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::mir::interpret::ConstValue;
8 use crate::traits::{self, PredicateObligation};
9 use crate::ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder, TypeVisitor};
10 use crate::ty::subst::{InternalSubsts, Kind, SubstsRef, UnpackedKind};
11 use crate::ty::{self, GenericParamDefKind, Ty, TyCtxt};
12 use crate::util::nodemap::DefIdMap;
13 use errors::DiagnosticBuilder;
14 use rustc::session::config::nightly_options;
15 use rustc_data_structures::fx::FxHashMap;
16 use rustc_data_structures::sync::Lrc;
19 pub type OpaqueTypeMap<'tcx> = DefIdMap<OpaqueTypeDecl<'tcx>>;
21 /// Information about the opaque, abstract 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 substitutions that we apply to the abstract that this
27 /// `impl Trait` desugars to. e.g., if:
29 /// fn foo<'a, 'b, T>() -> impl Trait<'a>
31 /// winds up desugared to:
33 /// abstract type Foo<'x, X>: Trait<'x>
34 /// fn foo<'a, 'b, T>() -> Foo<'a, T>
36 /// then `substs` would be `['a, T]`.
37 pub substs: SubstsRef<'tcx>,
39 /// The span of this particular definition of the opaque type. So
43 /// existential type Foo;
45 /// ^^^ This is the span we are looking for!
48 /// In cases where the fn returns `(impl Trait, impl Trait)` or
49 /// other such combinations, the result is currently
50 /// over-approximated, but better than nothing.
51 pub definition_span: Span,
53 /// The type variable that represents the value of the abstract type
54 /// that we require. In other words, after we compile this function,
55 /// we will be created a constraint like:
59 /// where `?C` is the value of this type variable. =) It may
60 /// naturally refer to the type and lifetime parameters in scope
61 /// in this function, though ultimately it should only reference
62 /// those that are arguments to `Foo` in the constraint above. (In
63 /// other words, `?C` should not include `'b`, even though it's a
64 /// lifetime parameter on `foo`.)
65 pub concrete_ty: Ty<'tcx>,
67 /// Returns `true` if the `impl Trait` bounds include region bounds.
68 /// For example, this would be true for:
70 /// fn foo<'a, 'b, 'c>() -> impl Trait<'c> + 'a + 'b
74 /// fn foo<'c>() -> impl Trait<'c>
76 /// unless `Trait` was declared like:
78 /// trait Trait<'c>: 'c
80 /// in which case it would be true.
82 /// This is used during regionck to decide whether we need to
83 /// impose any additional constraints to ensure that region
84 /// variables in `concrete_ty` wind up being constrained to
85 /// something from `substs` (or, at minimum, things that outlive
86 /// the fn body). (Ultimately, writeback is responsible for this
88 pub has_required_region_bounds: bool,
90 /// The origin of the existential type
91 pub origin: hir::ExistTyOrigin,
94 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
95 /// Replaces all opaque types in `value` with fresh inference variables
96 /// and creates appropriate obligations. For example, given the input:
98 /// impl Iterator<Item = impl Debug>
100 /// this method would create two type variables, `?0` and `?1`. It would
101 /// return the type `?0` but also the obligations:
103 /// ?0: Iterator<Item = ?1>
106 /// Moreover, it returns a `OpaqueTypeMap` that would map `?0` to
107 /// info about the `impl Iterator<..>` type and `?1` to info about
108 /// the `impl Debug` type.
112 /// - `parent_def_id` -- the `DefId` of the function in which the opaque type
114 /// - `body_id` -- the body-id with which the resulting obligations should
116 /// - `param_env` -- the in-scope parameter environment to be used for
118 /// - `value` -- the value within which we are instantiating opaque types
119 /// - `value_span` -- the span where the value came from, used in error reporting
120 pub fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
122 parent_def_id: DefId,
124 param_env: ty::ParamEnv<'tcx>,
127 ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)> {
129 "instantiate_opaque_types(value={:?}, parent_def_id={:?}, body_id={:?}, \
131 value, parent_def_id, body_id, param_env,
133 let mut instantiator = Instantiator {
139 opaque_types: Default::default(),
142 let value = instantiator.instantiate_opaque_types_in_map(value);
143 InferOk { value: (value, instantiator.opaque_types), obligations: instantiator.obligations }
146 /// Given the map `opaque_types` containing the existential `impl
147 /// Trait` types whose underlying, hidden types are being
148 /// inferred, this method adds constraints to the regions
149 /// appearing in those underlying hidden types to ensure that they
150 /// at least do not refer to random scopes within the current
151 /// function. These constraints are not (quite) sufficient to
152 /// guarantee that the regions are actually legal values; that
153 /// final condition is imposed after region inference is done.
