1 use crate::infer::InferCtxtExt as _;
2 use crate::traits::{self, ObligationCause, PredicateObligation};
3 use rustc_data_structures::fx::FxHashMap;
4 use rustc_data_structures::sync::Lrc;
6 use rustc_hir::def_id::{DefId, LocalDefId};
7 use rustc_infer::infer::error_reporting::unexpected_hidden_region_diagnostic;
8 use rustc_infer::infer::free_regions::FreeRegionRelations;
9 use rustc_infer::infer::opaque_types::OpaqueTypeDecl;
10 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
11 use rustc_infer::infer::{self, InferCtxt, InferOk};
12 use rustc_middle::ty::fold::{BottomUpFolder, TypeFoldable, TypeFolder, TypeVisitor};
13 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, InternalSubsts, Subst};
14 use rustc_middle::ty::{self, OpaqueTypeKey, Ty, TyCtxt};
17 use std::ops::ControlFlow;
19 /// Whether member constraints should be generated for all opaque types
21 pub enum GenerateMemberConstraints {
22 /// The default, used by typeck
24 /// The borrow checker needs member constraints in any case where we don't
25 /// have a `'static` bound. This is because the borrow checker has more
26 /// flexibility in the values of regions. For example, given `f<'a, 'b>`
27 /// the borrow checker can have an inference variable outlive `'a` and `'b`,
28 /// but not be equal to `'static`.
32 pub trait InferCtxtExt<'tcx> {
33 fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
36 param_env: ty::ParamEnv<'tcx>,
39 ) -> InferOk<'tcx, T>;
41 fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(&self, free_region_relations: &FRR);
43 fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
45 opaque_type_key: OpaqueTypeKey<'tcx>,
46 opaque_defn: &OpaqueTypeDecl<'tcx>,
47 mode: GenerateMemberConstraints,
48 free_region_relations: &FRR,
52 fn generate_member_constraint(
54 concrete_ty: Ty<'tcx>,
55 opaque_defn: &OpaqueTypeDecl<'tcx>,
56 opaque_type_key: OpaqueTypeKey<'tcx>,
57 first_own_region_index: usize,
60 fn infer_opaque_definition_from_instantiation(
62 opaque_type_key: OpaqueTypeKey<'tcx>,
63 instantiated_ty: Ty<'tcx>,
68 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
69 /// Replaces all opaque types in `value` with fresh inference variables
70 /// and creates appropriate obligations. For example, given the input:
72 /// impl Iterator<Item = impl Debug>
74 /// this method would create two type variables, `?0` and `?1`. It would
75 /// return the type `?0` but also the obligations:
77 /// ?0: Iterator<Item = ?1>
80 /// Moreover, it returns a `OpaqueTypeMap` that would map `?0` to
81 /// info about the `impl Iterator<..>` type and `?1` to info about
82 /// the `impl Debug` type.
86 /// - `parent_def_id` -- the `DefId` of the function in which the opaque type
88 /// - `body_id` -- the body-id with which the resulting obligations should
90 /// - `param_env` -- the in-scope parameter environment to be used for
92 /// - `value` -- the value within which we are instantiating opaque types
93 /// - `value_span` -- the span where the value came from, used in error reporting
94 fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
97 param_env: ty::ParamEnv<'tcx>,
100 ) -> InferOk<'tcx, T> {
102 "instantiate_opaque_types(value={:?}, body_id={:?}, \
103 param_env={:?}, value_span={:?})",
104 value, body_id, param_env, value_span,
106 let mut instantiator =
107 Instantiator { infcx: self, body_id, param_env, value_span, obligations: vec![] };
108 let value = instantiator.instantiate_opaque_types_in_map(value);
109 InferOk { value, obligations: instantiator.obligations }
112 /// Given the map `opaque_types` containing the opaque
113 /// `impl Trait` types whose underlying, hidden types are being
114 /// inferred, this method adds constraints to the regions
115 /// appearing in those underlying hidden types to ensure that they
116 /// at least do not refer to random scopes within the current
117 /// function. These constraints are not (quite) sufficient to
118 /// guarantee that the regions are actually legal values; that
119 /// final condition is imposed after region inference is done.
123 /// Let's work through an example to explain how it works. Assume
124 /// the current function is as follows:
127 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
130 /// Here, we have two `impl Trait` types whose values are being
131 /// inferred (the `impl Bar<'a>` and the `impl
132 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
133 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
134 /// the return type of `foo`, we *reference* those definitions:
137 /// type Foo1<'x> = impl Bar<'x>;
138 /// type Foo2<'x> = impl Bar<'x>;
139 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
146 /// As indicating in the comments above, each of those references
147 /// is (in the compiler) basically a substitution (`substs`)
148 /// applied to the type of a suitable `def_id` (which identifies
149 /// `Foo1` or `Foo2`).
