1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
//! Code for type-checking closure expressions.

use std::iter;
use std::ops::ControlFlow;

use rustc_errors::ErrorGuaranteed;
use rustc_hir as hir;
use rustc_hir::lang_items::LangItem;
use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
use rustc_infer::infer::{BoundRegionConversionTime, DefineOpaqueTypes, InferOk, InferResult};
use rustc_infer::traits::ObligationCauseCode;
use rustc_macros::{TypeFoldable, TypeVisitable};
use rustc_middle::span_bug;
use rustc_middle::ty::visit::{TypeVisitable, TypeVisitableExt};
use rustc_middle::ty::{self, GenericArgs, Ty, TyCtxt, TypeSuperVisitable, TypeVisitor};
use rustc_span::def_id::LocalDefId;
use rustc_span::{Span, DUMMY_SP};
use rustc_target::spec::abi::Abi;
use rustc_trait_selection::error_reporting::traits::ArgKind;
use rustc_trait_selection::traits;
use rustc_type_ir::ClosureKind;
use tracing::{debug, instrument, trace};

use super::{check_fn, CoroutineTypes, Expectation, FnCtxt};

/// What signature do we *expect* the closure to have from context?
#[derive(Debug, Clone, TypeFoldable, TypeVisitable)]
struct ExpectedSig<'tcx> {
    /// Span that gave us this expectation, if we know that.
    cause_span: Option<Span>,
    sig: ty::PolyFnSig<'tcx>,
}

#[derive(Debug)]
struct ClosureSignatures<'tcx> {
    /// The signature users of the closure see.
    bound_sig: ty::PolyFnSig<'tcx>,
    /// The signature within the function body.
    /// This mostly differs in the sense that lifetimes are now early bound and any
    /// opaque types from the signature expectation are overridden in case there are
    /// explicit hidden types written by the user in the closure signature.
    liberated_sig: ty::FnSig<'tcx>,
}

impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
    #[instrument(skip(self, closure), level = "debug")]
    pub fn check_expr_closure(
        &self,
        closure: &hir::Closure<'tcx>,
        expr_span: Span,
        expected: Expectation<'tcx>,
    ) -> Ty<'tcx> {
        let tcx = self.tcx;
        let body = tcx.hir().body(closure.body);
        let expr_def_id = closure.def_id;

        // It's always helpful for inference if we know the kind of
        // closure sooner rather than later, so first examine the expected
        // type, and see if can glean a closure kind from there.
        let (expected_sig, expected_kind) = match expected.to_option(self) {
            Some(ty) => self.deduce_closure_signature(
                self.try_structurally_resolve_type(expr_span, ty),
                closure.kind,
            ),
            None => (None, None),
        };

        let ClosureSignatures { bound_sig, mut liberated_sig } =
            self.sig_of_closure(expr_def_id, closure.fn_decl, closure.kind, expected_sig);

        debug!(?bound_sig, ?liberated_sig);

        let parent_args =
            GenericArgs::identity_for_item(tcx, tcx.typeck_root_def_id(expr_def_id.to_def_id()));

        let tupled_upvars_ty = self.next_ty_var(expr_span);

        // FIXME: We could probably actually just unify this further --
        // instead of having a `FnSig` and a `Option<CoroutineTypes>`,
        // we can have a `ClosureSignature { Coroutine { .. }, Closure { .. } }`,
        // similar to how `ty::GenSig` is a distinct data structure.
        let (closure_ty, coroutine_types) = match closure.kind {
            hir::ClosureKind::Closure => {
                // Tuple up the arguments and insert the resulting function type into
                // the `closures` table.
                let sig = bound_sig.map_bound(|sig| {
                    tcx.mk_fn_sig(
                        [Ty::new_tup(tcx, sig.inputs())],
                        sig.output(),
                        sig.c_variadic,
                        sig.safety,
                        sig.abi,
                    )
                });

                debug!(?sig, ?expected_kind);

                let closure_kind_ty = match expected_kind {
                    Some(kind) => Ty::from_closure_kind(tcx, kind),

                    // Create a type variable (for now) to represent the closure kind.
                    // It will be unified during the upvar inference phase (`upvar.rs`)
                    None => self.next_ty_var(expr_span),
                };

                let closure_args = ty::ClosureArgs::new(
                    tcx,
                    ty::ClosureArgsParts {
                        parent_args,
                        closure_kind_ty,
                        closure_sig_as_fn_ptr_ty: Ty::new_fn_ptr(tcx, sig),
                        tupled_upvars_ty,
                    },
                );

                (Ty::new_closure(tcx, expr_def_id.to_def_id(), closure_args.args), None)
            }
            hir::ClosureKind::Coroutine(kind) => {
                let yield_ty = match kind {
                    hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Gen, _)
                    | hir::CoroutineKind::Coroutine(_) => {
                        let yield_ty = self.next_ty_var(expr_span);
                        self.require_type_is_sized(
                            yield_ty,
                            expr_span,
                            ObligationCauseCode::SizedYieldType,
                        );
                        yield_ty
                    }
                    // HACK(-Ztrait-solver=next): In the *old* trait solver, we must eagerly
                    // guide inference on the yield type so that we can handle `AsyncIterator`
                    // in this block in projection correctly. In the new trait solver, it is
                    // not a problem.
                    hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::AsyncGen, _) => {
                        let yield_ty = self.next_ty_var(expr_span);
                        self.require_type_is_sized(
                            yield_ty,
                            expr_span,
                            ObligationCauseCode::SizedYieldType,
                        );