157 /// Let's work through an example to explain how it works. Assume
158 /// the current function is as follows:
161 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
164 /// Here, we have two `impl Trait` types whose values are being
165 /// inferred (the `impl Bar<'a>` and the `impl
166 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
167 /// define underlying abstract types (`Foo1`, `Foo2`) and then, in
168 /// the return type of `foo`, we *reference* those definitions:
171 /// abstract type Foo1<'x>: Bar<'x>;
172 /// abstract type Foo2<'x>: Bar<'x>;
173 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
180 /// As indicating in the comments above, each of those references
181 /// is (in the compiler) basically a substitution (`substs`)
182 /// applied to the type of a suitable `def_id` (which identifies
183 /// `Foo1` or `Foo2`).
185 /// Now, at this point in compilation, what we have done is to
186 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
187 /// fresh inference variables C1 and C2. We wish to use the values
188 /// of these variables to infer the underlying types of `Foo1` and
189 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
190 /// constraints like:
193 /// for<'a> (Foo1<'a> = C1)
194 /// for<'b> (Foo1<'b> = C2)
197 /// For these equation to be satisfiable, the types `C1` and `C2`
198 /// can only refer to a limited set of regions. For example, `C1`
199 /// can only refer to `'static` and `'a`, and `C2` can only refer
200 /// to `'static` and `'b`. The job of this function is to impose that
203 /// Up to this point, C1 and C2 are basically just random type
204 /// inference variables, and hence they may contain arbitrary
205 /// regions. In fact, it is fairly likely that they do! Consider
206 /// this possible definition of `foo`:
209 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
214 /// Here, the values for the concrete types of the two impl
215 /// traits will include inference variables:
222 /// Ordinarily, the subtyping rules would ensure that these are
223 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
224 /// type per se, we don't get such constraints by default. This
225 /// is where this function comes into play. It adds extra
226 /// constraints to ensure that all the regions which appear in the
227 /// inferred type are regions that could validly appear.
229 /// This is actually a bit of a tricky constraint in general. We
230 /// want to say that each variable (e.g., `'0`) can only take on
231 /// values that were supplied as arguments to the abstract type
232 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
233 /// scope. We don't have a constraint quite of this kind in the current
238 /// We generally prefer to make `<=` constraints, since they
239 /// integrate best into the region solver. To do that, we find the
240 /// "minimum" of all the arguments that appear in the substs: that
241 /// is, some region which is less than all the others. In the case
242 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
243 /// all). Then we apply that as a least bound to the variables
244 /// (e.g., `'a <= '0`).
246 /// In some cases, there is no minimum. Consider this example:
249 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
252 /// Here we would report a more complex "in constraint", like `'r
253 /// in ['a, 'b, 'static]` (where `'r` is some regon appearing in
254 /// the hidden type).
256 /// # Constrain regions, not the hidden concrete type
258 /// Note that generating constraints on each region `Rc` is *not*
259 /// the same as generating an outlives constraint on `Tc` iself.
260 /// For example, if we had a function like this:
263 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
267 /// // Equivalent to:
268 /// existential type FooReturn<'a, T>: Foo<'a>;
269 /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
272 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
273 /// is an inference variable). If we generated a constraint that
274 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
275 /// but this is not necessary, because the existential type we
276 /// create will be allowed to reference `T`. So we only generate a
277 /// constraint that `'0: 'a`.
279 /// # The `free_region_relations` parameter
281 /// The `free_region_relations` argument is used to find the
282 /// "minimum" of the regions supplied to a given abstract type.
283 /// It must be a relation that can answer whether `'a <= 'b`,
284 /// where `'a` and `'b` are regions that appear in the "substs"
285 /// for the abstract type references (the `<'a>` in `Foo1<'a>`).
