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
// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Code for type-checking closure expressions.

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

use astconv::AstConv;
use rustc::hir::def_id::DefId;
use rustc::infer::{InferOk, InferResult};
use rustc::infer::LateBoundRegionConversionTime;
use rustc::infer::type_variable::TypeVariableOrigin;
use rustc::traits::error_reporting::ArgKind;
use rustc::ty::{self, ToPolyTraitRef, Ty, GenericParamDefKind};
use rustc::ty::fold::TypeFoldable;
use rustc::ty::subst::Substs;
use std::cmp;
use std::iter;
use rustc_target::spec::abi::Abi;
use syntax::codemap::Span;
use rustc::hir;

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

struct ClosureSignatures<'tcx> {
    bound_sig: ty::PolyFnSig<'tcx>,
    liberated_sig: ty::FnSig<'tcx>,
}

impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
    pub fn check_expr_closure(
        &self,
        expr: &hir::Expr,
        _capture: hir::CaptureClause,
        decl: &'gcx hir::FnDecl,
        body_id: hir::BodyId,
        gen: Option<hir::GeneratorMovability>,
        expected: Expectation<'tcx>,
    ) -> Ty<'tcx> {
        debug!(
            "check_expr_closure(expr={:?},expected={:?})",
            expr, expected
        );

        // 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_expectations_from_expected_type(ty),
            None => (None, None),
        };
        let body = self.tcx.hir.body(body_id);
        self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
    }

    fn check_closure(
        &self,
        expr: &hir::Expr,
        opt_kind: Option<ty::ClosureKind>,
        decl: &'gcx hir::FnDecl,
        body: &'gcx hir::Body,
        gen: Option<hir::GeneratorMovability>,
        expected_sig: Option<ExpectedSig<'tcx>>,
    ) -> Ty<'tcx> {
        debug!(
            "check_closure(opt_kind={:?}, expected_sig={:?})",
            opt_kind, expected_sig
        );

        let expr_def_id = self.tcx.hir.local_def_id(expr.id);

        let ClosureSignatures {
            bound_sig,
            liberated_sig,
        } = self.sig_of_closure(expr_def_id, decl, body, expected_sig);

        debug!("check_closure: ty_of_closure returns {:?}", liberated_sig);

        let generator_types = check_fn(
            self,
            self.param_env,
            liberated_sig,
            decl,
            expr.id,
            body,
            gen,
        ).1;

        // Create type variables (for now) to represent the transformed
        // types of upvars. These will be unified during the upvar
        // inference phase (`upvar.rs`).
        let base_substs =
            Substs::identity_for_item(self.tcx, self.tcx.closure_base_def_id(expr_def_id));
        let substs = base_substs.extend_to(self.tcx,expr_def_id, |param, _| {
            match param.kind {
                GenericParamDefKind::Lifetime => {
                    span_bug!(expr.span, "closure has region param")
                }
                GenericParamDefKind::Type {..} => {
                    self.infcx
                        .next_ty_var(TypeVariableOrigin::ClosureSynthetic(expr.span)).into()
                }
            }
        });
        if let Some(GeneratorTypes { yield_ty, interior, movability }) = generator_types {
            let substs = ty::GeneratorSubsts { substs };
            self.demand_eqtype(
                expr.span,
                yield_ty,
                substs.yield_ty(expr_def_id, self.tcx),
            );
            self.demand_eqtype(
                expr.span,
                liberated_sig.output(),
                substs.return_ty(expr_def_id, self.tcx),
            );
            self.demand_eqtype(
                expr.span,
                interior,
                substs.witness(expr_def_id, self.tcx),
            );
            return self.tcx.mk_generator(expr_def_id, substs, movability);
        }

        let substs = ty::ClosureSubsts { substs };
        let closure_type = self.tcx.mk_closure(expr_def_id, substs);

        debug!(
            "check_closure: expr.id={:?} closure_type={:?}",
            expr.id, closure_type
        );

