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> {
27 /// The substitutions that we apply to the opaque type that this
28 /// `impl Trait` desugars to. e.g., if:
30 /// fn foo<'a, 'b, T>() -> impl Trait<'a>
32 /// winds up desugared to:
34 /// type Foo<'x, X> = impl Trait<'x>
35 /// fn foo<'a, 'b, T>() -> Foo<'a, T>
37 /// then `substs` would be `['a, T]`.
38 pub substs: SubstsRef<'tcx>,
40 /// The span of this particular definition of the opaque type. So
44 /// type Foo = impl Baz;
46 /// ^^^ This is the span we are looking for!
49 /// In cases where the fn returns `(impl Trait, impl Trait)` or
50 /// other such combinations, the result is currently
51 /// over-approximated, but better than nothing.
52 pub definition_span: Span,
54 /// The type variable that represents the value of the opaque type
55 /// that we require. In other words, after we compile this function,
56 /// we will be created a constraint like:
60 /// where `?C` is the value of this type variable. =) It may
61 /// naturally refer to the type and lifetime parameters in scope
62 /// in this function, though ultimately it should only reference
63 /// those that are arguments to `Foo` in the constraint above. (In
64 /// other words, `?C` should not include `'b`, even though it's a
65 /// lifetime parameter on `foo`.)
66 pub concrete_ty: Ty<'tcx>,
68 /// Returns `true` if the `impl Trait` bounds include region bounds.
69 /// For example, this would be true for:
71 /// fn foo<'a, 'b, 'c>() -> impl Trait<'c> + 'a + 'b
75 /// fn foo<'c>() -> impl Trait<'c>
77 /// unless `Trait` was declared like:
79 /// trait Trait<'c>: 'c
81 /// in which case it would be true.
83 /// This is used during regionck to decide whether we need to
84 /// impose any additional constraints to ensure that region
85 /// variables in `concrete_ty` wind up being constrained to
86 /// something from `substs` (or, at minimum, things that outlive
87 /// the fn body). (Ultimately, writeback is responsible for this
89 pub has_required_region_bounds: bool,
91 /// The origin of the opaque type.
92 pub origin: hir::OpaqueTyOrigin,
95 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
96 /// Replaces all opaque types in `value` with fresh inference variables
97 /// and creates appropriate obligations. For example, given the input:
99 /// impl Iterator<Item = impl Debug>
101 /// this method would create two type variables, `?0` and `?1`. It would
102 /// return the type `?0` but also the obligations:
104 /// ?0: Iterator<Item = ?1>
107 /// Moreover, it returns a `OpaqueTypeMap` that would map `?0` to
108 /// info about the `impl Iterator<..>` type and `?1` to info about
109 /// the `impl Debug` type.
113 /// - `parent_def_id` -- the `DefId` of the function in which the opaque type
115 /// - `body_id` -- the body-id with which the resulting obligations should
117 /// - `param_env` -- the in-scope parameter environment to be used for
119 /// - `value` -- the value within which we are instantiating opaque types
120 /// - `value_span` -- the span where the value came from, used in error reporting
121 pub fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
123 parent_def_id: DefId,
125 param_env: ty::ParamEnv<'tcx>,
128 ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)> {
130 "instantiate_opaque_types(value={:?}, parent_def_id={:?}, body_id={:?}, \
131 param_env={:?}, value_span={:?})",
132 value, parent_def_id, body_id, param_env, value_span,
134 let mut instantiator = Instantiator {
140 opaque_types: Default::default(),
143 let value = instantiator.instantiate_opaque_types_in_map(value);
144 InferOk { value: (value, instantiator.opaque_types), obligations: instantiator.obligations }
147 /// Given the map `opaque_types` containing the opaque
148 /// `impl Trait` types whose underlying, hidden types are being
149 /// inferred, this method adds constraints to the regions
150 /// appearing in those underlying hidden types to ensure that they
151 /// at least do not refer to random scopes within the current
152 /// function. These constraints are not (quite) sufficient to
153 /// guarantee that the regions are actually legal values; that
154 /// final condition is imposed after region inference is done.
