1 use crate::infer::InferCtxtExt as _;
2 use crate::traits::{self, PredicateObligation};
3 use rustc_data_structures::fx::FxHashMap;
4 use rustc_data_structures::sync::Lrc;
6 use rustc_hir::def_id::{DefId, DefIdMap, 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, SubstsRef};
14 use rustc_middle::ty::{self, Ty, TyCtxt};
17 use std::ops::ControlFlow;
19 pub type OpaqueTypeMap<'tcx> = DefIdMap<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 substitutions that we apply to the opaque type that this
30 /// `impl Trait` desugars to. e.g., if:
32 /// fn foo<'a, 'b, T>() -> impl Trait<'a>
34 /// winds up desugared to:
36 /// type Foo<'x, X> = impl Trait<'x>
37 /// fn foo<'a, 'b, T>() -> Foo<'a, T>
39 /// then `substs` would be `['a, T]`.
40 pub substs: SubstsRef<'tcx>,
42 /// The span of this particular definition of the opaque type. So
45 /// ```ignore (incomplete snippet)
46 /// type Foo = impl Baz;
48 /// // ^^^ This is the span we are looking for!
52 /// In cases where the fn returns `(impl Trait, impl Trait)` or
53 /// other such combinations, the result is currently
54 /// over-approximated, but better than nothing.
55 pub definition_span: Span,
57 /// The type variable that represents the value of the opaque type
58 /// that we require. In other words, after we compile this function,
59 /// we will be created a constraint like:
63 /// where `?C` is the value of this type variable. =) It may
64 /// naturally refer to the type and lifetime parameters in scope
65 /// in this function, though ultimately it should only reference
66 /// those that are arguments to `Foo` in the constraint above. (In
67 /// other words, `?C` should not include `'b`, even though it's a
68 /// lifetime parameter on `foo`.)
69 pub concrete_ty: Ty<'tcx>,
71 /// Returns `true` if the `impl Trait` bounds include region bounds.
72 /// For example, this would be true for:
74 /// fn foo<'a, 'b, 'c>() -> impl Trait<'c> + 'a + 'b
78 /// fn foo<'c>() -> impl Trait<'c>
80 /// unless `Trait` was declared like:
82 /// trait Trait<'c>: 'c
84 /// in which case it would be true.
86 /// This is used during regionck to decide whether we need to
87 /// impose any additional constraints to ensure that region
88 /// variables in `concrete_ty` wind up being constrained to
89 /// something from `substs` (or, at minimum, things that outlive
90 /// the fn body). (Ultimately, writeback is responsible for this
92 pub has_required_region_bounds: bool,
94 /// The origin of the opaque type.
95 pub origin: hir::OpaqueTyOrigin,
98 /// Whether member constraints should be generated for all opaque types
99 pub enum GenerateMemberConstraints {
100 /// The default, used by typeck
102 /// The borrow checker needs member constraints in any case where we don't
103 /// have a `'static` bound. This is because the borrow checker has more
104 /// flexibility in the values of regions. For example, given `f<'a, 'b>`
105 /// the borrow checker can have an inference variable outlive `'a` and `'b`,
106 /// but not be equal to `'static`.
110 pub trait InferCtxtExt<'tcx> {
111 fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
113 parent_def_id: LocalDefId,
115 param_env: ty::ParamEnv<'tcx>,
118 ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)>;
120 fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(
122 opaque_types: &OpaqueTypeMap<'tcx>,
123 free_region_relations: &FRR,
126 fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
129 opaque_defn: &OpaqueTypeDecl<'tcx>,
130 mode: GenerateMemberConstraints,
131 free_region_relations: &FRR,
135 fn generate_member_constraint(
137 concrete_ty: Ty<'tcx>,
138 opaque_defn: &OpaqueTypeDecl<'tcx>,
139 opaque_type_def_id: DefId,
140 first_own_region_index: usize,
143 fn infer_opaque_definition_from_instantiation(
146 substs: SubstsRef<'tcx>,
147 instantiated_ty: Ty<'tcx>,
152 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
153 /// Replaces all opaque types in `value` with fresh inference variables
154 /// and creates appropriate obligations. For example, given the input:
156 /// impl Iterator<Item = impl Debug>
158 /// this method would create two type variables, `?0` and `?1`. It would
159 /// return the type `?0` but also the obligations:
161 /// ?0: Iterator<Item = ?1>
164 /// Moreover, it returns a `OpaqueTypeMap` that would map `?0` to
165 /// info about the `impl Iterator<..>` type and `?1` to info about
166 /// the `impl Debug` type.
