compiler/rustc_infer/src/infer/opaque_types.rs

   1 use crate::infer::{InferCtxt, InferOk};
   2 use crate::traits;
   3 use hir::def_id::{DefId, LocalDefId};
   4 use hir::{HirId, OpaqueTyOrigin};
   5 use rustc_data_structures::sync::Lrc;
   6 use rustc_data_structures::vec_map::VecMap;
   7 use rustc_hir as hir;
   8 use rustc_middle::traits::ObligationCause;
   9 use rustc_middle::ty::fold::BottomUpFolder;
  10 use rustc_middle::ty::subst::{GenericArgKind, Subst};
  11 use rustc_middle::ty::{
  12     self, OpaqueHiddenType, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeVisitor,
  13 };
  14 use rustc_span::Span;
  15
  16 use std::ops::ControlFlow;
  17
  18 pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;
  19
  20 mod table;
  21
  22 pub use table::{OpaqueTypeStorage, OpaqueTypeTable};
  23
  24 use super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  25 use super::InferResult;
  26
  27 /// Information about the opaque types whose values we
  28 /// are inferring in this function (these are the `impl Trait` that
  29 /// appear in the return type).
  30 #[derive(Clone, Debug)]
  31 pub struct OpaqueTypeDecl<'tcx> {
  32     /// The hidden types that have been inferred for this opaque type.
  33     /// There can be multiple, but they are all `lub`ed together at the end
  34     /// to obtain the canonical hidden type.
  35     pub hidden_type: OpaqueHiddenType<'tcx>,
  36
  37     /// The origin of the opaque type.
  38     pub origin: hir::OpaqueTyOrigin,
  39 }
  40
  41 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
  42     /// This is a backwards compatibility hack to prevent breaking changes from
  43     /// lazy TAIT around RPIT handling.
  44     pub fn replace_opaque_types_with_inference_vars<T: TypeFoldable<'tcx>>(
  45         &self,
  46         value: T,
  47         body_id: HirId,
  48         span: Span,
  49         param_env: ty::ParamEnv<'tcx>,
  50     ) -> InferOk<'tcx, T> {
  51         if !value.has_opaque_types() {
  52             return InferOk { value, obligations: vec![] };
  53         }
  54         let mut obligations = vec![];
  55         let value = value.fold_with(&mut ty::fold::BottomUpFolder {
  56             tcx: self.tcx,
  57             lt_op: |lt| lt,
  58             ct_op: |ct| ct,
  59             ty_op: |ty| match *ty.kind() {
  60                 // Closures can't create hidden types for opaque types of their parent, as they
  61                 // do not have all the outlives information available. Also `type_of` looks for
  62                 // hidden types in the owner (so the closure's parent), so it would not find these
  63                 // definitions.
  64                 ty::Opaque(def_id, _substs)
  65                     if matches!(
  66                         self.opaque_type_origin(def_id, span),
  67                         Some(OpaqueTyOrigin::FnReturn(..))
  68                     ) =>
  69                 {
  70                     let span = if span.is_dummy() { self.tcx.def_span(def_id) } else { span };
  71                     let cause = ObligationCause::misc(span, body_id);
  72                     let ty_var = self.next_ty_var(TypeVariableOrigin {
  73                         kind: TypeVariableOriginKind::TypeInference,
  74                         span: cause.span,
  75                     });
  76                     obligations.extend(
  77                         self.handle_opaque_type(ty, ty_var, true, &cause, param_env)
  78                             .unwrap()
  79                             .obligations,
  80                     );
  81                     ty_var
  82                 }
  83                 _ => ty,
  84             },
  85         });
  86         InferOk { value, obligations }
  87     }
  88
  89     pub fn handle_opaque_type(
  90         &self,
  91         a: Ty<'tcx>,
  92         b: Ty<'tcx>,
  93         a_is_expected: bool,
  94         cause: &ObligationCause<'tcx>,
  95         param_env: ty::ParamEnv<'tcx>,
  96     ) -> InferResult<'tcx, ()> {
  97         if a.references_error() || b.references_error() {
  98             return Ok(InferOk { value: (), obligations: vec![] });
  99         }
 100         let (a, b) = if a_is_expected { (a, b) } else { (b, a) };
 101         let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() {
 102             ty::Opaque(def_id, substs) => {
 103                 let origin = if self.defining_use_anchor.is_some() {
 104                     // Check that this is `impl Trait` type is
 105                     // declared by `parent_def_id` -- i.e., one whose
 106                     // value we are inferring.  At present, this is
 107                     // always true during the first phase of
 108                     // type-check, but not always true later on during
 109                     // NLL. Once we support named opaque types more fully,
 110                     // this same scenario will be able to arise during all phases.
