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
use crate::infer::{DefiningAnchor, InferCtxt, InferOk};
use crate::traits;
use hir::def_id::{DefId, LocalDefId};
use hir::{HirId, OpaqueTyOrigin};
use rustc_data_structures::sync::Lrc;
use rustc_data_structures::vec_map::VecMap;
use rustc_hir as hir;
use rustc_middle::traits::ObligationCause;
use rustc_middle::ty::fold::BottomUpFolder;
use rustc_middle::ty::subst::{GenericArgKind, Subst};
use rustc_middle::ty::{
    self, OpaqueHiddenType, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable,
    TypeVisitable, TypeVisitor,
};
use rustc_span::Span;

use std::ops::ControlFlow;

pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;

mod table;

pub use table::{OpaqueTypeStorage, OpaqueTypeTable};

use super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
use super::InferResult;

/// Information about the opaque types whose values we
/// are inferring in this function (these are the `impl Trait` that
/// appear in the return type).
#[derive(Clone, Debug)]
pub struct OpaqueTypeDecl<'tcx> {
    /// The hidden types that have been inferred for this opaque type.
    /// There can be multiple, but they are all `lub`ed together at the end
    /// to obtain the canonical hidden type.
    pub hidden_type: OpaqueHiddenType<'tcx>,

    /// The origin of the opaque type.
    pub origin: hir::OpaqueTyOrigin,
}

impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
    /// This is a backwards compatibility hack to prevent breaking changes from
    /// lazy TAIT around RPIT handling.
    pub fn replace_opaque_types_with_inference_vars<T: TypeFoldable<'tcx>>(
        &self,
        value: T,
        body_id: HirId,
        span: Span,
        param_env: ty::ParamEnv<'tcx>,
    ) -> InferOk<'tcx, T> {
        if !value.has_opaque_types() {
            return InferOk { value, obligations: vec![] };
        }
        let mut obligations = vec![];
        let replace_opaque_type = |def_id: DefId| {
            def_id
                .as_local()
                .map_or(false, |def_id| self.opaque_type_origin(def_id, span).is_some())
        };
        let value = value.fold_with(&mut ty::fold::BottomUpFolder {
            tcx: self.tcx,
            lt_op: |lt| lt,
            ct_op: |ct| ct,
            ty_op: |ty| match *ty.kind() {
                ty::Opaque(def_id, _substs) if replace_opaque_type(def_id) => {
                    let def_span = self.tcx.def_span(def_id);
                    let span = if span.contains(def_span) { def_span } else { span };
                    let code = traits::ObligationCauseCode::OpaqueReturnType(None);
                    let cause = ObligationCause::new(span, body_id, code);
                    // FIXME(compiler-errors): We probably should add a new TypeVariableOriginKind
                    // for opaque types, and then use that kind to fix the spans for type errors
                    // that we see later on.
                    let ty_var = self.next_ty_var(TypeVariableOrigin {
                        kind: TypeVariableOriginKind::TypeInference,
                        span,
                    });
                    obligations.extend(
                        self.handle_opaque_type(ty, ty_var, true, &cause, param_env)
                            .unwrap()
                            .obligations,
                    );
                    ty_var
                }
                _ => ty,
            },
        });
        InferOk { value, obligations }
    }

