1 use crate::infer::InferCtxtExt as _;
2 use crate::traits::{self, ObligationCause, PredicateObligation};
3 use rustc_data_structures::fx::FxHashMap;
4 use rustc_data_structures::sync::Lrc;
5 use rustc_data_structures::vec_map::VecMap;
7 use rustc_hir::def_id::{DefId, LocalDefId};
8 use rustc_infer::infer::error_reporting::unexpected_hidden_region_diagnostic;
9 use rustc_infer::infer::free_regions::FreeRegionRelations;
10 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
11 use rustc_infer::infer::{self, InferCtxt, InferOk};
12 use rustc_middle::ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder, TypeVisitor};
13 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst};
14 use rustc_middle::ty::{self, OpaqueTypeKey, Ty, TyCtxt};
17 use std::ops::ControlFlow;
19 pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;
21 /// Information about the opaque types whose values we
22 /// are inferring in this function (these are the `impl Trait` that
23 /// appear in the return type).
24 #[derive(Copy, Clone, Debug)]
25 pub struct OpaqueTypeDecl<'tcx> {
26 /// The opaque type (`ty::Opaque`) for this declaration.
27 pub opaque_type: Ty<'tcx>,
29 /// The span of this particular definition of the opaque type. So
32 /// ```ignore (incomplete snippet)
33 /// type Foo = impl Baz;
35 /// // ^^^ This is the span we are looking for!
39 /// In cases where the fn returns `(impl Trait, impl Trait)` or
40 /// other such combinations, the result is currently
41 /// over-approximated, but better than nothing.
42 pub definition_span: Span,
44 /// The type variable that represents the value of the opaque type
45 /// that we require. In other words, after we compile this function,
46 /// we will be created a constraint like:
50 /// where `?C` is the value of this type variable. =) It may
51 /// naturally refer to the type and lifetime parameters in scope
52 /// in this function, though ultimately it should only reference
53 /// those that are arguments to `Foo` in the constraint above. (In
54 /// other words, `?C` should not include `'b`, even though it's a
55 /// lifetime parameter on `foo`.)
56 pub concrete_ty: Ty<'tcx>,
58 /// Returns `true` if the `impl Trait` bounds include region bounds.
59 /// For example, this would be true for:
61 /// fn foo<'a, 'b, 'c>() -> impl Trait<'c> + 'a + 'b
65 /// fn foo<'c>() -> impl Trait<'c>
67 /// unless `Trait` was declared like:
69 /// trait Trait<'c>: 'c
71 /// in which case it would be true.
73 /// This is used during regionck to decide whether we need to
74 /// impose any additional constraints to ensure that region
75 /// variables in `concrete_ty` wind up being constrained to
76 /// something from `substs` (or, at minimum, things that outlive
77 /// the fn body). (Ultimately, writeback is responsible for this
79 pub has_required_region_bounds: bool,
81 /// The origin of the opaque type.
82 pub origin: hir::OpaqueTyOrigin,
85 /// Whether member constraints should be generated for all opaque types
86 pub enum GenerateMemberConstraints {
87 /// The default, used by typeck
89 /// The borrow checker needs member constraints in any case where we don't
90 /// have a `'static` bound. This is because the borrow checker has more
91 /// flexibility in the values of regions. For example, given `f<'a, 'b>`
92 /// the borrow checker can have an inference variable outlive `'a` and `'b`,
93 /// but not be equal to `'static`.
97 pub trait InferCtxtExt<'tcx> {
98 fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
100 parent_def_id: LocalDefId,
102 param_env: ty::ParamEnv<'tcx>,
105 ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)>;
107 fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(
109 opaque_types: &OpaqueTypeMap<'tcx>,
110 free_region_relations: &FRR,
113 fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
115 opaque_type_key: OpaqueTypeKey<'tcx>,
116 opaque_defn: &OpaqueTypeDecl<'tcx>,
117 mode: GenerateMemberConstraints,
118 free_region_relations: &FRR,
122 fn generate_member_constraint(
124 concrete_ty: Ty<'tcx>,
125 opaque_defn: &OpaqueTypeDecl<'tcx>,
126 opaque_type_key: OpaqueTypeKey<'tcx>,
127 first_own_region_index: usize,
130 fn infer_opaque_definition_from_instantiation(
132 opaque_type_key: OpaqueTypeKey<'tcx>,
133 instantiated_ty: Ty<'tcx>,
138 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
139 /// Replaces all opaque types in `value` with fresh inference variables
140 /// and creates appropriate obligations. For example, given the input:
142 /// impl Iterator<Item = impl Debug>
144 /// this method would create two type variables, `?0` and `?1`. It would
145 /// return the type `?0` but also the obligations:
147 /// ?0: Iterator<Item = ?1>
150 /// Moreover, it returns a `OpaqueTypeMap` that would map `?0` to
151 /// info about the `impl Iterator<..>` type and `?1` to info about
152 /// the `impl Debug` type.