158 /// Let's work through an example to explain how it works. Assume
159 /// the current function is as follows:
162 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
165 /// Here, we have two `impl Trait` types whose values are being
166 /// inferred (the `impl Bar<'a>` and the `impl
167 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
168 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
169 /// the return type of `foo`, we *reference* those definitions:
172 /// type Foo1<'x> = impl Bar<'x>;
173 /// type Foo2<'x> = impl Bar<'x>;
174 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
181 /// As indicating in the comments above, each of those references
182 /// is (in the compiler) basically a substitution (`substs`)
183 /// applied to the type of a suitable `def_id` (which identifies
184 /// `Foo1` or `Foo2`).
186 /// Now, at this point in compilation, what we have done is to
187 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
188 /// fresh inference variables C1 and C2. We wish to use the values
189 /// of these variables to infer the underlying types of `Foo1` and
190 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
191 /// constraints like:
194 /// for<'a> (Foo1<'a> = C1)
195 /// for<'b> (Foo1<'b> = C2)
198 /// For these equation to be satisfiable, the types `C1` and `C2`
199 /// can only refer to a limited set of regions. For example, `C1`
200 /// can only refer to `'static` and `'a`, and `C2` can only refer
201 /// to `'static` and `'b`. The job of this function is to impose that
204 /// Up to this point, C1 and C2 are basically just random type
205 /// inference variables, and hence they may contain arbitrary
206 /// regions. In fact, it is fairly likely that they do! Consider
207 /// this possible definition of `foo`:
210 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
215 /// Here, the values for the concrete types of the two impl
216 /// traits will include inference variables:
223 /// Ordinarily, the subtyping rules would ensure that these are
224 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
225 /// type per se, we don't get such constraints by default. This
226 /// is where this function comes into play. It adds extra
227 /// constraints to ensure that all the regions which appear in the
228 /// inferred type are regions that could validly appear.
230 /// This is actually a bit of a tricky constraint in general. We
231 /// want to say that each variable (e.g., `'0`) can only take on
232 /// values that were supplied as arguments to the opaque type
233 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
234 /// scope. We don't have a constraint quite of this kind in the current
239 /// We generally prefer to make `<=` constraints, since they
240 /// integrate best into the region solver. To do that, we find the
241 /// "minimum" of all the arguments that appear in the substs: that
242 /// is, some region which is less than all the others. In the case
243 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
244 /// all). Then we apply that as a least bound to the variables
245 /// (e.g., `'a <= '0`).
247 /// In some cases, there is no minimum. Consider this example:
250 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
253 /// Here we would report a more complex "in constraint", like `'r
254 /// in ['a, 'b, 'static]` (where `'r` is some regon appearing in
255 /// the hidden type).
257 /// # Constrain regions, not the hidden concrete type
259 /// Note that generating constraints on each region `Rc` is *not*
260 /// the same as generating an outlives constraint on `Tc` iself.
261 /// For example, if we had a function like this:
264 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
268 /// // Equivalent to:
269 /// type FooReturn<'a, T> = impl Foo<'a>;
270 /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
273 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
274 /// is an inference variable). If we generated a constraint that
275 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
276 /// but this is not necessary, because the opaque type we
277 /// create will be allowed to reference `T`. So we only generate a
278 /// constraint that `'0: 'a`.
280 /// # The `free_region_relations` parameter
282 /// The `free_region_relations` argument is used to find the
283 /// "minimum" of the regions supplied to a given opaque type.
284 /// It must be a relation that can answer whether `'a <= 'b`,
285 /// where `'a` and `'b` are regions that appear in the "substs"
286 /// for the opaque type references (the `<'a>` in `Foo1<'a>`).
288 /// Note that we do not impose the constraints based on the
289 /// generic regions from the `Foo1` definition (e.g., `'x`). This
290 /// is because the constraints we are imposing here is basically
291 /// the concern of the one generating the constraining type C1,
292 /// which is the current function. It also means that we can
293 /// take "implied bounds" into account in some cases:
296 /// trait SomeTrait<'a, 'b> { }
297 /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
300 /// Here, the fact that `'b: 'a` is known only because of the
301 /// implied bounds from the `&'a &'b u32` parameter, and is not
302 /// "inherent" to the opaque type definition.