151 /// Now, at this point in compilation, what we have done is to
152 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
153 /// fresh inference variables C1 and C2. We wish to use the values
154 /// of these variables to infer the underlying types of `Foo1` and
155 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
156 /// constraints like:
159 /// for<'a> (Foo1<'a> = C1)
160 /// for<'b> (Foo1<'b> = C2)
163 /// For these equation to be satisfiable, the types `C1` and `C2`
164 /// can only refer to a limited set of regions. For example, `C1`
165 /// can only refer to `'static` and `'a`, and `C2` can only refer
166 /// to `'static` and `'b`. The job of this function is to impose that
169 /// Up to this point, C1 and C2 are basically just random type
170 /// inference variables, and hence they may contain arbitrary
171 /// regions. In fact, it is fairly likely that they do! Consider
172 /// this possible definition of `foo`:
175 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
180 /// Here, the values for the concrete types of the two impl
181 /// traits will include inference variables:
188 /// Ordinarily, the subtyping rules would ensure that these are
189 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
190 /// type per se, we don't get such constraints by default. This
191 /// is where this function comes into play. It adds extra
192 /// constraints to ensure that all the regions which appear in the
193 /// inferred type are regions that could validly appear.
195 /// This is actually a bit of a tricky constraint in general. We
196 /// want to say that each variable (e.g., `'0`) can only take on
197 /// values that were supplied as arguments to the opaque type
198 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
199 /// scope. We don't have a constraint quite of this kind in the current
204 /// We generally prefer to make `<=` constraints, since they
205 /// integrate best into the region solver. To do that, we find the
206 /// "minimum" of all the arguments that appear in the substs: that
207 /// is, some region which is less than all the others. In the case
208 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
209 /// all). Then we apply that as a least bound to the variables
210 /// (e.g., `'a <= '0`).
212 /// In some cases, there is no minimum. Consider this example:
215 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
218 /// Here we would report a more complex "in constraint", like `'r
219 /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
220 /// the hidden type).
222 /// # Constrain regions, not the hidden concrete type
224 /// Note that generating constraints on each region `Rc` is *not*
225 /// the same as generating an outlives constraint on `Tc` iself.
226 /// For example, if we had a function like this:
229 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
233 /// // Equivalent to:
234 /// type FooReturn<'a, T> = impl Foo<'a>;
235 /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
238 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
239 /// is an inference variable). If we generated a constraint that
240 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
241 /// but this is not necessary, because the opaque type we
242 /// create will be allowed to reference `T`. So we only generate a
243 /// constraint that `'0: 'a`.
245 /// # The `free_region_relations` parameter
247 /// The `free_region_relations` argument is used to find the
248 /// "minimum" of the regions supplied to a given opaque type.
249 /// It must be a relation that can answer whether `'a <= 'b`,
250 /// where `'a` and `'b` are regions that appear in the "substs"
251 /// for the opaque type references (the `<'a>` in `Foo1<'a>`).
253 /// Note that we do not impose the constraints based on the
254 /// generic regions from the `Foo1` definition (e.g., `'x`). This
255 /// is because the constraints we are imposing here is basically
256 /// the concern of the one generating the constraining type C1,
257 /// which is the current function. It also means that we can
258 /// take "implied bounds" into account in some cases:
261 /// trait SomeTrait<'a, 'b> { }
262 /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
265 /// Here, the fact that `'b: 'a` is known only because of the
266 /// implied bounds from the `&'a &'b u32` parameter, and is not
267 /// "inherent" to the opaque type definition.
271 /// - `opaque_types` -- the map produced by `instantiate_opaque_types`
272 /// - `free_region_relations` -- something that can be used to relate
273 /// the free regions (`'a`) that appear in the impl trait.
274 fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(&self, free_region_relations: &FRR) {
275 let opaque_types = self.inner.borrow().opaque_types.clone();
276 for (opaque_type_key, opaque_defn) in opaque_types {
277 self.constrain_opaque_type(
280 GenerateMemberConstraints::WhenRequired,
281 free_region_relations,
286 /// See `constrain_opaque_types` for documentation.