                        Ty::new_adt(
                            tcx,
                            tcx.adt_def(
                                tcx.require_lang_item(hir::LangItem::Poll, Some(expr_span)),
                            ),
                            tcx.mk_args(&[Ty::new_adt(
                                tcx,
                                tcx.adt_def(
                                    tcx.require_lang_item(hir::LangItem::Option, Some(expr_span)),
                                ),
                                tcx.mk_args(&[yield_ty.into()]),
                            )
                            .into()]),
                        )
                    }
                    hir::CoroutineKind::Desugared(hir::CoroutineDesugaring::Async, _) => {
                        tcx.types.unit
                    }
                };

                // Resume type defaults to `()` if the coroutine has no argument.
                let resume_ty = liberated_sig.inputs().get(0).copied().unwrap_or(tcx.types.unit);

                let interior = self.next_ty_var(expr_span);
                self.deferred_coroutine_interiors.borrow_mut().push((
                    expr_def_id,
                    body.id(),
                    interior,
                ));

                // Coroutines that come from coroutine closures have not yet determined
                // their kind ty, so make a fresh infer var which will be constrained
                // later during upvar analysis. Regular coroutines always have the kind
                // ty of `().`
                let kind_ty = match kind {
                    hir::CoroutineKind::Desugared(_, hir::CoroutineSource::Closure) => {
                        self.next_ty_var(expr_span)
                    }
                    _ => tcx.types.unit,
                };

                let coroutine_args = ty::CoroutineArgs::new(
                    tcx,
                    ty::CoroutineArgsParts {
                        parent_args,
                        kind_ty,
                        resume_ty,
                        yield_ty,
                        return_ty: liberated_sig.output(),
                        witness: interior,
                        tupled_upvars_ty,
                    },
                );

                (
                    Ty::new_coroutine(tcx, expr_def_id.to_def_id(), coroutine_args.args),
                    Some(CoroutineTypes { resume_ty, yield_ty }),
                )
            }
            hir::ClosureKind::CoroutineClosure(kind) => {
                // async closures always return the type ascribed after the `->` (if present),
                // and yield `()`.
                let (bound_return_ty, bound_yield_ty) = match kind {
                    hir::CoroutineDesugaring::Async => {
                        (bound_sig.skip_binder().output(), tcx.types.unit)
                    }
                    hir::CoroutineDesugaring::Gen | hir::CoroutineDesugaring::AsyncGen => {
                        todo!("`gen` and `async gen` closures not supported yet")
                    }
                };
                // Compute all of the variables that will be used to populate the coroutine.
                let resume_ty = self.next_ty_var(expr_span);
                let interior = self.next_ty_var(expr_span);

                let closure_kind_ty = match expected_kind {
                    Some(kind) => Ty::from_closure_kind(tcx, kind),

                    // Create a type variable (for now) to represent the closure kind.
                    // It will be unified during the upvar inference phase (`upvar.rs`)
                    None => self.next_ty_var(expr_span),
                };

                let coroutine_captures_by_ref_ty = self.next_ty_var(expr_span);
                let closure_args = ty::CoroutineClosureArgs::new(
                    tcx,
                    ty::CoroutineClosureArgsParts {
                        parent_args,
                        closure_kind_ty,
                        signature_parts_ty: Ty::new_fn_ptr(
                            tcx,
                            bound_sig.map_bound(|sig| {
                                tcx.mk_fn_sig(
                                    [
                                        resume_ty,
                                        Ty::new_tup_from_iter(tcx, sig.inputs().iter().copied()),
                                    ],
                                    Ty::new_tup(tcx, &[bound_yield_ty, bound_return_ty]),
                                    sig.c_variadic,
                                    sig.safety,
                                    sig.abi,
                                )
                            }),
                        ),
                        tupled_upvars_ty,
                        coroutine_captures_by_ref_ty,
                        coroutine_witness_ty: interior,
                    },
                );

                let coroutine_kind_ty = match expected_kind {
                    Some(kind) => Ty::from_coroutine_closure_kind(tcx, kind),

                    // Create a type variable (for now) to represent the closure kind.
                    // It will be unified during the upvar inference phase (`upvar.rs`)
                    None => self.next_ty_var(expr_span),
                };

                let coroutine_upvars_ty = self.next_ty_var(expr_span);