    pub fn handle_opaque_type(
        &self,
        a: Ty<'tcx>,
        b: Ty<'tcx>,
        a_is_expected: bool,
        cause: &ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
    ) -> InferResult<'tcx, ()> {
        if a.references_error() || b.references_error() {
            return Ok(InferOk { value: (), obligations: vec![] });
        }
        let (a, b) = if a_is_expected { (a, b) } else { (b, a) };
        let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() {
            ty::Opaque(def_id, substs) if def_id.is_local() => {
                let def_id = def_id.expect_local();
                let origin = match self.defining_use_anchor {
                    DefiningAnchor::Bind(_) => {
                        // Check that this is `impl Trait` type is
                        // declared by `parent_def_id` -- i.e., one whose
                        // value we are inferring.  At present, this is
                        // always true during the first phase of
                        // type-check, but not always true later on during
                        // NLL. Once we support named opaque types more fully,
                        // this same scenario will be able to arise during all phases.
                        //
                        // Here is an example using type alias `impl Trait`
                        // that indicates the distinction we are checking for:
                        //
                        // ```rust
                        // mod a {
                        //   pub type Foo = impl Iterator;
                        //   pub fn make_foo() -> Foo { .. }
                        // }
                        //
                        // mod b {
                        //   fn foo() -> a::Foo { a::make_foo() }
                        // }
                        // ```
                        //
                        // Here, the return type of `foo` references an
                        // `Opaque` indeed, but not one whose value is
                        // presently being inferred. You can get into a
                        // similar situation with closure return types
                        // today:
                        //
                        // ```rust
                        // fn foo() -> impl Iterator { .. }
                        // fn bar() {
                        //     let x = || foo(); // returns the Opaque assoc with `foo`
                        // }
                        // ```
                        self.opaque_type_origin(def_id, cause.span)?
                    }
                    DefiningAnchor::Bubble => self.opaque_ty_origin_unchecked(def_id, cause.span),
                    DefiningAnchor::Error => return None,
                };
                if let ty::Opaque(did2, _) = *b.kind() {
                    // We could accept this, but there are various ways to handle this situation, and we don't
                    // want to make a decision on it right now. Likely this case is so super rare anyway, that
                    // no one encounters it in practice.
                    // It does occur however in `fn fut() -> impl Future<Output = i32> { async { 42 } }`,
                    // where it is of no concern, so we only check for TAITs.
                    if let Some(OpaqueTyOrigin::TyAlias) =
                        did2.as_local().and_then(|did2| self.opaque_type_origin(did2, cause.span))
                    {
                        self.tcx
                                .sess
                                .struct_span_err(
                                    cause.span,
                                    "opaque type's hidden type cannot be another opaque type from the same scope",
                                )
                                .span_label(cause.span, "one of the two opaque types used here has to be outside its defining scope")
                                .span_note(
                                    self.tcx.def_span(def_id),
                                    "opaque type whose hidden type is being assigned",
                                )
                                .span_note(
                                    self.tcx.def_span(did2),
                                    "opaque type being used as hidden type",
                                )
                                .emit();
                    }
                }
                Some(self.register_hidden_type(
                    OpaqueTypeKey { def_id, substs },
                    cause.clone(),
                    param_env,
                    b,
                    origin,
                ))
            }
            _ => None,
        };
        if let Some(res) = process(a, b) {
            res
        } else if let Some(res) = process(b, a) {
            res
        } else {
            // Rerun equality check, but this time error out due to
            // different types.
            match self.at(cause, param_env).define_opaque_types(false).eq(a, b) {
                Ok(_) => span_bug!(
                    cause.span,
                    "opaque types are never equal to anything but themselves: {:#?}",
                    (a.kind(), b.kind())
                ),
                Err(e) => Err(e),
            }
        }
    }

