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::{InternalSubsts, GenericArg, SubstsRef, GenericArgKind};
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> {
28 /// The opaque type (`ty::Opaque`) for this declaration.
29 pub opaque_type: Ty<'tcx>,
31 /// The substitutions that we apply to the opaque type that this
32 /// `impl Trait` desugars to. e.g., if:
34 /// fn foo<'a, 'b, T>() -> impl Trait<'a>
36 /// winds up desugared to:
38 /// type Foo<'x, X> = impl Trait<'x>
39 /// fn foo<'a, 'b, T>() -> Foo<'a, T>
41 /// then `substs` would be `['a, T]`.
42 pub substs: SubstsRef<'tcx>,
44 /// The span of this particular definition of the opaque type. So
48 /// type Foo = impl Baz;
50 /// ^^^ This is the span we are looking for!
53 /// In cases where the fn returns `(impl Trait, impl Trait)` or
54 /// other such combinations, the result is currently
55 /// over-approximated, but better than nothing.
56 pub definition_span: Span,
58 /// The type variable that represents the value of the opaque type
59 /// that we require. In other words, after we compile this function,
60 /// we will be created a constraint like:
64 /// where `?C` is the value of this type variable. =) It may
65 /// naturally refer to the type and lifetime parameters in scope
66 /// in this function, though ultimately it should only reference
67 /// those that are arguments to `Foo` in the constraint above. (In
68 /// other words, `?C` should not include `'b`, even though it's a
69 /// lifetime parameter on `foo`.)
70 pub concrete_ty: Ty<'tcx>,
72 /// Returns `true` if the `impl Trait` bounds include region bounds.
73 /// For example, this would be true for:
75 /// fn foo<'a, 'b, 'c>() -> impl Trait<'c> + 'a + 'b
79 /// fn foo<'c>() -> impl Trait<'c>
81 /// unless `Trait` was declared like:
83 /// trait Trait<'c>: 'c
85 /// in which case it would be true.
87 /// This is used during regionck to decide whether we need to
88 /// impose any additional constraints to ensure that region
89 /// variables in `concrete_ty` wind up being constrained to
90 /// something from `substs` (or, at minimum, things that outlive
91 /// the fn body). (Ultimately, writeback is responsible for this
93 pub has_required_region_bounds: bool,
95 /// The origin of the opaque type.
96 pub origin: hir::OpaqueTyOrigin,
99 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
100 /// Replaces all opaque types in `value` with fresh inference variables
101 /// and creates appropriate obligations. For example, given the input:
103 /// impl Iterator<Item = impl Debug>
105 /// this method would create two type variables, `?0` and `?1`. It would
106 /// return the type `?0` but also the obligations:
108 /// ?0: Iterator<Item = ?1>
111 /// Moreover, it returns a `OpaqueTypeMap` that would map `?0` to
112 /// info about the `impl Iterator<..>` type and `?1` to info about
113 /// the `impl Debug` type.
117 /// - `parent_def_id` -- the `DefId` of the function in which the opaque type
119 /// - `body_id` -- the body-id with which the resulting obligations should
121 /// - `param_env` -- the in-scope parameter environment to be used for
123 /// - `value` -- the value within which we are instantiating opaque types
124 /// - `value_span` -- the span where the value came from, used in error reporting
125 pub fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
127 parent_def_id: DefId,
129 param_env: ty::ParamEnv<'tcx>,
132 ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)> {
134 "instantiate_opaque_types(value={:?}, parent_def_id={:?}, body_id={:?}, \
135 param_env={:?}, value_span={:?})",
136 value, parent_def_id, body_id, param_env, value_span,
138 let mut instantiator = Instantiator {
144 opaque_types: Default::default(),
147 let value = instantiator.instantiate_opaque_types_in_map(value);
148 InferOk { value: (value, instantiator.opaque_types), obligations: instantiator.obligations }
151 /// Given the map `opaque_types` containing the opaque
152 /// `impl Trait` types whose underlying, hidden types are being
153 /// inferred, this method adds constraints to the regions
154 /// appearing in those underlying hidden types to ensure that they
155 /// at least do not refer to random scopes within the current
156 /// function. These constraints are not (quite) sufficient to
157 /// guarantee that the regions are actually legal values; that
158 /// final condition is imposed after region inference is done.