                // We need to turn the liberated signature that we got from HIR, which
                // looks something like `|Args...| -> T`, into a signature that is suitable
                // for type checking the inner body of the closure, which always returns a
                // coroutine. To do so, we use the `CoroutineClosureSignature` to compute
                // the coroutine type, filling in the tupled_upvars_ty and kind_ty with infer
                // vars which will get constrained during upvar analysis.
                let coroutine_output_ty = tcx.liberate_late_bound_regions(
                    expr_def_id.to_def_id(),
                    closure_args.coroutine_closure_sig().map_bound(|sig| {
                        sig.to_coroutine(
                            tcx,
                            parent_args,
                            coroutine_kind_ty,
                            tcx.coroutine_for_closure(expr_def_id),
                            coroutine_upvars_ty,
                        )
                    }),
                );
                liberated_sig = tcx.mk_fn_sig(
                    liberated_sig.inputs().iter().copied(),
                    coroutine_output_ty,
                    liberated_sig.c_variadic,
                    liberated_sig.safety,
                    liberated_sig.abi,
                );

                (Ty::new_coroutine_closure(tcx, expr_def_id.to_def_id(), closure_args.args), None)
            }
        };

        check_fn(
            &mut FnCtxt::new(self, self.param_env, closure.def_id),
            liberated_sig,
            coroutine_types,
            closure.fn_decl,
            expr_def_id,
            body,
            // Closure "rust-call" ABI doesn't support unsized params
            false,
        );

        closure_ty
    }

    /// Given the expected type, figures out what it can about this closure we
    /// are about to type check:
    #[instrument(skip(self), level = "debug")]
    fn deduce_closure_signature(
        &self,
        expected_ty: Ty<'tcx>,
        closure_kind: hir::ClosureKind,
    ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
        match *expected_ty.kind() {
            ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => self
                .deduce_closure_signature_from_predicates(
                    expected_ty,
                    closure_kind,
                    self.tcx
                        .explicit_item_super_predicates(def_id)
                        .iter_instantiated_copied(self.tcx, args)
                        .map(|(c, s)| (c.as_predicate(), s)),
                ),
            ty::Dynamic(object_type, ..) => {
                let sig = object_type.projection_bounds().find_map(|pb| {
                    let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
                    self.deduce_sig_from_projection(None, closure_kind, pb)
                });
                let kind = object_type
                    .principal_def_id()
                    .and_then(|did| self.tcx.fn_trait_kind_from_def_id(did));
                (sig, kind)
            }
            ty::Infer(ty::TyVar(vid)) => self.deduce_closure_signature_from_predicates(
                Ty::new_var(self.tcx, self.root_var(vid)),
                closure_kind,
                self.obligations_for_self_ty(vid)
                    .into_iter()
                    .map(|obl| (obl.predicate, obl.cause.span)),
            ),
            ty::FnPtr(sig_tys, hdr) => match closure_kind {
                hir::ClosureKind::Closure => {
                    let expected_sig = ExpectedSig { cause_span: None, sig: sig_tys.with(hdr) };
                    (Some(expected_sig), Some(ty::ClosureKind::Fn))
                }
                hir::ClosureKind::Coroutine(_) | hir::ClosureKind::CoroutineClosure(_) => {
                    (None, None)
                }
            },
            _ => (None, None),
        }
    }

    fn deduce_closure_signature_from_predicates(
        &self,
        expected_ty: Ty<'tcx>,
        closure_kind: hir::ClosureKind,
        predicates: impl DoubleEndedIterator<Item = (ty::Predicate<'tcx>, Span)>,
    ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
        let mut expected_sig = None;
        let mut expected_kind = None;

        for (pred, span) in traits::elaborate(
            self.tcx,
            // Reverse the obligations here, since `elaborate_*` uses a stack,
            // and we want to keep inference generally in the same order of
            // the registered obligations.
            predicates.rev(),
        )
        // We only care about self bounds
        .filter_only_self()
        {
            debug!(?pred);
            let bound_predicate = pred.kind();

            // Given a Projection predicate, we can potentially infer
            // the complete signature.
            if expected_sig.is_none()
                && let ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj_predicate)) =
                    bound_predicate.skip_binder()
            {
                let inferred_sig = self.normalize(
                    span,
                    self.deduce_sig_from_projection(
                        Some(span),
                        closure_kind,
                        bound_predicate.rebind(proj_predicate),
                    ),
                );
                // Make sure that we didn't infer a signature that mentions itself.
                // This can happen when we elaborate certain supertrait bounds that
                // mention projections containing the `Self` type. See #105401.
                struct MentionsTy<'tcx> {
                    expected_ty: Ty<'tcx>,
                }
                impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for MentionsTy<'tcx> {
                    type Result = ControlFlow<()>;

                    fn visit_ty(&mut self, t: Ty<'tcx>) -> Self::Result {
                        if t == self.expected_ty {
                            ControlFlow::Break(())
                        } else {
                            t.super_visit_with(self)
                        }
                    }
                }
                if inferred_sig.visit_with(&mut MentionsTy { expected_ty }).is_continue() {
                    expected_sig = inferred_sig;
                }
            }