 111                     //
 112                     // Here is an example using type alias `impl Trait`
 113                     // that indicates the distinction we are checking for:
 114                     //
 115                     // ```rust
 116                     // mod a {
 117                     //   pub type Foo = impl Iterator;
 118                     //   pub fn make_foo() -> Foo { .. }
 119                     // }
 120                     //
 121                     // mod b {
 122                     //   fn foo() -> a::Foo { a::make_foo() }
 123                     // }
 124                     // ```
 125                     //
 126                     // Here, the return type of `foo` references an
 127                     // `Opaque` indeed, but not one whose value is
 128                     // presently being inferred. You can get into a
 129                     // similar situation with closure return types
 130                     // today:
 131                     //
 132                     // ```rust
 133                     // fn foo() -> impl Iterator { .. }
 134                     // fn bar() {
 135                     //     let x = || foo(); // returns the Opaque assoc with `foo`
 136                     // }
 137                     // ```
 138                     self.opaque_type_origin(def_id, cause.span)?
 139                 } else {
 140                     self.opaque_ty_origin_unchecked(def_id, cause.span)
 141                 };
 142                 if let ty::Opaque(did2, _) = *b.kind() {
 143                     // We could accept this, but there are various ways to handle this situation, and we don't
 144                     // want to make a decision on it right now. Likely this case is so super rare anyway, that
 145                     // no one encounters it in practice.
 146                     // It does occur however in `fn fut() -> impl Future<Output = i32> { async { 42 } }`,
 147                     // where it is of no concern, so we only check for TAITs.
 148                     if let Some(OpaqueTyOrigin::TyAlias) = self.opaque_type_origin(did2, cause.span)
 149                     {
 150                         self.tcx
 151                                 .sess
 152                                 .struct_span_err(
 153                                     cause.span,
 154                                     "opaque type's hidden type cannot be another opaque type from the same scope",
 155                                 )
 156                                 .span_label(cause.span, "one of the two opaque types used here has to be outside its defining scope")
 157                                 .span_note(
 158                                     self.tcx.def_span(def_id),
 159                                     "opaque type whose hidden type is being assigned",
 160                                 )
 161                                 .span_note(
 162                                     self.tcx.def_span(did2),
 163                                     "opaque type being used as hidden type",
 164                                 )
 165                                 .emit();
 166                     }
 167                 }
 168                 Some(self.register_hidden_type(
 169                     OpaqueTypeKey { def_id, substs },
 170                     cause.clone(),
 171                     param_env,
 172                     b,
 173                     origin,
 174                 ))
 175             }
 176             _ => None,
 177         };
 178         if let Some(res) = process(a, b) {
 179             res
 180         } else if let Some(res) = process(b, a) {
 181             res
 182         } else {
 183             // Rerun equality check, but this time error out due to
 184             // different types.
 185             match self.at(cause, param_env).define_opaque_types(false).eq(a, b) {
 186                 Ok(_) => span_bug!(
 187                     cause.span,
 188                     "opaque types are never equal to anything but themselves: {:#?}",
 189                     (a.kind(), b.kind())
 190                 ),
 191                 Err(e) => Err(e),
 192             }
 193         }
 194     }
 195
 196     /// Given the map `opaque_types` containing the opaque
 197     /// `impl Trait` types whose underlying, hidden types are being
 198     /// inferred, this method adds constraints to the regions
 199     /// appearing in those underlying hidden types to ensure that they
 200     /// at least do not refer to random scopes within the current
 201     /// function. These constraints are not (quite) sufficient to
 202     /// guarantee that the regions are actually legal values; that
 203     /// final condition is imposed after region inference is done.