287 /// Note that we do not impose the constraints based on the
288 /// generic regions from the `Foo1` definition (e.g., `'x`). This
289 /// is because the constraints we are imposing here is basically
290 /// the concern of the one generating the constraining type C1,
291 /// which is the current function. It also means that we can
292 /// take "implied bounds" into account in some cases:
295 /// trait SomeTrait<'a, 'b> { }
296 /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
299 /// Here, the fact that `'b: 'a` is known only because of the
300 /// implied bounds from the `&'a &'b u32` parameter, and is not
301 /// "inherent" to the abstract type definition.
305 /// - `opaque_types` -- the map produced by `instantiate_opaque_types`
306 /// - `free_region_relations` -- something that can be used to relate
307 /// the free regions (`'a`) that appear in the impl trait.
308 pub fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(
310 opaque_types: &OpaqueTypeMap<'tcx>,
311 free_region_relations: &FRR,
313 debug!("constrain_opaque_types()");
315 for (&def_id, opaque_defn) in opaque_types {
316 self.constrain_opaque_type(def_id, opaque_defn, free_region_relations);
320 /// See `constrain_opaque_types` for documentation.
321 pub fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
324 opaque_defn: &OpaqueTypeDecl<'tcx>,
325 free_region_relations: &FRR,
327 debug!("constrain_opaque_type()");
328 debug!("constrain_opaque_type: def_id={:?}", def_id);
329 debug!("constrain_opaque_type: opaque_defn={:#?}", opaque_defn);
333 let concrete_ty = self.resolve_vars_if_possible(&opaque_defn.concrete_ty);
335 debug!("constrain_opaque_type: concrete_ty={:?}", concrete_ty);
337 let opaque_type_generics = tcx.generics_of(def_id);
339 let span = tcx.def_span(def_id);
341 // If there are required region bounds, we can use them.
342 if opaque_defn.has_required_region_bounds {
343 let predicates_of = tcx.predicates_of(def_id);
344 debug!("constrain_opaque_type: predicates: {:#?}", predicates_of,);
345 let bounds = predicates_of.instantiate(tcx, opaque_defn.substs);
346 debug!("constrain_opaque_type: bounds={:#?}", bounds);
347 let opaque_type = tcx.mk_opaque(def_id, opaque_defn.substs);
349 let required_region_bounds = tcx.required_region_bounds(opaque_type, bounds.predicates);
350 debug_assert!(!required_region_bounds.is_empty());
352 for required_region in required_region_bounds {
353 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
355 op: |r| self.sub_regions(infer::CallReturn(span), required_region, r),
361 // There were no `required_region_bounds`,
362 // so we have to search for a `least_region`.
363 // Go through all the regions used as arguments to the
364 // abstract type. These are the parameters to the abstract
365 // type; so in our example above, `substs` would contain
366 // `['a]` for the first impl trait and `'b` for the
368 let mut least_region = None;
369 for param in &opaque_type_generics.params {
371 GenericParamDefKind::Lifetime => {}
375 // Get the value supplied for this region from the substs.
376 let subst_arg = opaque_defn.substs.region_at(param.index as usize);
378 // Compute the least upper bound of it with the other regions.
379 debug!("constrain_opaque_types: least_region={:?}", least_region);
380 debug!("constrain_opaque_types: subst_arg={:?}", subst_arg);
382 None => least_region = Some(subst_arg),
384 if free_region_relations.sub_free_regions(lr, subst_arg) {
385 // keep the current least region
386 } else if free_region_relations.sub_free_regions(subst_arg, lr) {
387 // switch to `subst_arg`
388 least_region = Some(subst_arg);
390 // There are two regions (`lr` and
391 // `subst_arg`) which are not relatable. We
392 // can't find a best choice. Therefore,
393 // instead of creating a single bound like
394 // `'r: 'a` (which is our preferred choice),
395 // we will create a "in bound" like `'r in
396 // ['a, 'b, 'c]`, where `'a..'c` are the
397 // regions that appear in the impl trait.
398 return self.generate_member_constraint(
400 opaque_type_generics,
411 let least_region = least_region.unwrap_or(tcx.lifetimes.re_static);
412 debug!("constrain_opaque_types: least_region={:?}", least_region);
414 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
416 op: |r| self.sub_regions(infer::CallReturn(span), least_region, r),
420 /// As a fallback, we sometimes generate an "in constraint". For
421 /// a case like `impl Foo<'a, 'b>`, where `'a` and `'b` cannot be
422 /// related, we would generate a constraint `'r in ['a, 'b,
423 /// 'static]` for each region `'r` that appears in the hidden type
424 /// (i.e., it must be equal to `'a`, `'b`, or `'static`).