156 /// - `parent_def_id` -- the `DefId` of the function in which the opaque type
158 /// - `body_id` -- the body-id with which the resulting obligations should
160 /// - `param_env` -- the in-scope parameter environment to be used for
162 /// - `value` -- the value within which we are instantiating opaque types
163 /// - `value_span` -- the span where the value came from, used in error reporting
164 fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
166 parent_def_id: LocalDefId,
168 param_env: ty::ParamEnv<'tcx>,
171 ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)> {
173 "instantiate_opaque_types(value={:?}, parent_def_id={:?}, body_id={:?}, \
174 param_env={:?}, value_span={:?})",
175 value, parent_def_id, body_id, param_env, value_span,
177 let mut instantiator = Instantiator {
183 opaque_types: Default::default(),
186 let value = instantiator.instantiate_opaque_types_in_map(value);
187 InferOk { value: (value, instantiator.opaque_types), obligations: instantiator.obligations }
190 /// Given the map `opaque_types` containing the opaque
191 /// `impl Trait` types whose underlying, hidden types are being
192 /// inferred, this method adds constraints to the regions
193 /// appearing in those underlying hidden types to ensure that they
194 /// at least do not refer to random scopes within the current
195 /// function. These constraints are not (quite) sufficient to
196 /// guarantee that the regions are actually legal values; that
197 /// final condition is imposed after region inference is done.
201 /// Let's work through an example to explain how it works. Assume
202 /// the current function is as follows:
205 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
208 /// Here, we have two `impl Trait` types whose values are being
209 /// inferred (the `impl Bar<'a>` and the `impl
210 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
211 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
212 /// the return type of `foo`, we *reference* those definitions:
215 /// type Foo1<'x> = impl Bar<'x>;
216 /// type Foo2<'x> = impl Bar<'x>;
217 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
224 /// As indicating in the comments above, each of those references
225 /// is (in the compiler) basically a substitution (`substs`)
226 /// applied to the type of a suitable `def_id` (which identifies
227 /// `Foo1` or `Foo2`).
229 /// Now, at this point in compilation, what we have done is to
230 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
231 /// fresh inference variables C1 and C2. We wish to use the values
232 /// of these variables to infer the underlying types of `Foo1` and
233 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
234 /// constraints like:
237 /// for<'a> (Foo1<'a> = C1)
238 /// for<'b> (Foo1<'b> = C2)
241 /// For these equation to be satisfiable, the types `C1` and `C2`
242 /// can only refer to a limited set of regions. For example, `C1`
243 /// can only refer to `'static` and `'a`, and `C2` can only refer
244 /// to `'static` and `'b`. The job of this function is to impose that
247 /// Up to this point, C1 and C2 are basically just random type
248 /// inference variables, and hence they may contain arbitrary
249 /// regions. In fact, it is fairly likely that they do! Consider
250 /// this possible definition of `foo`:
253 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
258 /// Here, the values for the concrete types of the two impl
259 /// traits will include inference variables:
266 /// Ordinarily, the subtyping rules would ensure that these are
267 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
268 /// type per se, we don't get such constraints by default. This
269 /// is where this function comes into play. It adds extra
270 /// constraints to ensure that all the regions which appear in the
271 /// inferred type are regions that could validly appear.
273 /// This is actually a bit of a tricky constraint in general. We
274 /// want to say that each variable (e.g., `'0`) can only take on
275 /// values that were supplied as arguments to the opaque type
276 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
277 /// scope. We don't have a constraint quite of this kind in the current
282 /// We generally prefer to make `<=` constraints, since they
283 /// integrate best into the region solver. To do that, we find the
284 /// "minimum" of all the arguments that appear in the substs: that
285 /// is, some region which is less than all the others. In the case
286 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
287 /// all). Then we apply that as a least bound to the variables
288 /// (e.g., `'a <= '0`).
290 /// In some cases, there is no minimum. Consider this example:
293 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
296 /// Here we would report a more complex "in constraint", like `'r
297 /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
298 /// the hidden type).