170 /// - `parent_def_id` -- the `DefId` of the function in which the opaque type
172 /// - `body_id` -- the body-id with which the resulting obligations should
174 /// - `param_env` -- the in-scope parameter environment to be used for
176 /// - `value` -- the value within which we are instantiating opaque types
177 /// - `value_span` -- the span where the value came from, used in error reporting
178 fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
180 parent_def_id: LocalDefId,
182 param_env: ty::ParamEnv<'tcx>,
185 ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)> {
187 "instantiate_opaque_types(value={:?}, parent_def_id={:?}, body_id={:?}, \
188 param_env={:?}, value_span={:?})",
189 value, parent_def_id, body_id, param_env, value_span,
191 let mut instantiator = Instantiator {
197 opaque_types: Default::default(),
200 let value = instantiator.instantiate_opaque_types_in_map(value);
201 InferOk { value: (value, instantiator.opaque_types), obligations: instantiator.obligations }
204 /// Given the map `opaque_types` containing the opaque
205 /// `impl Trait` types whose underlying, hidden types are being
206 /// inferred, this method adds constraints to the regions
207 /// appearing in those underlying hidden types to ensure that they
208 /// at least do not refer to random scopes within the current
209 /// function. These constraints are not (quite) sufficient to
210 /// guarantee that the regions are actually legal values; that
211 /// final condition is imposed after region inference is done.
215 /// Let's work through an example to explain how it works. Assume
216 /// the current function is as follows:
219 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
222 /// Here, we have two `impl Trait` types whose values are being
223 /// inferred (the `impl Bar<'a>` and the `impl
224 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
225 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
226 /// the return type of `foo`, we *reference* those definitions:
229 /// type Foo1<'x> = impl Bar<'x>;
230 /// type Foo2<'x> = impl Bar<'x>;
231 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
238 /// As indicating in the comments above, each of those references
239 /// is (in the compiler) basically a substitution (`substs`)
240 /// applied to the type of a suitable `def_id` (which identifies
241 /// `Foo1` or `Foo2`).
243 /// Now, at this point in compilation, what we have done is to
244 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
245 /// fresh inference variables C1 and C2. We wish to use the values
246 /// of these variables to infer the underlying types of `Foo1` and
247 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
248 /// constraints like:
251 /// for<'a> (Foo1<'a> = C1)
252 /// for<'b> (Foo1<'b> = C2)
255 /// For these equation to be satisfiable, the types `C1` and `C2`
256 /// can only refer to a limited set of regions. For example, `C1`
257 /// can only refer to `'static` and `'a`, and `C2` can only refer
258 /// to `'static` and `'b`. The job of this function is to impose that
261 /// Up to this point, C1 and C2 are basically just random type
262 /// inference variables, and hence they may contain arbitrary
263 /// regions. In fact, it is fairly likely that they do! Consider
264 /// this possible definition of `foo`:
267 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
272 /// Here, the values for the concrete types of the two impl
273 /// traits will include inference variables:
280 /// Ordinarily, the subtyping rules would ensure that these are
281 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
282 /// type per se, we don't get such constraints by default. This
283 /// is where this function comes into play. It adds extra
284 /// constraints to ensure that all the regions which appear in the
285 /// inferred type are regions that could validly appear.
287 /// This is actually a bit of a tricky constraint in general. We
288 /// want to say that each variable (e.g., `'0`) can only take on
289 /// values that were supplied as arguments to the opaque type
290 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
291 /// scope. We don't have a constraint quite of this kind in the current
296 /// We generally prefer to make `<=` constraints, since they
297 /// integrate best into the region solver. To do that, we find the
298 /// "minimum" of all the arguments that appear in the substs: that
299 /// is, some region which is less than all the others. In the case
300 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
301 /// all). Then we apply that as a least bound to the variables
302 /// (e.g., `'a <= '0`).
304 /// In some cases, there is no minimum. Consider this example:
307 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
310 /// Here we would report a more complex "in constraint", like `'r
311 /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
312 /// the hidden type).
314 /// # Constrain regions, not the hidden concrete type
316 /// Note that generating constraints on each region `Rc` is *not*
317 /// the same as generating an outlives constraint on `Tc` iself.