            // Even if we can't infer the full signature, we may be able to
            // infer the kind. This can occur when we elaborate a predicate
            // like `F : Fn<A>`. Note that due to subtyping we could encounter
            // many viable options, so pick the most restrictive.
            let trait_def_id = match bound_predicate.skip_binder() {
                ty::PredicateKind::Clause(ty::ClauseKind::Projection(data)) => {
                    Some(data.projection_term.trait_def_id(self.tcx))
                }
                ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => Some(data.def_id()),
                _ => None,
            };

            if let Some(trait_def_id) = trait_def_id {
                let found_kind = match closure_kind {
                    hir::ClosureKind::Closure => self.tcx.fn_trait_kind_from_def_id(trait_def_id),
                    hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async) => self
                        .tcx
                        .async_fn_trait_kind_from_def_id(trait_def_id)
                        .or_else(|| self.tcx.fn_trait_kind_from_def_id(trait_def_id)),
                    _ => None,
                };

                if let Some(found_kind) = found_kind {
                    // always use the closure kind that is more permissive.
                    match (expected_kind, found_kind) {
                        (None, _) => expected_kind = Some(found_kind),
                        (Some(ClosureKind::FnMut), ClosureKind::Fn) => {
                            expected_kind = Some(ClosureKind::Fn)
                        }
                        (Some(ClosureKind::FnOnce), ClosureKind::Fn | ClosureKind::FnMut) => {
                            expected_kind = Some(found_kind)
                        }
                        _ => {}
                    }
                }
            }
        }

        (expected_sig, expected_kind)
    }

    /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
    /// everything we need to know about a closure or coroutine.
    ///
    /// The `cause_span` should be the span that caused us to
    /// have this expected signature, or `None` if we can't readily
    /// know that.
    #[instrument(level = "debug", skip(self, cause_span), ret)]
    fn deduce_sig_from_projection(
        &self,
        cause_span: Option<Span>,
        closure_kind: hir::ClosureKind,
        projection: ty::PolyProjectionPredicate<'tcx>,
    ) -> Option<ExpectedSig<'tcx>> {
        let tcx = self.tcx;

        let trait_def_id = projection.trait_def_id(tcx);

        // For now, we only do signature deduction based off of the `Fn` and `AsyncFn` traits,
        // for closures and async closures, respectively.
        match closure_kind {
            hir::ClosureKind::Closure
                if self.tcx.fn_trait_kind_from_def_id(trait_def_id).is_some() =>
            {
                self.extract_sig_from_projection(cause_span, projection)
            }
            hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async)
                if self.tcx.async_fn_trait_kind_from_def_id(trait_def_id).is_some() =>
            {
                self.extract_sig_from_projection(cause_span, projection)
            }
            // It's possible we've passed the closure to a (somewhat out-of-fashion)
            // `F: FnOnce() -> Fut, Fut: Future<Output = T>` style bound. Let's still
            // guide inference here, since it's beneficial for the user.
            hir::ClosureKind::CoroutineClosure(hir::CoroutineDesugaring::Async)
                if self.tcx.fn_trait_kind_from_def_id(trait_def_id).is_some() =>
            {
                self.extract_sig_from_projection_and_future_bound(cause_span, projection)
            }
            _ => None,
        }
    }

    /// Given an `FnOnce::Output` or `AsyncFn::Output` projection, extract the args
    /// and return type to infer a [`ty::PolyFnSig`] for the closure.
    fn extract_sig_from_projection(
        &self,
        cause_span: Option<Span>,
        projection: ty::PolyProjectionPredicate<'tcx>,
    ) -> Option<ExpectedSig<'tcx>> {
        let projection = self.resolve_vars_if_possible(projection);

        let arg_param_ty = projection.skip_binder().projection_term.args.type_at(1);
        debug!(?arg_param_ty);

        let ty::Tuple(input_tys) = *arg_param_ty.kind() else {
            return None;
        };

        // Since this is a return parameter type it is safe to unwrap.
        let ret_param_ty = projection.skip_binder().term.expect_type();
        debug!(?ret_param_ty);

        let sig = projection.rebind(self.tcx.mk_fn_sig(
            input_tys,
            ret_param_ty,
            false,
            hir::Safety::Safe,
            Abi::Rust,
        ));

        Some(ExpectedSig { cause_span, sig })
    }

    /// When an async closure is passed to a function that has a "two-part" `Fn`
    /// and `Future` trait bound, like:
    ///
    /// ```rust
    /// use std::future::Future;
    ///
    /// fn not_exactly_an_async_closure<F, Fut>(_f: F)
    /// where
    ///     F: FnOnce(String, u32) -> Fut,
    ///     Fut: Future<Output = i32>,
    /// {}
    /// ```
    ///
    /// The we want to be able to extract the signature to guide inference in the async
    /// closure. We will have two projection predicates registered in this case. First,
    /// we identify the `FnOnce<Args, Output = ?Fut>` bound, and if the output type is
    /// an inference variable `?Fut`, we check if that is bounded by a `Future<Output = Ty>`
    /// projection.
    ///
    /// This function is actually best-effort with the return type; if we don't find a
    /// `Future` projection, we still will return arguments that we extracted from the `FnOnce`
    /// projection, and the output will be an unconstrained type variable instead.
    fn extract_sig_from_projection_and_future_bound(
        &self,
        cause_span: Option<Span>,
        projection: ty::PolyProjectionPredicate<'tcx>,
    ) -> Option<ExpectedSig<'tcx>> {
        let projection = self.resolve_vars_if_possible(projection);

        let arg_param_ty = projection.skip_binder().projection_term.args.type_at(1);
        debug!(?arg_param_ty);

        let ty::Tuple(input_tys) = *arg_param_ty.kind() else {
            return None;
        };

