }

            Rvalue::Cast(
                CastKind::Pointer(
                    PointerCast::MutToConstPointer
                    | PointerCast::ArrayToPointer
                    | PointerCast::UnsafeFnPointer
                    | PointerCast::ClosureFnPointer(_)
                    | PointerCast::ReifyFnPointer,
                ),
                _,
                _,
            ) => {
                // These are all okay; they only change the type, not the data.
            }

            Rvalue::Cast(CastKind::Pointer(PointerCast::Unsize), _, _) => {
                // Unsizing is implemented for CTFE.
            }

            Rvalue::Cast(CastKind::PointerExposeAddress, _, _) => {
                self.check_op(ops::RawPtrToIntCast);
            }
            Rvalue::Cast(CastKind::PointerFromExposedAddress, _, _) => {
                // Since no pointer can ever get exposed (rejected above), this is easy to support.
            }

            Rvalue::Cast(CastKind::DynStar, _, _) => {
                unimplemented!()
            }

            Rvalue::Cast(CastKind::Misc, _, _) => {}

            Rvalue::NullaryOp(NullOp::SizeOf | NullOp::AlignOf, _) => {}
            Rvalue::ShallowInitBox(_, _) => {}

            Rvalue::UnaryOp(_, ref operand) => {
                let ty = operand.ty(self.body, self.tcx);
                if is_int_bool_or_char(ty) {
                    // Int, bool, and char operations are fine.
                } else if ty.is_floating_point() {
                    self.check_op(ops::FloatingPointOp);
                } else {
                    span_bug!(self.span, "non-primitive type in `Rvalue::UnaryOp`: {:?}", ty);
                }
            }

            Rvalue::BinaryOp(op, box (ref lhs, ref rhs))
            | Rvalue::CheckedBinaryOp(op, box (ref lhs, ref rhs)) => {
                let lhs_ty = lhs.ty(self.body, self.tcx);
                let rhs_ty = rhs.ty(self.body, self.tcx);

                if is_int_bool_or_char(lhs_ty) && is_int_bool_or_char(rhs_ty) {
                    // Int, bool, and char operations are fine.
                } else if lhs_ty.is_fn_ptr() || lhs_ty.is_unsafe_ptr() {
                    assert_eq!(lhs_ty, rhs_ty);
                    assert!(
                        op == BinOp::Eq
                            || op == BinOp::Ne
                            || op == BinOp::Le
                            || op == BinOp::Lt
                            || op == BinOp::Ge
                            || op == BinOp::Gt
                            || op == BinOp::Offset
                    );

                    self.check_op(ops::RawPtrComparison);
                } else if lhs_ty.is_floating_point() || rhs_ty.is_floating_point() {
                    self.check_op(ops::FloatingPointOp);
                } else {
                    span_bug!(
                        self.span,
                        "non-primitive type in `Rvalue::BinaryOp`: {:?} ⚬ {:?}",
                        lhs_ty,
                        rhs_ty
                    );
                }
            }
        }
    }

    fn visit_operand(&mut self, op: &Operand<'tcx>, location: Location) {
        self.super_operand(op, location);
        if let Operand::Constant(c) = op {
            if let Some(def_id) = c.check_static_ptr(self.tcx) {
                self.check_static(def_id, self.span);
            }
        }
    }
    fn visit_projection_elem(
        &mut self,
        place_local: Local,
        proj_base: &[PlaceElem<'tcx>],
        elem: PlaceElem<'tcx>,
        context: PlaceContext,
        location: Location,
    ) {
        trace!(
            "visit_projection_elem: place_local={:?} proj_base={:?} elem={:?} \
            context={:?} location={:?}",
            place_local,
            proj_base,
            elem,
            context,
            location,
        );

        self.super_projection_elem(place_local, proj_base, elem, context, location);

        match elem {
            ProjectionElem::Deref => {
                let base_ty = Place::ty_from(place_local, proj_base, self.body, self.tcx).ty;
                if base_ty.is_unsafe_ptr() {
                    if proj_base.is_empty() {
                        let decl = &self.body.local_decls[place_local];
                        if let Some(box LocalInfo::StaticRef { def_id, .. }) = decl.local_info {
                            let span = decl.source_info.span;
                            self.check_static(def_id, span);
                            return;
                        }
                    }

                    // `*const T` is stable, `*mut T` is not
                    if !base_ty.is_mutable_ptr() {
                        return;
                    }

                    self.check_op(ops::RawMutPtrDeref);
                }

                if context.is_mutating_use() {
                    self.check_op(ops::MutDeref);
                }
            }