300 /// # Constrain regions, not the hidden concrete type
302 /// Note that generating constraints on each region `Rc` is *not*
303 /// the same as generating an outlives constraint on `Tc` iself.
304 /// For example, if we had a function like this:
307 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
311 /// // Equivalent to:
312 /// type FooReturn<'a, T> = impl Foo<'a>;
313 /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
316 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
317 /// is an inference variable). If we generated a constraint that
318 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
319 /// but this is not necessary, because the opaque type we
320 /// create will be allowed to reference `T`. So we only generate a
321 /// constraint that `'0: 'a`.
323 /// # The `free_region_relations` parameter
325 /// The `free_region_relations` argument is used to find the
326 /// "minimum" of the regions supplied to a given opaque type.
327 /// It must be a relation that can answer whether `'a <= 'b`,
328 /// where `'a` and `'b` are regions that appear in the "substs"
329 /// for the opaque type references (the `<'a>` in `Foo1<'a>`).
331 /// Note that we do not impose the constraints based on the
332 /// generic regions from the `Foo1` definition (e.g., `'x`). This
333 /// is because the constraints we are imposing here is basically
334 /// the concern of the one generating the constraining type C1,
335 /// which is the current function. It also means that we can
336 /// take "implied bounds" into account in some cases:
339 /// trait SomeTrait<'a, 'b> { }
340 /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
343 /// Here, the fact that `'b: 'a` is known only because of the
344 /// implied bounds from the `&'a &'b u32` parameter, and is not
345 /// "inherent" to the opaque type definition.
349 /// - `opaque_types` -- the map produced by `instantiate_opaque_types`
350 /// - `free_region_relations` -- something that can be used to relate
351 /// the free regions (`'a`) that appear in the impl trait.
352 fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(
354 opaque_types: &OpaqueTypeMap<'tcx>,
355 free_region_relations: &FRR,
357 debug!("constrain_opaque_types()");
359 for &(opaque_type_key, opaque_defn) in opaque_types {
360 self.constrain_opaque_type(
363 GenerateMemberConstraints::WhenRequired,
364 free_region_relations,
369 /// See `constrain_opaque_types` for documentation.
370 fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
372 opaque_type_key: OpaqueTypeKey<'tcx>,
373 opaque_defn: &OpaqueTypeDecl<'tcx>,
374 mode: GenerateMemberConstraints,
375 free_region_relations: &FRR,
377 let def_id = opaque_type_key.def_id;
379 debug!("constrain_opaque_type()");
380 debug!("constrain_opaque_type: def_id={:?}", def_id);
381 debug!("constrain_opaque_type: opaque_defn={:#?}", opaque_defn);
385 let concrete_ty = self.resolve_vars_if_possible(opaque_defn.concrete_ty);
387 debug!("constrain_opaque_type: concrete_ty={:?}", concrete_ty);
389 let first_own_region = match opaque_defn.origin {
390 hir::OpaqueTyOrigin::FnReturn | hir::OpaqueTyOrigin::AsyncFn => {
393 // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
397 // type foo::<'p0..'pn>::Foo<'q0..'qm>
398 // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
400 // For these types we onlt iterate over `'l0..lm` below.
401 tcx.generics_of(def_id).parent_count
403 // These opaque type inherit all lifetime parameters from their
404 // parent, so we have to check them all.
405 hir::OpaqueTyOrigin::Binding
406 | hir::OpaqueTyOrigin::TyAlias
407 | hir::OpaqueTyOrigin::Misc => 0,
410 let span = tcx.def_span(def_id);
412 // If there are required region bounds, we can use them.
413 if opaque_defn.has_required_region_bounds {
414 let bounds = tcx.explicit_item_bounds(def_id);
415 debug!("constrain_opaque_type: predicates: {:#?}", bounds);
417 bounds.iter().map(|(bound, _)| bound.subst(tcx, opaque_type_key.substs)).collect();
418 debug!("constrain_opaque_type: bounds={:#?}", bounds);
419 let opaque_type = tcx.mk_opaque(def_id, opaque_type_key.substs);
421 let required_region_bounds =
422 required_region_bounds(tcx, opaque_type, bounds.into_iter());
423 debug_assert!(!required_region_bounds.is_empty());
425 for required_region in required_region_bounds {
426 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
427 op: |r| self.sub_regions(infer::CallReturn(span), required_region, r),
430 if let GenerateMemberConstraints::IfNoStaticBound = mode {
431 self.generate_member_constraint(
441 // There were no `required_region_bounds`,
442 // so we have to search for a `least_region`.