306 /// - `opaque_types` -- the map produced by `instantiate_opaque_types`
307 /// - `free_region_relations` -- something that can be used to relate
308 /// the free regions (`'a`) that appear in the impl trait.
309 pub fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(
311 opaque_types: &OpaqueTypeMap<'tcx>,
312 free_region_relations: &FRR,
314 debug!("constrain_opaque_types()");
316 for (&def_id, opaque_defn) in opaque_types {
317 self.constrain_opaque_type(def_id, opaque_defn, free_region_relations);
321 /// See `constrain_opaque_types` for documentation.
322 pub fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
325 opaque_defn: &OpaqueTypeDecl<'tcx>,
326 free_region_relations: &FRR,
328 debug!("constrain_opaque_type()");
329 debug!("constrain_opaque_type: def_id={:?}", def_id);
330 debug!("constrain_opaque_type: opaque_defn={:#?}", opaque_defn);
334 let concrete_ty = self.resolve_vars_if_possible(&opaque_defn.concrete_ty);
336 debug!("constrain_opaque_type: concrete_ty={:?}", concrete_ty);
338 let opaque_type_generics = tcx.generics_of(def_id);
340 let span = tcx.def_span(def_id);
342 // If there are required region bounds, we can use them.
343 if opaque_defn.has_required_region_bounds {
344 let predicates_of = tcx.predicates_of(def_id);
345 debug!("constrain_opaque_type: predicates: {:#?}", predicates_of,);
346 let bounds = predicates_of.instantiate(tcx, opaque_defn.substs);
347 debug!("constrain_opaque_type: bounds={:#?}", bounds);
348 let opaque_type = tcx.mk_opaque(def_id, opaque_defn.substs);
350 let required_region_bounds = tcx.required_region_bounds(opaque_type, bounds.predicates);
351 debug_assert!(!required_region_bounds.is_empty());
353 for required_region in required_region_bounds {
354 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
356 op: |r| self.sub_regions(infer::CallReturn(span), required_region, r),
362 // There were no `required_region_bounds`,
363 // so we have to search for a `least_region`.
364 // Go through all the regions used as arguments to the
365 // opaque type. These are the parameters to the opaque
366 // type; so in our example above, `substs` would contain
367 // `['a]` for the first impl trait and `'b` for the
369 let mut least_region = None;
370 for param in &opaque_type_generics.params {
372 GenericParamDefKind::Lifetime => {}
376 // Get the value supplied for this region from the substs.
377 let subst_arg = opaque_defn.substs.region_at(param.index as usize);
379 // Compute the least upper bound of it with the other regions.
380 debug!("constrain_opaque_types: least_region={:?}", least_region);
381 debug!("constrain_opaque_types: subst_arg={:?}", subst_arg);
383 None => least_region = Some(subst_arg),
385 if free_region_relations.sub_free_regions(lr, subst_arg) {
386 // keep the current least region
387 } else if free_region_relations.sub_free_regions(subst_arg, lr) {
388 // switch to `subst_arg`
389 least_region = Some(subst_arg);
391 // There are two regions (`lr` and
392 // `subst_arg`) which are not relatable. We
393 // can't find a best choice. Therefore,
394 // instead of creating a single bound like
395 // `'r: 'a` (which is our preferred choice),
396 // we will create a "in bound" like `'r in
397 // ['a, 'b, 'c]`, where `'a..'c` are the
398 // regions that appear in the impl trait.
399 return self.generate_member_constraint(
401 opaque_type_generics,
412 let least_region = least_region.unwrap_or(tcx.lifetimes.re_static);
413 debug!("constrain_opaque_types: least_region={:?}", least_region);
415 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
417 op: |r| self.sub_regions(infer::CallReturn(span), least_region, r),
421 /// As a fallback, we sometimes generate an "in constraint". For
422 /// a case like `impl Foo<'a, 'b>`, where `'a` and `'b` cannot be
423 /// related, we would generate a constraint `'r in ['a, 'b,
424 /// 'static]` for each region `'r` that appears in the hidden type
425 /// (i.e., it must be equal to `'a`, `'b`, or `'static`).