        // If the return type is a type variable, look for bounds on it.
        // We could theoretically support other kinds of return types here,
        // but none of them would be useful, since async closures return
        // concrete anonymous future types, and their futures are not coerced
        // into any other type within the body of the async closure.
        let ty::Infer(ty::TyVar(return_vid)) = *projection.skip_binder().term.expect_type().kind()
        else {
            return None;
        };

        // FIXME: We may want to elaborate here, though I assume this will be exceedingly rare.
        let mut return_ty = None;
        for bound in self.obligations_for_self_ty(return_vid) {
            if let Some(ret_projection) = bound.predicate.as_projection_clause()
                && let Some(ret_projection) = ret_projection.no_bound_vars()
                && self.tcx.is_lang_item(ret_projection.def_id(), LangItem::FutureOutput)
            {
                return_ty = Some(ret_projection.term.expect_type());
                break;
            }
        }

        // SUBTLE: If we didn't find a `Future<Output = ...>` bound for the return
        // vid, we still want to attempt to provide inference guidance for the async
        // closure's arguments. Instantiate a new vid to plug into the output type.
        //
        // You may be wondering, what if it's higher-ranked? Well, given that we
        // found a type variable for the `FnOnce::Output` projection above, we know
        // that the output can't mention any of the vars.
        //
        // Also note that we use a fresh var here for the signature since the signature
        // records the output of the *future*, and `return_vid` above is the type
        // variable of the future, not its output.
        //
        // FIXME: We probably should store this signature inference output in a way
        // that does not misuse a `FnSig` type, but that can be done separately.
        let return_ty =
            return_ty.unwrap_or_else(|| self.next_ty_var(cause_span.unwrap_or(DUMMY_SP)));

        let sig = projection.rebind(self.tcx.mk_fn_sig(
            input_tys,
            return_ty,
            false,
            hir::Safety::Safe,
            Abi::Rust,
        ));

        return Some(ExpectedSig { cause_span, sig });
    }

    fn sig_of_closure(
        &self,
        expr_def_id: LocalDefId,
        decl: &hir::FnDecl<'tcx>,
        closure_kind: hir::ClosureKind,
        expected_sig: Option<ExpectedSig<'tcx>>,
    ) -> ClosureSignatures<'tcx> {
        if let Some(e) = expected_sig {
            self.sig_of_closure_with_expectation(expr_def_id, decl, closure_kind, e)
        } else {
            self.sig_of_closure_no_expectation(expr_def_id, decl, closure_kind)
        }
    }

    /// If there is no expected signature, then we will convert the
    /// types that the user gave into a signature.
    #[instrument(skip(self, expr_def_id, decl), level = "debug")]
    fn sig_of_closure_no_expectation(
        &self,
        expr_def_id: LocalDefId,
        decl: &hir::FnDecl<'tcx>,
        closure_kind: hir::ClosureKind,
    ) -> ClosureSignatures<'tcx> {
        let bound_sig = self.supplied_sig_of_closure(expr_def_id, decl, closure_kind);

        self.closure_sigs(expr_def_id, bound_sig)
    }

    /// Invoked to compute the signature of a closure expression. This
    /// combines any user-provided type annotations (e.g., `|x: u32|
    /// -> u32 { .. }`) with the expected signature.
    ///
    /// The approach is as follows:
    ///
    /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
    /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
    ///   - If we have no expectation `E`, then the signature of the closure is `S`.
    ///   - Otherwise, the signature of the closure is E. Moreover:
    ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
    ///     - Instantiate all the late-bound regions bound in the closure within `S`
    ///       with fresh (existential) variables, yielding `S'`
    ///     - Require that `E' = S'`
    ///       - We could use some kind of subtyping relationship here,
    ///         I imagine, but equality is easier and works fine for
    ///         our purposes.
    ///
    /// The key intuition here is that the user's types must be valid
    /// from "the inside" of the closure, but the expectation
    /// ultimately drives the overall signature.
    ///
    /// # Examples
    ///
    /// ```ignore (illustrative)
    /// fn with_closure<F>(_: F)
    ///   where F: Fn(&u32) -> &u32 { .. }
    ///
    /// with_closure(|x: &u32| { ... })
    /// ```
    ///
    /// Here:
    /// - E would be `fn(&u32) -> &u32`.
    /// - S would be `fn(&u32) -> ?T`
    /// - E' is `&'!0 u32 -> &'!0 u32`
    /// - S' is `&'?0 u32 -> ?T`
    ///
    /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
    ///
    /// # Arguments
    ///
    /// - `expr_def_id`: the `LocalDefId` of the closure expression
    /// - `decl`: the HIR declaration of the closure
    /// - `body`: the body of the closure
    /// - `expected_sig`: the expected signature (if any). Note that
    ///   this is missing a binder: that is, there may be late-bound
    ///   regions with depth 1, which are bound then by the closure.
    #[instrument(skip(self, expr_def_id, decl), level = "debug")]
    fn sig_of_closure_with_expectation(
        &self,
        expr_def_id: LocalDefId,
        decl: &hir::FnDecl<'tcx>,
        closure_kind: hir::ClosureKind,
        expected_sig: ExpectedSig<'tcx>,
    ) -> ClosureSignatures<'tcx> {
        // Watch out for some surprises and just ignore the
        // expectation if things don't see to match up with what we
        // expect.
        if expected_sig.sig.c_variadic() != decl.c_variadic {
            return self.sig_of_closure_no_expectation(expr_def_id, decl, closure_kind);
        } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
            return self.sig_of_closure_with_mismatched_number_of_arguments(
                expr_def_id,
                decl,
                expected_sig,
            );
        }