        // Tuple up the arguments and insert the resulting function type into
        // the `closures` table.
        let sig = bound_sig.map_bound(|sig| {
            self.tcx.mk_fn_sig(
                iter::once(self.tcx.intern_tup(sig.inputs())),
                sig.output(),
                sig.variadic,
                sig.unsafety,
                sig.abi,
            )
        });

        debug!(
            "check_closure: expr_def_id={:?}, sig={:?}, opt_kind={:?}",
            expr_def_id, sig, opt_kind
        );

        let sig_fn_ptr_ty = self.tcx.mk_fn_ptr(sig);
        self.demand_eqtype(
            expr.span,
            sig_fn_ptr_ty,
            substs.closure_sig_ty(expr_def_id, self.tcx),
        );

        if let Some(kind) = opt_kind {
            self.demand_eqtype(
                expr.span,
                kind.to_ty(self.tcx),
                substs.closure_kind_ty(expr_def_id, self.tcx),
            );
        }

        closure_type
    }

    /// Given the expected type, figures out what it can about this closure we
    /// are about to type check:
    fn deduce_expectations_from_expected_type(
        &self,
        expected_ty: Ty<'tcx>,
    ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
        debug!(
            "deduce_expectations_from_expected_type(expected_ty={:?})",
            expected_ty
        );

        match expected_ty.sty {
            ty::TyDynamic(ref object_type, ..) => {
                let sig = object_type
                    .projection_bounds()
                    .filter_map(|pb| {
                        let pb = pb.with_self_ty(self.tcx, self.tcx.types.err);
                        self.deduce_sig_from_projection(None, &pb)
                    })
                    .next();
                let kind = object_type
                    .principal()
                    .and_then(|p| self.tcx.lang_items().fn_trait_kind(p.def_id()));
                (sig, kind)
            }
            ty::TyInfer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
            ty::TyFnPtr(sig) => {
                let expected_sig = ExpectedSig {
                    cause_span: None,
                    sig: sig.skip_binder().clone(),
                };
                (Some(expected_sig), Some(ty::ClosureKind::Fn))
            }
            _ => (None, None),
        }
    }

    fn deduce_expectations_from_obligations(
        &self,
        expected_vid: ty::TyVid,
    ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
        let fulfillment_cx = self.fulfillment_cx.borrow();
        // Here `expected_ty` is known to be a type inference variable.

        let expected_sig = fulfillment_cx
            .pending_obligations()
            .iter()
            .filter_map(|obligation| {
                debug!(
                    "deduce_expectations_from_obligations: obligation.predicate={:?}",
                    obligation.predicate
                );

                match obligation.predicate {
                    // Given a Projection predicate, we can potentially infer
                    // the complete signature.
                    ty::Predicate::Projection(ref proj_predicate) => {
                        let trait_ref = proj_predicate.to_poly_trait_ref(self.tcx);
                        self.self_type_matches_expected_vid(trait_ref, expected_vid)
                            .and_then(|_| {
                                self.deduce_sig_from_projection(
                                    Some(obligation.cause.span),
                                    proj_predicate,
                                )
                            })
                    }
                    _ => None,
                }
            })
            .next();

        // Even if we can't infer the full signature, we may be able to
        // infer the kind. This can occur if there is a trait-reference
        // like `F : Fn<A>`. Note that due to subtyping we could encounter
        // many viable options, so pick the most restrictive.
        let expected_kind = fulfillment_cx
            .pending_obligations()
            .iter()
            .filter_map(|obligation| {
                let opt_trait_ref = match obligation.predicate {
                    ty::Predicate::Projection(ref data) => Some(data.to_poly_trait_ref(self.tcx)),
                    ty::Predicate::Trait(ref data) => Some(data.to_poly_trait_ref()),
                    ty::Predicate::Subtype(..) => None,
                    ty::Predicate::RegionOutlives(..) => None,
                    ty::Predicate::TypeOutlives(..) => None,
                    ty::Predicate::WellFormed(..) => None,
                    ty::Predicate::ObjectSafe(..) => None,
                    ty::Predicate::ConstEvaluatable(..) => None,

                    // NB: This predicate is created by breaking down a
                    // `ClosureType: FnFoo()` predicate, where
                    // `ClosureType` represents some `TyClosure`. It can't
                    // possibly be referring to the current closure,
                    // because we haven't produced the `TyClosure` for
                    // this closure yet; this is exactly why the other
                    // code is looking for a self type of a unresolved
                    // inference variable.
                    ty::Predicate::ClosureKind(..) => None,
                };
                opt_trait_ref
                    .and_then(|tr| self.self_type_matches_expected_vid(tr, expected_vid))
                    .and_then(|tr| self.tcx.lang_items().fn_trait_kind(tr.def_id()))
            })
            .fold(None, |best, cur| {
                Some(best.map_or(cur, |best| cmp::min(best, cur)))
            });