162 /// Let's work through an example to explain how it works. Assume
163 /// the current function is as follows:
166 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
169 /// Here, we have two `impl Trait` types whose values are being
170 /// inferred (the `impl Bar<'a>` and the `impl
171 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
172 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
173 /// the return type of `foo`, we *reference* those definitions:
176 /// type Foo1<'x> = impl Bar<'x>;
177 /// type Foo2<'x> = impl Bar<'x>;
178 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
185 /// As indicating in the comments above, each of those references
186 /// is (in the compiler) basically a substitution (`substs`)
187 /// applied to the type of a suitable `def_id` (which identifies
188 /// `Foo1` or `Foo2`).
190 /// Now, at this point in compilation, what we have done is to
191 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
192 /// fresh inference variables C1 and C2. We wish to use the values
193 /// of these variables to infer the underlying types of `Foo1` and
194 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
195 /// constraints like:
198 /// for<'a> (Foo1<'a> = C1)
199 /// for<'b> (Foo1<'b> = C2)
202 /// For these equation to be satisfiable, the types `C1` and `C2`
203 /// can only refer to a limited set of regions. For example, `C1`
204 /// can only refer to `'static` and `'a`, and `C2` can only refer
205 /// to `'static` and `'b`. The job of this function is to impose that
208 /// Up to this point, C1 and C2 are basically just random type
209 /// inference variables, and hence they may contain arbitrary
210 /// regions. In fact, it is fairly likely that they do! Consider
211 /// this possible definition of `foo`:
214 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
219 /// Here, the values for the concrete types of the two impl
220 /// traits will include inference variables:
227 /// Ordinarily, the subtyping rules would ensure that these are
228 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
229 /// type per se, we don't get such constraints by default. This
230 /// is where this function comes into play. It adds extra
231 /// constraints to ensure that all the regions which appear in the
232 /// inferred type are regions that could validly appear.
234 /// This is actually a bit of a tricky constraint in general. We
235 /// want to say that each variable (e.g., `'0`) can only take on
236 /// values that were supplied as arguments to the opaque type
237 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
238 /// scope. We don't have a constraint quite of this kind in the current
243 /// We generally prefer to make `<=` constraints, since they
244 /// integrate best into the region solver. To do that, we find the
245 /// "minimum" of all the arguments that appear in the substs: that
246 /// is, some region which is less than all the others. In the case
247 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
248 /// all). Then we apply that as a least bound to the variables
249 /// (e.g., `'a <= '0`).
251 /// In some cases, there is no minimum. Consider this example:
254 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
257 /// Here we would report a more complex "in constraint", like `'r
258 /// in ['a, 'b, 'static]` (where `'r` is some regon appearing in
259 /// the hidden type).
261 /// # Constrain regions, not the hidden concrete type
263 /// Note that generating constraints on each region `Rc` is *not*
264 /// the same as generating an outlives constraint on `Tc` iself.
265 /// For example, if we had a function like this:
268 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
272 /// // Equivalent to:
273 /// type FooReturn<'a, T> = impl Foo<'a>;
274 /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
277 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
278 /// is an inference variable). If we generated a constraint that
279 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
280 /// but this is not necessary, because the opaque type we
281 /// create will be allowed to reference `T`. So we only generate a
282 /// constraint that `'0: 'a`.
284 /// # The `free_region_relations` parameter
286 /// The `free_region_relations` argument is used to find the
287 /// "minimum" of the regions supplied to a given opaque type.
288 /// It must be a relation that can answer whether `'a <= 'b`,
289 /// where `'a` and `'b` are regions that appear in the "substs"
290 /// for the opaque type references (the `<'a>` in `Foo1<'a>`).
292 /// Note that we do not impose the constraints based on the
293 /// generic regions from the `Foo1` definition (e.g., `'x`). This
294 /// is because the constraints we are imposing here is basically
295 /// the concern of the one generating the constraining type C1,
296 /// which is the current function. It also means that we can
297 /// take "implied bounds" into account in some cases:
300 /// trait SomeTrait<'a, 'b> { }
301 /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
304 /// Here, the fact that `'b: 'a` is known only because of the
305 /// implied bounds from the `&'a &'b u32` parameter, and is not
306 /// "inherent" to the opaque type definition.