443 // Go through all the regions used as arguments to the
444 // opaque type. These are the parameters to the opaque
445 // type; so in our example above, `substs` would contain
446 // `['a]` for the first impl trait and `'b` for the
448 let mut least_region = None;
450 for subst_arg in &opaque_type_key.substs[first_own_region..] {
451 let subst_region = match subst_arg.unpack() {
452 GenericArgKind::Lifetime(r) => r,
453 GenericArgKind::Type(_) | GenericArgKind::Const(_) => continue,
456 // Compute the least upper bound of it with the other regions.
457 debug!("constrain_opaque_types: least_region={:?}", least_region);
458 debug!("constrain_opaque_types: subst_region={:?}", subst_region);
460 None => least_region = Some(subst_region),
462 if free_region_relations.sub_free_regions(self.tcx, lr, subst_region) {
463 // keep the current least region
464 } else if free_region_relations.sub_free_regions(self.tcx, subst_region, lr) {
465 // switch to `subst_region`
466 least_region = Some(subst_region);
468 // There are two regions (`lr` and
469 // `subst_region`) which are not relatable. We
470 // can't find a best choice. Therefore,
471 // instead of creating a single bound like
472 // `'r: 'a` (which is our preferred choice),
473 // we will create a "in bound" like `'r in
474 // ['a, 'b, 'c]`, where `'a..'c` are the
475 // regions that appear in the impl trait.
477 return self.generate_member_constraint(
488 let least_region = least_region.unwrap_or(tcx.lifetimes.re_static);
489 debug!("constrain_opaque_types: least_region={:?}", least_region);
491 if let GenerateMemberConstraints::IfNoStaticBound = mode {
492 if least_region != tcx.lifetimes.re_static {
493 self.generate_member_constraint(
501 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
502 op: |r| self.sub_regions(infer::CallReturn(span), least_region, r),
506 /// As a fallback, we sometimes generate an "in constraint". For
507 /// a case like `impl Foo<'a, 'b>`, where `'a` and `'b` cannot be
508 /// related, we would generate a constraint `'r in ['a, 'b,
509 /// 'static]` for each region `'r` that appears in the hidden type
510 /// (i.e., it must be equal to `'a`, `'b`, or `'static`).
512 /// `conflict1` and `conflict2` are the two region bounds that we
513 /// detected which were unrelated. They are used for diagnostics.
514 fn generate_member_constraint(
516 concrete_ty: Ty<'tcx>,
517 opaque_defn: &OpaqueTypeDecl<'tcx>,
518 opaque_type_key: OpaqueTypeKey<'tcx>,
519 first_own_region: usize,
521 // Create the set of choice regions: each region in the hidden
522 // type can be equal to any of the region parameters of the
523 // opaque type definition.
524 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
525 opaque_type_key.substs[first_own_region..]
527 .filter_map(|arg| match arg.unpack() {
528 GenericArgKind::Lifetime(r) => Some(r),
529 GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
531 .chain(std::iter::once(self.tcx.lifetimes.re_static))
535 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
537 self.member_constraint(
538 opaque_type_key.def_id,
539 opaque_defn.definition_span,
548 /// Given the fully resolved, instantiated type for an opaque
549 /// type, i.e., the value of an inference variable like C1 or C2
550 /// (*), computes the "definition type" for an opaque type
551 /// definition -- that is, the inferred value of `Foo1<'x>` or
552 /// `Foo2<'x>` that we would conceptually use in its definition:
554 /// type Foo1<'x> = impl Bar<'x> = AAA; <-- this type AAA
555 /// type Foo2<'x> = impl Bar<'x> = BBB; <-- or this type BBB
556 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
558 /// Note that these values are defined in terms of a distinct set of
559 /// generic parameters (`'x` instead of `'a`) from C1 or C2. The main
560 /// purpose of this function is to do that translation.
562 /// (*) C1 and C2 were introduced in the comments on
563 /// `constrain_opaque_types`. Read that comment for more context.