            ProjectionElem::ConstantIndex { .. }
            | ProjectionElem::Downcast(..)
            | ProjectionElem::Subslice { .. }
            | ProjectionElem::Field(..)
            | ProjectionElem::Index(_) => {}
        }
    }

    fn visit_source_info(&mut self, source_info: &SourceInfo) {
        trace!("visit_source_info: source_info={:?}", source_info);
        self.span = source_info.span;
    }

    fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
        trace!("visit_statement: statement={:?} location={:?}", statement, location);

        self.super_statement(statement, location);

        match statement.kind {
            StatementKind::Assign(..)
            | StatementKind::SetDiscriminant { .. }
            | StatementKind::Deinit(..)
            | StatementKind::FakeRead(..)
            | StatementKind::StorageLive(_)
            | StatementKind::StorageDead(_)
            | StatementKind::Retag { .. }
            | StatementKind::AscribeUserType(..)
            | StatementKind::Coverage(..)
            | StatementKind::Intrinsic(..)
            | StatementKind::Nop => {}
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location) {
        self.super_terminator(terminator, location);

        match &terminator.kind {
            TerminatorKind::Call { func, args, fn_span, from_hir_call, .. } => {
                let ConstCx { tcx, body, param_env, .. } = *self.ccx;
                let caller = self.def_id();

                let fn_ty = func.ty(body, tcx);

                let (mut callee, mut substs) = match *fn_ty.kind() {
                    ty::FnDef(def_id, substs) => (def_id, substs),

                    ty::FnPtr(_) => {
                        self.check_op(ops::FnCallIndirect);
                        return;
                    }
                    _ => {
                        span_bug!(terminator.source_info.span, "invalid callee of type {:?}", fn_ty)
                    }
                };

                // Attempting to call a trait method?
                if let Some(trait_id) = tcx.trait_of_item(callee) {
                    trace!("attempting to call a trait method");
                    if !self.tcx.features().const_trait_impl {
                        self.check_op(ops::FnCallNonConst {
                            caller,
                            callee,
                            substs,
                            span: *fn_span,
                            from_hir_call: *from_hir_call,
                        });
                        return;
                    }

                    let trait_ref = TraitRef::from_method(tcx, trait_id, substs);
                    let poly_trait_pred = Binder::dummy(TraitPredicate {
                        trait_ref,
                        constness: ty::BoundConstness::ConstIfConst,
                        polarity: ty::ImplPolarity::Positive,
                    });
                    let obligation =
                        Obligation::new(ObligationCause::dummy(), param_env, poly_trait_pred);

                    let implsrc = tcx.infer_ctxt().enter(|infcx| {
                        let mut selcx = SelectionContext::new(&infcx);
                        selcx.select(&obligation)
                    });

                    match implsrc {
                        Ok(Some(ImplSource::Param(_, ty::BoundConstness::ConstIfConst))) => {
                            debug!(
                                "const_trait_impl: provided {:?} via where-clause in {:?}",
                                trait_ref, param_env
                            );
                            return;
                        }
                        Ok(Some(ImplSource::UserDefined(data))) => {
                            let callee_name = tcx.item_name(callee);
                            if let Some(&did) = tcx
                                .associated_item_def_ids(data.impl_def_id)
                                .iter()
                                .find(|did| tcx.item_name(**did) == callee_name)
                            {
                                // using internal substs is ok here, since this is only
                                // used for the `resolve` call below
                                substs = InternalSubsts::identity_for_item(tcx, did);
                                callee = did;
                            }

                            if let hir::Constness::NotConst = tcx.constness(data.impl_def_id) {
                                self.check_op(ops::FnCallNonConst {
                                    caller,
                                    callee,
                                    substs,
                                    span: *fn_span,
                                    from_hir_call: *from_hir_call,
                                });
                                return;
                            }
                        }
                        _ if !tcx.is_const_fn_raw(callee) => {
                            // At this point, it is only legal when the caller is in a trait
                            // marked with #[const_trait], and the callee is in the same trait.
                            let mut nonconst_call_permission = false;
                            if let Some(callee_trait) = tcx.trait_of_item(callee)
                                && tcx.has_attr(callee_trait, sym::const_trait)
                                && Some(callee_trait) == tcx.trait_of_item(caller.to_def_id())
                                // Can only call methods when it's `<Self as TheTrait>::f`.
                                && tcx.types.self_param == substs.type_at(0)
                            {
                                nonconst_call_permission = true;
                            }