310 /// - `opaque_types` -- the map produced by `instantiate_opaque_types`
311 /// - `free_region_relations` -- something that can be used to relate
312 /// the free regions (`'a`) that appear in the impl trait.
313 pub fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(
315 opaque_types: &OpaqueTypeMap<'tcx>,
316 free_region_relations: &FRR,
318 debug!("constrain_opaque_types()");
320 for (&def_id, opaque_defn) in opaque_types {
321 self.constrain_opaque_type(def_id, opaque_defn, free_region_relations);
325 /// See `constrain_opaque_types` for documentation.
326 pub fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
329 opaque_defn: &OpaqueTypeDecl<'tcx>,
330 free_region_relations: &FRR,
332 debug!("constrain_opaque_type()");
333 debug!("constrain_opaque_type: def_id={:?}", def_id);
334 debug!("constrain_opaque_type: opaque_defn={:#?}", opaque_defn);
338 let concrete_ty = self.resolve_vars_if_possible(&opaque_defn.concrete_ty);
340 debug!("constrain_opaque_type: concrete_ty={:?}", concrete_ty);
342 let opaque_type_generics = tcx.generics_of(def_id);
344 let span = tcx.def_span(def_id);
346 // If there are required region bounds, we can use them.
347 if opaque_defn.has_required_region_bounds {
348 let predicates_of = tcx.predicates_of(def_id);
349 debug!("constrain_opaque_type: predicates: {:#?}", predicates_of,);
350 let bounds = predicates_of.instantiate(tcx, opaque_defn.substs);
351 debug!("constrain_opaque_type: bounds={:#?}", bounds);
352 let opaque_type = tcx.mk_opaque(def_id, opaque_defn.substs);
354 let required_region_bounds = tcx.required_region_bounds(opaque_type, bounds.predicates);
355 debug_assert!(!required_region_bounds.is_empty());
357 for required_region in required_region_bounds {
358 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
360 op: |r| self.sub_regions(infer::CallReturn(span), required_region, r),
366 // There were no `required_region_bounds`,
367 // so we have to search for a `least_region`.
368 // Go through all the regions used as arguments to the
369 // opaque type. These are the parameters to the opaque
370 // type; so in our example above, `substs` would contain
371 // `['a]` for the first impl trait and `'b` for the
373 let mut least_region = None;
374 for param in &opaque_type_generics.params {
376 GenericParamDefKind::Lifetime => {}
380 // Get the value supplied for this region from the substs.
381 let subst_arg = opaque_defn.substs.region_at(param.index as usize);
383 // Compute the least upper bound of it with the other regions.
384 debug!("constrain_opaque_types: least_region={:?}", least_region);
385 debug!("constrain_opaque_types: subst_arg={:?}", subst_arg);
387 None => least_region = Some(subst_arg),
389 if free_region_relations.sub_free_regions(lr, subst_arg) {
390 // keep the current least region
391 } else if free_region_relations.sub_free_regions(subst_arg, lr) {
392 // switch to `subst_arg`
393 least_region = Some(subst_arg);
395 // There are two regions (`lr` and
396 // `subst_arg`) which are not relatable. We
397 // can't find a best choice. Therefore,
398 // instead of creating a single bound like
399 // `'r: 'a` (which is our preferred choice),
400 // we will create a "in bound" like `'r in
401 // ['a, 'b, 'c]`, where `'a..'c` are the
402 // regions that appear in the impl trait.
403 return self.generate_member_constraint(
405 opaque_type_generics,
416 let least_region = least_region.unwrap_or(tcx.lifetimes.re_static);
417 debug!("constrain_opaque_types: least_region={:?}", least_region);
419 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
421 op: |r| self.sub_regions(infer::CallReturn(span), least_region, r),
425 /// As a fallback, we sometimes generate an "in constraint". For
426 /// a case like `impl Foo<'a, 'b>`, where `'a` and `'b` cannot be
427 /// related, we would generate a constraint `'r in ['a, 'b,
428 /// 'static]` for each region `'r` that appears in the hidden type
429 /// (i.e., it must be equal to `'a`, `'b`, or `'static`).
431 /// `conflict1` and `conflict2` are the two region bounds that we
432 /// detected which were unrelated. They are used for diagnostics.