    /// Given the map `opaque_types` containing the opaque
    /// `impl Trait` types whose underlying, hidden types are being
    /// inferred, this method adds constraints to the regions
    /// appearing in those underlying hidden types to ensure that they
    /// at least do not refer to random scopes within the current
    /// function. These constraints are not (quite) sufficient to
    /// guarantee that the regions are actually legal values; that
    /// final condition is imposed after region inference is done.
    ///
    /// # The Problem
    ///
    /// Let's work through an example to explain how it works. Assume
    /// the current function is as follows:
    ///
    /// ```text
    /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
    /// ```
    ///
    /// Here, we have two `impl Trait` types whose values are being
    /// inferred (the `impl Bar<'a>` and the `impl
    /// Bar<'b>`). Conceptually, this is sugar for a setup where we
    /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
    /// the return type of `foo`, we *reference* those definitions:
    ///
    /// ```text
    /// type Foo1<'x> = impl Bar<'x>;
    /// type Foo2<'x> = impl Bar<'x>;
    /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
    ///                    //  ^^^^ ^^
    ///                    //  |    |
    ///                    //  |    substs
    ///                    //  def_id
    /// ```
    ///
    /// As indicating in the comments above, each of those references
    /// is (in the compiler) basically a substitution (`substs`)
    /// applied to the type of a suitable `def_id` (which identifies
    /// `Foo1` or `Foo2`).
    ///
    /// Now, at this point in compilation, what we have done is to
    /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
    /// fresh inference variables C1 and C2. We wish to use the values
    /// of these variables to infer the underlying types of `Foo1` and
    /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
    /// constraints like:
    ///
    /// ```text
    /// for<'a> (Foo1<'a> = C1)
    /// for<'b> (Foo1<'b> = C2)
    /// ```
    ///
    /// For these equation to be satisfiable, the types `C1` and `C2`
    /// can only refer to a limited set of regions. For example, `C1`
    /// can only refer to `'static` and `'a`, and `C2` can only refer
    /// to `'static` and `'b`. The job of this function is to impose that
    /// constraint.
    ///
    /// Up to this point, C1 and C2 are basically just random type
    /// inference variables, and hence they may contain arbitrary
    /// regions. In fact, it is fairly likely that they do! Consider
    /// this possible definition of `foo`:
    ///
    /// ```text
    /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
    ///         (&*x, &*y)
    ///     }
    /// ```
    ///
    /// Here, the values for the concrete types of the two impl
    /// traits will include inference variables:
    ///
    /// ```text
    /// &'0 i32
    /// &'1 i32
    /// ```
    ///
    /// Ordinarily, the subtyping rules would ensure that these are
    /// sufficiently large. But since `impl Bar<'a>` isn't a specific
    /// type per se, we don't get such constraints by default. This
    /// is where this function comes into play. It adds extra
    /// constraints to ensure that all the regions which appear in the
    /// inferred type are regions that could validly appear.
    ///
    /// This is actually a bit of a tricky constraint in general. We
    /// want to say that each variable (e.g., `'0`) can only take on
    /// values that were supplied as arguments to the opaque type
    /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
    /// scope. We don't have a constraint quite of this kind in the current
    /// region checker.
    ///
    /// # The Solution
    ///
    /// We generally prefer to make `<=` constraints, since they
    /// integrate best into the region solver. To do that, we find the
    /// "minimum" of all the arguments that appear in the substs: that
    /// is, some region which is less than all the others. In the case
    /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
    /// all). Then we apply that as a least bound to the variables
    /// (e.g., `'a <= '0`).
    ///
    /// In some cases, there is no minimum. Consider this example:
    ///
    /// ```text
    /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
    /// ```
    ///
    /// Here we would report a more complex "in constraint", like `'r
    /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
    /// the hidden type).
    ///
    /// # Constrain regions, not the hidden concrete type
    ///
    /// Note that generating constraints on each region `Rc` is *not*
    /// the same as generating an outlives constraint on `Tc` itself.
    /// For example, if we had a function like this:
    ///
    /// ```
    /// # #![feature(type_alias_impl_trait)]
    /// # fn main() {}
    /// # trait Foo<'a> {}
    /// # impl<'a, T> Foo<'a> for (&'a u32, T) {}
    /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
    ///   (x, y)
    /// }
    ///
    /// // Equivalent to:
    /// # mod dummy { use super::*;
    /// type FooReturn<'a, T> = impl Foo<'a>;
    /// fn foo<'a, T>(x: &'a u32, y: T) -> FooReturn<'a, T> {
    ///   (x, y)
    /// }
    /// # }
    /// ```
    ///
    /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
    /// is an inference variable). If we generated a constraint that
    /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
    /// but this is not necessary, because the opaque type we
    /// create will be allowed to reference `T`. So we only generate a
    /// constraint that `'0: 'a`.
    #[instrument(level = "debug", skip(self))]
    pub fn register_member_constraints(
        &self,
        param_env: ty::ParamEnv<'tcx>,
        opaque_type_key: OpaqueTypeKey<'tcx>,
        concrete_ty: Ty<'tcx>,
        span: Span,
    ) {
        let def_id = opaque_type_key.def_id;

        let tcx = self.tcx;

        let concrete_ty = self.resolve_vars_if_possible(concrete_ty);

        debug!(?concrete_ty);

        let first_own_region = match self.opaque_ty_origin_unchecked(def_id, span) {
            hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {
                // We lower
                //
                // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
                //
                // into
                //
                // type foo::<'p0..'pn>::Foo<'q0..'qm>
                // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
                //
                // For these types we only iterate over `'l0..lm` below.
                tcx.generics_of(def_id).parent_count
            }
            // These opaque type inherit all lifetime parameters from their
            // parent, so we have to check them all.
            hir::OpaqueTyOrigin::TyAlias => 0,
        };

        // For a case like `impl Foo<'a, 'b>`, we would generate a constraint
        // `'r in ['a, 'b, 'static]` for each region `'r` that appears in the
        // hidden type (i.e., it must be equal to `'a`, `'b`, or `'static`).
        //
        // `conflict1` and `conflict2` are the two region bounds that we
        // detected which were unrelated. They are used for diagnostics.