 204     ///
 205     /// # The Problem
 206     ///
 207     /// Let's work through an example to explain how it works. Assume
 208     /// the current function is as follows:
 209     ///
 210     /// ```text
 211     /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
 212     /// ```
 213     ///
 214     /// Here, we have two `impl Trait` types whose values are being
 215     /// inferred (the `impl Bar<'a>` and the `impl
 216     /// Bar<'b>`). Conceptually, this is sugar for a setup where we
 217     /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
 218     /// the return type of `foo`, we *reference* those definitions:
 219     ///
 220     /// ```text
 221     /// type Foo1<'x> = impl Bar<'x>;
 222     /// type Foo2<'x> = impl Bar<'x>;
 223     /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
 224     ///                    //  ^^^^ ^^
 225     ///                    //  |    |
 226     ///                    //  |    substs
 227     ///                    //  def_id
 228     /// ```
 229     ///
 230     /// As indicating in the comments above, each of those references
 231     /// is (in the compiler) basically a substitution (`substs`)
 232     /// applied to the type of a suitable `def_id` (which identifies
 233     /// `Foo1` or `Foo2`).
 234     ///
 235     /// Now, at this point in compilation, what we have done is to
 236     /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
 237     /// fresh inference variables C1 and C2. We wish to use the values
 238     /// of these variables to infer the underlying types of `Foo1` and
 239     /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
 240     /// constraints like:
 241     ///
 242     /// ```text
 243     /// for<'a> (Foo1<'a> = C1)
 244     /// for<'b> (Foo1<'b> = C2)
 245     /// ```
 246     ///
 247     /// For these equation to be satisfiable, the types `C1` and `C2`
 248     /// can only refer to a limited set of regions. For example, `C1`
 249     /// can only refer to `'static` and `'a`, and `C2` can only refer
 250     /// to `'static` and `'b`. The job of this function is to impose that
 251     /// constraint.
 252     ///
 253     /// Up to this point, C1 and C2 are basically just random type
 254     /// inference variables, and hence they may contain arbitrary
 255     /// regions. In fact, it is fairly likely that they do! Consider
 256     /// this possible definition of `foo`:
 257     ///
 258     /// ```text
 259     /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
 260     ///         (&*x, &*y)
 261     ///     }
 262     /// ```
 263     ///
 264     /// Here, the values for the concrete types of the two impl
 265     /// traits will include inference variables:
 266     ///
 267     /// ```text
 268     /// &'0 i32
 269     /// &'1 i32
 270     /// ```
 271     ///
 272     /// Ordinarily, the subtyping rules would ensure that these are
 273     /// sufficiently large. But since `impl Bar<'a>` isn't a specific
 274     /// type per se, we don't get such constraints by default. This
 275     /// is where this function comes into play. It adds extra
 276     /// constraints to ensure that all the regions which appear in the
 277     /// inferred type are regions that could validly appear.
 278     ///
 279     /// This is actually a bit of a tricky constraint in general. We
 280     /// want to say that each variable (e.g., `'0`) can only take on
 281     /// values that were supplied as arguments to the opaque type
 282     /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
 283     /// scope. We don't have a constraint quite of this kind in the current
 284     /// region checker.
 285     ///
 286     /// # The Solution
 287     ///
 288     /// We generally prefer to make `<=` constraints, since they
 289     /// integrate best into the region solver. To do that, we find the
 290     /// "minimum" of all the arguments that appear in the substs: that
 291     /// is, some region which is less than all the others. In the case
 292     /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
 293     /// all). Then we apply that as a least bound to the variables
 294     /// (e.g., `'a <= '0`).
 295     ///
 296     /// In some cases, there is no minimum. Consider this example:
 297     ///
 298     /// ```text
 299     /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
 300     /// ```
 301     ///
 302     /// Here we would report a more complex "in constraint", like `'r
 303     /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
 304     /// the hidden type).