        (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.
    ///
    /// The `cause_span` should be the span that caused us to
    /// have this expected signature, or `None` if we can't readily
    /// know that.
    fn deduce_sig_from_projection(
        &self,
        cause_span: Option<Span>,
        projection: &ty::PolyProjectionPredicate<'tcx>,
    ) -> Option<ExpectedSig<'tcx>> {
        let tcx = self.tcx;

        debug!("deduce_sig_from_projection({:?})", projection);

        let trait_ref = projection.to_poly_trait_ref(tcx);

        if tcx.lang_items().fn_trait_kind(trait_ref.def_id()).is_none() {
            return None;
        }

        let arg_param_ty = trait_ref.skip_binder().substs.type_at(1);
        let arg_param_ty = self.resolve_type_vars_if_possible(&arg_param_ty);
        debug!(
            "deduce_sig_from_projection: arg_param_ty {:?}",
            arg_param_ty
        );

        let input_tys = match arg_param_ty.sty {
            ty::TyTuple(tys) => tys.into_iter(),
            _ => {
                return None;
            }
        };

        let ret_param_ty = projection.skip_binder().ty;
        let ret_param_ty = self.resolve_type_vars_if_possible(&ret_param_ty);
        debug!(
            "deduce_sig_from_projection: ret_param_ty {:?}",
            ret_param_ty
        );

        let sig = self.tcx.mk_fn_sig(
            input_tys.cloned(),
            ret_param_ty,
            false,
            hir::Unsafety::Normal,
            Abi::Rust,
        );
        debug!("deduce_sig_from_projection: sig {:?}", sig);

        Some(ExpectedSig { cause_span, sig })
    }

    fn self_type_matches_expected_vid(
        &self,
        trait_ref: ty::PolyTraitRef<'tcx>,
        expected_vid: ty::TyVid,
    ) -> Option<ty::PolyTraitRef<'tcx>> {
        let self_ty = self.shallow_resolve(trait_ref.self_ty());
        debug!(
            "self_type_matches_expected_vid(trait_ref={:?}, self_ty={:?})",
            trait_ref, self_ty
        );
        match self_ty.sty {
            ty::TyInfer(ty::TyVar(v)) if expected_vid == v => Some(trait_ref),
            _ => None,
        }
    }

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

    /// If there is no expected signature, then we will convert the
    /// types that the user gave into a signature.
    fn sig_of_closure_no_expectation(
        &self,
        expr_def_id: DefId,
        decl: &hir::FnDecl,
        body: &hir::Body,
    ) -> ClosureSignatures<'tcx> {
        debug!("sig_of_closure_no_expectation()");

        let bound_sig = self.supplied_sig_of_closure(decl);

        self.closure_sigs(expr_def_id, body, 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
    ///
    /// ```
    /// 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) ->
    /// - 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 def-id 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.
    fn sig_of_closure_with_expectation(
        &self,
        expr_def_id: DefId,
        decl: &hir::FnDecl,
        body: &hir::Body,
        expected_sig: ExpectedSig<'tcx>,
    ) -> ClosureSignatures<'tcx> {
        debug!(
            "sig_of_closure_with_expectation(expected_sig={:?})",
            expected_sig
        );

        // 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.variadic != decl.variadic {
            return self.sig_of_closure_no_expectation(expr_def_id, decl, body);
        } else if expected_sig.sig.inputs_and_output.len() != decl.inputs.len() + 1 {
            return self.sig_of_closure_with_mismatched_number_of_arguments(
                expr_def_id,
                decl,
                body,
                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.has_regions_bound_above(ty::INNERMOST));
        let bound_sig = ty::Binder::bind(self.tcx.mk_fn_sig(
            expected_sig.sig.inputs().iter().cloned(),
            expected_sig.sig.output(),
            decl.variadic,
            hir::Unsafety::Normal,
            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_late_bound_regions(&bound_sig);

        let closure_sigs = self.closure_sigs(expr_def_id, body, 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 tables, which are then later used by the privacy
        // check.
        match self.check_supplied_sig_against_expectation(decl, &closure_sigs) {
            Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
            Err(_) => return self.sig_of_closure_no_expectation(expr_def_id, decl, body),
        }