426 /// `conflict1` and `conflict2` are the two region bounds that we
427 /// detected which were unrelated. They are used for diagnostics.
428 fn generate_member_constraint(
430 concrete_ty: Ty<'tcx>,
431 opaque_type_generics: &ty::Generics,
432 opaque_defn: &OpaqueTypeDecl<'tcx>,
433 opaque_type_def_id: DefId,
434 conflict1: ty::Region<'tcx>,
435 conflict2: ty::Region<'tcx>,
437 // For now, enforce a feature gate outside of async functions.
438 if self.member_constraint_feature_gate(
447 // Create the set of choice regions: each region in the hidden
448 // type can be equal to any of the region parameters of the
449 // opaque type definition.
450 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
454 .filter(|param| match param.kind {
455 GenericParamDefKind::Lifetime => true,
456 GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => false,
458 .map(|param| opaque_defn.substs.region_at(param.index as usize))
459 .chain(std::iter::once(self.tcx.lifetimes.re_static))
463 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
465 op: |r| self.member_constraint(
467 opaque_defn.definition_span,
475 /// Member constraints are presently feature-gated except for
476 /// async-await. We expect to lift this once we've had a bit more
478 fn member_constraint_feature_gate(
480 opaque_defn: &OpaqueTypeDecl<'tcx>,
481 opaque_type_def_id: DefId,
482 conflict1: ty::Region<'tcx>,
483 conflict2: ty::Region<'tcx>,
485 // If we have `#![feature(member_constraints)]`, no problems.
486 if self.tcx.features().member_constraints {
490 let span = self.tcx.def_span(opaque_type_def_id);
492 // Without a feature-gate, we only generate member-constraints for async-await.
493 let context_name = match opaque_defn.origin {
494 // No feature-gate required for `async fn`.
495 hir::ExistTyOrigin::AsyncFn => return false,
497 // Otherwise, generate the label we'll use in the error message.
498 hir::ExistTyOrigin::ExistentialType => "existential type",
499 hir::ExistTyOrigin::ReturnImplTrait => "impl Trait",
501 let msg = format!("ambiguous lifetime bound in `{}`", context_name);
502 let mut err = self.tcx.sess.struct_span_err(span, &msg);
504 let conflict1_name = conflict1.to_string();
505 let conflict2_name = conflict2.to_string();
507 let label = match (&*conflict1_name, &*conflict2_name) {
508 ("'_", "'_") => "the elided lifetimes here do not outlive one another",
510 label_owned = format!(
511 "neither `{}` nor `{}` outlives the other",
512 conflict1_name, conflict2_name,
517 err.span_label(span, label);
519 if nightly_options::is_nightly_build() {
521 "add #![feature(member_constraints)] to the crate attributes \
529 /// Given the fully resolved, instantiated type for an opaque
530 /// type, i.e., the value of an inference variable like C1 or C2
531 /// (*), computes the "definition type" for an abstract type
532 /// definition -- that is, the inferred value of `Foo1<'x>` or
533 /// `Foo2<'x>` that we would conceptually use in its definition:
535 /// abstract type Foo1<'x>: Bar<'x> = AAA; <-- this type AAA
536 /// abstract type Foo2<'x>: Bar<'x> = BBB; <-- or this type BBB
537 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
539 /// Note that these values are defined in terms of a distinct set of
540 /// generic parameters (`'x` instead of `'a`) from C1 or C2. The main
541 /// purpose of this function is to do that translation.
543 /// (*) C1 and C2 were introduced in the comments on
544 /// `constrain_opaque_types`. Read that comment for more context.