287 #[instrument(level = "debug", skip(self, free_region_relations))]
288 fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
290 opaque_type_key: OpaqueTypeKey<'tcx>,
291 opaque_defn: &OpaqueTypeDecl<'tcx>,
292 mode: GenerateMemberConstraints,
293 free_region_relations: &FRR,
295 let def_id = opaque_type_key.def_id;
299 let concrete_ty = self.resolve_vars_if_possible(opaque_defn.concrete_ty);
301 debug!(?concrete_ty);
303 let first_own_region = match opaque_defn.origin {
304 hir::OpaqueTyOrigin::FnReturn | hir::OpaqueTyOrigin::AsyncFn => {
307 // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
311 // type foo::<'p0..'pn>::Foo<'q0..'qm>
312 // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
314 // For these types we only iterate over `'l0..lm` below.
315 tcx.generics_of(def_id).parent_count
317 // These opaque type inherit all lifetime parameters from their
318 // parent, so we have to check them all.
319 hir::OpaqueTyOrigin::TyAlias => 0,
322 let span = tcx.def_span(def_id);
324 // Check if the `impl Trait` bounds include region bounds.
325 // For example, this would be true for:
327 // fn foo<'a, 'b, 'c>() -> impl Trait<'c> + 'a + 'b
331 // fn foo<'c>() -> impl Trait<'c>
333 // unless `Trait` was declared like:
335 // trait Trait<'c>: 'c
337 // in which case it would be true.
339 // This is used during regionck to decide whether we need to
340 // impose any additional constraints to ensure that region
341 // variables in `concrete_ty` wind up being constrained to
342 // something from `substs` (or, at minimum, things that outlive
343 // the fn body). (Ultimately, writeback is responsible for this
345 let bounds = tcx.explicit_item_bounds(def_id);
346 debug!("{:#?}", bounds);
348 bounds.iter().map(|(bound, _)| bound.subst(tcx, opaque_type_key.substs)).collect();
349 debug!("{:#?}", bounds);
350 let opaque_type = tcx.mk_opaque(def_id, opaque_type_key.substs);
352 let required_region_bounds = required_region_bounds(tcx, opaque_type, bounds.into_iter());
353 if !required_region_bounds.is_empty() {
354 for required_region in required_region_bounds {
355 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
356 op: |r| self.sub_regions(infer::CallReturn(span), required_region, r),
359 if let GenerateMemberConstraints::IfNoStaticBound = mode {
360 self.generate_member_constraint(
370 // There were no `required_region_bounds`,
371 // so we have to search for a `least_region`.
372 // Go through all the regions used as arguments to the
373 // opaque type. These are the parameters to the opaque
374 // type; so in our example above, `substs` would contain
375 // `['a]` for the first impl trait and `'b` for the
377 let mut least_region = None;
379 for subst_arg in &opaque_type_key.substs[first_own_region..] {
380 let subst_region = match subst_arg.unpack() {
381 GenericArgKind::Lifetime(r) => r,
382 GenericArgKind::Type(_) | GenericArgKind::Const(_) => continue,
385 // Compute the least upper bound of it with the other regions.
386 debug!(?least_region);
387 debug!(?subst_region);
389 None => least_region = Some(subst_region),
391 if free_region_relations.sub_free_regions(self.tcx, lr, subst_region) {
392 // keep the current least region
393 } else if free_region_relations.sub_free_regions(self.tcx, subst_region, lr) {
394 // switch to `subst_region`
395 least_region = Some(subst_region);
397 // There are two regions (`lr` and
398 // `subst_region`) which are not relatable. We
399 // can't find a best choice. Therefore,
400 // instead of creating a single bound like
401 // `'r: 'a` (which is our preferred choice),
402 // we will create a "in bound" like `'r in
403 // ['a, 'b, 'c]`, where `'a..'c` are the
404 // regions that appear in the impl trait.
406 return self.generate_member_constraint(
417 let least_region = least_region.unwrap_or(tcx.lifetimes.re_static);
418 debug!(?least_region);
420 if let GenerateMemberConstraints::IfNoStaticBound = mode {
421 if least_region != tcx.lifetimes.re_static {
422 self.generate_member_constraint(
430 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
431 op: |r| self.sub_regions(infer::CallReturn(span), least_region, r),
435 /// As a fallback, we sometimes generate an "in constraint". For
436 /// a case like `impl Foo<'a, 'b>`, where `'a` and `'b` cannot be
437 /// related, we would generate a constraint `'r in ['a, 'b,
438 /// 'static]` for each region `'r` that appears in the hidden type
439 /// (i.e., it must be equal to `'a`, `'b`, or `'static`).
441 /// `conflict1` and `conflict2` are the two region bounds that we
442 /// detected which were unrelated. They are used for diagnostics.
