1 use crate::infer::error_reporting::{note_and_explain_free_region, note_and_explain_region};
2 use crate::infer::{self, InferCtxt, InferOk, TypeVariableOrigin, TypeVariableOriginKind};
3 use crate::middle::region;
4 use crate::traits::{self, PredicateObligation};
5 use crate::ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder, TypeVisitor};
6 use crate::ty::free_region_map::FreeRegionRelations;
7 use crate::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, SubstsRef};
8 use crate::ty::{self, GenericParamDefKind, Ty, TyCtxt};
9 use errors::{struct_span_err, DiagnosticBuilder};
10 use rustc::session::config::nightly_options;
11 use rustc_data_structures::fx::FxHashMap;
12 use rustc_data_structures::sync::Lrc;
14 use rustc_hir::def_id::{DefId, DefIdMap};
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 =
354 required_region_bounds(tcx, 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 {
471 self.member_constraint(
473 opaque_defn.definition_span,
482 /// Member constraints are presently feature-gated except for
483 /// async-await. We expect to lift this once we've had a bit more
485 fn member_constraint_feature_gate(
487 opaque_defn: &OpaqueTypeDecl<'tcx>,
488 opaque_type_def_id: DefId,
489 conflict1: ty::Region<'tcx>,
490 conflict2: ty::Region<'tcx>,
492 // If we have `#![feature(member_constraints)]`, no problems.
493 if self.tcx.features().member_constraints {
497 let span = self.tcx.def_span(opaque_type_def_id);
499 // Without a feature-gate, we only generate member-constraints for async-await.
500 let context_name = match opaque_defn.origin {
501 // No feature-gate required for `async fn`.
502 hir::OpaqueTyOrigin::AsyncFn => return false,
504 // Otherwise, generate the label we'll use in the error message.
505 hir::OpaqueTyOrigin::TypeAlias => "impl Trait",
506 hir::OpaqueTyOrigin::FnReturn => "impl Trait",
508 let msg = format!("ambiguous lifetime bound in `{}`", context_name);
509 let mut err = self.tcx.sess.struct_span_err(span, &msg);
511 let conflict1_name = conflict1.to_string();
512 let conflict2_name = conflict2.to_string();
514 let label = match (&*conflict1_name, &*conflict2_name) {
515 ("'_", "'_") => "the elided lifetimes here do not outlive one another",
517 label_owned = format!(
518 "neither `{}` nor `{}` outlives the other",
519 conflict1_name, conflict2_name,
524 err.span_label(span, label);
526 if nightly_options::is_nightly_build() {
527 err.help("add #![feature(member_constraints)] to the crate attributes to enable");
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 note_and_explain_free_region(
628 &format!("hidden type `{}` captures ", hidden_ty),
633 // Ugh. This is a painful case: the hidden region is not one
634 // that we can easily summarize or explain. This can happen
636 // `src/test/ui/multiple-lifetimes/ordinary-bounds-unsuited.rs`:
639 // fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b> {
640 // if condition() { a } else { b }
644 // Here the captured lifetime is the intersection of `'a` and
645 // `'b`, which we can't quite express.
647 if let Some(region_scope_tree) = region_scope_tree {
648 // If the `region_scope_tree` is available, this is being
649 // invoked from the "region inferencer error". We can at
650 // least report a really cryptic error for now.
651 note_and_explain_region(
655 &format!("hidden type `{}` captures ", hidden_ty),
660 // If the `region_scope_tree` is *unavailable*, this is
661 // being invoked by the code that comes *after* region
662 // inferencing. This is a bug, as the region inferencer
663 // ought to have noticed the failed constraint and invoked
664 // error reporting, which in turn should have prevented us
665 // from getting trying to infer the hidden type
667 tcx.sess.delay_span_bug(
670 "hidden type captures unexpected lifetime `{:?}` \
671 but no region inference failure",
681 // Visitor that requires that (almost) all regions in the type visited outlive
682 // `least_region`. We cannot use `push_outlives_components` because regions in
683 // closure signatures are not included in their outlives components. We need to
684 // ensure all regions outlive the given bound so that we don't end up with,
685 // say, `ReScope` appearing in a return type and causing ICEs when other
686 // functions end up with region constraints involving regions from other
689 // We also cannot use `for_each_free_region` because for closures it includes
690 // the regions parameters from the enclosing item.