        // Create a `PolyFnSig`. Note the oddity that late bound
        // regions appearing free in `expected_sig` are now bound up
        // in this binder we are creating.
        assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
        let bound_sig = expected_sig.sig.map_bound(|sig| {
            self.tcx.mk_fn_sig(
                sig.inputs().iter().cloned(),
                sig.output(),
                sig.c_variadic,
                hir::Safety::Safe,
                Abi::RustCall,
            )
        });

        // `deduce_expectations_from_expected_type` introduces
        // late-bound lifetimes defined elsewhere, which we now
        // anonymize away, so as not to confuse the user.
        let bound_sig = self.tcx.anonymize_bound_vars(bound_sig);

        let closure_sigs = self.closure_sigs(expr_def_id, bound_sig);

        // Up till this point, we have ignored the annotations that the user
        // gave. This function will check that they unify successfully.
        // Along the way, it also writes out entries for types that the user
        // wrote into our typeck results, which are then later used by the privacy
        // check.
        match self.merge_supplied_sig_with_expectation(
            expr_def_id,
            decl,
            closure_kind,
            closure_sigs,
        ) {
            Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
            Err(_) => self.sig_of_closure_no_expectation(expr_def_id, decl, closure_kind),
        }
    }

    fn sig_of_closure_with_mismatched_number_of_arguments(
        &self,
        expr_def_id: LocalDefId,
        decl: &hir::FnDecl<'tcx>,
        expected_sig: ExpectedSig<'tcx>,
    ) -> ClosureSignatures<'tcx> {
        let expr_map_node = self.tcx.hir_node_by_def_id(expr_def_id);
        let expected_args: Vec<_> = expected_sig
            .sig
            .skip_binder()
            .inputs()
            .iter()
            .map(|ty| ArgKind::from_expected_ty(*ty, None))
            .collect();
        let (closure_span, closure_arg_span, found_args) =
            match self.err_ctxt().get_fn_like_arguments(expr_map_node) {
                Some((sp, arg_sp, args)) => (Some(sp), arg_sp, args),
                None => (None, None, Vec::new()),
            };
        let expected_span =
            expected_sig.cause_span.unwrap_or_else(|| self.tcx.def_span(expr_def_id));
        let guar = self
            .err_ctxt()
            .report_arg_count_mismatch(
                expected_span,
                closure_span,
                expected_args,
                found_args,
                true,
                closure_arg_span,
            )
            .emit();

        let error_sig = self.error_sig_of_closure(decl, guar);

        self.closure_sigs(expr_def_id, error_sig)
    }

    /// Enforce the user's types against the expectation. See
    /// `sig_of_closure_with_expectation` for details on the overall
    /// strategy.
    #[instrument(level = "debug", skip(self, expr_def_id, decl, expected_sigs))]
    fn merge_supplied_sig_with_expectation(
        &self,
        expr_def_id: LocalDefId,
        decl: &hir::FnDecl<'tcx>,
        closure_kind: hir::ClosureKind,
        mut expected_sigs: ClosureSignatures<'tcx>,
    ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
        // Get the signature S that the user gave.
        //
        // (See comment on `sig_of_closure_with_expectation` for the
        // meaning of these letters.)
        let supplied_sig = self.supplied_sig_of_closure(expr_def_id, decl, closure_kind);

        debug!(?supplied_sig);

        // FIXME(#45727): As discussed in [this comment][c1], naively
        // forcing equality here actually results in suboptimal error
        // messages in some cases. For now, if there would have been
        // an obvious error, we fallback to declaring the type of the
        // closure to be the one the user gave, which allows other
        // error message code to trigger.
        //
        // However, I think [there is potential to do even better
        // here][c2], since in *this* code we have the precise span of
        // the type parameter in question in hand when we report the
        // error.
        //
        // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
        // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
        self.commit_if_ok(|_| {
            let mut all_obligations = vec![];
            let supplied_sig = self.instantiate_binder_with_fresh_vars(
                self.tcx.def_span(expr_def_id),
                BoundRegionConversionTime::FnCall,
                supplied_sig,
            );