318 /// For example, if we had a function like this:
321 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
325 /// // Equivalent to:
326 /// type FooReturn<'a, T> = impl Foo<'a>;
327 /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
330 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
331 /// is an inference variable). If we generated a constraint that
332 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
333 /// but this is not necessary, because the opaque type we
334 /// create will be allowed to reference `T`. So we only generate a
335 /// constraint that `'0: 'a`.
337 /// # The `free_region_relations` parameter
339 /// The `free_region_relations` argument is used to find the
340 /// "minimum" of the regions supplied to a given opaque type.
341 /// It must be a relation that can answer whether `'a <= 'b`,
342 /// where `'a` and `'b` are regions that appear in the "substs"
343 /// for the opaque type references (the `<'a>` in `Foo1<'a>`).
345 /// Note that we do not impose the constraints based on the
346 /// generic regions from the `Foo1` definition (e.g., `'x`). This
347 /// is because the constraints we are imposing here is basically
348 /// the concern of the one generating the constraining type C1,
349 /// which is the current function. It also means that we can
350 /// take "implied bounds" into account in some cases:
353 /// trait SomeTrait<'a, 'b> { }
354 /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
357 /// Here, the fact that `'b: 'a` is known only because of the
358 /// implied bounds from the `&'a &'b u32` parameter, and is not
359 /// "inherent" to the opaque type definition.
363 /// - `opaque_types` -- the map produced by `instantiate_opaque_types`
364 /// - `free_region_relations` -- something that can be used to relate
365 /// the free regions (`'a`) that appear in the impl trait.
366 fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(
368 opaque_types: &OpaqueTypeMap<'tcx>,
369 free_region_relations: &FRR,
371 debug!("constrain_opaque_types()");
373 for (&def_id, opaque_defn) in opaque_types {
374 self.constrain_opaque_type(
377 GenerateMemberConstraints::WhenRequired,
378 free_region_relations,
383 /// See `constrain_opaque_types` for documentation.
384 fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
387 opaque_defn: &OpaqueTypeDecl<'tcx>,
388 mode: GenerateMemberConstraints,
389 free_region_relations: &FRR,
391 debug!("constrain_opaque_type()");
392 debug!("constrain_opaque_type: def_id={:?}", def_id);
393 debug!("constrain_opaque_type: opaque_defn={:#?}", opaque_defn);
397 let concrete_ty = self.resolve_vars_if_possible(opaque_defn.concrete_ty);
399 debug!("constrain_opaque_type: concrete_ty={:?}", concrete_ty);
401 let first_own_region = match opaque_defn.origin {
402 hir::OpaqueTyOrigin::FnReturn | hir::OpaqueTyOrigin::AsyncFn => {
405 // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
409 // type foo::<'p0..'pn>::Foo<'q0..'qm>
410 // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
412 // For these types we onlt iterate over `'l0..lm` below.
413 tcx.generics_of(def_id).parent_count
415 // These opaque type inherit all lifetime parameters from their
416 // parent, so we have to check them all.
417 hir::OpaqueTyOrigin::Binding
418 | hir::OpaqueTyOrigin::TyAlias
419 | hir::OpaqueTyOrigin::Misc => 0,
422 let span = tcx.def_span(def_id);
424 // If there are required region bounds, we can use them.
425 if opaque_defn.has_required_region_bounds {
426 let bounds = tcx.explicit_item_bounds(def_id);
427 debug!("constrain_opaque_type: predicates: {:#?}", bounds);
429 bounds.iter().map(|(bound, _)| bound.subst(tcx, opaque_defn.substs)).collect();
430 debug!("constrain_opaque_type: bounds={:#?}", bounds);
431 let opaque_type = tcx.mk_opaque(def_id, opaque_defn.substs);
433 let required_region_bounds =
434 required_region_bounds(tcx, opaque_type, bounds.into_iter());
435 debug_assert!(!required_region_bounds.is_empty());
437 for required_region in required_region_bounds {
438 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
439 op: |r| self.sub_regions(infer::CallReturn(span), required_region, r),
442 if let GenerateMemberConstraints::IfNoStaticBound = mode {
443 self.generate_member_constraint(concrete_ty, opaque_defn, def_id, first_own_region);
448 // There were no `required_region_bounds`,
449 // so we have to search for a `least_region`.