427 /// `conflict1` and `conflict2` are the two region bounds that we
428 /// detected which were unrelated. They are used for diagnostics.
429 fn generate_member_constraint(
431 concrete_ty: Ty<'tcx>,
432 opaque_type_generics: &ty::Generics,
433 opaque_defn: &OpaqueTypeDecl<'tcx>,
434 opaque_type_def_id: DefId,
435 conflict1: ty::Region<'tcx>,
436 conflict2: ty::Region<'tcx>,
438 // For now, enforce a feature gate outside of async functions.
439 if self.member_constraint_feature_gate(
448 // Create the set of choice regions: each region in the hidden
449 // type can be equal to any of the region parameters of the
450 // opaque type definition.
451 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
455 .filter(|param| match param.kind {
456 GenericParamDefKind::Lifetime => true,
457 GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => false,
459 .map(|param| opaque_defn.substs.region_at(param.index as usize))
460 .chain(std::iter::once(self.tcx.lifetimes.re_static))
464 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
466 op: |r| self.member_constraint(
468 opaque_defn.definition_span,
476 /// Member constraints are presently feature-gated except for
477 /// async-await. We expect to lift this once we've had a bit more
479 fn member_constraint_feature_gate(
481 opaque_defn: &OpaqueTypeDecl<'tcx>,
482 opaque_type_def_id: DefId,
483 conflict1: ty::Region<'tcx>,
484 conflict2: ty::Region<'tcx>,
486 // If we have `#![feature(member_constraints)]`, no problems.
487 if self.tcx.features().member_constraints {
491 let span = self.tcx.def_span(opaque_type_def_id);
493 // Without a feature-gate, we only generate member-constraints for async-await.
494 let context_name = match opaque_defn.origin {
495 // No feature-gate required for `async fn`.
496 hir::OpaqueTyOrigin::AsyncFn => return false,
498 // Otherwise, generate the label we'll use in the error message.
499 hir::OpaqueTyOrigin::TypeAlias => "impl Trait",
500 hir::OpaqueTyOrigin::FnReturn => "impl Trait",
502 let msg = format!("ambiguous lifetime bound in `{}`", context_name);
503 let mut err = self.tcx.sess.struct_span_err(span, &msg);
505 let conflict1_name = conflict1.to_string();
506 let conflict2_name = conflict2.to_string();
508 let label = match (&*conflict1_name, &*conflict2_name) {
509 ("'_", "'_") => "the elided lifetimes here do not outlive one another",
511 label_owned = format!(
512 "neither `{}` nor `{}` outlives the other",
513 conflict1_name, conflict2_name,
518 err.span_label(span, label);
520 if nightly_options::is_nightly_build() {
522 "add #![feature(member_constraints)] to the crate attributes \
530 /// Given the fully resolved, instantiated type for an opaque
531 /// type, i.e., the value of an inference variable like C1 or C2
532 /// (*), computes the "definition type" for an opaque type
533 /// definition -- that is, the inferred value of `Foo1<'x>` or
534 /// `Foo2<'x>` that we would conceptually use in its definition:
536 /// type Foo1<'x> = impl Bar<'x> = AAA; <-- this type AAA
537 /// type Foo2<'x> = impl Bar<'x> = BBB; <-- or this type BBB
538 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
540 /// Note that these values are defined in terms of a distinct set of
541 /// generic parameters (`'x` instead of `'a`) from C1 or C2. The main
542 /// purpose of this function is to do that translation.
544 /// (*) C1 and C2 were introduced in the comments on
545 /// `constrain_opaque_types`. Read that comment for more context.