567 /// - `def_id`, the `impl Trait` type
568 /// - `substs`, the substs used to instantiate this opaque type
569 /// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of
570 /// `opaque_defn.concrete_ty`
571 #[instrument(skip(self))]
572 fn infer_opaque_definition_from_instantiation(
574 opaque_type_key: OpaqueTypeKey<'tcx>,
575 instantiated_ty: Ty<'tcx>,
578 let OpaqueTypeKey { def_id, substs } = opaque_type_key;
580 // Use substs to build up a reverse map from regions to their
581 // identity mappings. This is necessary because of `impl
582 // Trait` lifetimes are computed by replacing existing
583 // lifetimes with 'static and remapping only those used in the
584 // `impl Trait` return type, resulting in the parameters
586 let id_substs = InternalSubsts::identity_for_item(self.tcx, def_id);
588 let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> =
589 substs.iter().enumerate().map(|(index, subst)| (subst, id_substs[index])).collect();
591 // Convert the type from the function into a type valid outside
592 // the function, by replacing invalid regions with 'static,
593 // after producing an error for each of them.
594 let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new(
596 self.is_tainted_by_errors(),
602 debug!(?definition_ty);
608 // Visitor that requires that (almost) all regions in the type visited outlive
609 // `least_region`. We cannot use `push_outlives_components` because regions in
610 // closure signatures are not included in their outlives components. We need to
611 // ensure all regions outlive the given bound so that we don't end up with,
612 // say, `ReVar` appearing in a return type and causing ICEs when other
613 // functions end up with region constraints involving regions from other
616 // We also cannot use `for_each_free_region` because for closures it includes
617 // the regions parameters from the enclosing item.
619 // We ignore any type parameters because impl trait values are assumed to
620 // capture all the in-scope type parameters.
621 struct ConstrainOpaqueTypeRegionVisitor<OP> {
625 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
627 OP: FnMut(ty::Region<'tcx>),
629 fn visit_binder<T: TypeFoldable<'tcx>>(
631 t: &ty::Binder<'tcx, T>,
632 ) -> ControlFlow<Self::BreakTy> {
633 t.as_ref().skip_binder().visit_with(self);
634 ControlFlow::CONTINUE
637 fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
639 // ignore bound regions, keep visiting
640 ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
643 ControlFlow::CONTINUE
648 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
649 // We're only interested in types involving regions
650 if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
651 return ControlFlow::CONTINUE;
655 ty::Closure(_, ref substs) => {
656 // Skip lifetime parameters of the enclosing item(s)
658 substs.as_closure().tupled_upvars_ty().visit_with(self);
659 substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
662 ty::Generator(_, ref substs, _) => {
663 // Skip lifetime parameters of the enclosing item(s)
664 // Also skip the witness type, because that has no free regions.
666 substs.as_generator().tupled_upvars_ty().visit_with(self);
667 substs.as_generator().return_ty().visit_with(self);
668 substs.as_generator().yield_ty().visit_with(self);
669 substs.as_generator().resume_ty().visit_with(self);
672 ty.super_visit_with(self);
676 ControlFlow::CONTINUE
680 struct ReverseMapper<'tcx> {
683 /// If errors have already been reported in this fn, we suppress
684 /// our own errors because they are sometimes derivative.
685 tainted_by_errors: bool,
687 opaque_type_def_id: DefId,
688 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
689 map_missing_regions_to_empty: bool,
691 /// initially `Some`, set to `None` once error has been reported
692 hidden_ty: Option<Ty<'tcx>>,
694 /// Span of function being checked.
698 impl ReverseMapper<'tcx> {
701 tainted_by_errors: bool,
702 opaque_type_def_id: DefId,
703 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
712 map_missing_regions_to_empty: false,
713 hidden_ty: Some(hidden_ty),
718 fn fold_kind_mapping_missing_regions_to_empty(
720 kind: GenericArg<'tcx>,
721 ) -> GenericArg<'tcx> {
722 assert!(!self.map_missing_regions_to_empty);
723 self.map_missing_regions_to_empty = true;
724 let kind = kind.fold_with(self);
725 self.map_missing_regions_to_empty = false;
729 fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
730 assert!(!self.map_missing_regions_to_empty);
735 impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
736 fn tcx(&self) -> TyCtxt<'tcx> {
740 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
742 // Ignore bound regions and `'static` regions that appear in the
743 // type, we only need to remap regions that reference lifetimes
744 // from the function declaraion.
745 // This would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
746 ty::ReLateBound(..) | ty::ReStatic => return r,
748 // If regions have been erased (by writeback), don't try to unerase
750 ty::ReErased => return r,
752 // The regions that we expect from borrow checking.