        // Create the set of choice regions: each region in the hidden
        // type can be equal to any of the region parameters of the
        // opaque type definition.
        let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
            opaque_type_key.substs[first_own_region..]
                .iter()
                .filter_map(|arg| match arg.unpack() {
                    GenericArgKind::Lifetime(r) => Some(r),
                    GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
                })
                .chain(std::iter::once(self.tcx.lifetimes.re_static))
                .collect(),
        );

        concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
            op: |r| self.member_constraint(opaque_type_key, span, concrete_ty, r, &choice_regions),
        });
    }

    #[instrument(skip(self), level = "trace")]
    pub fn opaque_type_origin(&self, def_id: LocalDefId, span: Span) -> Option<OpaqueTyOrigin> {
        let opaque_hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
        let parent_def_id = match self.defining_use_anchor {
            DefiningAnchor::Bubble | DefiningAnchor::Error => return None,
            DefiningAnchor::Bind(bind) => bind,
        };
        let item_kind = &self.tcx.hir().expect_item(def_id).kind;

        let hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, ..  }) = item_kind else {
            span_bug!(
                span,
                "weird opaque type: {:#?}, {:#?}",
                def_id,
                item_kind
            )
        };
        let in_definition_scope = match *origin {
            // Async `impl Trait`
            hir::OpaqueTyOrigin::AsyncFn(parent) => parent == parent_def_id,
            // Anonymous `impl Trait`
            hir::OpaqueTyOrigin::FnReturn(parent) => parent == parent_def_id,
            // Named `type Foo = impl Bar;`
            hir::OpaqueTyOrigin::TyAlias => {
                may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id)
            }
        };
        trace!(?origin);
        in_definition_scope.then_some(*origin)
    }

    #[instrument(skip(self), level = "trace")]
    fn opaque_ty_origin_unchecked(&self, def_id: LocalDefId, span: Span) -> OpaqueTyOrigin {
        let origin = match self.tcx.hir().expect_item(def_id).kind {
            hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => origin,
            ref itemkind => {
                span_bug!(span, "weird opaque type: {:?}, {:#?}", def_id, itemkind)
            }
        };
        trace!(?origin);
        origin
    }
}

// Visitor that requires that (almost) all regions in the type visited outlive
// `least_region`. We cannot use `push_outlives_components` because regions in
// closure signatures are not included in their outlives components. We need to
// ensure all regions outlive the given bound so that we don't end up with,
// say, `ReVar` appearing in a return type and causing ICEs when other
// functions end up with region constraints involving regions from other
// functions.
//
// We also cannot use `for_each_free_region` because for closures it includes
// the regions parameters from the enclosing item.
//
// We ignore any type parameters because impl trait values are assumed to
// capture all the in-scope type parameters.
struct ConstrainOpaqueTypeRegionVisitor<OP> {
    op: OP,
}

impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
where
    OP: FnMut(ty::Region<'tcx>),
{
    fn visit_binder<T: TypeVisitable<'tcx>>(
        &mut self,
        t: &ty::Binder<'tcx, T>,
    ) -> ControlFlow<Self::BreakTy> {
        t.super_visit_with(self);
        ControlFlow::CONTINUE
    }

    fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
        match *r {
            // ignore bound regions, keep visiting
            ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
            _ => {
                (self.op)(r);
                ControlFlow::CONTINUE
            }
        }
    }

    fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
        // We're only interested in types involving regions
        if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
            return ControlFlow::CONTINUE;
        }

        match ty.kind() {
            ty::Closure(_, ref substs) => {
                // Skip lifetime parameters of the enclosing item(s)

                substs.as_closure().tupled_upvars_ty().visit_with(self);
                substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
            }

            ty::Generator(_, ref substs, _) => {
                // Skip lifetime parameters of the enclosing item(s)
                // Also skip the witness type, because that has no free regions.

                substs.as_generator().tupled_upvars_ty().visit_with(self);
                substs.as_generator().return_ty().visit_with(self);
                substs.as_generator().yield_ty().visit_with(self);
                substs.as_generator().resume_ty().visit_with(self);
            }
            _ => {
                ty.super_visit_with(self);
            }
        }