 305     ///
 306     /// # Constrain regions, not the hidden concrete type
 307     ///
 308     /// Note that generating constraints on each region `Rc` is *not*
 309     /// the same as generating an outlives constraint on `Tc` itself.
 310     /// For example, if we had a function like this:
 311     ///
 312     /// ```rust
 313     /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
 314     ///   (x, y)
 315     /// }
 316     ///
 317     /// // Equivalent to:
 318     /// type FooReturn<'a, T> = impl Foo<'a>;
 319     /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
 320     /// ```
 321     ///
 322     /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
 323     /// is an inference variable). If we generated a constraint that
 324     /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
 325     /// but this is not necessary, because the opaque type we
 326     /// create will be allowed to reference `T`. So we only generate a
 327     /// constraint that `'0: 'a`.
 328     #[instrument(level = "debug", skip(self))]
 329     pub fn register_member_constraints(
 330         &self,
 331         param_env: ty::ParamEnv<'tcx>,
 332         opaque_type_key: OpaqueTypeKey<'tcx>,
 333         concrete_ty: Ty<'tcx>,
 334         span: Span,
 335     ) {
 336         let def_id = opaque_type_key.def_id;
 337
 338         let tcx = self.tcx;
 339
 340         let concrete_ty = self.resolve_vars_if_possible(concrete_ty);
 341
 342         debug!(?concrete_ty);
 343
 344         let first_own_region = match self.opaque_ty_origin_unchecked(def_id, span) {
 345             hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {
 346                 // We lower
 347                 //
 348                 // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
 349                 //
 350                 // into
 351                 //
 352                 // type foo::<'p0..'pn>::Foo<'q0..'qm>
 353                 // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
 354                 //
 355                 // For these types we only iterate over `'l0..lm` below.
 356                 tcx.generics_of(def_id).parent_count
 357             }
 358             // These opaque type inherit all lifetime parameters from their
 359             // parent, so we have to check them all.
 360             hir::OpaqueTyOrigin::TyAlias => 0,
 361         };
 362
 363         // For a case like `impl Foo<'a, 'b>`, we would generate a constraint
 364         // `'r in ['a, 'b, 'static]` for each region `'r` that appears in the
 365         // hidden type (i.e., it must be equal to `'a`, `'b`, or `'static`).
 366         //
 367         // `conflict1` and `conflict2` are the two region bounds that we
 368         // detected which were unrelated. They are used for diagnostics.
 369
 370         // Create the set of choice regions: each region in the hidden
 371         // type can be equal to any of the region parameters of the
 372         // opaque type definition.
 373         let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
 374             opaque_type_key.substs[first_own_region..]
 375                 .iter()
 376                 .filter_map(|arg| match arg.unpack() {
 377                     GenericArgKind::Lifetime(r) => Some(r),
 378                     GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
 379                 })
 380                 .chain(std::iter::once(self.tcx.lifetimes.re_static))
 381                 .collect(),
 382         );
 383
 384         concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
 385             op: |r| {
 386                 self.member_constraint(
 387                     opaque_type_key.def_id,
 388                     span,
 389                     concrete_ty,
 390                     r,
 391                     &choice_regions,
 392                 )
 393             },
 394         });
 395     }
 396
 397     #[instrument(skip(self), level = "trace")]
 398     pub fn opaque_type_origin(&self, opaque_def_id: DefId, span: Span) -> Option<OpaqueTyOrigin> {
 399         let def_id = opaque_def_id.as_local()?;
 400         let opaque_hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
 401         let parent_def_id = self.defining_use_anchor?;
 402         let item_kind = &self.tcx.hir().expect_item(def_id).kind;
 403
 404         let hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, ..  }) = item_kind else {
 405             span_bug!(
 406                 span,
 407                 "weird opaque type: {:#?}, {:#?}",
 408                 opaque_def_id,
 409                 item_kind
 410             )
 411         };
 412         let in_definition_scope = match *origin {
 413             // Async `impl Trait`
 414             hir::OpaqueTyOrigin::AsyncFn(parent) => parent == parent_def_id,
 415             // Anonymous `impl Trait`
 416             hir::OpaqueTyOrigin::FnReturn(parent) => parent == parent_def_id,