692 // We ignore any type parameters because impl trait values are assumed to
693 // capture all the in-scope type parameters.
694 struct ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
696 OP: FnMut(ty::Region<'tcx>),
702 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
704 OP: FnMut(ty::Region<'tcx>),
706 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> bool {
707 t.skip_binder().visit_with(self);
708 false // keep visiting
711 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
713 // ignore bound regions, keep visiting
714 ty::ReLateBound(_, _) => false,
722 fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
723 // We're only interested in types involving regions
724 if !ty.flags.intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
725 return false; // keep visiting
729 ty::Closure(def_id, ref substs) => {
730 // Skip lifetime parameters of the enclosing item(s)
732 for upvar_ty in substs.as_closure().upvar_tys(def_id, self.tcx) {
733 upvar_ty.visit_with(self);
736 substs.as_closure().sig_ty(def_id, self.tcx).visit_with(self);
739 ty::Generator(def_id, ref substs, _) => {
740 // Skip lifetime parameters of the enclosing item(s)
741 // Also skip the witness type, because that has no free regions.
743 for upvar_ty in substs.as_generator().upvar_tys(def_id, self.tcx) {
744 upvar_ty.visit_with(self);
747 substs.as_generator().return_ty(def_id, self.tcx).visit_with(self);
748 substs.as_generator().yield_ty(def_id, self.tcx).visit_with(self);
751 ty.super_visit_with(self);
759 struct ReverseMapper<'tcx> {
762 /// If errors have already been reported in this fn, we suppress
763 /// our own errors because they are sometimes derivative.
764 tainted_by_errors: bool,
766 opaque_type_def_id: DefId,
767 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
768 map_missing_regions_to_empty: bool,
770 /// initially `Some`, set to `None` once error has been reported
771 hidden_ty: Option<Ty<'tcx>>,
773 /// Span of function being checked.
777 impl ReverseMapper<'tcx> {
780 tainted_by_errors: bool,
781 opaque_type_def_id: DefId,
782 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
791 map_missing_regions_to_empty: false,
792 hidden_ty: Some(hidden_ty),
797 fn fold_kind_mapping_missing_regions_to_empty(
799 kind: GenericArg<'tcx>,
800 ) -> GenericArg<'tcx> {
801 assert!(!self.map_missing_regions_to_empty);
802 self.map_missing_regions_to_empty = true;
803 let kind = kind.fold_with(self);
804 self.map_missing_regions_to_empty = false;
808 fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
809 assert!(!self.map_missing_regions_to_empty);
814 impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
815 fn tcx(&self) -> TyCtxt<'tcx> {
819 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
821 // ignore bound regions that appear in the type (e.g., this
822 // would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
823 ty::ReLateBound(..) |
825 // ignore `'static`, as that can appear anywhere
826 ty::ReStatic => return r,
831 let generics = self.tcx().generics_of(self.opaque_type_def_id);
832 match self.map.get(&r.into()).map(|k| k.unpack()) {
833 Some(GenericArgKind::Lifetime(r1)) => r1,
834 Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
835 None if generics.parent.is_some() => {
836 if !self.map_missing_regions_to_empty && !self.tainted_by_errors {
837 if let Some(hidden_ty) = self.hidden_ty.take() {
838 unexpected_hidden_region_diagnostic(
841 self.opaque_type_def_id,
848 self.tcx.lifetimes.re_empty
853 .struct_span_err(self.span, "non-defining opaque type use in defining scope")
857 "lifetime `{}` is part of concrete type but not used in \
858 parameter list of the `impl Trait` type alias",
864 self.tcx().mk_region(ty::ReStatic)
869 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
871 ty::Closure(def_id, substs) => {
872 // I am a horrible monster and I pray for death. When
873 // we encounter a closure here, it is always a closure
874 // from within the function that we are currently
875 // type-checking -- one that is now being encapsulated
876 // in an opaque type. Ideally, we would
877 // go through the types/lifetimes that it references
878 // and treat them just like we would any other type,
879 // which means we would error out if we find any
880 // reference to a type/region that is not in the
883 // **However,** in the case of closures, there is a
884 // somewhat subtle (read: hacky) consideration. The
885 // problem is that our closure types currently include
886 // all the lifetime parameters declared on the
887 // enclosing function, even if they are unused by the
888 // closure itself. We can't readily filter them out,
889 // so here we replace those values with `'empty`. This
890 // can't really make a difference to the rest of the
891 // compiler; those regions are ignored for the
892 // outlives relation, and hence don't affect trait
893 // selection or auto traits, and they are erased
896 let generics = self.tcx.generics_of(def_id);
897 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, &kind)| {
898 if index < generics.parent_count {
899 // Accommodate missing regions in the parent kinds...