549 /// - `def_id`, the `impl Trait` type
550 /// - `opaque_defn`, the opaque definition created in `instantiate_opaque_types`
551 /// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of
552 /// `opaque_defn.concrete_ty`
553 pub fn infer_opaque_definition_from_instantiation(
556 opaque_defn: &OpaqueTypeDecl<'tcx>,
557 instantiated_ty: Ty<'tcx>,
561 "infer_opaque_definition_from_instantiation(def_id={:?}, instantiated_ty={:?})",
562 def_id, instantiated_ty
565 // Use substs to build up a reverse map from regions to their
566 // identity mappings. This is necessary because of `impl
567 // Trait` lifetimes are computed by replacing existing
568 // lifetimes with 'static and remapping only those used in the
569 // `impl Trait` return type, resulting in the parameters
571 let id_substs = InternalSubsts::identity_for_item(self.tcx, def_id);
572 let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> = opaque_defn
576 .map(|(index, subst)| (*subst, id_substs[index]))
579 // Convert the type from the function into a type valid outside
580 // the function, by replacing invalid regions with 'static,
581 // after producing an error for each of them.
582 let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new(
584 self.is_tainted_by_errors(),
590 debug!("infer_opaque_definition_from_instantiation: definition_ty={:?}", definition_ty);
596 pub fn unexpected_hidden_region_diagnostic(
598 region_scope_tree: Option<®ion::ScopeTree>,
599 opaque_type_def_id: DefId,
601 hidden_region: ty::Region<'tcx>,
602 ) -> DiagnosticBuilder<'tcx> {
603 let span = tcx.def_span(opaque_type_def_id);
604 let mut err = struct_span_err!(
608 "hidden type for `impl Trait` captures lifetime that does not appear in bounds",
611 // Explain the region we are capturing.
612 if let ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReStatic | ty::ReEmpty = hidden_region {
613 // Assuming regionck succeeded (*), we ought to always be
614 // capturing *some* region from the fn header, and hence it
615 // ought to be free. So under normal circumstances, we will go
616 // down this path which gives a decent human readable
619 // (*) if not, the `tainted_by_errors` flag would be set to
620 // true in any case, so we wouldn't be here at all.
621 tcx.note_and_explain_free_region(
623 &format!("hidden type `{}` captures ", hidden_ty),
628 // Ugh. This is a painful case: the hidden region is not one
629 // that we can easily summarize or explain. This can happen
631 // `src/test/ui/multiple-lifetimes/ordinary-bounds-unsuited.rs`:
634 // fn upper_bounds<'a, 'b>(a: Ordinary<'a>, b: Ordinary<'b>) -> impl Trait<'a, 'b> {
635 // if condition() { a } else { b }
639 // Here the captured lifetime is the intersection of `'a` and
640 // `'b`, which we can't quite express.
642 if let Some(region_scope_tree) = region_scope_tree {
643 // If the `region_scope_tree` is available, this is being
644 // invoked from the "region inferencer error". We can at
645 // least report a really cryptic error for now.
646 tcx.note_and_explain_region(
649 &format!("hidden type `{}` captures ", hidden_ty),
654 // If the `region_scope_tree` is *unavailable*, this is
655 // being invoked by the code that comes *after* region
656 // inferencing. This is a bug, as the region inferencer
657 // ought to have noticed the failed constraint and invoked
658 // error reporting, which in turn should have prevented us
659 // from getting trying to infer the hidden type
661 tcx.sess.delay_span_bug(
664 "hidden type captures unexpected lifetime `{:?}` \
665 but no region inference failure",
675 // Visitor that requires that (almost) all regions in the type visited outlive
676 // `least_region`. We cannot use `push_outlives_components` because regions in
677 // closure signatures are not included in their outlives components. We need to
678 // ensure all regions outlive the given bound so that we don't end up with,
679 // say, `ReScope` appearing in a return type and causing ICEs when other
680 // functions end up with region constraints involving regions from other
683 // We also cannot use `for_each_free_region` because for closures it includes
684 // the regions parameters from the enclosing item.
686 // We ignore any type parameters because impl trait values are assumed to
687 // capture all the in-scope type parameters.