753 ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReEmpty(ty::UniverseIndex::ROOT) => {}
755 ty::ReEmpty(_) | ty::RePlaceholder(_) | ty::ReVar(_) => {
756 // All of the regions in the type should either have been
757 // erased by writeback, or mapped back to named regions by
759 bug!("unexpected region kind in opaque type: {:?}", r);
763 let generics = self.tcx().generics_of(self.opaque_type_def_id);
764 match self.map.get(&r.into()).map(|k| k.unpack()) {
765 Some(GenericArgKind::Lifetime(r1)) => r1,
766 Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
767 None if self.map_missing_regions_to_empty || self.tainted_by_errors => {
768 self.tcx.lifetimes.re_root_empty
770 None if generics.parent.is_some() => {
771 if let Some(hidden_ty) = self.hidden_ty.take() {
772 unexpected_hidden_region_diagnostic(
774 self.tcx.def_span(self.opaque_type_def_id),
780 self.tcx.lifetimes.re_root_empty
785 .struct_span_err(self.span, "non-defining opaque type use in defining scope")
789 "lifetime `{}` is part of concrete type but not used in \
790 parameter list of the `impl Trait` type alias",
796 self.tcx().lifetimes.re_static
801 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
803 ty::Closure(def_id, substs) => {
804 // I am a horrible monster and I pray for death. When
805 // we encounter a closure here, it is always a closure
806 // from within the function that we are currently
807 // type-checking -- one that is now being encapsulated
808 // in an opaque type. Ideally, we would
809 // go through the types/lifetimes that it references
810 // and treat them just like we would any other type,
811 // which means we would error out if we find any
812 // reference to a type/region that is not in the
815 // **However,** in the case of closures, there is a
816 // somewhat subtle (read: hacky) consideration. The
817 // problem is that our closure types currently include
818 // all the lifetime parameters declared on the
819 // enclosing function, even if they are unused by the
820 // closure itself. We can't readily filter them out,
821 // so here we replace those values with `'empty`. This
822 // can't really make a difference to the rest of the
823 // compiler; those regions are ignored for the
824 // outlives relation, and hence don't affect trait
825 // selection or auto traits, and they are erased
828 let generics = self.tcx.generics_of(def_id);
829 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
830 if index < generics.parent_count {
831 // Accommodate missing regions in the parent kinds...
832 self.fold_kind_mapping_missing_regions_to_empty(kind)
834 // ...but not elsewhere.
835 self.fold_kind_normally(kind)
839 self.tcx.mk_closure(def_id, substs)
842 ty::Generator(def_id, substs, movability) => {
843 let generics = self.tcx.generics_of(def_id);
844 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
845 if index < generics.parent_count {
846 // Accommodate missing regions in the parent kinds...
847 self.fold_kind_mapping_missing_regions_to_empty(kind)
849 // ...but not elsewhere.
850 self.fold_kind_normally(kind)
854 self.tcx.mk_generator(def_id, substs, movability)
857 ty::Param(param) => {
858 // Look it up in the substitution list.
859 match self.map.get(&ty.into()).map(|k| k.unpack()) {
860 // Found it in the substitution list; replace with the parameter from the
862 Some(GenericArgKind::Type(t1)) => t1,
863 Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
865 debug!(?param, ?self.map);
871 "type parameter `{}` is part of concrete type but not \
872 used in parameter list for the `impl Trait` type alias",
878 self.tcx().ty_error()
883 _ => ty.super_fold_with(self),
887 fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
888 trace!("checking const {:?}", ct);
889 // Find a const parameter
891 ty::ConstKind::Param(..) => {
892 // Look it up in the substitution list.
893 match self.map.get(&ct.into()).map(|k| k.unpack()) {
894 // Found it in the substitution list, replace with the parameter from the
896 Some(GenericArgKind::Const(c1)) => c1,
897 Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
904 "const parameter `{}` is part of concrete type but not \
905 used in parameter list for the `impl Trait` type alias",
911 self.tcx().const_error(ct.ty)
921 struct Instantiator<'a, 'tcx> {
922 infcx: &'a InferCtxt<'a, 'tcx>,
923 parent_def_id: LocalDefId,
925 param_env: ty::ParamEnv<'tcx>,
927 opaque_types: OpaqueTypeMap<'tcx>,
928 obligations: Vec<PredicateObligation<'tcx>>,
931 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
932 #[instrument(skip(self))]
933 fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
934 let tcx = self.infcx.tcx;
935 value.fold_with(&mut BottomUpFolder {
938 if ty.references_error() {
939 return tcx.ty_error();
940 } else if let ty::Opaque(def_id, substs) = ty.kind() {
941 // Check that this is `impl Trait` type is
942 // declared by `parent_def_id` -- i.e., one whose
943 // value we are inferring. At present, this is
944 // always true during the first phase of
945 // type-check, but not always true later on during
946 // NLL. Once we support named opaque types more fully,
947 // this same scenario will be able to arise during all phases.