548 /// - `def_id`, the `impl Trait` type
549 /// - `opaque_defn`, the opaque definition created in `instantiate_opaque_types`
550 /// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of
551 /// `opaque_defn.concrete_ty`
552 pub fn infer_opaque_definition_from_instantiation(
555 opaque_defn: &OpaqueTypeDecl<'tcx>,
556 instantiated_ty: Ty<'tcx>,
560 "infer_opaque_definition_from_instantiation(def_id={:?}, instantiated_ty={:?})",
561 def_id, instantiated_ty
564 let gcx = self.tcx.global_tcx();
566 // Use substs to build up a reverse map from regions to their
567 // identity mappings. This is necessary because of `impl
568 // Trait` lifetimes are computed by replacing existing
569 // lifetimes with 'static and remapping only those used in the
570 // `impl Trait` return type, resulting in the parameters
572 let id_substs = InternalSubsts::identity_for_item(gcx, def_id);
573 let map: FxHashMap<Kind<'tcx>, Kind<'tcx>> = opaque_defn
577 .map(|(index, subst)| (*subst, id_substs[index]))
580 // Convert the type from the function into a type valid outside
581 // the function, by replacing invalid regions with 'static,
582 // after producing an error for each of them.
583 let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new(
585 self.is_tainted_by_errors(),
591 debug!("infer_opaque_definition_from_instantiation: definition_ty={:?}", definition_ty);
597 pub fn unexpected_hidden_region_diagnostic(
599 region_scope_tree: Option<®ion::ScopeTree>,
600 opaque_type_def_id: DefId,
602 hidden_region: ty::Region<'tcx>,
603 ) -> DiagnosticBuilder<'tcx> {
604 let span = tcx.def_span(opaque_type_def_id);
605 let mut err = struct_span_err!(
609 "hidden type for `impl Trait` captures lifetime that does not appear in bounds",
612 // Explain the region we are capturing.
613 if let ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic | ty::ReEmpty = hidden_region {
614 // Assuming regionck succeeded (*), we ought to always be
615 // capturing *some* region from the fn header, and hence it
616 // ought to be free. So under normal circumstances, we will go
617 // down this path which gives a decent human readable
620 // (*) if not, the `tainted_by_errors` flag would be set to
621 // true in any case, so we wouldn't be here at all.
622 tcx.note_and_explain_free_region(
624 &format!("hidden type `{}` captures ", hidden_ty),
629 // Ugh. This is a painful case: the hidden region is not one
630 // that we can easily summarize or explain. This can happen
632 // `src/test/ui/multiple-lifetimes/ordinary-bounds-unsuited.rs`:
635 // fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b> {
636 // if condition() { a } else { b }
640 // Here the captured lifetime is the intersection of `'a` and
641 // `'b`, which we can't quite express.
643 if let Some(region_scope_tree) = region_scope_tree {
644 // If the `region_scope_tree` is available, this is being
645 // invoked from the "region inferencer error". We can at
646 // least report a really cryptic error for now.
647 tcx.note_and_explain_region(
650 &format!("hidden type `{}` captures ", hidden_ty),
655 // If the `region_scope_tree` is *unavailable*, this is
656 // being invoked by the code that comes *after* region
657 // inferencing. This is a bug, as the region inferencer
658 // ought to have noticed the failed constraint and invoked
659 // error reporting, which in turn should have prevented us
660 // from getting trying to infer the hidden type
662 tcx.sess.delay_span_bug(
665 "hidden type captures unexpected lifetime `{:?}` \
666 but no region inference failure",
676 // Visitor that requires that (almost) all regions in the type visited outlive
677 // `least_region`. We cannot use `push_outlives_components` because regions in
678 // closure signatures are not included in their outlives components. We need to
679 // ensure all regions outlive the given bound so that we don't end up with,
680 // say, `ReScope` appearing in a return type and causing ICEs when other
681 // functions end up with region constraints involving regions from other
684 // We also cannot use `for_each_free_region` because for closures it includes
685 // the regions parameters from the enclosing item.
687 // We ignore any type parameters because impl trait values are assumed to
688 // capture all the in-scope type parameters.
689 struct ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
691 OP: FnMut(ty::Region<'tcx>),
697 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
699 OP: FnMut(ty::Region<'tcx>),
701 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> bool {
702 t.skip_binder().visit_with(self);
703 false // keep visiting
706 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
708 // ignore bound regions, keep visiting
709 ty::ReLateBound(_, _) => false,
717 fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
718 // We're only interested in types involving regions
719 if !ty.flags.intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
720 return false; // keep visiting
724 ty::Closure(def_id, ref substs) => {
725 // Skip lifetime parameters of the enclosing item(s)
727 for upvar_ty in substs.upvar_tys(def_id, self.tcx) {
728 upvar_ty.visit_with(self);
731 substs.closure_sig_ty(def_id, self.tcx).visit_with(self);
734 ty::Generator(def_id, ref substs, _) => {
735 // Skip lifetime parameters of the enclosing item(s)
736 // Also skip the witness type, because that has no free regions.