 417             // Named `type Foo = impl Bar;`
 418             hir::OpaqueTyOrigin::TyAlias => {
 419                 may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id)
 420             }
 421         };
 422         trace!(?origin);
 423         in_definition_scope.then_some(*origin)
 424     }
 425
 426     #[instrument(skip(self), level = "trace")]
 427     fn opaque_ty_origin_unchecked(&self, opaque_def_id: DefId, span: Span) -> OpaqueTyOrigin {
 428         let def_id = opaque_def_id.as_local().unwrap();
 429         let origin = match self.tcx.hir().expect_item(def_id).kind {
 430             hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => origin,
 431             ref itemkind => {
 432                 span_bug!(span, "weird opaque type: {:?}, {:#?}", opaque_def_id, itemkind)
 433             }
 434         };
 435         trace!(?origin);
 436         origin
 437     }
 438 }
 439
 440 // Visitor that requires that (almost) all regions in the type visited outlive
 441 // `least_region`. We cannot use `push_outlives_components` because regions in
 442 // closure signatures are not included in their outlives components. We need to
 443 // ensure all regions outlive the given bound so that we don't end up with,
 444 // say, `ReVar` appearing in a return type and causing ICEs when other
 445 // functions end up with region constraints involving regions from other
 446 // functions.
 447 //
 448 // We also cannot use `for_each_free_region` because for closures it includes
 449 // the regions parameters from the enclosing item.
 450 //
 451 // We ignore any type parameters because impl trait values are assumed to
 452 // capture all the in-scope type parameters.
 453 struct ConstrainOpaqueTypeRegionVisitor<OP> {
 454     op: OP,
 455 }
 456
 457 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
 458 where
 459     OP: FnMut(ty::Region<'tcx>),
 460 {
 461     fn visit_binder<T: TypeFoldable<'tcx>>(
 462         &mut self,
 463         t: &ty::Binder<'tcx, T>,
 464     ) -> ControlFlow<Self::BreakTy> {
 465         t.as_ref().skip_binder().visit_with(self);
 466         ControlFlow::CONTINUE
 467     }
 468
 469     fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
 470         match *r {
 471             // ignore bound regions, keep visiting
 472             ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
 473             _ => {
 474                 (self.op)(r);
 475                 ControlFlow::CONTINUE
 476             }
 477         }
 478     }
 479
 480     fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 481         // We're only interested in types involving regions
 482         if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
 483             return ControlFlow::CONTINUE;
 484         }
 485
 486         match ty.kind() {
 487             ty::Closure(_, ref substs) => {
 488                 // Skip lifetime parameters of the enclosing item(s)
 489
 490                 substs.as_closure().tupled_upvars_ty().visit_with(self);
 491                 substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
 492             }
 493
 494             ty::Generator(_, ref substs, _) => {
 495                 // Skip lifetime parameters of the enclosing item(s)
 496                 // Also skip the witness type, because that has no free regions.
 497
 498                 substs.as_generator().tupled_upvars_ty().visit_with(self);
 499                 substs.as_generator().return_ty().visit_with(self);
 500                 substs.as_generator().yield_ty().visit_with(self);
 501                 substs.as_generator().resume_ty().visit_with(self);
 502             }
 503             _ => {
 504                 ty.super_visit_with(self);
 505             }
 506         }
 507
 508         ControlFlow::CONTINUE
 509     }
 510 }
 511
 512 pub enum UseKind {
 513     DefiningUse,
 514     OpaqueUse,
 515 }
 516
 517 impl UseKind {
 518     pub fn is_defining(self) -> bool {
 519         match self {
 520             UseKind::DefiningUse => true,
 521             UseKind::OpaqueUse => false,
 522         }
 523     }
 524 }
 525
 526 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 527     #[instrument(skip(self), level = "debug")]
 528     pub fn register_hidden_type(
 529         &self,
 530         opaque_type_key: OpaqueTypeKey<'tcx>,
 531         cause: ObligationCause<'tcx>,
 532         param_env: ty::ParamEnv<'tcx>,
 533         hidden_ty: Ty<'tcx>,
 534         origin: hir::OpaqueTyOrigin,
 535     ) -> InferResult<'tcx, ()> {
 536         let tcx = self.tcx;
 537         let OpaqueTypeKey { def_id, substs } = opaque_type_key;
 538
 539         // Ideally, we'd get the span where *this specific `ty` came
 540         // from*, but right now we just use the span from the overall
 541         // value being folded. In simple cases like `-> impl Foo`,
 542         // these are the same span, but not in cases like `-> (impl
 543         // Foo, impl Bar)`.