900 self.fold_kind_mapping_missing_regions_to_empty(kind)
902 // ...but not elsewhere.
903 self.fold_kind_normally(kind)
907 self.tcx.mk_closure(def_id, substs)
910 ty::Generator(def_id, substs, movability) => {
911 let generics = self.tcx.generics_of(def_id);
912 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, &kind)| {
913 if index < generics.parent_count {
914 // Accommodate missing regions in the parent kinds...
915 self.fold_kind_mapping_missing_regions_to_empty(kind)
917 // ...but not elsewhere.
918 self.fold_kind_normally(kind)
922 self.tcx.mk_generator(def_id, substs, movability)
926 // Look it up in the substitution list.
927 match self.map.get(&ty.into()).map(|k| k.unpack()) {
928 // Found it in the substitution list; replace with the parameter from the
930 Some(GenericArgKind::Type(t1)) => t1,
931 Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
938 "type parameter `{}` is part of concrete type but not \
939 used in parameter list for the `impl Trait` type alias",
950 _ => ty.super_fold_with(self),
954 fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
955 trace!("checking const {:?}", ct);
956 // Find a const parameter
958 ty::ConstKind::Param(..) => {
959 // Look it up in the substitution list.
960 match self.map.get(&ct.into()).map(|k| k.unpack()) {
961 // Found it in the substitution list, replace with the parameter from the
963 Some(GenericArgKind::Const(c1)) => c1,
964 Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
971 "const parameter `{}` is part of concrete type but not \
972 used in parameter list for the `impl Trait` type alias",
978 self.tcx().consts.err
988 struct Instantiator<'a, 'tcx> {
989 infcx: &'a InferCtxt<'a, 'tcx>,
990 parent_def_id: DefId,
992 param_env: ty::ParamEnv<'tcx>,
994 opaque_types: OpaqueTypeMap<'tcx>,
995 obligations: Vec<PredicateObligation<'tcx>>,
998 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
999 fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
1000 debug!("instantiate_opaque_types_in_map(value={:?})", value);
1001 let tcx = self.infcx.tcx;
1002 value.fold_with(&mut BottomUpFolder {
1005 if ty.references_error() {
1006 return tcx.types.err;
1007 } else if let ty::Opaque(def_id, substs) = ty.kind {
1008 // Check that this is `impl Trait` type is
1009 // declared by `parent_def_id` -- i.e., one whose
1010 // value we are inferring. At present, this is
1011 // always true during the first phase of
1012 // type-check, but not always true later on during
1013 // NLL. Once we support named opaque types more fully,
1014 // this same scenario will be able to arise during all phases.