433 fn generate_member_constraint(
435 concrete_ty: Ty<'tcx>,
436 opaque_type_generics: &ty::Generics,
437 opaque_defn: &OpaqueTypeDecl<'tcx>,
438 opaque_type_def_id: DefId,
439 conflict1: ty::Region<'tcx>,
440 conflict2: ty::Region<'tcx>,
442 // For now, enforce a feature gate outside of async functions.
443 if self.member_constraint_feature_gate(
452 // Create the set of choice regions: each region in the hidden
453 // type can be equal to any of the region parameters of the
454 // opaque type definition.
455 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
459 .filter(|param| match param.kind {
460 GenericParamDefKind::Lifetime => true,
461 GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => false,
463 .map(|param| opaque_defn.substs.region_at(param.index as usize))
464 .chain(std::iter::once(self.tcx.lifetimes.re_static))
468 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
470 op: |r| self.member_constraint(
472 opaque_defn.definition_span,
480 /// Member constraints are presently feature-gated except for
481 /// async-await. We expect to lift this once we've had a bit more
483 fn member_constraint_feature_gate(
485 opaque_defn: &OpaqueTypeDecl<'tcx>,
486 opaque_type_def_id: DefId,
487 conflict1: ty::Region<'tcx>,
488 conflict2: ty::Region<'tcx>,
490 // If we have `#![feature(member_constraints)]`, no problems.
491 if self.tcx.features().member_constraints {
495 let span = self.tcx.def_span(opaque_type_def_id);
497 // Without a feature-gate, we only generate member-constraints for async-await.
498 let context_name = match opaque_defn.origin {
499 // No feature-gate required for `async fn`.
500 hir::OpaqueTyOrigin::AsyncFn => return false,
502 // Otherwise, generate the label we'll use in the error message.
503 hir::OpaqueTyOrigin::TypeAlias => "impl Trait",
504 hir::OpaqueTyOrigin::FnReturn => "impl Trait",
506 let msg = format!("ambiguous lifetime bound in `{}`", context_name);
507 let mut err = self.tcx.sess.struct_span_err(span, &msg);
509 let conflict1_name = conflict1.to_string();
510 let conflict2_name = conflict2.to_string();
512 let label = match (&*conflict1_name, &*conflict2_name) {
513 ("'_", "'_") => "the elided lifetimes here do not outlive one another",
515 label_owned = format!(
516 "neither `{}` nor `{}` outlives the other",
517 conflict1_name, conflict2_name,
522 err.span_label(span, label);
524 if nightly_options::is_nightly_build() {
526 "add #![feature(member_constraints)] to the crate attributes \
534 /// Given the fully resolved, instantiated type for an opaque
535 /// type, i.e., the value of an inference variable like C1 or C2
536 /// (*), computes the "definition type" for an opaque type
537 /// definition -- that is, the inferred value of `Foo1<'x>` or
538 /// `Foo2<'x>` that we would conceptually use in its definition:
540 /// type Foo1<'x> = impl Bar<'x> = AAA; <-- this type AAA
541 /// type Foo2<'x> = impl Bar<'x> = BBB; <-- or this type BBB
542 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
544 /// Note that these values are defined in terms of a distinct set of
545 /// generic parameters (`'x` instead of `'a`) from C1 or C2. The main
546 /// purpose of this function is to do that translation.
548 /// (*) C1 and C2 were introduced in the comments on
549 /// `constrain_opaque_types`. Read that comment for more context.
553 /// - `def_id`, the `impl Trait` type
554 /// - `opaque_defn`, the opaque definition created in `instantiate_opaque_types`
555 /// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of
556 /// `opaque_defn.concrete_ty`
557 pub fn infer_opaque_definition_from_instantiation(
560 opaque_defn: &OpaqueTypeDecl<'tcx>,
561 instantiated_ty: Ty<'tcx>,
565 "infer_opaque_definition_from_instantiation(def_id={:?}, instantiated_ty={:?})",
566 def_id, instantiated_ty
569 // Use substs to build up a reverse map from regions to their
570 // identity mappings. This is necessary because of `impl
571 // Trait` lifetimes are computed by replacing existing
572 // lifetimes with 'static and remapping only those used in the
573 // `impl Trait` return type, resulting in the parameters
575 let id_substs = InternalSubsts::identity_for_item(self.tcx, def_id);
576 let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> = opaque_defn
580 .map(|(index, subst)| (*subst, id_substs[index]))
583 // Convert the type from the function into a type valid outside
584 // the function, by replacing invalid regions with 'static,
585 // after producing an error for each of them.