443 fn generate_member_constraint(
445 concrete_ty: Ty<'tcx>,
446 opaque_defn: &OpaqueTypeDecl<'tcx>,
447 opaque_type_key: OpaqueTypeKey<'tcx>,
448 first_own_region: usize,
450 // Create the set of choice regions: each region in the hidden
451 // type can be equal to any of the region parameters of the
452 // opaque type definition.
453 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
454 opaque_type_key.substs[first_own_region..]
456 .filter_map(|arg| match arg.unpack() {
457 GenericArgKind::Lifetime(r) => Some(r),
458 GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
460 .chain(std::iter::once(self.tcx.lifetimes.re_static))
464 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
466 self.member_constraint(
467 opaque_type_key.def_id,
468 opaque_defn.definition_span,
477 /// Given the fully resolved, instantiated type for an opaque
478 /// type, i.e., the value of an inference variable like C1 or C2
479 /// (*), computes the "definition type" for an opaque type
480 /// definition -- that is, the inferred value of `Foo1<'x>` or
481 /// `Foo2<'x>` that we would conceptually use in its definition:
483 /// type Foo1<'x> = impl Bar<'x> = AAA; <-- this type AAA
484 /// type Foo2<'x> = impl Bar<'x> = BBB; <-- or this type BBB
485 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
487 /// Note that these values are defined in terms of a distinct set of
488 /// generic parameters (`'x` instead of `'a`) from C1 or C2. The main
489 /// purpose of this function is to do that translation.
491 /// (*) C1 and C2 were introduced in the comments on
492 /// `constrain_opaque_types`. Read that comment for more context.
496 /// - `def_id`, the `impl Trait` type
497 /// - `substs`, the substs used to instantiate this opaque type
498 /// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of
499 /// `opaque_defn.concrete_ty`
500 #[instrument(skip(self))]
501 fn infer_opaque_definition_from_instantiation(
503 opaque_type_key: OpaqueTypeKey<'tcx>,
504 instantiated_ty: Ty<'tcx>,
507 let OpaqueTypeKey { def_id, substs } = opaque_type_key;
509 // Use substs to build up a reverse map from regions to their
510 // identity mappings. This is necessary because of `impl
511 // Trait` lifetimes are computed by replacing existing
512 // lifetimes with 'static and remapping only those used in the
513 // `impl Trait` return type, resulting in the parameters
515 let id_substs = InternalSubsts::identity_for_item(self.tcx, def_id);
517 let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> =
518 substs.iter().enumerate().map(|(index, subst)| (subst, id_substs[index])).collect();
520 // Convert the type from the function into a type valid outside
521 // the function, by replacing invalid regions with 'static,
522 // after producing an error for each of them.
523 let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new(
525 self.is_tainted_by_errors(),
531 debug!(?definition_ty);
537 // Visitor that requires that (almost) all regions in the type visited outlive
538 // `least_region`. We cannot use `push_outlives_components` because regions in
539 // closure signatures are not included in their outlives components. We need to
540 // ensure all regions outlive the given bound so that we don't end up with,
541 // say, `ReVar` appearing in a return type and causing ICEs when other
542 // functions end up with region constraints involving regions from other
545 // We also cannot use `for_each_free_region` because for closures it includes
546 // the regions parameters from the enclosing item.
548 // We ignore any type parameters because impl trait values are assumed to
549 // capture all the in-scope type parameters.
550 struct ConstrainOpaqueTypeRegionVisitor<OP> {
554 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
556 OP: FnMut(ty::Region<'tcx>),
558 fn visit_binder<T: TypeFoldable<'tcx>>(
560 t: &ty::Binder<'tcx, T>,
561 ) -> ControlFlow<Self::BreakTy> {
562 t.as_ref().skip_binder().visit_with(self);
563 ControlFlow::CONTINUE
566 fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
568 // ignore bound regions, keep visiting
569 ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
572 ControlFlow::CONTINUE
577 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
578 // We're only interested in types involving regions
579 if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
580 return ControlFlow::CONTINUE;
584 ty::Closure(_, ref substs) => {
585 // Skip lifetime parameters of the enclosing item(s)
587 substs.as_closure().tupled_upvars_ty().visit_with(self);
588 substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
591 ty::Generator(_, ref substs, _) => {
592 // Skip lifetime parameters of the enclosing item(s)
593 // Also skip the witness type, because that has no free regions.