            // The liberated version of this signature should be a subtype
            // of the liberated form of the expectation.
            for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
                iter::zip(decl.inputs, supplied_sig.inputs()),
                expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
            ) {
                // Check that E' = S'.
                let cause = self.misc(hir_ty.span);
                let InferOk { value: (), obligations } = self.at(&cause, self.param_env).eq(
                    DefineOpaqueTypes::Yes,
                    *expected_ty,
                    supplied_ty,
                )?;
                all_obligations.extend(obligations);
            }

            let supplied_output_ty = supplied_sig.output();
            let cause = &self.misc(decl.output.span());
            let InferOk { value: (), obligations } = self.at(cause, self.param_env).eq(
                DefineOpaqueTypes::Yes,
                expected_sigs.liberated_sig.output(),
                supplied_output_ty,
            )?;
            all_obligations.extend(obligations);

            let inputs =
                supplied_sig.inputs().into_iter().map(|&ty| self.resolve_vars_if_possible(ty));

            expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
                inputs,
                supplied_output_ty,
                expected_sigs.liberated_sig.c_variadic,
                hir::Safety::Safe,
                Abi::RustCall,
            );

            Ok(InferOk { value: expected_sigs, obligations: all_obligations })
        })
    }

    /// If there is no expected signature, then we will convert the
    /// types that the user gave into a signature.
    ///
    /// Also, record this closure signature for later.
    #[instrument(skip(self, decl), level = "debug", ret)]
    fn supplied_sig_of_closure(
        &self,
        expr_def_id: LocalDefId,
        decl: &hir::FnDecl<'tcx>,
        closure_kind: hir::ClosureKind,
    ) -> ty::PolyFnSig<'tcx> {
        let lowerer = self.lowerer();

        trace!("decl = {:#?}", decl);
        debug!(?closure_kind);

        let hir_id = self.tcx.local_def_id_to_hir_id(expr_def_id);
        let bound_vars = self.tcx.late_bound_vars(hir_id);

        // First, convert the types that the user supplied (if any).
        let supplied_arguments = decl.inputs.iter().map(|a| lowerer.lower_ty(a));
        let supplied_return = match decl.output {
            hir::FnRetTy::Return(ref output) => lowerer.lower_ty(output),
            hir::FnRetTy::DefaultReturn(_) => match closure_kind {
                // In the case of the async block that we create for a function body,
                // we expect the return type of the block to match that of the enclosing
                // function.
                hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                    hir::CoroutineDesugaring::Async,
                    hir::CoroutineSource::Fn,
                )) => {
                    debug!("closure is async fn body");
                    self.deduce_future_output_from_obligations(expr_def_id).unwrap_or_else(|| {
                        // AFAIK, deducing the future output
                        // always succeeds *except* in error cases
                        // like #65159. I'd like to return Error
                        // here, but I can't because I can't
                        // easily (and locally) prove that we
                        // *have* reported an
                        // error. --nikomatsakis
                        lowerer.ty_infer(None, decl.output.span())
                    })
                }
                // All `gen {}` and `async gen {}` must return unit.
                hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                    hir::CoroutineDesugaring::Gen | hir::CoroutineDesugaring::AsyncGen,
                    _,
                )) => self.tcx.types.unit,

                // For async blocks, we just fall back to `_` here.
                // For closures/coroutines, we know nothing about the return
                // type unless it was supplied.
                hir::ClosureKind::Coroutine(hir::CoroutineKind::Desugared(
                    hir::CoroutineDesugaring::Async,
                    _,
                ))
                | hir::ClosureKind::Coroutine(hir::CoroutineKind::Coroutine(_))
                | hir::ClosureKind::Closure
                | hir::ClosureKind::CoroutineClosure(_) => {
                    lowerer.ty_infer(None, decl.output.span())
                }
            },
        };

        let result = ty::Binder::bind_with_vars(
            self.tcx.mk_fn_sig(
                supplied_arguments,
                supplied_return,
                decl.c_variadic,
                hir::Safety::Safe,
                Abi::RustCall,
            ),
            bound_vars,
        );

        let c_result = self.infcx.canonicalize_response(result);
        self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);

        // Normalize only after registering in `user_provided_sigs`.
        self.normalize(self.tcx.hir().span(hir_id), result)
    }

    /// Invoked when we are translating the coroutine that results
    /// from desugaring an `async fn`. Returns the "sugared" return
    /// type of the `async fn` -- that is, the return type that the
    /// user specified. The "desugared" return type is an `impl
    /// Future<Output = T>`, so we do this by searching through the
    /// obligations to extract the `T`.
    #[instrument(skip(self), level = "debug", ret)]
    fn deduce_future_output_from_obligations(&self, body_def_id: LocalDefId) -> Option<Ty<'tcx>> {
        let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
            span_bug!(self.tcx.def_span(body_def_id), "async fn coroutine outside of a fn")
        });

        let closure_span = self.tcx.def_span(body_def_id);
        let ret_ty = ret_coercion.borrow().expected_ty();
        let ret_ty = self.try_structurally_resolve_type(closure_span, ret_ty);