586 let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new(
588 self.is_tainted_by_errors(),
594 debug!("infer_opaque_definition_from_instantiation: definition_ty={:?}", definition_ty);
600 pub fn unexpected_hidden_region_diagnostic(
602 region_scope_tree: Option<®ion::ScopeTree>,
603 opaque_type_def_id: DefId,
605 hidden_region: ty::Region<'tcx>,
606 ) -> DiagnosticBuilder<'tcx> {
607 let span = tcx.def_span(opaque_type_def_id);
608 let mut err = struct_span_err!(
612 "hidden type for `impl Trait` captures lifetime that does not appear in bounds",
615 // Explain the region we are capturing.
616 if let ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic | ty::ReEmpty = hidden_region {
617 // Assuming regionck succeeded (*), we ought to always be
618 // capturing *some* region from the fn header, and hence it
619 // ought to be free. So under normal circumstances, we will go
620 // down this path which gives a decent human readable
623 // (*) if not, the `tainted_by_errors` flag would be set to
624 // true in any case, so we wouldn't be here at all.
625 tcx.note_and_explain_free_region(
627 &format!("hidden type `{}` captures ", hidden_ty),
632 // Ugh. This is a painful case: the hidden region is not one
633 // that we can easily summarize or explain. This can happen
635 // `src/test/ui/multiple-lifetimes/ordinary-bounds-unsuited.rs`:
638 // fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b> {
639 // if condition() { a } else { b }
643 // Here the captured lifetime is the intersection of `'a` and
644 // `'b`, which we can't quite express.
646 if let Some(region_scope_tree) = region_scope_tree {
647 // If the `region_scope_tree` is available, this is being
648 // invoked from the "region inferencer error". We can at
649 // least report a really cryptic error for now.
650 tcx.note_and_explain_region(
653 &format!("hidden type `{}` captures ", hidden_ty),
658 // If the `region_scope_tree` is *unavailable*, this is
659 // being invoked by the code that comes *after* region
660 // inferencing. This is a bug, as the region inferencer
661 // ought to have noticed the failed constraint and invoked
662 // error reporting, which in turn should have prevented us
663 // from getting trying to infer the hidden type
665 tcx.sess.delay_span_bug(
668 "hidden type captures unexpected lifetime `{:?}` \
669 but no region inference failure",
679 // Visitor that requires that (almost) all regions in the type visited outlive
680 // `least_region`. We cannot use `push_outlives_components` because regions in
681 // closure signatures are not included in their outlives components. We need to
682 // ensure all regions outlive the given bound so that we don't end up with,
683 // say, `ReScope` appearing in a return type and causing ICEs when other
684 // functions end up with region constraints involving regions from other
687 // We also cannot use `for_each_free_region` because for closures it includes
688 // the regions parameters from the enclosing item.
690 // We ignore any type parameters because impl trait values are assumed to
691 // capture all the in-scope type parameters.
692 struct ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
694 OP: FnMut(ty::Region<'tcx>),
700 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
702 OP: FnMut(ty::Region<'tcx>),
704 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> bool {
705 t.skip_binder().visit_with(self);
706 false // keep visiting
709 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
711 // ignore bound regions, keep visiting
712 ty::ReLateBound(_, _) => false,
720 fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
721 // We're only interested in types involving regions
722 if !ty.flags.intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
723 return false; // keep visiting
727 ty::Closure(def_id, ref substs) => {
728 // Skip lifetime parameters of the enclosing item(s)
730 for upvar_ty in substs.as_closure().upvar_tys(def_id, self.tcx) {
731 upvar_ty.visit_with(self);
734 substs.as_closure().sig_ty(def_id, self.tcx).visit_with(self);
737 ty::Generator(def_id, ref substs, _) => {
738 // Skip lifetime parameters of the enclosing item(s)
739 // Also skip the witness type, because that has no free regions.