 544         let span = cause.span;
 545
 546         let mut obligations = vec![];
 547         let prev = self.inner.borrow_mut().opaque_types().register(
 548             OpaqueTypeKey { def_id, substs },
 549             OpaqueHiddenType { ty: hidden_ty, span },
 550             origin,
 551         );
 552         if let Some(prev) = prev {
 553             obligations = self.at(&cause, param_env).eq(prev, hidden_ty)?.obligations;
 554         }
 555
 556         let item_bounds = tcx.explicit_item_bounds(def_id);
 557
 558         for (predicate, _) in item_bounds {
 559             debug!(?predicate);
 560             let predicate = predicate.subst(tcx, substs);
 561
 562             let predicate = predicate.fold_with(&mut BottomUpFolder {
 563                 tcx,
 564                 ty_op: |ty| match *ty.kind() {
 565                     // We can't normalize associated types from `rustc_infer`,
 566                     // but we can eagerly register inference variables for them.
 567                     ty::Projection(projection_ty) if !projection_ty.has_escaping_bound_vars() => {
 568                         self.infer_projection(
 569                             param_env,
 570                             projection_ty,
 571                             cause.clone(),
 572                             0,
 573                             &mut obligations,
 574                         )
 575                     }
 576                     // Replace all other mentions of the same opaque type with the hidden type,
 577                     // as the bounds must hold on the hidden type after all.
 578                     ty::Opaque(def_id2, substs2) if def_id == def_id2 && substs == substs2 => {
 579                         hidden_ty
 580                     }
 581                     _ => ty,
 582                 },
 583                 lt_op: |lt| lt,
 584                 ct_op: |ct| ct,
 585             });
 586
 587             if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
 588                 if projection.term.references_error() {
 589                     // No point on adding these obligations since there's a type error involved.
 590                     return Ok(InferOk { value: (), obligations: vec![] });
 591                 }
 592                 trace!("{:#?}", projection.term);
 593             }
 594             // Require that the predicate holds for the concrete type.
 595             debug!(?predicate);
 596             obligations.push(traits::Obligation::new(cause.clone(), param_env, predicate));
 597         }
 598         Ok(InferOk { value: (), obligations })
 599     }
 600 }
 601
 602 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
 603 ///
 604 /// Example:
 605 /// ```rust
 606 /// pub mod foo {
 607 ///     pub mod bar {
 608 ///         pub trait Bar { .. }
 609 ///
 610 ///         pub type Baz = impl Bar;
 611 ///
 612 ///         fn f1() -> Baz { .. }
 613 ///     }
 614 ///
 615 ///     fn f2() -> bar::Baz { .. }
 616 /// }
 617 /// ```
 618 ///
 619 /// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
 620 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
 621 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
 622 fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: LocalDefId, opaque_hir_id: hir::HirId) -> bool {
 623     let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 624
 625     // Named opaque types can be defined by any siblings or children of siblings.
 626     let scope = tcx.hir().get_defining_scope(opaque_hir_id);
 627     // We walk up the node tree until we hit the root or the scope of the opaque type.
 628     while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
 629         hir_id = tcx.hir().local_def_id_to_hir_id(tcx.hir().get_parent_item(hir_id));
 630     }
 631     // Syntactically, we are allowed to define the concrete type if:
 632     let res = hir_id == scope;
 633     trace!(
 634         "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
 635         tcx.hir().find(hir_id),
 636         tcx.hir().get(opaque_hir_id),
 637         res
 638     );
 639     res
 640 }