738 for upvar_ty in substs.upvar_tys(def_id, self.tcx) {
739 upvar_ty.visit_with(self);
742 substs.return_ty(def_id, self.tcx).visit_with(self);
743 substs.yield_ty(def_id, self.tcx).visit_with(self);
746 ty.super_visit_with(self);
754 struct ReverseMapper<'tcx> {
757 /// If errors have already been reported in this fn, we suppress
758 /// our own errors because they are sometimes derivative.
759 tainted_by_errors: bool,
761 opaque_type_def_id: DefId,
762 map: FxHashMap<Kind<'tcx>, Kind<'tcx>>,
763 map_missing_regions_to_empty: bool,
765 /// initially `Some`, set to `None` once error has been reported
766 hidden_ty: Option<Ty<'tcx>>,
768 /// Span of function being checked.
772 impl ReverseMapper<'tcx> {
775 tainted_by_errors: bool,
776 opaque_type_def_id: DefId,
777 map: FxHashMap<Kind<'tcx>, Kind<'tcx>>,
786 map_missing_regions_to_empty: false,
787 hidden_ty: Some(hidden_ty),
792 fn fold_kind_mapping_missing_regions_to_empty(&mut self, kind: Kind<'tcx>) -> Kind<'tcx> {
793 assert!(!self.map_missing_regions_to_empty);
794 self.map_missing_regions_to_empty = true;
795 let kind = kind.fold_with(self);
796 self.map_missing_regions_to_empty = false;
800 fn fold_kind_normally(&mut self, kind: Kind<'tcx>) -> Kind<'tcx> {
801 assert!(!self.map_missing_regions_to_empty);
806 impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
807 fn tcx(&self) -> TyCtxt<'tcx> {
811 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
813 // ignore bound regions that appear in the type (e.g., this
814 // would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
815 ty::ReLateBound(..) |
817 // ignore `'static`, as that can appear anywhere
818 ty::ReStatic => return r,
823 let generics = self.tcx().generics_of(self.opaque_type_def_id);
824 match self.map.get(&r.into()).map(|k| k.unpack()) {
825 Some(UnpackedKind::Lifetime(r1)) => r1,
826 Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
827 None if generics.parent.is_some() => {
828 if !self.map_missing_regions_to_empty && !self.tainted_by_errors {
829 if let Some(hidden_ty) = self.hidden_ty.take() {
830 unexpected_hidden_region_diagnostic(
833 self.opaque_type_def_id,
839 self.tcx.lifetimes.re_empty
845 "non-defining existential type use in defining scope"
849 format!("lifetime `{}` is part of concrete type but not used in \
850 parameter list of existential type", r),
854 self.tcx().global_tcx().mk_region(ty::ReStatic)
859 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
861 ty::Closure(def_id, substs) => {
862 // I am a horrible monster and I pray for death. When
863 // we encounter a closure here, it is always a closure
864 // from within the function that we are currently
865 // type-checking -- one that is now being encapsulated
866 // in an existential abstract type. Ideally, we would
867 // go through the types/lifetimes that it references
868 // and treat them just like we would any other type,
869 // which means we would error out if we find any
870 // reference to a type/region that is not in the
873 // **However,** in the case of closures, there is a
874 // somewhat subtle (read: hacky) consideration. The
875 // problem is that our closure types currently include
876 // all the lifetime parameters declared on the
877 // enclosing function, even if they are unused by the
878 // closure itself. We can't readily filter them out,
879 // so here we replace those values with `'empty`. This
880 // can't really make a difference to the rest of the
881 // compiler; those regions are ignored for the
882 // outlives relation, and hence don't affect trait
883 // selection or auto traits, and they are erased
886 let generics = self.tcx.generics_of(def_id);
888 self.tcx.mk_substs(substs.substs.iter().enumerate().map(|(index, &kind)| {
889 if index < generics.parent_count {
890 // Accommodate missing regions in the parent kinds...