741 for upvar_ty in substs.as_generator().upvar_tys(def_id, self.tcx) {
742 upvar_ty.visit_with(self);
745 substs.as_generator().return_ty(def_id, self.tcx).visit_with(self);
746 substs.as_generator().yield_ty(def_id, self.tcx).visit_with(self);
749 ty.super_visit_with(self);
757 struct ReverseMapper<'tcx> {
760 /// If errors have already been reported in this fn, we suppress
761 /// our own errors because they are sometimes derivative.
762 tainted_by_errors: bool,
764 opaque_type_def_id: DefId,
765 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
766 map_missing_regions_to_empty: bool,
768 /// initially `Some`, set to `None` once error has been reported
769 hidden_ty: Option<Ty<'tcx>>,
771 /// Span of function being checked.
775 impl ReverseMapper<'tcx> {
778 tainted_by_errors: bool,
779 opaque_type_def_id: DefId,
780 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
789 map_missing_regions_to_empty: false,
790 hidden_ty: Some(hidden_ty),
795 fn fold_kind_mapping_missing_regions_to_empty(
797 kind: GenericArg<'tcx>,
798 ) -> GenericArg<'tcx> {
799 assert!(!self.map_missing_regions_to_empty);
800 self.map_missing_regions_to_empty = true;
801 let kind = kind.fold_with(self);
802 self.map_missing_regions_to_empty = false;
806 fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
807 assert!(!self.map_missing_regions_to_empty);
812 impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
813 fn tcx(&self) -> TyCtxt<'tcx> {
817 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
819 // ignore bound regions that appear in the type (e.g., this
820 // would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
821 ty::ReLateBound(..) |
823 // ignore `'static`, as that can appear anywhere
824 ty::ReStatic => return r,
829 let generics = self.tcx().generics_of(self.opaque_type_def_id);
830 match self.map.get(&r.into()).map(|k| k.unpack()) {
831 Some(GenericArgKind::Lifetime(r1)) => r1,
832 Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
833 None if generics.parent.is_some() => {
834 if !self.map_missing_regions_to_empty && !self.tainted_by_errors {
835 if let Some(hidden_ty) = self.hidden_ty.take() {
836 unexpected_hidden_region_diagnostic(
839 self.opaque_type_def_id,
845 self.tcx.lifetimes.re_empty
851 "non-defining opaque type use in defining scope"
855 format!("lifetime `{}` is part of concrete type but not used in \
856 parameter list of the `impl Trait` type alias", r),
860 self.tcx().mk_region(ty::ReStatic)
865 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
867 ty::Closure(def_id, substs) => {
868 // I am a horrible monster and I pray for death. When
869 // we encounter a closure here, it is always a closure
870 // from within the function that we are currently
871 // type-checking -- one that is now being encapsulated
872 // in an opaque type. Ideally, we would
873 // go through the types/lifetimes that it references
874 // and treat them just like we would any other type,
875 // which means we would error out if we find any
876 // reference to a type/region that is not in the
879 // **However,** in the case of closures, there is a
880 // somewhat subtle (read: hacky) consideration. The
881 // problem is that our closure types currently include
882 // all the lifetime parameters declared on the
883 // enclosing function, even if they are unused by the
884 // closure itself. We can't readily filter them out,
885 // so here we replace those values with `'empty`. This
886 // can't really make a difference to the rest of the
887 // compiler; those regions are ignored for the
888 // outlives relation, and hence don't affect trait
889 // selection or auto traits, and they are erased
892 let generics = self.tcx.generics_of(def_id);
894 self.tcx.mk_substs(substs.iter().enumerate().map(|(index, &kind)| {
895 if index < generics.parent_count {
896 // Accommodate missing regions in the parent kinds...
897 self.fold_kind_mapping_missing_regions_to_empty(kind)
899 // ...but not elsewhere.
900 self.fold_kind_normally(kind)
904 self.tcx.mk_closure(def_id, substs)
907 ty::Generator(def_id, substs, movability) => {
908 let generics = self.tcx.generics_of(def_id);
910 self.tcx.mk_substs(substs.iter().enumerate().map(|(index, &kind)| {
911 if index < generics.parent_count {
912 // Accommodate missing regions in the parent kinds...