688 struct ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
690 OP: FnMut(ty::Region<'tcx>),
696 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<'tcx, OP>
698 OP: FnMut(ty::Region<'tcx>),
700 fn visit_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> bool {
701 t.skip_binder().visit_with(self);
702 false // keep visiting
705 fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
707 // ignore bound regions, keep visiting
708 ty::ReLateBound(_, _) => false,
716 fn visit_ty(&mut self, ty: Ty<'tcx>) -> bool {
717 // We're only interested in types involving regions
718 if !ty.flags.intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
719 return false; // keep visiting
723 ty::Closure(def_id, ref substs) => {
724 // Skip lifetime parameters of the enclosing item(s)
726 for upvar_ty in substs.as_closure().upvar_tys(def_id, self.tcx) {
727 upvar_ty.visit_with(self);
730 substs.as_closure().sig_ty(def_id, self.tcx).visit_with(self);
733 ty::Generator(def_id, ref substs, _) => {
734 // Skip lifetime parameters of the enclosing item(s)
735 // Also skip the witness type, because that has no free regions.
737 for upvar_ty in substs.as_generator().upvar_tys(def_id, self.tcx) {
738 upvar_ty.visit_with(self);
741 substs.as_generator().return_ty(def_id, self.tcx).visit_with(self);
742 substs.as_generator().yield_ty(def_id, self.tcx).visit_with(self);
745 ty.super_visit_with(self);
753 struct ReverseMapper<'tcx> {
756 /// If errors have already been reported in this fn, we suppress
757 /// our own errors because they are sometimes derivative.
758 tainted_by_errors: bool,
760 opaque_type_def_id: DefId,
761 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
762 map_missing_regions_to_empty: bool,
764 /// initially `Some`, set to `None` once error has been reported
765 hidden_ty: Option<Ty<'tcx>>,
767 /// Span of function being checked.
771 impl ReverseMapper<'tcx> {
774 tainted_by_errors: bool,
775 opaque_type_def_id: DefId,
776 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
785 map_missing_regions_to_empty: false,
786 hidden_ty: Some(hidden_ty),
791 fn fold_kind_mapping_missing_regions_to_empty(
793 kind: GenericArg<'tcx>,
794 ) -> GenericArg<'tcx> {
795 assert!(!self.map_missing_regions_to_empty);
796 self.map_missing_regions_to_empty = true;
797 let kind = kind.fold_with(self);
798 self.map_missing_regions_to_empty = false;
802 fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
803 assert!(!self.map_missing_regions_to_empty);
808 impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
809 fn tcx(&self) -> TyCtxt<'tcx> {
813 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
815 // ignore bound regions that appear in the type (e.g., this
816 // would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
817 ty::ReLateBound(..) |
819 // ignore `'static`, as that can appear anywhere
820 ty::ReStatic => return r,
825 let generics = self.tcx().generics_of(self.opaque_type_def_id);
826 match self.map.get(&r.into()).map(|k| k.unpack()) {
827 Some(GenericArgKind::Lifetime(r1)) => r1,
828 Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
829 None if generics.parent.is_some() => {
830 if !self.map_missing_regions_to_empty && !self.tainted_by_errors {
831 if let Some(hidden_ty) = self.hidden_ty.take() {
832 unexpected_hidden_region_diagnostic(
835 self.opaque_type_def_id,
841 self.tcx.lifetimes.re_empty
847 "non-defining opaque type use in defining scope"
851 format!("lifetime `{}` is part of concrete type but not used in \
852 parameter list of the `impl Trait` type alias", r),
856 self.tcx().mk_region(ty::ReStatic)
861 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
863 ty::Closure(def_id, substs) => {
864 // I am a horrible monster and I pray for death. When
865 // we encounter a closure here, it is always a closure
866 // from within the function that we are currently
867 // type-checking -- one that is now being encapsulated
868 // in an opaque type. Ideally, we would
869 // go through the types/lifetimes that it references
870 // and treat them just like we would any other type,
871 // which means we would error out if we find any
872 // reference to a type/region that is not in the
875 // **However,** in the case of closures, there is a
876 // somewhat subtle (read: hacky) consideration. The
877 // problem is that our closure types currently include
878 // all the lifetime parameters declared on the
879 // enclosing function, even if they are unused by the
880 // closure itself. We can't readily filter them out,
881 // so here we replace those values with `'empty`. This
882 // can't really make a difference to the rest of the
883 // compiler; those regions are ignored for the
884 // outlives relation, and hence don't affect trait
885 // selection or auto traits, and they are erased
888 let generics = self.tcx.generics_of(def_id);
890 self.tcx.mk_substs(substs.iter().enumerate().map(|(index, &kind)| {
891 if index < generics.parent_count {
892 // Accommodate missing regions in the parent kinds...