450 // Go through all the regions used as arguments to the
451 // opaque type. These are the parameters to the opaque
452 // type; so in our example above, `substs` would contain
453 // `['a]` for the first impl trait and `'b` for the
455 let mut least_region = None;
457 for subst_arg in &opaque_defn.substs[first_own_region..] {
458 let subst_region = match subst_arg.unpack() {
459 GenericArgKind::Lifetime(r) => r,
460 GenericArgKind::Type(_) | GenericArgKind::Const(_) => continue,
463 // Compute the least upper bound of it with the other regions.
464 debug!("constrain_opaque_types: least_region={:?}", least_region);
465 debug!("constrain_opaque_types: subst_region={:?}", subst_region);
467 None => least_region = Some(subst_region),
469 if free_region_relations.sub_free_regions(self.tcx, lr, subst_region) {
470 // keep the current least region
471 } else if free_region_relations.sub_free_regions(self.tcx, subst_region, lr) {
472 // switch to `subst_region`
473 least_region = Some(subst_region);
475 // There are two regions (`lr` and
476 // `subst_region`) which are not relatable. We
477 // can't find a best choice. Therefore,
478 // instead of creating a single bound like
479 // `'r: 'a` (which is our preferred choice),
480 // we will create a "in bound" like `'r in
481 // ['a, 'b, 'c]`, where `'a..'c` are the
482 // regions that appear in the impl trait.
484 return self.generate_member_constraint(
495 let least_region = least_region.unwrap_or(tcx.lifetimes.re_static);
496 debug!("constrain_opaque_types: least_region={:?}", least_region);
498 if let GenerateMemberConstraints::IfNoStaticBound = mode {
499 if least_region != tcx.lifetimes.re_static {
500 self.generate_member_constraint(concrete_ty, opaque_defn, def_id, first_own_region);
503 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
504 op: |r| self.sub_regions(infer::CallReturn(span), least_region, r),
508 /// As a fallback, we sometimes generate an "in constraint". For
509 /// a case like `impl Foo<'a, 'b>`, where `'a` and `'b` cannot be
510 /// related, we would generate a constraint `'r in ['a, 'b,
511 /// 'static]` for each region `'r` that appears in the hidden type
512 /// (i.e., it must be equal to `'a`, `'b`, or `'static`).
514 /// `conflict1` and `conflict2` are the two region bounds that we
515 /// detected which were unrelated. They are used for diagnostics.
516 fn generate_member_constraint(
518 concrete_ty: Ty<'tcx>,
519 opaque_defn: &OpaqueTypeDecl<'tcx>,
520 opaque_type_def_id: DefId,
521 first_own_region: usize,
523 // Create the set of choice regions: each region in the hidden
524 // type can be equal to any of the region parameters of the
525 // opaque type definition.
526 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
527 opaque_defn.substs[first_own_region..]
529 .filter_map(|arg| match arg.unpack() {
530 GenericArgKind::Lifetime(r) => Some(r),
531 GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
533 .chain(std::iter::once(self.tcx.lifetimes.re_static))
537 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
539 self.member_constraint(
541 opaque_defn.definition_span,
550 /// Given the fully resolved, instantiated type for an opaque
551 /// type, i.e., the value of an inference variable like C1 or C2
552 /// (*), computes the "definition type" for an opaque type
553 /// definition -- that is, the inferred value of `Foo1<'x>` or
554 /// `Foo2<'x>` that we would conceptually use in its definition:
556 /// type Foo1<'x> = impl Bar<'x> = AAA; <-- this type AAA
557 /// type Foo2<'x> = impl Bar<'x> = BBB; <-- or this type BBB
558 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
560 /// Note that these values are defined in terms of a distinct set of
561 /// generic parameters (`'x` instead of `'a`) from C1 or C2. The main
562 /// purpose of this function is to do that translation.
564 /// (*) C1 and C2 were introduced in the comments on
565 /// `constrain_opaque_types`. Read that comment for more context.