949 // Here is an example using type alias `impl Trait`
950 // that indicates the distinction we are checking for:
954 // pub type Foo = impl Iterator;
955 // pub fn make_foo() -> Foo { .. }
959 // fn foo() -> a::Foo { a::make_foo() }
963 // Here, the return type of `foo` references a
964 // `Opaque` indeed, but not one whose value is
965 // presently being inferred. You can get into a
966 // similar situation with closure return types
970 // fn foo() -> impl Iterator { .. }
972 // let x = || foo(); // returns the Opaque assoc with `foo`
975 if let Some(def_id) = def_id.as_local() {
976 let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
977 let parent_def_id = self.parent_def_id;
978 let def_scope_default = || {
979 let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
980 parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
982 let (in_definition_scope, origin) =
983 match tcx.hir().expect_item(opaque_hir_id).kind {
984 // Anonymous `impl Trait`
985 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
986 impl_trait_fn: Some(parent),
989 }) => (parent == self.parent_def_id.to_def_id(), origin),
990 // Named `type Foo = impl Bar;`
991 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
996 may_define_opaque_type(tcx, self.parent_def_id, opaque_hir_id),
999 _ => (def_scope_default(), hir::OpaqueTyOrigin::Misc),
1001 if in_definition_scope {
1002 let opaque_type_key =
1003 OpaqueTypeKey { def_id: def_id.to_def_id(), substs };
1004 return self.fold_opaque_ty(ty, opaque_type_key, origin);
1008 "instantiate_opaque_types_in_map: \
1009 encountered opaque outside its definition scope \
1026 opaque_type_key: OpaqueTypeKey<'tcx>,
1027 origin: hir::OpaqueTyOrigin,
1029 let infcx = self.infcx;
1030 let tcx = infcx.tcx;
1031 let OpaqueTypeKey { def_id, substs } = opaque_type_key;
1033 debug!("instantiate_opaque_types: Opaque(def_id={:?}, substs={:?})", def_id, substs);
1035 // Use the same type variable if the exact same opaque type appears more
1036 // than once in the return type (e.g., if it's passed to a type alias).
1037 if let Some(opaque_defn) = self.opaque_types.get(&opaque_type_key) {
1038 debug!("instantiate_opaque_types: returning concrete ty {:?}", opaque_defn.concrete_ty);
1039 return opaque_defn.concrete_ty;
1041 let span = tcx.def_span(def_id);
1042 debug!("fold_opaque_ty {:?} {:?}", self.value_span, span);
1044 .next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span });
1046 let item_bounds = tcx.explicit_item_bounds(def_id);
1047 debug!("instantiate_opaque_types: bounds={:#?}", item_bounds);
1048 let bounds: Vec<_> =
1049 item_bounds.iter().map(|(bound, _)| bound.subst(tcx, substs)).collect();
1051 let param_env = tcx.param_env(def_id);
1052 let InferOk { value: bounds, obligations } = infcx.partially_normalize_associated_types_in(
1053 ObligationCause::misc(span, self.body_id),
1057 self.obligations.extend(obligations);
1059 debug!("instantiate_opaque_types: bounds={:?}", bounds);
1061 let required_region_bounds = required_region_bounds(tcx, ty, bounds.iter().copied());
1062 debug!("instantiate_opaque_types: required_region_bounds={:?}", required_region_bounds);
1064 // Make sure that we are in fact defining the *entire* type
1065 // (e.g., `type Foo<T: Bound> = impl Bar;` needs to be
1066 // defined by a function like `fn foo<T: Bound>() -> Foo<T>`).