913 self.fold_kind_mapping_missing_regions_to_empty(kind)
915 // ...but not elsewhere.
916 self.fold_kind_normally(kind)
920 self.tcx.mk_generator(def_id, substs, movability)
924 // Look it up in the substitution list.
925 match self.map.get(&ty.into()).map(|k| k.unpack()) {
926 // Found it in the substitution list; replace with the parameter from the
928 Some(GenericArgKind::Type(t1)) => t1,
929 Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
934 &format!("type parameter `{}` is part of concrete type but not \
935 used in parameter list for the `impl Trait` type alias",
945 _ => ty.super_fold_with(self),
949 fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
950 trace!("checking const {:?}", ct);
951 // Find a const parameter
953 ty::ConstKind::Param(..) => {
954 // Look it up in the substitution list.
955 match self.map.get(&ct.into()).map(|k| k.unpack()) {
956 // Found it in the substitution list, replace with the parameter from the
958 Some(GenericArgKind::Const(c1)) => c1,
959 Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
964 &format!("const parameter `{}` is part of concrete type but not \
965 used in parameter list for the `impl Trait` type alias",
970 self.tcx().consts.err
980 struct Instantiator<'a, 'tcx> {
981 infcx: &'a InferCtxt<'a, 'tcx>,
982 parent_def_id: DefId,
984 param_env: ty::ParamEnv<'tcx>,
986 opaque_types: OpaqueTypeMap<'tcx>,
987 obligations: Vec<PredicateObligation<'tcx>>,
990 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
991 fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
992 debug!("instantiate_opaque_types_in_map(value={:?})", value);
993 let tcx = self.infcx.tcx;
994 value.fold_with(&mut BottomUpFolder {
997 if ty.references_error() {
998 return tcx.types.err;
999 } else if let ty::Opaque(def_id, substs) = ty.kind {
1000 // Check that this is `impl Trait` type is
1001 // declared by `parent_def_id` -- i.e., one whose
1002 // value we are inferring. At present, this is
1003 // always true during the first phase of
1004 // type-check, but not always true later on during
1005 // NLL. Once we support named opaque types more fully,
1006 // this same scenario will be able to arise during all phases.
1008 // Here is an example using type alias `impl Trait`
1009 // that indicates the distinction we are checking for:
1013 // pub type Foo = impl Iterator;
1014 // pub fn make_foo() -> Foo { .. }
1018 // fn foo() -> a::Foo { a::make_foo() }
1022 // Here, the return type of `foo` references a
1023 // `Opaque` indeed, but not one whose value is
1024 // presently being inferred. You can get into a
1025 // similar situation with closure return types
1029 // fn foo() -> impl Iterator { .. }
1031 // let x = || foo(); // returns the Opaque assoc with `foo`
1034 if let Some(opaque_hir_id) = tcx.hir().as_local_hir_id(def_id) {
1035 let parent_def_id = self.parent_def_id;
1036 let def_scope_default = || {
1037 let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
1038 parent_def_id == tcx.hir()
1039 .local_def_id(opaque_parent_hir_id)
1041 let (in_definition_scope, origin) = match tcx.hir().find(opaque_hir_id) {
1042 Some(Node::Item(item)) => match item.kind {
1043 // Anonymous `impl Trait`
1044 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1045 impl_trait_fn: Some(parent),
1048 }) => (parent == self.parent_def_id, origin),
1049 // Named `type Foo = impl Bar;`
1050 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1051 impl_trait_fn: None,
1055 may_define_opaque_type(
1063 (def_scope_default(), hir::OpaqueTyOrigin::TypeAlias)
1066 Some(Node::ImplItem(item)) => match item.kind {
1067 hir::ImplItemKind::OpaqueTy(_) => (
1068 may_define_opaque_type(
1073 hir::OpaqueTyOrigin::TypeAlias,
1076 (def_scope_default(), hir::OpaqueTyOrigin::TypeAlias)
1080 "expected (impl) item, found {}",
1081 tcx.hir().node_to_string(opaque_hir_id),
1084 if in_definition_scope {
1085 return self.fold_opaque_ty(ty, def_id, substs, origin);
1089 "instantiate_opaque_types_in_map: \
1090 encountered opaque outside its definition scope \
1108 substs: SubstsRef<'tcx>,
1109 origin: hir::OpaqueTyOrigin,
1111 let infcx = self.infcx;
1112 let tcx = infcx.tcx;
1114 debug!("instantiate_opaque_types: Opaque(def_id={:?}, substs={:?})", def_id, substs);
1116 // Use the same type variable if the exact same opaque type appears more
1117 // than once in the return type (e.g., if it's passed to a type alias).