                            if !nonconst_call_permission {
                                let obligation = Obligation::new(
                                    ObligationCause::dummy_with_span(*fn_span),
                                    param_env,
                                    tcx.mk_predicate(
                                        poly_trait_pred.map_bound(ty::PredicateKind::Trait),
                                    ),
                                );

                                // improve diagnostics by showing what failed. Our requirements are stricter this time
                                // as we are going to error again anyways.
                                tcx.infer_ctxt().enter(|infcx| {
                                    if let Err(e) = implsrc {
                                        infcx.report_selection_error(
                                            obligation.clone(),
                                            &obligation,
                                            &e,
                                            false,
                                        );
                                    }
                                });

                                self.check_op(ops::FnCallNonConst {
                                    caller,
                                    callee,
                                    substs,
                                    span: *fn_span,
                                    from_hir_call: *from_hir_call,
                                });
                                return;
                            }
                        }
                        _ => {}
                    }

                    // Resolve a trait method call to its concrete implementation, which may be in a
                    // `const` trait impl.
                    let instance = Instance::resolve(tcx, param_env, callee, substs);
                    debug!("Resolving ({:?}) -> {:?}", callee, instance);
                    if let Ok(Some(func)) = instance {
                        if let InstanceDef::Item(def) = func.def {
                            callee = def.did;
                        }
                    }
                }

                // At this point, we are calling a function, `callee`, whose `DefId` is known...

                // `begin_panic` and `panic_display` are generic functions that accept
                // types other than str. Check to enforce that only str can be used in
                // const-eval.

                // const-eval of the `begin_panic` fn assumes the argument is `&str`
                if Some(callee) == tcx.lang_items().begin_panic_fn() {
                    match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
                        ty::Ref(_, ty, _) if ty.is_str() => return,
                        _ => self.check_op(ops::PanicNonStr),
                    }
                }

                // const-eval of the `panic_display` fn assumes the argument is `&&str`
                if Some(callee) == tcx.lang_items().panic_display() {
                    match args[0].ty(&self.ccx.body.local_decls, tcx).kind() {
                        ty::Ref(_, ty, _) if matches!(ty.kind(), ty::Ref(_, ty, _) if ty.is_str()) =>
                        {
                            return;
                        }
                        _ => self.check_op(ops::PanicNonStr),
                    }
                }

                if Some(callee) == tcx.lang_items().exchange_malloc_fn() {
                    self.check_op(ops::HeapAllocation);
                    return;
                }

                // `async` blocks get lowered to `std::future::from_generator(/* a closure */)`.
                let is_async_block = Some(callee) == tcx.lang_items().from_generator_fn();
                if is_async_block {
                    let kind = hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block);
                    self.check_op(ops::Generator(kind));
                    return;
                }

                let is_intrinsic = tcx.is_intrinsic(callee);

                if !tcx.is_const_fn_raw(callee) {
                    if !tcx.is_const_default_method(callee) {
                        // To get to here we must have already found a const impl for the
                        // trait, but for it to still be non-const can be that the impl is
                        // using default method bodies.
                        self.check_op(ops::FnCallNonConst {
                            caller,
                            callee,
                            substs,
                            span: *fn_span,
                            from_hir_call: *from_hir_call,
                        });
                        return;
                    }
                }

                // If the `const fn` we are trying to call is not const-stable, ensure that we have
                // the proper feature gate enabled.
                if let Some(gate) = is_unstable_const_fn(tcx, callee) {
                    trace!(?gate, "calling unstable const fn");
                    if self.span.allows_unstable(gate) {
                        return;
                    }

                    // Calling an unstable function *always* requires that the corresponding gate
                    // be enabled, even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
                    if !tcx.features().declared_lib_features.iter().any(|&(sym, _)| sym == gate) {
                        self.check_op(ops::FnCallUnstable(callee, Some(gate)));
                        return;
                    }

                    // If this crate is not using stability attributes, or the caller is not claiming to be a
                    // stable `const fn`, that is all that is required.
                    if !self.ccx.is_const_stable_const_fn() {
                        trace!("crate not using stability attributes or caller not stably const");
                        return;
                    }

                    // Otherwise, we are something const-stable calling a const-unstable fn.

                    if super::rustc_allow_const_fn_unstable(tcx, caller, gate) {
                        trace!("rustc_allow_const_fn_unstable gate active");
                        return;
                    }

                    self.check_op(ops::FnCallUnstable(callee, Some(gate)));
                    return;
                }