891 self.fold_kind_mapping_missing_regions_to_empty(kind)
893 // ...but not elsewhere.
894 self.fold_kind_normally(kind)
898 self.tcx.mk_closure(def_id, ty::ClosureSubsts { substs })
901 ty::Generator(def_id, substs, movability) => {
902 let generics = self.tcx.generics_of(def_id);
904 self.tcx.mk_substs(substs.substs.iter().enumerate().map(|(index, &kind)| {
905 if index < generics.parent_count {
906 // Accommodate missing regions in the parent kinds...
907 self.fold_kind_mapping_missing_regions_to_empty(kind)
909 // ...but not elsewhere.
910 self.fold_kind_normally(kind)
914 self.tcx.mk_generator(def_id, ty::GeneratorSubsts { substs }, movability)
918 // Look it up in the substitution list.
919 match self.map.get(&ty.into()).map(|k| k.unpack()) {
920 // Found it in the substitution list; replace with the parameter from the
922 Some(UnpackedKind::Type(t1)) => t1,
923 Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
928 &format!("type parameter `{}` is part of concrete type but not \
929 used in parameter list for existential type", ty),
938 _ => ty.super_fold_with(self),
942 fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
943 trace!("checking const {:?}", ct);
944 // Find a const parameter
946 ConstValue::Param(..) => {
947 // Look it up in the substitution list.
948 match self.map.get(&ct.into()).map(|k| k.unpack()) {
949 // Found it in the substitution list, replace with the parameter from the
951 Some(UnpackedKind::Const(c1)) => c1,
952 Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
957 &format!("const parameter `{}` is part of concrete type but not \
958 used in parameter list for existential type", ct)
962 self.tcx().consts.err
972 struct Instantiator<'a, 'tcx> {
973 infcx: &'a InferCtxt<'a, 'tcx>,
974 parent_def_id: DefId,
976 param_env: ty::ParamEnv<'tcx>,
978 opaque_types: OpaqueTypeMap<'tcx>,
979 obligations: Vec<PredicateObligation<'tcx>>,
982 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
983 fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
984 debug!("instantiate_opaque_types_in_map(value={:?})", value);
985 let tcx = self.infcx.tcx;
986 value.fold_with(&mut BottomUpFolder {
989 if let ty::Opaque(def_id, substs) = ty.sty {
990 // Check that this is `impl Trait` type is
991 // declared by `parent_def_id` -- i.e., one whose
992 // value we are inferring. At present, this is
993 // always true during the first phase of
994 // type-check, but not always true later on during
995 // NLL. Once we support named abstract types more fully,
996 // this same scenario will be able to arise during all phases.
998 // Here is an example using `abstract type` that indicates
999 // the distinction we are checking for:
1003 // pub abstract type Foo: Iterator;
1004 // pub fn make_foo() -> Foo { .. }
1008 // fn foo() -> a::Foo { a::make_foo() }
1012 // Here, the return type of `foo` references a
1013 // `Opaque` indeed, but not one whose value is
1014 // presently being inferred. You can get into a
1015 // similar situation with closure return types
1019 // fn foo() -> impl Iterator { .. }
1021 // let x = || foo(); // returns the Opaque assoc with `foo`
1024 if let Some(opaque_hir_id) = tcx.hir().as_local_hir_id(def_id) {
1025 let parent_def_id = self.parent_def_id;
1026 let def_scope_default = || {
1027 let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
1028 parent_def_id == tcx.hir()
1029 .local_def_id(opaque_parent_hir_id)
1031 let (in_definition_scope, origin) = match tcx.hir().find(opaque_hir_id) {
1032 Some(Node::Item(item)) => match item.node {
1033 // Anonymous `impl Trait`
1034 hir::ItemKind::Existential(hir::ExistTy {
1035 impl_trait_fn: Some(parent),
1038 }) => (parent == self.parent_def_id, origin),
1039 // Named `existential type`
1040 hir::ItemKind::Existential(hir::ExistTy {
1041 impl_trait_fn: None,
1045 may_define_existential_type(
1052 _ => (def_scope_default(), hir::ExistTyOrigin::ExistentialType),
1054 Some(Node::ImplItem(item)) => match item.node {
1055 hir::ImplItemKind::Existential(_) => (
1056 may_define_existential_type(
1061 hir::ExistTyOrigin::ExistentialType,
1063 _ => (def_scope_default(), hir::ExistTyOrigin::ExistentialType),
1066 "expected (impl) item, found {}",
1067 tcx.hir().node_to_string(opaque_hir_id),
1070 if in_definition_scope {
1071 return self.fold_opaque_ty(ty, def_id, substs, origin);
1075 "instantiate_opaque_types_in_map: \
1076 encountered opaque outside its definition scope \
1094 substs: SubstsRef<'tcx>,
1095 origin: hir::ExistTyOrigin,
1097 let infcx = self.infcx;
1098 let tcx = infcx.tcx;
1100 debug!("instantiate_opaque_types: Opaque(def_id={:?}, substs={:?})", def_id, substs);
1102 // Use the same type variable if the exact same opaque type appears more
1103 // than once in the return type (e.g., if it's passed to a type alias).