1016 // Here is an example using type alias `impl Trait`
1017 // that indicates the distinction we are checking for:
1021 // pub type Foo = impl Iterator;
1022 // pub fn make_foo() -> Foo { .. }
1026 // fn foo() -> a::Foo { a::make_foo() }
1030 // Here, the return type of `foo` references a
1031 // `Opaque` indeed, but not one whose value is
1032 // presently being inferred. You can get into a
1033 // similar situation with closure return types
1037 // fn foo() -> impl Iterator { .. }
1039 // let x = || foo(); // returns the Opaque assoc with `foo`
1042 if let Some(opaque_hir_id) = tcx.hir().as_local_hir_id(def_id) {
1043 let parent_def_id = self.parent_def_id;
1044 let def_scope_default = || {
1045 let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
1046 parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
1048 let (in_definition_scope, origin) = match tcx.hir().find(opaque_hir_id) {
1049 Some(Node::Item(item)) => match item.kind {
1050 // Anonymous `impl Trait`
1051 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1052 impl_trait_fn: Some(parent),
1055 }) => (parent == self.parent_def_id, origin),
1056 // Named `type Foo = impl Bar;`
1057 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1058 impl_trait_fn: None,
1062 may_define_opaque_type(tcx, self.parent_def_id, opaque_hir_id),
1065 _ => (def_scope_default(), hir::OpaqueTyOrigin::TypeAlias),
1067 Some(Node::ImplItem(item)) => match item.kind {
1068 hir::ImplItemKind::OpaqueTy(_) => (
1069 may_define_opaque_type(tcx, self.parent_def_id, opaque_hir_id),
1070 hir::OpaqueTyOrigin::TypeAlias,
1072 _ => (def_scope_default(), hir::OpaqueTyOrigin::TypeAlias),
1075 "expected (impl) item, found {}",
1076 tcx.hir().node_to_string(opaque_hir_id),
1079 if in_definition_scope {
1080 return self.fold_opaque_ty(ty, def_id, substs, origin);
1084 "instantiate_opaque_types_in_map: \
1085 encountered opaque outside its definition scope \
1103 substs: SubstsRef<'tcx>,
1104 origin: hir::OpaqueTyOrigin,
1106 let infcx = self.infcx;
1107 let tcx = infcx.tcx;
1109 debug!("instantiate_opaque_types: Opaque(def_id={:?}, substs={:?})", def_id, substs);
1111 // Use the same type variable if the exact same opaque type appears more
1112 // than once in the return type (e.g., if it's passed to a type alias).
1113 if let Some(opaque_defn) = self.opaque_types.get(&def_id) {
1114 debug!("instantiate_opaque_types: returning concrete ty {:?}", opaque_defn.concrete_ty);
1115 return opaque_defn.concrete_ty;
1117 let span = tcx.def_span(def_id);
1118 debug!("fold_opaque_ty {:?} {:?}", self.value_span, span);
1120 .next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span });
1122 let predicates_of = tcx.predicates_of(def_id);
1123 debug!("instantiate_opaque_types: predicates={:#?}", predicates_of,);
1124 let bounds = predicates_of.instantiate(tcx, substs);
1126 let param_env = tcx.param_env(def_id);
1127 let InferOk { value: bounds, obligations } =
1128 infcx.partially_normalize_associated_types_in(span, self.body_id, param_env, &bounds);
1129 self.obligations.extend(obligations);
1131 debug!("instantiate_opaque_types: bounds={:?}", bounds);
1133 let required_region_bounds = required_region_bounds(tcx, ty, bounds.predicates.clone());
1134 debug!("instantiate_opaque_types: required_region_bounds={:?}", required_region_bounds);
1136 // Make sure that we are in fact defining the *entire* type
1137 // (e.g., `type Foo<T: Bound> = impl Bar;` needs to be
1138 // defined by a function like `fn foo<T: Bound>() -> Foo<T>`).
1139 debug!("instantiate_opaque_types: param_env={:#?}", self.param_env,);
1140 debug!("instantiate_opaque_types: generics={:#?}", tcx.generics_of(def_id),);
1142 // Ideally, we'd get the span where *this specific `ty` came
1143 // from*, but right now we just use the span from the overall
1144 // value being folded. In simple cases like `-> impl Foo`,
1145 // these are the same span, but not in cases like `-> (impl
1147 let definition_span = self.value_span;
1149 self.opaque_types.insert(
1155 concrete_ty: ty_var,
1156 has_required_region_bounds: !required_region_bounds.is_empty(),
1160 debug!("instantiate_opaque_types: ty_var={:?}", ty_var);
1162 for predicate in &bounds.predicates {
1163 if let ty::Predicate::Projection(projection) = &predicate {
1164 if projection.skip_binder().ty.references_error() {
1165 // No point on adding these obligations since there's a type error involved.