1118 if let Some(opaque_defn) = self.opaque_types.get(&def_id) {
1119 debug!("instantiate_opaque_types: returning concrete ty {:?}", opaque_defn.concrete_ty);
1120 return opaque_defn.concrete_ty;
1122 let span = tcx.def_span(def_id);
1123 debug!("fold_opaque_ty {:?} {:?}", self.value_span, span);
1125 .next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span });
1127 let predicates_of = tcx.predicates_of(def_id);
1128 debug!("instantiate_opaque_types: predicates={:#?}", predicates_of,);
1129 let bounds = predicates_of.instantiate(tcx, substs);
1131 let param_env = tcx.param_env(def_id);
1132 let InferOk { value: bounds, obligations } =
1133 infcx.partially_normalize_associated_types_in(span, self.body_id, param_env, &bounds);
1134 self.obligations.extend(obligations);
1136 debug!("instantiate_opaque_types: bounds={:?}", bounds);
1138 let required_region_bounds = tcx.required_region_bounds(ty, bounds.predicates.clone());
1139 debug!("instantiate_opaque_types: required_region_bounds={:?}", required_region_bounds);
1141 // Make sure that we are in fact defining the *entire* type
1142 // (e.g., `type Foo<T: Bound> = impl Bar;` needs to be
1143 // defined by a function like `fn foo<T: Bound>() -> Foo<T>`).
1144 debug!("instantiate_opaque_types: param_env={:#?}", self.param_env,);
1145 debug!("instantiate_opaque_types: generics={:#?}", tcx.generics_of(def_id),);
1147 // Ideally, we'd get the span where *this specific `ty` came
1148 // from*, but right now we just use the span from the overall
1149 // value being folded. In simple cases like `-> impl Foo`,
1150 // these are the same span, but not in cases like `-> (impl
1152 let definition_span = self.value_span;
1154 self.opaque_types.insert(
1160 concrete_ty: ty_var,
1161 has_required_region_bounds: !required_region_bounds.is_empty(),
1165 debug!("instantiate_opaque_types: ty_var={:?}", ty_var);
1167 for predicate in &bounds.predicates {
1168 if let ty::Predicate::Projection(projection) = &predicate {
1169 if projection.skip_binder().ty.references_error() {
1170 // No point on adding these obligations since there's a type error involved.
1176 self.obligations.reserve(bounds.predicates.len());
1177 for predicate in bounds.predicates {
1178 // Change the predicate to refer to the type variable,
1179 // which will be the concrete type instead of the opaque type.
1180 // This also instantiates nested instances of `impl Trait`.
1181 let predicate = self.instantiate_opaque_types_in_map(&predicate);
1183 let cause = traits::ObligationCause::new(span, self.body_id, traits::SizedReturnType);
1185 // Require that the predicate holds for the concrete type.
1186 debug!("instantiate_opaque_types: predicate={:?}", predicate);
1187 self.obligations.push(traits::Obligation::new(cause, self.param_env, predicate));
1194 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
1200 /// pub trait Bar { .. }
1202 /// pub type Baz = impl Bar;
1204 /// fn f1() -> Baz { .. }
1207 /// fn f2() -> bar::Baz { .. }
1211 /// Here, `def_id` is the `DefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
1212 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
1213 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
1214 pub fn may_define_opaque_type(
1217 opaque_hir_id: hir::HirId,
1219 let mut hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
1221 // Named opaque types can be defined by any siblings or children of siblings.
1222 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
1223 // We walk up the node tree until we hit the root or the scope of the opaque type.
1224 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
1225 hir_id = tcx.hir().get_parent_item(hir_id);
1227 // Syntactically, we are allowed to define the concrete type if:
1228 let res = hir_id == scope;
1230 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
1231 tcx.hir().get(hir_id),
1232 tcx.hir().get(opaque_hir_id),