893 self.fold_kind_mapping_missing_regions_to_empty(kind)
895 // ...but not elsewhere.
896 self.fold_kind_normally(kind)
900 self.tcx.mk_closure(def_id, substs)
903 ty::Generator(def_id, substs, movability) => {
904 let generics = self.tcx.generics_of(def_id);
906 self.tcx.mk_substs(substs.iter().enumerate().map(|(index, &kind)| {
907 if index < generics.parent_count {
908 // Accommodate missing regions in the parent kinds...
909 self.fold_kind_mapping_missing_regions_to_empty(kind)
911 // ...but not elsewhere.
912 self.fold_kind_normally(kind)
916 self.tcx.mk_generator(def_id, substs, movability)
920 // Look it up in the substitution list.
921 match self.map.get(&ty.into()).map(|k| k.unpack()) {
922 // Found it in the substitution list; replace with the parameter from the
924 Some(GenericArgKind::Type(t1)) => t1,
925 Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
930 &format!("type parameter `{}` is part of concrete type but not \
931 used in parameter list for the `impl Trait` type alias",
941 _ => ty.super_fold_with(self),
945 fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
946 trace!("checking const {:?}", ct);
947 // Find a const parameter
949 ty::ConstKind::Param(..) => {
950 // Look it up in the substitution list.
951 match self.map.get(&ct.into()).map(|k| k.unpack()) {
952 // Found it in the substitution list, replace with the parameter from the
954 Some(GenericArgKind::Const(c1)) => c1,
955 Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
960 &format!("const parameter `{}` is part of concrete type but not \
961 used in parameter list for the `impl Trait` type alias",
966 self.tcx().consts.err
976 struct Instantiator<'a, 'tcx> {
977 infcx: &'a InferCtxt<'a, 'tcx>,
978 parent_def_id: DefId,
980 param_env: ty::ParamEnv<'tcx>,
982 opaque_types: OpaqueTypeMap<'tcx>,
983 obligations: Vec<PredicateObligation<'tcx>>,
986 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
987 fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
988 debug!("instantiate_opaque_types_in_map(value={:?})", value);
989 let tcx = self.infcx.tcx;
990 value.fold_with(&mut BottomUpFolder {
993 if ty.references_error() {
994 return tcx.types.err;
995 } else if let ty::Opaque(def_id, substs) = ty.kind {
996 // Check that this is `impl Trait` type is
997 // declared by `parent_def_id` -- i.e., one whose
998 // value we are inferring. At present, this is
999 // always true during the first phase of
1000 // type-check, but not always true later on during
1001 // NLL. Once we support named opaque types more fully,
1002 // this same scenario will be able to arise during all phases.