569 /// - `def_id`, the `impl Trait` type
570 /// - `substs`, the substs used to instantiate this opaque type
571 /// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of
572 /// `opaque_defn.concrete_ty`
573 fn infer_opaque_definition_from_instantiation(
576 substs: SubstsRef<'tcx>,
577 instantiated_ty: Ty<'tcx>,
581 "infer_opaque_definition_from_instantiation(def_id={:?}, instantiated_ty={:?})",
582 def_id, instantiated_ty
585 // Use substs to build up a reverse map from regions to their
586 // identity mappings. This is necessary because of `impl
587 // Trait` lifetimes are computed by replacing existing
588 // lifetimes with 'static and remapping only those used in the
589 // `impl Trait` return type, resulting in the parameters
591 let id_substs = InternalSubsts::identity_for_item(self.tcx, def_id);
592 let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> =
593 substs.iter().enumerate().map(|(index, subst)| (subst, id_substs[index])).collect();
595 // Convert the type from the function into a type valid outside
596 // the function, by replacing invalid regions with 'static,
597 // after producing an error for each of them.
598 let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new(
600 self.is_tainted_by_errors(),
606 debug!("infer_opaque_definition_from_instantiation: definition_ty={:?}", definition_ty);
612 // Visitor that requires that (almost) all regions in the type visited outlive
613 // `least_region`. We cannot use `push_outlives_components` because regions in
614 // closure signatures are not included in their outlives components. We need to
615 // ensure all regions outlive the given bound so that we don't end up with,
616 // say, `ReVar` appearing in a return type and causing ICEs when other
617 // functions end up with region constraints involving regions from other
620 // We also cannot use `for_each_free_region` because for closures it includes
621 // the regions parameters from the enclosing item.
623 // We ignore any type parameters because impl trait values are assumed to
624 // capture all the in-scope type parameters.
625 struct ConstrainOpaqueTypeRegionVisitor<OP> {
629 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
631 OP: FnMut(ty::Region<'tcx>),
633 fn visit_binder<T: TypeFoldable<'tcx>>(
635 t: &ty::Binder<'tcx, T>,
636 ) -> ControlFlow<Self::BreakTy> {
637 t.as_ref().skip_binder().visit_with(self);
638 ControlFlow::CONTINUE
641 fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
643 // ignore bound regions, keep visiting
644 ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
647 ControlFlow::CONTINUE
652 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
653 // We're only interested in types involving regions
654 if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
655 return ControlFlow::CONTINUE;
659 ty::Closure(_, ref substs) => {
660 // Skip lifetime parameters of the enclosing item(s)
662 substs.as_closure().tupled_upvars_ty().visit_with(self);
663 substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
666 ty::Generator(_, ref substs, _) => {
667 // Skip lifetime parameters of the enclosing item(s)
668 // Also skip the witness type, because that has no free regions.
670 substs.as_generator().tupled_upvars_ty().visit_with(self);
671 substs.as_generator().return_ty().visit_with(self);
672 substs.as_generator().yield_ty().visit_with(self);
673 substs.as_generator().resume_ty().visit_with(self);
676 ty.super_visit_with(self);
680 ControlFlow::CONTINUE
684 struct ReverseMapper<'tcx> {
687 /// If errors have already been reported in this fn, we suppress
688 /// our own errors because they are sometimes derivative.
689 tainted_by_errors: bool,
691 opaque_type_def_id: DefId,
692 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
693 map_missing_regions_to_empty: bool,
695 /// initially `Some`, set to `None` once error has been reported
696 hidden_ty: Option<Ty<'tcx>>,
698 /// Span of function being checked.
702 impl ReverseMapper<'tcx> {
705 tainted_by_errors: bool,
706 opaque_type_def_id: DefId,
707 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
716 map_missing_regions_to_empty: false,
717 hidden_ty: Some(hidden_ty),
722 fn fold_kind_mapping_missing_regions_to_empty(
724 kind: GenericArg<'tcx>,
725 ) -> GenericArg<'tcx> {
726 assert!(!self.map_missing_regions_to_empty);
727 self.map_missing_regions_to_empty = true;
728 let kind = kind.fold_with(self);
729 self.map_missing_regions_to_empty = false;
733 fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
734 assert!(!self.map_missing_regions_to_empty);
739 impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
740 fn tcx(&self) -> TyCtxt<'tcx> {
744 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
746 // Ignore bound regions and `'static` regions that appear in the
747 // type, we only need to remap regions that reference lifetimes
748 // from the function declaraion.
749 // This would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
750 ty::ReLateBound(..) | ty::ReStatic => return r,
752 // If regions have been erased (by writeback), don't try to unerase
754 ty::ReErased => return r,
756 // The regions that we expect from borrow checking.