1171 self.obligations.reserve(bounds.predicates.len());
1172 for predicate in bounds.predicates {
1173 // Change the predicate to refer to the type variable,
1174 // which will be the concrete type instead of the opaque type.
1175 // This also instantiates nested instances of `impl Trait`.
1176 let predicate = self.instantiate_opaque_types_in_map(&predicate);
1178 let cause = traits::ObligationCause::new(span, self.body_id, traits::SizedReturnType);
1180 // Require that the predicate holds for the concrete type.
1181 debug!("instantiate_opaque_types: predicate={:?}", predicate);
1182 self.obligations.push(traits::Obligation::new(cause, self.param_env, predicate));
1189 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
1195 /// pub trait Bar { .. }
1197 /// pub type Baz = impl Bar;
1199 /// fn f1() -> Baz { .. }
1202 /// fn f2() -> bar::Baz { .. }
1206 /// Here, `def_id` is the `DefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
1207 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
1208 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
1209 pub fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: DefId, opaque_hir_id: hir::HirId) -> bool {
1210 let mut hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
1212 // Named opaque types can be defined by any siblings or children of siblings.
1213 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
1214 // We walk up the node tree until we hit the root or the scope of the opaque type.
1215 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
1216 hir_id = tcx.hir().get_parent_item(hir_id);
1218 // Syntactically, we are allowed to define the concrete type if:
1219 let res = hir_id == scope;
1221 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
1222 tcx.hir().get(hir_id),
1223 tcx.hir().get(opaque_hir_id),
1229 /// Given a set of predicates that apply to an object type, returns
1230 /// the region bounds that the (erased) `Self` type must
1231 /// outlive. Precisely *because* the `Self` type is erased, the
1232 /// parameter `erased_self_ty` must be supplied to indicate what type
1233 /// has been used to represent `Self` in the predicates
1234 /// themselves. This should really be a unique type; `FreshTy(0)` is a
1237 /// N.B., in some cases, particularly around higher-ranked bounds,
1238 /// this function returns a kind of conservative approximation.
1239 /// That is, all regions returned by this function are definitely
1240 /// required, but there may be other region bounds that are not
1241 /// returned, as well as requirements like `for<'a> T: 'a`.
1243 /// Requires that trait definitions have been processed so that we can
1244 /// elaborate predicates and walk supertraits.
1246 // FIXME: callers may only have a `&[Predicate]`, not a `Vec`, so that's
1247 // what this code should accept.
1248 crate fn required_region_bounds(
1250 erased_self_ty: Ty<'tcx>,
1251 predicates: Vec<ty::Predicate<'tcx>>,
1252 ) -> Vec<ty::Region<'tcx>> {
1254 "required_region_bounds(erased_self_ty={:?}, predicates={:?})",
1255 erased_self_ty, predicates
1258 assert!(!erased_self_ty.has_escaping_bound_vars());
1260 traits::elaborate_predicates(tcx, predicates)
1261 .filter_map(|predicate| {
1263 ty::Predicate::Projection(..)
1264 | ty::Predicate::Trait(..)
1265 | ty::Predicate::Subtype(..)
1266 | ty::Predicate::WellFormed(..)
1267 | ty::Predicate::ObjectSafe(..)
1268 | ty::Predicate::ClosureKind(..)
1269 | ty::Predicate::RegionOutlives(..)
1270 | ty::Predicate::ConstEvaluatable(..) => None,
1271 ty::Predicate::TypeOutlives(predicate) => {
1272 // Search for a bound of the form `erased_self_ty
1273 // : 'a`, but be wary of something like `for<'a>
1274 // erased_self_ty : 'a` (we interpret a
1275 // higher-ranked bound like that as 'static,
1276 // though at present the code in `fulfill.rs`
1277 // considers such bounds to be unsatisfiable, so
1278 // it's kind of a moot point since you could never
1279 // construct such an object, but this seems
1280 // correct even if that code changes).
1281 let ty::OutlivesPredicate(ref t, ref r) = predicate.skip_binder();
1282 if t == &erased_self_ty && !r.has_escaping_bound_vars() {