        closure_sigs
    }

    fn sig_of_closure_with_mismatched_number_of_arguments(
        &self,
        expr_def_id: DefId,
        decl: &hir::FnDecl,
        body: &hir::Body,
        expected_sig: ExpectedSig<'tcx>,
    ) -> ClosureSignatures<'tcx> {
        let expr_map_node = self.tcx.hir.get_if_local(expr_def_id).unwrap();
        let expected_args: Vec<_> = expected_sig
            .sig
            .inputs()
            .iter()
            .map(|ty| ArgKind::from_expected_ty(ty))
            .collect();
        let (closure_span, found_args) = self.get_fn_like_arguments(expr_map_node);
        let expected_span = expected_sig.cause_span.unwrap_or(closure_span);
        self.report_arg_count_mismatch(
            expected_span,
            Some(closure_span),
            expected_args,
            found_args,
            true,
        ).emit();

        let error_sig = self.error_sig_of_closure(decl);

        self.closure_sigs(expr_def_id, body, error_sig)
    }

    /// Enforce the user's types against the expectation.  See
    /// `sig_of_closure_with_expectation` for details on the overall
    /// strategy.
    fn check_supplied_sig_against_expectation(
        &self,
        decl: &hir::FnDecl,
        expected_sigs: &ClosureSignatures<'tcx>,
    ) -> InferResult<'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(decl);

        debug!(
            "check_supplied_sig_against_expectation: supplied_sig={:?}",
            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.infcx.commit_if_ok(|_| {
            let mut all_obligations = vec![];

            // The liberated version of this signature should be be a subtype
            // of the liberated form of the expectation.
            for ((hir_ty, &supplied_ty), expected_ty) in decl.inputs.iter()
                           .zip(*supplied_sig.inputs().skip_binder()) // binder moved to (*) below
                           .zip(expected_sigs.liberated_sig.inputs())
            // `liberated_sig` is E'.
            {
                // Instantiate (this part of..) S to S', i.e., with fresh variables.
                let (supplied_ty, _) = self.infcx.replace_late_bound_regions_with_fresh_var(
                    hir_ty.span,
                    LateBoundRegionConversionTime::FnCall,
                    &ty::Binder::bind(supplied_ty),
                ); // recreated from (*) above

                // Check that E' = S'.
                let cause = &self.misc(hir_ty.span);
                let InferOk {
                    value: (),
                    obligations,
                } = self.at(cause, self.param_env)
                    .eq(*expected_ty, supplied_ty)?;
                all_obligations.extend(obligations);
            }

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

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

    /// If there is no expected signature, then we will convert the
    /// types that the user gave into a signature.
    fn supplied_sig_of_closure(&self, decl: &hir::FnDecl) -> ty::PolyFnSig<'tcx> {
        let astconv: &dyn AstConv = self;

        // First, convert the types that the user supplied (if any).
        let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
        let supplied_return = match decl.output {
            hir::Return(ref output) => astconv.ast_ty_to_ty(&output),
            hir::DefaultReturn(_) => astconv.ty_infer(decl.output.span()),
        };

        let result = ty::Binder::bind(self.tcx.mk_fn_sig(
            supplied_arguments,
            supplied_return,
            decl.variadic,
            hir::Unsafety::Normal,
            Abi::RustCall,
        ));

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

        result
    }

    /// 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 `TyErr`.
    fn error_sig_of_closure(&self, decl: &hir::FnDecl) -> ty::PolyFnSig<'tcx> {
        let astconv: &dyn AstConv = self;

        let supplied_arguments = decl.inputs.iter().map(|a| {
            // Convert the types that the user supplied (if any), but ignore them.
            astconv.ast_ty_to_ty(a);
            self.tcx.types.err
        });

        match decl.output {
            hir::Return(ref output) => {
                astconv.ast_ty_to_ty(&output);
            }
            hir::DefaultReturn(_) => {}
        }

        let result = ty::Binder::bind(self.tcx.mk_fn_sig(
            supplied_arguments,
            self.tcx.types.err,
            decl.variadic,
            hir::Unsafety::Normal,
            Abi::RustCall,
        ));

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

        result
    }

    fn closure_sigs(
        &self,
        expr_def_id: DefId,
        body: &hir::Body,
        bound_sig: ty::PolyFnSig<'tcx>,
    ) -> ClosureSignatures<'tcx> {
        let liberated_sig = self.tcx()
            .liberate_late_bound_regions(expr_def_id, &bound_sig);
        let liberated_sig = self.inh.normalize_associated_types_in(
            body.value.span,
            body.value.id,
            self.param_env,
            &liberated_sig,
        );
        ClosureSignatures {
            bound_sig,
            liberated_sig,
        }
    }
}