1104 if let Some(opaque_defn) = self.opaque_types.get(&def_id) {
1105 return opaque_defn.concrete_ty;
1107 let span = tcx.def_span(def_id);
1109 .next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span });
1111 let predicates_of = tcx.predicates_of(def_id);
1112 debug!("instantiate_opaque_types: predicates={:#?}", predicates_of,);
1113 let bounds = predicates_of.instantiate(tcx, substs);
1114 debug!("instantiate_opaque_types: bounds={:?}", bounds);
1116 let required_region_bounds = tcx.required_region_bounds(ty, bounds.predicates.clone());
1117 debug!("instantiate_opaque_types: required_region_bounds={:?}", required_region_bounds);
1119 // Make sure that we are in fact defining the *entire* type
1120 // (e.g., `existential type Foo<T: Bound>: Bar;` needs to be
1121 // defined by a function like `fn foo<T: Bound>() -> Foo<T>`).
1122 debug!("instantiate_opaque_types: param_env={:#?}", self.param_env,);
1123 debug!("instantiate_opaque_types: generics={:#?}", tcx.generics_of(def_id),);
1125 // Ideally, we'd get the span where *this specific `ty` came
1126 // from*, but right now we just use the span from the overall
1127 // value being folded. In simple cases like `-> impl Foo`,
1128 // these are the same span, but not in cases like `-> (impl
1130 let definition_span = self.value_span;
1132 self.opaque_types.insert(
1137 concrete_ty: ty_var,
1138 has_required_region_bounds: !required_region_bounds.is_empty(),
1142 debug!("instantiate_opaque_types: ty_var={:?}", ty_var);
1144 self.obligations.reserve(bounds.predicates.len());
1145 for predicate in bounds.predicates {
1146 // Change the predicate to refer to the type variable,
1147 // which will be the concrete type instead of the opaque type.
1148 // This also instantiates nested instances of `impl Trait`.
1149 let predicate = self.instantiate_opaque_types_in_map(&predicate);
1151 let cause = traits::ObligationCause::new(span, self.body_id, traits::SizedReturnType);
1153 // Require that the predicate holds for the concrete type.
1154 debug!("instantiate_opaque_types: predicate={:?}", predicate);
1155 self.obligations.push(traits::Obligation::new(cause, self.param_env, predicate));
1162 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
1168 /// pub existential type Baz;
1170 /// fn f1() -> Baz { .. }
1173 /// fn f2() -> bar::Baz { .. }
1177 /// Here, `def_id` is the `DefId` of the defining use of the existential type (e.g., `f1` or `f2`),
1178 /// and `opaque_hir_id` is the `HirId` of the definition of the existential type `Baz`.
1179 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
1180 pub fn may_define_existential_type(
1183 opaque_hir_id: hir::HirId,
1185 let mut hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
1187 "may_define_existential_type(def={:?}, opaque_node={:?})",
1188 tcx.hir().get(hir_id),
1189 tcx.hir().get(opaque_hir_id)
1192 // Named existential types can be defined by any siblings or children of siblings.
1193 let scope = tcx.hir().get_defining_scope(opaque_hir_id).expect("could not get defining scope");
1194 // We walk up the node tree until we hit the root or the scope of the opaque type.
1195 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
1196 hir_id = tcx.hir().get_parent_item(hir_id);
1198 // Syntactically, we are allowed to define the concrete type if: