2 use crate::hir::def_id::DefId;
4 use crate::infer::outlives::free_region_map::FreeRegionRelations;
5 use crate::infer::{self, InferCtxt, InferOk, TypeVariableOrigin, TypeVariableOriginKind};
6 use crate::middle::region;
7 use crate::traits::{self, PredicateObligation};
8 use crate::ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder, TypeVisitor};
9 use crate::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, SubstsRef};
10 use crate::ty::{self, GenericParamDefKind, Ty, TyCtxt};
11 use crate::util::nodemap::DefIdMap;
12 use errors::DiagnosticBuilder;
13 use rustc::session::config::nightly_options;
14 use rustc_data_structures::fx::FxHashMap;
15 use rustc_data_structures::sync::Lrc;
18 use rustc_error_codes::*;
20 pub type OpaqueTypeMap<'tcx> = DefIdMap<OpaqueTypeDecl<'tcx>>;
22 /// Information about the opaque types whose values we
23 /// are inferring in this function (these are the `impl Trait` that
24 /// appear in the return type).
25 #[derive(Copy, Clone, Debug)]
26 pub struct OpaqueTypeDecl<'tcx> {
27 /// The opaque type (`ty::Opaque`) for this declaration.
28 pub opaque_type: Ty<'tcx>,
30 /// The substitutions that we apply to the opaque type that this
31 /// `impl Trait` desugars to. e.g., if:
33 /// fn foo<'a, 'b, T>() -> impl Trait<'a>
35 /// winds up desugared to:
37 /// type Foo<'x, X> = impl Trait<'x>
38 /// fn foo<'a, 'b, T>() -> Foo<'a, T>
40 /// then `substs` would be `['a, T]`.
41 pub substs: SubstsRef<'tcx>,
43 /// The span of this particular definition of the opaque type. So
47 /// type Foo = impl Baz;
49 /// ^^^ 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 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
99 /// Replaces all opaque types in `value` with fresh inference variables
100 /// and creates appropriate obligations. For example, given the input:
102 /// impl Iterator<Item = impl Debug>
104 /// this method would create two type variables, `?0` and `?1`. It would
105 /// return the type `?0` but also the obligations:
107 /// ?0: Iterator<Item = ?1>
110 /// Moreover, it returns a `OpaqueTypeMap` that would map `?0` to
111 /// info about the `impl Iterator<..>` type and `?1` to info about
112 /// the `impl Debug` type.
116 /// - `parent_def_id` -- the `DefId` of the function in which the opaque type
118 /// - `body_id` -- the body-id with which the resulting obligations should
120 /// - `param_env` -- the in-scope parameter environment to be used for
122 /// - `value` -- the value within which we are instantiating opaque types
123 /// - `value_span` -- the span where the value came from, used in error reporting
124 pub fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
126 parent_def_id: DefId,
128 param_env: ty::ParamEnv<'tcx>,
131 ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)> {
133 "instantiate_opaque_types(value={:?}, parent_def_id={:?}, body_id={:?}, \
134 param_env={:?}, value_span={:?})",
135 value, parent_def_id, body_id, param_env, value_span,
137 let mut instantiator = Instantiator {
143 opaque_types: Default::default(),
146 let value = instantiator.instantiate_opaque_types_in_map(value);
147 InferOk { value: (value, instantiator.opaque_types), obligations: instantiator.obligations }
150 /// Given the map `opaque_types` containing the opaque
151 /// `impl Trait` types whose underlying, hidden types are being
152 /// inferred, this method adds constraints to the regions
153 /// appearing in those underlying hidden types to ensure that they
154 /// at least do not refer to random scopes within the current
155 /// function. These constraints are not (quite) sufficient to
156 /// guarantee that the regions are actually legal values; that
157 /// final condition is imposed after region inference is done.
161 /// Let's work through an example to explain how it works. Assume
162 /// the current function is as follows:
165 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
168 /// Here, we have two `impl Trait` types whose values are being
169 /// inferred (the `impl Bar<'a>` and the `impl
170 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
171 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
172 /// the return type of `foo`, we *reference* those definitions:
175 /// type Foo1<'x> = impl Bar<'x>;
176 /// type Foo2<'x> = impl Bar<'x>;
177 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
184 /// As indicating in the comments above, each of those references
185 /// is (in the compiler) basically a substitution (`substs`)
186 /// applied to the type of a suitable `def_id` (which identifies
187 /// `Foo1` or `Foo2`).
189 /// Now, at this point in compilation, what we have done is to
190 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
191 /// fresh inference variables C1 and C2. We wish to use the values
192 /// of these variables to infer the underlying types of `Foo1` and
193 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
194 /// constraints like:
197 /// for<'a> (Foo1<'a> = C1)
198 /// for<'b> (Foo1<'b> = C2)
201 /// For these equation to be satisfiable, the types `C1` and `C2`
202 /// can only refer to a limited set of regions. For example, `C1`
203 /// can only refer to `'static` and `'a`, and `C2` can only refer
204 /// to `'static` and `'b`. The job of this function is to impose that
207 /// Up to this point, C1 and C2 are basically just random type
208 /// inference variables, and hence they may contain arbitrary
209 /// regions. In fact, it is fairly likely that they do! Consider
210 /// this possible definition of `foo`:
213 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
218 /// Here, the values for the concrete types of the two impl
219 /// traits will include inference variables:
226 /// Ordinarily, the subtyping rules would ensure that these are
227 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
228 /// type per se, we don't get such constraints by default. This
229 /// is where this function comes into play. It adds extra
230 /// constraints to ensure that all the regions which appear in the
231 /// inferred type are regions that could validly appear.
233 /// This is actually a bit of a tricky constraint in general. We
234 /// want to say that each variable (e.g., `'0`) can only take on
235 /// values that were supplied as arguments to the opaque type
236 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
237 /// scope. We don't have a constraint quite of this kind in the current
242 /// We generally prefer to make `<=` constraints, since they
243 /// integrate best into the region solver. To do that, we find the
244 /// "minimum" of all the arguments that appear in the substs: that
245 /// is, some region which is less than all the others. In the case
246 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
247 /// all). Then we apply that as a least bound to the variables
248 /// (e.g., `'a <= '0`).
250 /// In some cases, there is no minimum. Consider this example:
253 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
256 /// Here we would report a more complex "in constraint", like `'r
257 /// in ['a, 'b, 'static]` (where `'r` is some regon appearing in
258 /// the hidden type).
260 /// # Constrain regions, not the hidden concrete type
262 /// Note that generating constraints on each region `Rc` is *not*
263 /// the same as generating an outlives constraint on `Tc` iself.
264 /// For example, if we had a function like this:
267 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
271 /// // Equivalent to:
272 /// type FooReturn<'a, T> = impl Foo<'a>;
273 /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
276 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
277 /// is an inference variable). If we generated a constraint that
278 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
279 /// but this is not necessary, because the opaque type we
280 /// create will be allowed to reference `T`. So we only generate a
281 /// constraint that `'0: 'a`.
283 /// # The `free_region_relations` parameter
285 /// The `free_region_relations` argument is used to find the
286 /// "minimum" of the regions supplied to a given opaque type.
287 /// It must be a relation that can answer whether `'a <= 'b`,
288 /// where `'a` and `'b` are regions that appear in the "substs"
289 /// for the opaque type references (the `<'a>` in `Foo1<'a>`).
291 /// Note that we do not impose the constraints based on the
292 /// generic regions from the `Foo1` definition (e.g., `'x`). This
293 /// is because the constraints we are imposing here is basically
294 /// the concern of the one generating the constraining type C1,
295 /// which is the current function. It also means that we can
296 /// take "implied bounds" into account in some cases:
299 /// trait SomeTrait<'a, 'b> { }
300 /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
303 /// Here, the fact that `'b: 'a` is known only because of the
304 /// implied bounds from the `&'a &'b u32` parameter, and is not
305 /// "inherent" to the opaque type definition.
309 /// - `opaque_types` -- the map produced by `instantiate_opaque_types`
310 /// - `free_region_relations` -- something that can be used to relate
311 /// the free regions (`'a`) that appear in the impl trait.
312 pub fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(
314 opaque_types: &OpaqueTypeMap<'tcx>,
315 free_region_relations: &FRR,
317 debug!("constrain_opaque_types()");
319 for (&def_id, opaque_defn) in opaque_types {
320 self.constrain_opaque_type(def_id, opaque_defn, free_region_relations);
324 /// See `constrain_opaque_types` for documentation.
325 pub fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
328 opaque_defn: &OpaqueTypeDecl<'tcx>,
329 free_region_relations: &FRR,
331 debug!("constrain_opaque_type()");
332 debug!("constrain_opaque_type: def_id={:?}", def_id);
333 debug!("constrain_opaque_type: opaque_defn={:#?}", opaque_defn);
337 let concrete_ty = self.resolve_vars_if_possible(&opaque_defn.concrete_ty);
339 debug!("constrain_opaque_type: concrete_ty={:?}", concrete_ty);
341 let opaque_type_generics = tcx.generics_of(def_id);
343 let span = tcx.def_span(def_id);
345 // If there are required region bounds, we can use them.
346 if opaque_defn.has_required_region_bounds {
347 let predicates_of = tcx.predicates_of(def_id);
348 debug!("constrain_opaque_type: predicates: {:#?}", predicates_of,);
349 let bounds = predicates_of.instantiate(tcx, opaque_defn.substs);
350 debug!("constrain_opaque_type: bounds={:#?}", bounds);
351 let opaque_type = tcx.mk_opaque(def_id, opaque_defn.substs);
353 let required_region_bounds = tcx.required_region_bounds(opaque_type, bounds.predicates);
354 debug_assert!(!required_region_bounds.is_empty());
356 for required_region in required_region_bounds {
357 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
359 op: |r| self.sub_regions(infer::CallReturn(span), required_region, r),
365 // There were no `required_region_bounds`,
366 // so we have to search for a `least_region`.
367 // Go through all the regions used as arguments to the
368 // opaque type. These are the parameters to the opaque
369 // type; so in our example above, `substs` would contain
370 // `['a]` for the first impl trait and `'b` for the
372 let mut least_region = None;
373 for param in &opaque_type_generics.params {
375 GenericParamDefKind::Lifetime => {}
379 // Get the value supplied for this region from the substs.
380 let subst_arg = opaque_defn.substs.region_at(param.index as usize);
382 // Compute the least upper bound of it with the other regions.
383 debug!("constrain_opaque_types: least_region={:?}", least_region);
384 debug!("constrain_opaque_types: subst_arg={:?}", subst_arg);
386 None => least_region = Some(subst_arg),
388 if free_region_relations.sub_free_regions(lr, subst_arg) {
389 // keep the current least region
390 } else if free_region_relations.sub_free_regions(subst_arg, lr) {
391 // switch to `subst_arg`
392 least_region = Some(subst_arg);
394 // There are two regions (`lr` and
395 // `subst_arg`) which are not relatable. We
396 // can't find a best choice. Therefore,
397 // instead of creating a single bound like
398 // `'r: 'a` (which is our preferred choice),
399 // we will create a "in bound" like `'r in
400 // ['a, 'b, 'c]`, where `'a..'c` are the
401 // regions that appear in the impl trait.
402 return self.generate_member_constraint(
404 opaque_type_generics,
415 let least_region = least_region.unwrap_or(tcx.lifetimes.re_static);
416 debug!("constrain_opaque_types: least_region={:?}", least_region);
418 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
420 op: |r| self.sub_regions(infer::CallReturn(span), least_region, r),
424 /// As a fallback, we sometimes generate an "in constraint". For
425 /// a case like `impl Foo<'a, 'b>`, where `'a` and `'b` cannot be
426 /// related, we would generate a constraint `'r in ['a, 'b,
427 /// 'static]` for each region `'r` that appears in the hidden type
428 /// (i.e., it must be equal to `'a`, `'b`, or `'static`).
430 /// `conflict1` and `conflict2` are the two region bounds that we
431 /// detected which were unrelated. They are used for diagnostics.
432 fn generate_member_constraint(
434 concrete_ty: Ty<'tcx>,
435 opaque_type_generics: &ty::Generics,
436 opaque_defn: &OpaqueTypeDecl<'tcx>,
437 opaque_type_def_id: DefId,
438 conflict1: ty::Region<'tcx>,
439 conflict2: ty::Region<'tcx>,
441 // For now, enforce a feature gate outside of async functions.
442 if self.member_constraint_feature_gate(
451 // Create the set of choice regions: each region in the hidden
452 // type can be equal to any of the region parameters of the
453 // opaque type definition.
454 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
458 .filter(|param| match param.kind {
459 GenericParamDefKind::Lifetime => true,
460 GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => false,
462 .map(|param| opaque_defn.substs.region_at(param.index as usize))
463 .chain(std::iter::once(self.tcx.lifetimes.re_static))
467 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
470 self.member_constraint(
472 opaque_defn.definition_span,
481 /// Member constraints are presently feature-gated except for
482 /// async-await. We expect to lift this once we've had a bit more
484 fn member_constraint_feature_gate(
486 opaque_defn: &OpaqueTypeDecl<'tcx>,
487 opaque_type_def_id: DefId,
488 conflict1: ty::Region<'tcx>,
489 conflict2: ty::Region<'tcx>,
491 // If we have `#![feature(member_constraints)]`, no problems.
492 if self.tcx.features().member_constraints {
496 let span = self.tcx.def_span(opaque_type_def_id);
498 // Without a feature-gate, we only generate member-constraints for async-await.
499 let context_name = match opaque_defn.origin {
500 // No feature-gate required for `async fn`.
501 hir::OpaqueTyOrigin::AsyncFn => return false,
503 // Otherwise, generate the label we'll use in the error message.
504 hir::OpaqueTyOrigin::TypeAlias => "impl Trait",
505 hir::OpaqueTyOrigin::FnReturn => "impl Trait",
507 let msg = format!("ambiguous lifetime bound in `{}`", context_name);
508 let mut err = self.tcx.sess.struct_span_err(span, &msg);
510 let conflict1_name = conflict1.to_string();
511 let conflict2_name = conflict2.to_string();
513 let label = match (&*conflict1_name, &*conflict2_name) {
514 ("'_", "'_") => "the elided lifetimes here do not outlive one another",
516 label_owned = format!(
517 "neither `{}` nor `{}` outlives the other",
518 conflict1_name, conflict2_name,
523 err.span_label(span, label);
525 if nightly_options::is_nightly_build() {
528 "add #![feature(member_constraints)] to the crate attributes \
537 /// Given the fully resolved, instantiated type for an opaque
538 /// type, i.e., the value of an inference variable like C1 or C2
539 /// (*), computes the "definition type" for an opaque type
540 /// definition -- that is, the inferred value of `Foo1<'x>` or
541 /// `Foo2<'x>` that we would conceptually use in its definition:
543 /// type Foo1<'x> = impl Bar<'x> = AAA; <-- this type AAA
544 /// type Foo2<'x> = impl Bar<'x> = BBB; <-- or this type BBB
545 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
547 /// Note that these values are defined in terms of a distinct set of
548 /// generic parameters (`'x` instead of `'a`) from C1 or C2. The main
549 /// purpose of this function is to do that translation.
551 /// (*) C1 and C2 were introduced in the comments on
552 /// `constrain_opaque_types`. Read that comment for more context.
556 /// - `def_id`, the `impl Trait` type
557 /// - `opaque_defn`, the opaque definition created in `instantiate_opaque_types`
558 /// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of
559 /// `opaque_defn.concrete_ty`
560 pub fn infer_opaque_definition_from_instantiation(
563 opaque_defn: &OpaqueTypeDecl<'tcx>,
564 instantiated_ty: Ty<'tcx>,
568 "infer_opaque_definition_from_instantiation(def_id={:?}, instantiated_ty={:?})",
569 def_id, instantiated_ty
572 // Use substs to build up a reverse map from regions to their
573 // identity mappings. This is necessary because of `impl
574 // Trait` lifetimes are computed by replacing existing
575 // lifetimes with 'static and remapping only those used in the
576 // `impl Trait` return type, resulting in the parameters
578 let id_substs = InternalSubsts::identity_for_item(self.tcx, def_id);
579 let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> = opaque_defn
583 .map(|(index, subst)| (*subst, id_substs[index]))
586 // Convert the type from the function into a type valid outside
587 // the function, by replacing invalid regions with 'static,
588 // after producing an error for each of them.
589 let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new(
591 self.is_tainted_by_errors(),
597 debug!("infer_opaque_definition_from_instantiation: definition_ty={:?}", definition_ty);
603 pub fn unexpected_hidden_region_diagnostic(
605 region_scope_tree: Option<®ion::ScopeTree>,
606 opaque_type_def_id: DefId,
608 hidden_region: ty::Region<'tcx>,
609 ) -> DiagnosticBuilder<'tcx> {
610 let span = tcx.def_span(opaque_type_def_id);
611 let mut err = struct_span_err!(
615 "hidden type for `impl Trait` captures lifetime that does not appear in bounds",
618 // Explain the region we are capturing.
619 if let ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic | ty::ReEmpty = hidden_region {
620 // Assuming regionck succeeded (*), we ought to always be
621 // capturing *some* region from the fn header, and hence it
622 // ought to be free. So under normal circumstances, we will go
623 // down this path which gives a decent human readable
626 // (*) if not, the `tainted_by_errors` flag would be set to
627 // true in any case, so we wouldn't be here at all.
628 tcx.note_and_explain_free_region(
630 &format!("hidden type `{}` captures ", hidden_ty),
635 // Ugh. This is a painful case: the hidden region is not one
636 // that we can easily summarize or explain. This can happen
638 // `src/test/ui/multiple-lifetimes/ordinary-bounds-unsuited.rs`:
641 // fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b> {
642 // if condition() { a } else { b }
646 // Here the captured lifetime is the intersection of `'a` and
647 // `'b`, which we can't quite express.
649 if let Some(region_scope_tree) = region_scope_tree {
650 // If the `region_scope_tree` is available, this is being
651 // invoked from the "region inferencer error". We can at
652 // least report a really cryptic error for now.
653 tcx.note_and_explain_region(
656 &format!("hidden type `{}` captures ", hidden_ty),
661 // If the `region_scope_tree` is *unavailable*, this is
662 // being invoked by the code that comes *after* region
663 // inferencing. This is a bug, as the region inferencer
664 // ought to have noticed the failed constraint and invoked
665 // error reporting, which in turn should have prevented us
666 // from getting trying to infer the hidden type
668 tcx.sess.delay_span_bug(
671 "hidden type captures unexpected lifetime `{:?}` \
672 but no region inference failure",
682 // Visitor that requires that (almost) all regions in the type visited outlive
683 // `least_region`. We cannot use `push_outlives_components` because regions in
684 // closure signatures are not included in their outlives components. We need to
685 // ensure all regions outlive the given bound so that we don't end up with,
686 // say, `ReScope` appearing in a return type and causing ICEs when other
687 // functions end up with region constraints involving regions from other
690 // We also cannot use `for_each_free_region` because for closures it includes
691 // the regions parameters from the enclosing item.
693 // We ignore any type parameters because impl trait values are assumed to
694 // capture all the in-scope type parameters.
695 struct ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
697 OP: FnMut(ty::Region<'tcx>),
703 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
705 OP: FnMut(ty::Region<'tcx>),
707 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> bool {
708 t.skip_binder().visit_with(self);
709 false // keep visiting
712 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
714 // ignore bound regions, keep visiting
715 ty::ReLateBound(_, _) => false,
723 fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
724 // We're only interested in types involving regions
725 if !ty.flags.intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
726 return false; // keep visiting
730 ty::Closure(def_id, ref substs) => {
731 // Skip lifetime parameters of the enclosing item(s)
733 for upvar_ty in substs.as_closure().upvar_tys(def_id, self.tcx) {
734 upvar_ty.visit_with(self);
737 substs.as_closure().sig_ty(def_id, self.tcx).visit_with(self);
740 ty::Generator(def_id, ref substs, _) => {
741 // Skip lifetime parameters of the enclosing item(s)
742 // Also skip the witness type, because that has no free regions.
744 for upvar_ty in substs.as_generator().upvar_tys(def_id, self.tcx) {
745 upvar_ty.visit_with(self);
748 substs.as_generator().return_ty(def_id, self.tcx).visit_with(self);
749 substs.as_generator().yield_ty(def_id, self.tcx).visit_with(self);
752 ty.super_visit_with(self);
760 struct ReverseMapper<'tcx> {
763 /// If errors have already been reported in this fn, we suppress
764 /// our own errors because they are sometimes derivative.
765 tainted_by_errors: bool,
767 opaque_type_def_id: DefId,
768 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
769 map_missing_regions_to_empty: bool,
771 /// initially `Some`, set to `None` once error has been reported
772 hidden_ty: Option<Ty<'tcx>>,
774 /// Span of function being checked.
778 impl ReverseMapper<'tcx> {
781 tainted_by_errors: bool,
782 opaque_type_def_id: DefId,
783 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
792 map_missing_regions_to_empty: false,
793 hidden_ty: Some(hidden_ty),
798 fn fold_kind_mapping_missing_regions_to_empty(
800 kind: GenericArg<'tcx>,
801 ) -> GenericArg<'tcx> {
802 assert!(!self.map_missing_regions_to_empty);
803 self.map_missing_regions_to_empty = true;
804 let kind = kind.fold_with(self);
805 self.map_missing_regions_to_empty = false;
809 fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
810 assert!(!self.map_missing_regions_to_empty);
815 impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
816 fn tcx(&self) -> TyCtxt<'tcx> {
820 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
822 // ignore bound regions that appear in the type (e.g., this
823 // would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
824 ty::ReLateBound(..) |
826 // ignore `'static`, as that can appear anywhere
827 ty::ReStatic => return r,
832 let generics = self.tcx().generics_of(self.opaque_type_def_id);
833 match self.map.get(&r.into()).map(|k| k.unpack()) {
834 Some(GenericArgKind::Lifetime(r1)) => r1,
835 Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
836 None if generics.parent.is_some() => {
837 if !self.map_missing_regions_to_empty && !self.tainted_by_errors {
838 if let Some(hidden_ty) = self.hidden_ty.take() {
839 unexpected_hidden_region_diagnostic(
842 self.opaque_type_def_id,
849 self.tcx.lifetimes.re_empty
854 .struct_span_err(self.span, "non-defining opaque type use in defining scope")
858 "lifetime `{}` is part of concrete type but not used in \
859 parameter list of the `impl Trait` type alias",
865 self.tcx().mk_region(ty::ReStatic)
870 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
872 ty::Closure(def_id, substs) => {
873 // I am a horrible monster and I pray for death. When
874 // we encounter a closure here, it is always a closure
875 // from within the function that we are currently
876 // type-checking -- one that is now being encapsulated
877 // in an opaque type. Ideally, we would
878 // go through the types/lifetimes that it references
879 // and treat them just like we would any other type,
880 // which means we would error out if we find any
881 // reference to a type/region that is not in the
884 // **However,** in the case of closures, there is a
885 // somewhat subtle (read: hacky) consideration. The
886 // problem is that our closure types currently include
887 // all the lifetime parameters declared on the
888 // enclosing function, even if they are unused by the
889 // closure itself. We can't readily filter them out,
890 // so here we replace those values with `'empty`. This
891 // can't really make a difference to the rest of the
892 // compiler; those regions are ignored for the
893 // outlives relation, and hence don't affect trait
894 // selection or auto traits, and they are erased
897 let generics = self.tcx.generics_of(def_id);
898 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, &kind)| {
899 if index < generics.parent_count {
900 // Accommodate missing regions in the parent kinds...
901 self.fold_kind_mapping_missing_regions_to_empty(kind)
903 // ...but not elsewhere.
904 self.fold_kind_normally(kind)
908 self.tcx.mk_closure(def_id, substs)
911 ty::Generator(def_id, substs, movability) => {
912 let generics = self.tcx.generics_of(def_id);
913 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, &kind)| {
914 if index < generics.parent_count {
915 // Accommodate missing regions in the parent kinds...
916 self.fold_kind_mapping_missing_regions_to_empty(kind)
918 // ...but not elsewhere.
919 self.fold_kind_normally(kind)
923 self.tcx.mk_generator(def_id, substs, movability)
927 // Look it up in the substitution list.
928 match self.map.get(&ty.into()).map(|k| k.unpack()) {
929 // Found it in the substitution list; replace with the parameter from the
931 Some(GenericArgKind::Type(t1)) => t1,
932 Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
939 "type parameter `{}` is part of concrete type but not \
940 used in parameter list for the `impl Trait` type alias",
951 _ => ty.super_fold_with(self),
955 fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
956 trace!("checking const {:?}", ct);
957 // Find a const parameter
959 ty::ConstKind::Param(..) => {
960 // Look it up in the substitution list.
961 match self.map.get(&ct.into()).map(|k| k.unpack()) {
962 // Found it in the substitution list, replace with the parameter from the
964 Some(GenericArgKind::Const(c1)) => c1,
965 Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
972 "const parameter `{}` is part of concrete type but not \
973 used in parameter list for the `impl Trait` type alias",
979 self.tcx().consts.err
989 struct Instantiator<'a, 'tcx> {
990 infcx: &'a InferCtxt<'a, 'tcx>,
991 parent_def_id: DefId,
993 param_env: ty::ParamEnv<'tcx>,
995 opaque_types: OpaqueTypeMap<'tcx>,
996 obligations: Vec<PredicateObligation<'tcx>>,
999 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
1000 fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
1001 debug!("instantiate_opaque_types_in_map(value={:?})", value);
1002 let tcx = self.infcx.tcx;
1003 value.fold_with(&mut BottomUpFolder {
1006 if ty.references_error() {
1007 return tcx.types.err;
1008 } else if let ty::Opaque(def_id, substs) = ty.kind {
1009 // Check that this is `impl Trait` type is
1010 // declared by `parent_def_id` -- i.e., one whose
1011 // value we are inferring. At present, this is
1012 // always true during the first phase of
1013 // type-check, but not always true later on during
1014 // NLL. Once we support named opaque types more fully,
1015 // this same scenario will be able to arise during all phases.
1017 // Here is an example using type alias `impl Trait`
1018 // that indicates the distinction we are checking for:
1022 // pub type Foo = impl Iterator;
1023 // pub fn make_foo() -> Foo { .. }
1027 // fn foo() -> a::Foo { a::make_foo() }
1031 // Here, the return type of `foo` references a
1032 // `Opaque` indeed, but not one whose value is
1033 // presently being inferred. You can get into a
1034 // similar situation with closure return types
1038 // fn foo() -> impl Iterator { .. }
1040 // let x = || foo(); // returns the Opaque assoc with `foo`
1043 if let Some(opaque_hir_id) = tcx.hir().as_local_hir_id(def_id) {
1044 let parent_def_id = self.parent_def_id;
1045 let def_scope_default = || {
1046 let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
1047 parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
1049 let (in_definition_scope, origin) = match tcx.hir().find(opaque_hir_id) {
1050 Some(Node::Item(item)) => match item.kind {
1051 // Anonymous `impl Trait`
1052 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1053 impl_trait_fn: Some(parent),
1056 }) => (parent == self.parent_def_id, origin),
1057 // Named `type Foo = impl Bar;`
1058 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1059 impl_trait_fn: None,
1063 may_define_opaque_type(tcx, self.parent_def_id, opaque_hir_id),
1066 _ => (def_scope_default(), hir::OpaqueTyOrigin::TypeAlias),
1068 Some(Node::ImplItem(item)) => match item.kind {
1069 hir::ImplItemKind::OpaqueTy(_) => (
1070 may_define_opaque_type(tcx, self.parent_def_id, opaque_hir_id),
1071 hir::OpaqueTyOrigin::TypeAlias,
1073 _ => (def_scope_default(), hir::OpaqueTyOrigin::TypeAlias),
1076 "expected (impl) item, found {}",
1077 tcx.hir().node_to_string(opaque_hir_id),
1080 if in_definition_scope {
1081 return self.fold_opaque_ty(ty, def_id, substs, origin);
1085 "instantiate_opaque_types_in_map: \
1086 encountered opaque outside its definition scope \
1104 substs: SubstsRef<'tcx>,
1105 origin: hir::OpaqueTyOrigin,
1107 let infcx = self.infcx;
1108 let tcx = infcx.tcx;
1110 debug!("instantiate_opaque_types: Opaque(def_id={:?}, substs={:?})", def_id, substs);
1112 // Use the same type variable if the exact same opaque type appears more
1113 // than once in the return type (e.g., if it's passed to a type alias).
1114 if let Some(opaque_defn) = self.opaque_types.get(&def_id) {
1115 debug!("instantiate_opaque_types: returning concrete ty {:?}", opaque_defn.concrete_ty);
1116 return opaque_defn.concrete_ty;
1118 let span = tcx.def_span(def_id);
1119 debug!("fold_opaque_ty {:?} {:?}", self.value_span, span);
1121 .next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span });
1123 let predicates_of = tcx.predicates_of(def_id);
1124 debug!("instantiate_opaque_types: predicates={:#?}", predicates_of,);
1125 let bounds = predicates_of.instantiate(tcx, substs);
1127 let param_env = tcx.param_env(def_id);
1128 let InferOk { value: bounds, obligations } =
1129 infcx.partially_normalize_associated_types_in(span, self.body_id, param_env, &bounds);
1130 self.obligations.extend(obligations);
1132 debug!("instantiate_opaque_types: bounds={:?}", bounds);
1134 let required_region_bounds = tcx.required_region_bounds(ty, bounds.predicates.clone());
1135 debug!("instantiate_opaque_types: required_region_bounds={:?}", required_region_bounds);
1137 // Make sure that we are in fact defining the *entire* type
1138 // (e.g., `type Foo<T: Bound> = impl Bar;` needs to be
1139 // defined by a function like `fn foo<T: Bound>() -> Foo<T>`).
1140 debug!("instantiate_opaque_types: param_env={:#?}", self.param_env,);
1141 debug!("instantiate_opaque_types: generics={:#?}", tcx.generics_of(def_id),);
1143 // Ideally, we'd get the span where *this specific `ty` came
1144 // from*, but right now we just use the span from the overall
1145 // value being folded. In simple cases like `-> impl Foo`,
1146 // these are the same span, but not in cases like `-> (impl
1148 let definition_span = self.value_span;
1150 self.opaque_types.insert(
1156 concrete_ty: ty_var,
1157 has_required_region_bounds: !required_region_bounds.is_empty(),
1161 debug!("instantiate_opaque_types: ty_var={:?}", ty_var);
1163 for predicate in &bounds.predicates {
1164 if let ty::Predicate::Projection(projection) = &predicate {
1165 if projection.skip_binder().ty.references_error() {
1166 // No point on adding these obligations since there's a type error involved.
1172 self.obligations.reserve(bounds.predicates.len());
1173 for predicate in bounds.predicates {
1174 // Change the predicate to refer to the type variable,
1175 // which will be the concrete type instead of the opaque type.
1176 // This also instantiates nested instances of `impl Trait`.
1177 let predicate = self.instantiate_opaque_types_in_map(&predicate);
1179 let cause = traits::ObligationCause::new(span, self.body_id, traits::SizedReturnType);
1181 // Require that the predicate holds for the concrete type.
1182 debug!("instantiate_opaque_types: predicate={:?}", predicate);
1183 self.obligations.push(traits::Obligation::new(cause, self.param_env, predicate));
1190 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
1196 /// pub trait Bar { .. }
1198 /// pub type Baz = impl Bar;
1200 /// fn f1() -> Baz { .. }
1203 /// fn f2() -> bar::Baz { .. }
1207 /// Here, `def_id` is the `DefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
1208 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
1209 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
1210 pub fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: DefId, opaque_hir_id: hir::HirId) -> bool {
1211 let mut hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
1213 // Named opaque types can be defined by any siblings or children of siblings.
1214 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
1215 // We walk up the node tree until we hit the root or the scope of the opaque type.
1216 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
1217 hir_id = tcx.hir().get_parent_item(hir_id);
1219 // Syntactically, we are allowed to define the concrete type if:
1220 let res = hir_id == scope;
1222 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
1223 tcx.hir().get(hir_id),
1224 tcx.hir().get(opaque_hir_id),