1004 // Here is an example using type alias `impl Trait`
1005 // that indicates the distinction we are checking for:
1009 // pub type Foo = impl Iterator;
1010 // pub fn make_foo() -> Foo { .. }
1014 // fn foo() -> a::Foo { a::make_foo() }
1018 // Here, the return type of `foo` references a
1019 // `Opaque` indeed, but not one whose value is
1020 // presently being inferred. You can get into a
1021 // similar situation with closure return types
1025 // fn foo() -> impl Iterator { .. }
1027 // let x = || foo(); // returns the Opaque assoc with `foo`
1030 if let Some(opaque_hir_id) = tcx.hir().as_local_hir_id(def_id) {
1031 let parent_def_id = self.parent_def_id;
1032 let def_scope_default = || {
1033 let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
1034 parent_def_id == tcx.hir()
1035 .local_def_id(opaque_parent_hir_id)
1037 let (in_definition_scope, origin) = match tcx.hir().find(opaque_hir_id) {
1038 Some(Node::Item(item)) => match item.kind {
1039 // Anonymous `impl Trait`
1040 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1041 impl_trait_fn: Some(parent),
1044 }) => (parent == self.parent_def_id, origin),
1045 // Named `type Foo = impl Bar;`
1046 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1047 impl_trait_fn: None,
1051 may_define_opaque_type(
1059 (def_scope_default(), hir::OpaqueTyOrigin::TypeAlias)
1062 Some(Node::ImplItem(item)) => match item.kind {
1063 hir::ImplItemKind::OpaqueTy(_) => (
1064 may_define_opaque_type(
1069 hir::OpaqueTyOrigin::TypeAlias,
1072 (def_scope_default(), hir::OpaqueTyOrigin::TypeAlias)
1076 "expected (impl) item, found {}",
1077 tcx.hir().node_to_string(opaque_hir_id),
1080 if in_definition_scope {
1081 return self.fold_opaque_ty(ty, def_id, substs, origin);
1085 "instantiate_opaque_types_in_map: \
1086 encountered opaque outside its definition scope \
1104 substs: SubstsRef<'tcx>,
1105 origin: hir::OpaqueTyOrigin,
1107 let infcx = self.infcx;
1108 let tcx = infcx.tcx;
1110 debug!("instantiate_opaque_types: Opaque(def_id={:?}, substs={:?})", def_id, substs);
1112 // Use the same type variable if the exact same opaque type appears more
1113 // than once in the return type (e.g., if it's passed to a type alias).
1114 if let Some(opaque_defn) = self.opaque_types.get(&def_id) {
1115 debug!("instantiate_opaque_types: returning concrete ty {:?}", opaque_defn.concrete_ty);
1116 return opaque_defn.concrete_ty;
1118 let span = tcx.def_span(def_id);
1119 debug!("fold_opaque_ty {:?} {:?}", self.value_span, span);
1121 .next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span });
1123 let predicates_of = tcx.predicates_of(def_id);
1124 debug!("instantiate_opaque_types: predicates={:#?}", predicates_of,);
1125 let bounds = predicates_of.instantiate(tcx, substs);
1127 let param_env = tcx.param_env(def_id);
1128 let InferOk { value: bounds, obligations } =
1129 infcx.partially_normalize_associated_types_in(span, self.body_id, param_env, &bounds);
1130 self.obligations.extend(obligations);
1132 debug!("instantiate_opaque_types: bounds={:?}", bounds);
1134 let required_region_bounds = tcx.required_region_bounds(ty, bounds.predicates.clone());
1135 debug!("instantiate_opaque_types: required_region_bounds={:?}", required_region_bounds);
1137 // Make sure that we are in fact defining the *entire* type
1138 // (e.g., `type Foo<T: Bound> = impl Bar;` needs to be
1139 // defined by a function like `fn foo<T: Bound>() -> Foo<T>`).
1140 debug!("instantiate_opaque_types: param_env={:#?}", self.param_env,);
1141 debug!("instantiate_opaque_types: generics={:#?}", tcx.generics_of(def_id),);
1143 // Ideally, we'd get the span where *this specific `ty` came
1144 // from*, but right now we just use the span from the overall
1145 // value being folded. In simple cases like `-> impl Foo`,
1146 // these are the same span, but not in cases like `-> (impl
1148 let definition_span = self.value_span;
1150 self.opaque_types.insert(
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(
1212 opaque_hir_id: hir::HirId,
1214 let mut hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
1216 // Named opaque types can be defined by any siblings or children of siblings.
1217 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
1218 // We walk up the node tree until we hit the root or the scope of the opaque type.
1219 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
1220 hir_id = tcx.hir().get_parent_item(hir_id);
1222 // Syntactically, we are allowed to define the concrete type if:
1223 let res = hir_id == scope;
1225 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
1226 tcx.hir().get(hir_id),
1227 tcx.hir().get(opaque_hir_id),