                // FIXME(ecstaticmorse); For compatibility, we consider `unstable` callees that
                // have no `rustc_const_stable` attributes to be const-unstable as well. This
                // should be fixed later.
                let callee_is_unstable_unmarked = tcx.lookup_const_stability(callee).is_none()
                    && tcx.lookup_stability(callee).map_or(false, |s| s.is_unstable());
                if callee_is_unstable_unmarked {
                    trace!("callee_is_unstable_unmarked");
                    // We do not use `const` modifiers for intrinsic "functions", as intrinsics are
                    // `extern` functions, and these have no way to get marked `const`. So instead we
                    // use `rustc_const_(un)stable` attributes to mean that the intrinsic is `const`
                    if self.ccx.is_const_stable_const_fn() || is_intrinsic {
                        self.check_op(ops::FnCallUnstable(callee, None));
                        return;
                    }
                }
                trace!("permitting call");
            }

            // Forbid all `Drop` terminators unless the place being dropped is a local with no
            // projections that cannot be `NeedsNonConstDrop`.
            TerminatorKind::Drop { place: dropped_place, .. }
            | TerminatorKind::DropAndReplace { place: dropped_place, .. } => {
                // If we are checking live drops after drop-elaboration, don't emit duplicate
                // errors here.
                if super::post_drop_elaboration::checking_enabled(self.ccx) {
                    return;
                }

                let mut err_span = self.span;
                let ty_of_dropped_place = dropped_place.ty(self.body, self.tcx).ty;

                let ty_needs_non_const_drop =
                    qualifs::NeedsNonConstDrop::in_any_value_of_ty(self.ccx, ty_of_dropped_place);

                debug!(?ty_of_dropped_place, ?ty_needs_non_const_drop);

                if !ty_needs_non_const_drop {
                    return;
                }

                let needs_non_const_drop = if let Some(local) = dropped_place.as_local() {
                    // Use the span where the local was declared as the span of the drop error.
                    err_span = self.body.local_decls[local].source_info.span;
                    self.qualifs.needs_non_const_drop(self.ccx, local, location)
                } else {
                    true
                };

                if needs_non_const_drop {
                    self.check_op_spanned(
                        ops::LiveDrop { dropped_at: Some(terminator.source_info.span) },
                        err_span,
                    );
                }
            }

            TerminatorKind::InlineAsm { .. } => self.check_op(ops::InlineAsm),

            TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
                self.check_op(ops::Generator(hir::GeneratorKind::Gen))
            }

            TerminatorKind::Abort => {
                // Cleanup blocks are skipped for const checking (see `visit_basic_block_data`).
                span_bug!(self.span, "`Abort` terminator outside of cleanup block")
            }

            TerminatorKind::Assert { .. }
            | TerminatorKind::FalseEdge { .. }
            | TerminatorKind::FalseUnwind { .. }
            | TerminatorKind::Goto { .. }
            | TerminatorKind::Resume
            | TerminatorKind::Return
            | TerminatorKind::SwitchInt { .. }
            | TerminatorKind::Unreachable => {}
        }
    }
}

fn place_as_reborrow<'tcx>(
    tcx: TyCtxt<'tcx>,
    body: &Body<'tcx>,
    place: Place<'tcx>,
) -> Option<PlaceRef<'tcx>> {
    match place.as_ref().last_projection() {
        Some((place_base, ProjectionElem::Deref)) => {
            // A borrow of a `static` also looks like `&(*_1)` in the MIR, but `_1` is a `const`
            // that points to the allocation for the static. Don't treat these as reborrows.
            if body.local_decls[place_base.local].is_ref_to_static() {
                None
            } else {
                // Ensure the type being derefed is a reference and not a raw pointer.
                // This is sufficient to prevent an access to a `static mut` from being marked as a
                // reborrow, even if the check above were to disappear.
                let inner_ty = place_base.ty(body, tcx).ty;

                if let ty::Ref(..) = inner_ty.kind() {
                    return Some(place_base);
                } else {
                    return None;
                }
            }
        }
        _ => None,
    }
}

fn is_int_bool_or_char(ty: Ty<'_>) -> bool {
    ty.is_bool() || ty.is_integral() || ty.is_char()
}

fn emit_unstable_in_stable_error(ccx: &ConstCx<'_, '_>, span: Span, gate: Symbol) {
    let attr_span = ccx.tcx.def_span(ccx.def_id()).shrink_to_lo();

    ccx.tcx.sess.emit_err(UnstableInStable { gate: gate.to_string(), span, attr_span });
}