757 ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReEmpty(ty::UniverseIndex::ROOT) => {}
759 ty::ReEmpty(_) | ty::RePlaceholder(_) | ty::ReVar(_) => {
760 // All of the regions in the type should either have been
761 // erased by writeback, or mapped back to named regions by
763 bug!("unexpected region kind in opaque type: {:?}", r);
767 let generics = self.tcx().generics_of(self.opaque_type_def_id);
768 match self.map.get(&r.into()).map(|k| k.unpack()) {
769 Some(GenericArgKind::Lifetime(r1)) => r1,
770 Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
771 None if self.map_missing_regions_to_empty || self.tainted_by_errors => {
772 self.tcx.lifetimes.re_root_empty
774 None if generics.parent.is_some() => {
775 if let Some(hidden_ty) = self.hidden_ty.take() {
776 unexpected_hidden_region_diagnostic(
778 self.tcx.def_span(self.opaque_type_def_id),
784 self.tcx.lifetimes.re_root_empty
789 .struct_span_err(self.span, "non-defining opaque type use in defining scope")
793 "lifetime `{}` is part of concrete type but not used in \
794 parameter list of the `impl Trait` type alias",
800 self.tcx().lifetimes.re_static
805 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
807 ty::Closure(def_id, substs) => {
808 // I am a horrible monster and I pray for death. When
809 // we encounter a closure here, it is always a closure
810 // from within the function that we are currently
811 // type-checking -- one that is now being encapsulated
812 // in an opaque type. Ideally, we would
813 // go through the types/lifetimes that it references
814 // and treat them just like we would any other type,
815 // which means we would error out if we find any
816 // reference to a type/region that is not in the
819 // **However,** in the case of closures, there is a
820 // somewhat subtle (read: hacky) consideration. The
821 // problem is that our closure types currently include
822 // all the lifetime parameters declared on the
823 // enclosing function, even if they are unused by the
824 // closure itself. We can't readily filter them out,
825 // so here we replace those values with `'empty`. This
826 // can't really make a difference to the rest of the
827 // compiler; those regions are ignored for the
828 // outlives relation, and hence don't affect trait
829 // selection or auto traits, and they are erased
832 let generics = self.tcx.generics_of(def_id);
833 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
834 if index < generics.parent_count {
835 // Accommodate missing regions in the parent kinds...
836 self.fold_kind_mapping_missing_regions_to_empty(kind)
838 // ...but not elsewhere.
839 self.fold_kind_normally(kind)
843 self.tcx.mk_closure(def_id, substs)
846 ty::Generator(def_id, substs, movability) => {
847 let generics = self.tcx.generics_of(def_id);
848 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
849 if index < generics.parent_count {
850 // Accommodate missing regions in the parent kinds...
851 self.fold_kind_mapping_missing_regions_to_empty(kind)
853 // ...but not elsewhere.
854 self.fold_kind_normally(kind)
858 self.tcx.mk_generator(def_id, substs, movability)
862 // Look it up in the substitution list.
863 match self.map.get(&ty.into()).map(|k| k.unpack()) {
864 // Found it in the substitution list; replace with the parameter from the
866 Some(GenericArgKind::Type(t1)) => t1,
867 Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
874 "type parameter `{}` is part of concrete type but not \
875 used in parameter list for the `impl Trait` type alias",
881 self.tcx().ty_error()
886 _ => ty.super_fold_with(self),
890 fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
891 trace!("checking const {:?}", ct);
892 // Find a const parameter
894 ty::ConstKind::Param(..) => {
895 // Look it up in the substitution list.
896 match self.map.get(&ct.into()).map(|k| k.unpack()) {
897 // Found it in the substitution list, replace with the parameter from the
899 Some(GenericArgKind::Const(c1)) => c1,
900 Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
907 "const parameter `{}` is part of concrete type but not \
908 used in parameter list for the `impl Trait` type alias",
914 self.tcx().const_error(ct.ty)
924 struct Instantiator<'a, 'tcx> {
925 infcx: &'a InferCtxt<'a, 'tcx>,
926 parent_def_id: LocalDefId,
928 param_env: ty::ParamEnv<'tcx>,
930 opaque_types: OpaqueTypeMap<'tcx>,
931 obligations: Vec<PredicateObligation<'tcx>>,
934 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
935 fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
936 debug!("instantiate_opaque_types_in_map(value={:?})", value);
937 let tcx = self.infcx.tcx;
938 value.fold_with(&mut BottomUpFolder {
941 if ty.references_error() {
942 return tcx.ty_error();
943 } else if let ty::Opaque(def_id, substs) = ty.kind() {
944 // Check that this is `impl Trait` type is
945 // declared by `parent_def_id` -- i.e., one whose
946 // value we are inferring. At present, this is
947 // always true during the first phase of
948 // type-check, but not always true later on during
949 // NLL. Once we support named opaque types more fully,
950 // this same scenario will be able to arise during all phases.