        ControlFlow::CONTINUE
    }
}

pub enum UseKind {
    DefiningUse,
    OpaqueUse,
}

impl UseKind {
    pub fn is_defining(self) -> bool {
        match self {
            UseKind::DefiningUse => true,
            UseKind::OpaqueUse => false,
        }
    }
}

impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
    #[instrument(skip(self), level = "debug")]
    pub fn register_hidden_type(
        &self,
        opaque_type_key: OpaqueTypeKey<'tcx>,
        cause: ObligationCause<'tcx>,
        param_env: ty::ParamEnv<'tcx>,
        hidden_ty: Ty<'tcx>,
        origin: hir::OpaqueTyOrigin,
    ) -> InferResult<'tcx, ()> {
        let tcx = self.tcx;
        let OpaqueTypeKey { def_id, substs } = opaque_type_key;

        // Ideally, we'd get the span where *this specific `ty` came
        // from*, but right now we just use the span from the overall
        // value being folded. In simple cases like `-> impl Foo`,
        // these are the same span, but not in cases like `-> (impl
        // Foo, impl Bar)`.
        let span = cause.span;

        let mut obligations = vec![];
        let prev = self.inner.borrow_mut().opaque_types().register(
            OpaqueTypeKey { def_id, substs },
            OpaqueHiddenType { ty: hidden_ty, span },
            origin,
        );
        if let Some(prev) = prev {
            obligations = self.at(&cause, param_env).eq(prev, hidden_ty)?.obligations;
        }

        let item_bounds = tcx.bound_explicit_item_bounds(def_id.to_def_id());

        for predicate in item_bounds.transpose_iter().map(|e| e.map_bound(|(p, _)| *p)) {
            debug!(?predicate);
            let predicate = predicate.subst(tcx, substs);

            let predicate = predicate.fold_with(&mut BottomUpFolder {
                tcx,
                ty_op: |ty| match *ty.kind() {
                    // We can't normalize associated types from `rustc_infer`,
                    // but we can eagerly register inference variables for them.
                    ty::Projection(projection_ty) if !projection_ty.has_escaping_bound_vars() => {
                        self.infer_projection(
                            param_env,
                            projection_ty,
                            cause.clone(),
                            0,
                            &mut obligations,
                        )
                    }
                    // Replace all other mentions of the same opaque type with the hidden type,
                    // as the bounds must hold on the hidden type after all.
                    ty::Opaque(def_id2, substs2)
                        if def_id.to_def_id() == def_id2 && substs == substs2 =>
                    {
                        hidden_ty
                    }
                    _ => ty,
                },
                lt_op: |lt| lt,
                ct_op: |ct| ct,
            });

            if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
                if projection.term.references_error() {
                    // No point on adding these obligations since there's a type error involved.
                    return Ok(InferOk { value: (), obligations: vec![] });
                }
                trace!("{:#?}", projection.term);
            }
            // Require that the predicate holds for the concrete type.
            debug!(?predicate);
            obligations.push(traits::Obligation::new(cause.clone(), param_env, predicate));
        }
        Ok(InferOk { value: (), obligations })
    }
}

/// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
///
/// Example:
/// ```ignore UNSOLVED (is this a bug?)
/// # #![feature(type_alias_impl_trait)]
/// pub mod foo {
///     pub mod bar {
///         pub trait Bar { /* ... */ }
///         pub type Baz = impl Bar;
///
///         # impl Bar for () {}
///         fn f1() -> Baz { /* ... */ }
///     }
///     fn f2() -> bar::Baz { /* ... */ }
/// }
/// ```
///
/// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
/// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
/// For the above example, this function returns `true` for `f1` and `false` for `f2`.
fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: LocalDefId, opaque_hir_id: hir::HirId) -> bool {
    let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);

    // Named opaque types can be defined by any siblings or children of siblings.
    let scope = tcx.hir().get_defining_scope(opaque_hir_id);
    // We walk up the node tree until we hit the root or the scope of the opaque type.
    while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
        hir_id = tcx.hir().local_def_id_to_hir_id(tcx.hir().get_parent_item(hir_id));
    }
    // Syntactically, we are allowed to define the concrete type if:
    let res = hir_id == scope;
    trace!(
        "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
        tcx.hir().find(hir_id),
        tcx.hir().get(opaque_hir_id),
        res
    );
    res
}