1067 debug!("instantiate_opaque_types: param_env={:#?}", self.param_env,);
1068 debug!("instantiate_opaque_types: generics={:#?}", tcx.generics_of(def_id),);
1070 // Ideally, we'd get the span where *this specific `ty` came
1071 // from*, but right now we just use the span from the overall
1072 // value being folded. In simple cases like `-> impl Foo`,
1073 // these are the same span, but not in cases like `-> (impl
1075 let definition_span = self.value_span;
1077 self.opaque_types.insert(
1078 OpaqueTypeKey { def_id, substs },
1082 concrete_ty: ty_var,
1083 has_required_region_bounds: !required_region_bounds.is_empty(),
1087 debug!("instantiate_opaque_types: ty_var={:?}", ty_var);
1089 for predicate in &bounds {
1090 if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
1091 if projection.ty.references_error() {
1092 // No point on adding these obligations since there's a type error involved.
1098 self.obligations.reserve(bounds.len());
1099 for predicate in bounds {
1100 // Change the predicate to refer to the type variable,
1101 // which will be the concrete type instead of the opaque type.
1102 // This also instantiates nested instances of `impl Trait`.
1103 let predicate = self.instantiate_opaque_types_in_map(predicate);
1105 let cause = traits::ObligationCause::new(span, self.body_id, traits::OpaqueType);
1107 // Require that the predicate holds for the concrete type.
1108 debug!("instantiate_opaque_types: predicate={:?}", predicate);
1109 self.obligations.push(traits::Obligation::new(cause, self.param_env, predicate));
1116 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
1122 /// pub trait Bar { .. }
1124 /// pub type Baz = impl Bar;
1126 /// fn f1() -> Baz { .. }
1129 /// fn f2() -> bar::Baz { .. }
1133 /// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
1134 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
1135 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
1136 pub fn may_define_opaque_type(
1139 opaque_hir_id: hir::HirId,
1141 let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
1143 // Named opaque types can be defined by any siblings or children of siblings.
1144 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
1145 // We walk up the node tree until we hit the root or the scope of the opaque type.
1146 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
1147 hir_id = tcx.hir().get_parent_item(hir_id);
1149 // Syntactically, we are allowed to define the concrete type if:
1150 let res = hir_id == scope;
1152 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
1153 tcx.hir().find(hir_id),
1154 tcx.hir().get(opaque_hir_id),
1160 /// Given a set of predicates that apply to an object type, returns
1161 /// the region bounds that the (erased) `Self` type must
1162 /// outlive. Precisely *because* the `Self` type is erased, the
1163 /// parameter `erased_self_ty` must be supplied to indicate what type
1164 /// has been used to represent `Self` in the predicates
1165 /// themselves. This should really be a unique type; `FreshTy(0)` is a
1168 /// N.B., in some cases, particularly around higher-ranked bounds,
1169 /// this function returns a kind of conservative approximation.
1170 /// That is, all regions returned by this function are definitely
1171 /// required, but there may be other region bounds that are not
1172 /// returned, as well as requirements like `for<'a> T: 'a`.
1174 /// Requires that trait definitions have been processed so that we can
1175 /// elaborate predicates and walk supertraits.
1176 crate fn required_region_bounds(
1178 erased_self_ty: Ty<'tcx>,
1179 predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
1180 ) -> Vec<ty::Region<'tcx>> {
1181 debug!("required_region_bounds(erased_self_ty={:?})", erased_self_ty);
1183 assert!(!erased_self_ty.has_escaping_bound_vars());
1185 traits::elaborate_predicates(tcx, predicates)
1186 .filter_map(|obligation| {
1187 debug!("required_region_bounds(obligation={:?})", obligation);
1188 match obligation.predicate.kind().skip_binder() {
1189 ty::PredicateKind::Projection(..)
1190 | ty::PredicateKind::Trait(..)
1191 | ty::PredicateKind::Subtype(..)
1192 | ty::PredicateKind::WellFormed(..)
1193 | ty::PredicateKind::ObjectSafe(..)
1194 | ty::PredicateKind::ClosureKind(..)
1195 | ty::PredicateKind::RegionOutlives(..)
1196 | ty::PredicateKind::ConstEvaluatable(..)
1197 | ty::PredicateKind::ConstEquate(..)
1198 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
1199 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
1200 // Search for a bound of the form `erased_self_ty
1201 // : 'a`, but be wary of something like `for<'a>
1202 // erased_self_ty : 'a` (we interpret a
1203 // higher-ranked bound like that as 'static,
1204 // though at present the code in `fulfill.rs`
1205 // considers such bounds to be unsatisfiable, so
1206 // it's kind of a moot point since you could never
1207 // construct such an object, but this seems
1208 // correct even if that code changes).
1209 if t == &erased_self_ty && !r.has_escaping_bound_vars() {