952 // Here is an example using type alias `impl Trait`
953 // that indicates the distinction we are checking for:
957 // pub type Foo = impl Iterator;
958 // pub fn make_foo() -> Foo { .. }
962 // fn foo() -> a::Foo { a::make_foo() }
966 // Here, the return type of `foo` references a
967 // `Opaque` indeed, but not one whose value is
968 // presently being inferred. You can get into a
969 // similar situation with closure return types
973 // fn foo() -> impl Iterator { .. }
975 // let x = || foo(); // returns the Opaque assoc with `foo`
978 if let Some(def_id) = def_id.as_local() {
979 let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
980 let parent_def_id = self.parent_def_id;
981 let def_scope_default = || {
982 let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
983 parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
985 let (in_definition_scope, origin) = match tcx.hir().find(opaque_hir_id) {
986 Some(Node::Item(item)) => match item.kind {
987 // Anonymous `impl Trait`
988 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
989 impl_trait_fn: Some(parent),
992 }) => (parent == self.parent_def_id.to_def_id(), origin),
993 // Named `type Foo = impl Bar;`
994 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
999 may_define_opaque_type(tcx, self.parent_def_id, opaque_hir_id),
1002 _ => (def_scope_default(), hir::OpaqueTyOrigin::Misc),
1005 "expected item, found {}",
1006 tcx.hir().node_to_string(opaque_hir_id),
1009 if in_definition_scope {
1010 return self.fold_opaque_ty(ty, def_id.to_def_id(), substs, origin);
1014 "instantiate_opaque_types_in_map: \
1015 encountered opaque outside its definition scope \
1033 substs: SubstsRef<'tcx>,
1034 origin: hir::OpaqueTyOrigin,
1036 let infcx = self.infcx;
1037 let tcx = infcx.tcx;
1039 debug!("instantiate_opaque_types: Opaque(def_id={:?}, substs={:?})", def_id, substs);
1041 // Use the same type variable if the exact same opaque type appears more
1042 // than once in the return type (e.g., if it's passed to a type alias).
1043 if let Some(opaque_defn) = self.opaque_types.get(&def_id) {
1044 debug!("instantiate_opaque_types: returning concrete ty {:?}", opaque_defn.concrete_ty);
1045 return opaque_defn.concrete_ty;
1047 let span = tcx.def_span(def_id);
1048 debug!("fold_opaque_ty {:?} {:?}", self.value_span, span);
1050 .next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span });
1052 let item_bounds = tcx.explicit_item_bounds(def_id);
1053 debug!("instantiate_opaque_types: bounds={:#?}", item_bounds);
1054 let bounds: Vec<_> =
1055 item_bounds.iter().map(|(bound, _)| bound.subst(tcx, substs)).collect();
1057 let param_env = tcx.param_env(def_id);
1058 let InferOk { value: bounds, obligations } =
1059 infcx.partially_normalize_associated_types_in(span, self.body_id, param_env, bounds);
1060 self.obligations.extend(obligations);
1062 debug!("instantiate_opaque_types: bounds={:?}", bounds);
1064 let required_region_bounds = required_region_bounds(tcx, ty, bounds.iter().copied());
1065 debug!("instantiate_opaque_types: required_region_bounds={:?}", required_region_bounds);
1067 // Make sure that we are in fact defining the *entire* type
1068 // (e.g., `type Foo<T: Bound> = impl Bar;` needs to be
1069 // defined by a function like `fn foo<T: Bound>() -> Foo<T>`).