        let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
            // Search for a pending obligation like
            //
            // `<R as Future>::Output = T`
            //
            // where R is the return type we are expecting. This type `T`
            // will be our output.
            let bound_predicate = predicate.kind();
            if let ty::PredicateKind::Clause(ty::ClauseKind::Projection(proj_predicate)) =
                bound_predicate.skip_binder()
            {
                self.deduce_future_output_from_projection(
                    span,
                    bound_predicate.rebind(proj_predicate),
                )
            } else {
                None
            }
        };

        let output_ty = match *ret_ty.kind() {
            ty::Infer(ty::TyVar(ret_vid)) => {
                self.obligations_for_self_ty(ret_vid).into_iter().find_map(|obligation| {
                    get_future_output(obligation.predicate, obligation.cause.span)
                })?
            }
            ty::Alias(ty::Projection, _) => {
                return Some(Ty::new_error_with_message(
                    self.tcx,
                    closure_span,
                    "this projection should have been projected to an opaque type",
                ));
            }
            ty::Alias(ty::Opaque, ty::AliasTy { def_id, args, .. }) => self
                .tcx
                .explicit_item_super_predicates(def_id)
                .iter_instantiated_copied(self.tcx, args)
                .find_map(|(p, s)| get_future_output(p.as_predicate(), s))?,
            ty::Error(_) => return Some(ret_ty),
            _ => {
                span_bug!(closure_span, "invalid async fn coroutine return type: {ret_ty:?}")
            }
        };

        let output_ty = self.normalize(closure_span, output_ty);

        // async fn that have opaque types in their return type need to redo the conversion to inference variables
        // as they fetch the still opaque version from the signature.
        let InferOk { value: output_ty, obligations } = self
            .replace_opaque_types_with_inference_vars(
                output_ty,
                body_def_id,
                closure_span,
                self.param_env,
            );
        self.register_predicates(obligations);

        Some(output_ty)
    }

    /// Given a projection like
    ///
    /// `<X as Future>::Output = T`
    ///
    /// where `X` is some type that has no late-bound regions, returns
    /// `Some(T)`. If the projection is for some other trait, returns
    /// `None`.
    fn deduce_future_output_from_projection(
        &self,
        cause_span: Span,
        predicate: ty::PolyProjectionPredicate<'tcx>,
    ) -> Option<Ty<'tcx>> {
        debug!("deduce_future_output_from_projection(predicate={:?})", predicate);

        // We do not expect any bound regions in our predicate, so
        // skip past the bound vars.
        let Some(predicate) = predicate.no_bound_vars() else {
            debug!("deduce_future_output_from_projection: has late-bound regions");
            return None;
        };

        // Check that this is a projection from the `Future` trait.
        let trait_def_id = predicate.projection_term.trait_def_id(self.tcx);
        let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
        if trait_def_id != future_trait {
            debug!("deduce_future_output_from_projection: not a future");
            return None;
        }

        // The `Future` trait has only one associated item, `Output`,
        // so check that this is what we see.
        let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
        if output_assoc_item != predicate.projection_term.def_id {
            span_bug!(
                cause_span,
                "projecting associated item `{:?}` from future, which is not Output `{:?}`",
                predicate.projection_term.def_id,
                output_assoc_item,
            );
        }

        // Extract the type from the projection. Note that there can
        // be no bound variables in this type because the "self type"
        // does not have any regions in it.
        let output_ty = self.resolve_vars_if_possible(predicate.term);
        debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
        // This is a projection on a Fn trait so will always be a type.
        Some(output_ty.expect_type())
    }

    /// Converts the types that the user supplied, in case that doing
    /// so should yield an error, but returns back a signature where
    /// all parameters are of type `ty::Error`.
    fn error_sig_of_closure(
        &self,
        decl: &hir::FnDecl<'tcx>,
        guar: ErrorGuaranteed,
    ) -> ty::PolyFnSig<'tcx> {
        let lowerer = self.lowerer();
        let err_ty = Ty::new_error(self.tcx, guar);

        let supplied_arguments = decl.inputs.iter().map(|a| {
            // Convert the types that the user supplied (if any), but ignore them.
            lowerer.lower_ty(a);
            err_ty
        });

        if let hir::FnRetTy::Return(ref output) = decl.output {
            lowerer.lower_ty(output);
        }

        let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
            supplied_arguments,
            err_ty,
            decl.c_variadic,
            hir::Safety::Safe,
            Abi::RustCall,
        ));

        debug!("supplied_sig_of_closure: result={:?}", result);

        result
    }

    #[instrument(level = "debug", skip(self), ret)]
    fn closure_sigs(
        &self,
        expr_def_id: LocalDefId,
        bound_sig: ty::PolyFnSig<'tcx>,
    ) -> ClosureSignatures<'tcx> {
        let liberated_sig =
            self.tcx().liberate_late_bound_regions(expr_def_id.to_def_id(), bound_sig);
        let liberated_sig = self.normalize(self.tcx.def_span(expr_def_id), liberated_sig);
        ClosureSignatures { bound_sig, liberated_sig }
    }
}