1070 debug!("instantiate_opaque_types: param_env={:#?}", self.param_env,);
1071 debug!("instantiate_opaque_types: generics={:#?}", tcx.generics_of(def_id),);
1073 // Ideally, we'd get the span where *this specific `ty` came
1074 // from*, but right now we just use the span from the overall
1075 // value being folded. In simple cases like `-> impl Foo`,
1076 // these are the same span, but not in cases like `-> (impl
1078 let definition_span = self.value_span;
1080 self.opaque_types.insert(
1086 concrete_ty: ty_var,
1087 has_required_region_bounds: !required_region_bounds.is_empty(),
1091 debug!("instantiate_opaque_types: ty_var={:?}", ty_var);
1093 for predicate in &bounds {
1094 if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
1095 if projection.ty.references_error() {
1096 // No point on adding these obligations since there's a type error involved.
1102 self.obligations.reserve(bounds.len());
1103 for predicate in bounds {
1104 // Change the predicate to refer to the type variable,
1105 // which will be the concrete type instead of the opaque type.
1106 // This also instantiates nested instances of `impl Trait`.
1107 let predicate = self.instantiate_opaque_types_in_map(predicate);
1109 let cause = traits::ObligationCause::new(span, self.body_id, traits::OpaqueType);
1111 // Require that the predicate holds for the concrete type.
1112 debug!("instantiate_opaque_types: predicate={:?}", predicate);
1113 self.obligations.push(traits::Obligation::new(cause, self.param_env, predicate));
1120 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
1126 /// pub trait Bar { .. }
1128 /// pub type Baz = impl Bar;
1130 /// fn f1() -> Baz { .. }
1133 /// fn f2() -> bar::Baz { .. }
1137 /// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
1138 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
1139 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
1140 pub fn may_define_opaque_type(
1143 opaque_hir_id: hir::HirId,
1145 let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
1147 // Named opaque types can be defined by any siblings or children of siblings.
1148 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
1149 // We walk up the node tree until we hit the root or the scope of the opaque type.
1150 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
1151 hir_id = tcx.hir().get_parent_item(hir_id);
1153 // Syntactically, we are allowed to define the concrete type if:
1154 let res = hir_id == scope;
1156 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
1157 tcx.hir().find(hir_id),
1158 tcx.hir().get(opaque_hir_id),
1164 /// Given a set of predicates that apply to an object type, returns
1165 /// the region bounds that the (erased) `Self` type must
1166 /// outlive. Precisely *because* the `Self` type is erased, the
1167 /// parameter `erased_self_ty` must be supplied to indicate what type
1168 /// has been used to represent `Self` in the predicates
1169 /// themselves. This should really be a unique type; `FreshTy(0)` is a
1172 /// N.B., in some cases, particularly around higher-ranked bounds,
1173 /// this function returns a kind of conservative approximation.
1174 /// That is, all regions returned by this function are definitely
1175 /// required, but there may be other region bounds that are not
1176 /// returned, as well as requirements like `for<'a> T: 'a`.
1178 /// Requires that trait definitions have been processed so that we can
1179 /// elaborate predicates and walk supertraits.
1180 crate fn required_region_bounds(
1182 erased_self_ty: Ty<'tcx>,
1183 predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
1184 ) -> Vec<ty::Region<'tcx>> {
1185 debug!("required_region_bounds(erased_self_ty={:?})", erased_self_ty);
1187 assert!(!erased_self_ty.has_escaping_bound_vars());
1189 traits::elaborate_predicates(tcx, predicates)
1190 .filter_map(|obligation| {
1191 debug!("required_region_bounds(obligation={:?})", obligation);
1192 match obligation.predicate.kind().skip_binder() {
1193 ty::PredicateKind::Projection(..)
1194 | ty::PredicateKind::Trait(..)
1195 | ty::PredicateKind::Subtype(..)
1196 | ty::PredicateKind::WellFormed(..)
1197 | ty::PredicateKind::ObjectSafe(..)
1198 | ty::PredicateKind::ClosureKind(..)
1199 | ty::PredicateKind::RegionOutlives(..)
1200 | ty::PredicateKind::ConstEvaluatable(..)
1201 | ty::PredicateKind::ConstEquate(..)
1202 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
1203 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
1204 // Search for a bound of the form `erased_self_ty
1205 // : 'a`, but be wary of something like `for<'a>
1206 // erased_self_ty : 'a` (we interpret a
1207 // higher-ranked bound like that as 'static,
1208 // though at present the code in `fulfill.rs`
1209 // considers such bounds to be unsatisfiable, so
1210 // it's kind of a moot point since you could never
1211 // construct such an object, but this seems
1212 // correct even if that code changes).
1213 if t == &erased_self_ty && !r.has_escaping_bound_vars() {