595 substs.as_generator().tupled_upvars_ty().visit_with(self);
596 substs.as_generator().return_ty().visit_with(self);
597 substs.as_generator().yield_ty().visit_with(self);
598 substs.as_generator().resume_ty().visit_with(self);
601 ty.super_visit_with(self);
605 ControlFlow::CONTINUE
609 struct ReverseMapper<'tcx> {
612 /// If errors have already been reported in this fn, we suppress
613 /// our own errors because they are sometimes derivative.
614 tainted_by_errors: bool,
616 opaque_type_def_id: DefId,
617 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
618 map_missing_regions_to_empty: bool,
620 /// initially `Some`, set to `None` once error has been reported
621 hidden_ty: Option<Ty<'tcx>>,
623 /// Span of function being checked.
627 impl ReverseMapper<'tcx> {
630 tainted_by_errors: bool,
631 opaque_type_def_id: DefId,
632 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
641 map_missing_regions_to_empty: false,
642 hidden_ty: Some(hidden_ty),
647 fn fold_kind_mapping_missing_regions_to_empty(
649 kind: GenericArg<'tcx>,
650 ) -> GenericArg<'tcx> {
651 assert!(!self.map_missing_regions_to_empty);
652 self.map_missing_regions_to_empty = true;
653 let kind = kind.fold_with(self);
654 self.map_missing_regions_to_empty = false;
658 fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
659 assert!(!self.map_missing_regions_to_empty);
664 impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
665 fn tcx(&self) -> TyCtxt<'tcx> {
669 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
671 // Ignore bound regions and `'static` regions that appear in the
672 // type, we only need to remap regions that reference lifetimes
673 // from the function declaraion.
674 // This would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
675 ty::ReLateBound(..) | ty::ReStatic => return r,
677 // If regions have been erased (by writeback), don't try to unerase
679 ty::ReErased => return r,
681 // The regions that we expect from borrow checking.
682 ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReEmpty(ty::UniverseIndex::ROOT) => {}
684 ty::ReEmpty(_) | ty::RePlaceholder(_) | ty::ReVar(_) => {
685 // All of the regions in the type should either have been
686 // erased by writeback, or mapped back to named regions by
688 bug!("unexpected region kind in opaque type: {:?}", r);
692 let generics = self.tcx().generics_of(self.opaque_type_def_id);
693 match self.map.get(&r.into()).map(|k| k.unpack()) {
694 Some(GenericArgKind::Lifetime(r1)) => r1,
695 Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
696 None if self.map_missing_regions_to_empty || self.tainted_by_errors => {
697 self.tcx.lifetimes.re_root_empty
699 None if generics.parent.is_some() => {
700 if let Some(hidden_ty) = self.hidden_ty.take() {
701 unexpected_hidden_region_diagnostic(
703 self.tcx.def_span(self.opaque_type_def_id),
709 self.tcx.lifetimes.re_root_empty
714 .struct_span_err(self.span, "non-defining opaque type use in defining scope")
718 "lifetime `{}` is part of concrete type but not used in \
719 parameter list of the `impl Trait` type alias",
725 self.tcx().lifetimes.re_static
730 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
732 ty::Closure(def_id, substs) => {
733 // I am a horrible monster and I pray for death. When
734 // we encounter a closure here, it is always a closure
735 // from within the function that we are currently
736 // type-checking -- one that is now being encapsulated
737 // in an opaque type. Ideally, we would
738 // go through the types/lifetimes that it references
739 // and treat them just like we would any other type,
740 // which means we would error out if we find any
741 // reference to a type/region that is not in the
744 // **However,** in the case of closures, there is a
745 // somewhat subtle (read: hacky) consideration. The
746 // problem is that our closure types currently include
747 // all the lifetime parameters declared on the
748 // enclosing function, even if they are unused by the
749 // closure itself. We can't readily filter them out,
750 // so here we replace those values with `'empty`. This
751 // can't really make a difference to the rest of the
752 // compiler; those regions are ignored for the
753 // outlives relation, and hence don't affect trait
754 // selection or auto traits, and they are erased
757 let generics = self.tcx.generics_of(def_id);
758 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
759 if index < generics.parent_count {
760 // Accommodate missing regions in the parent kinds...
761 self.fold_kind_mapping_missing_regions_to_empty(kind)
763 // ...but not elsewhere.
764 self.fold_kind_normally(kind)
768 self.tcx.mk_closure(def_id, substs)
771 ty::Generator(def_id, substs, movability) => {
772 let generics = self.tcx.generics_of(def_id);
773 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
774 if index < generics.parent_count {
775 // Accommodate missing regions in the parent kinds...
776 self.fold_kind_mapping_missing_regions_to_empty(kind)
778 // ...but not elsewhere.
779 self.fold_kind_normally(kind)
783 self.tcx.mk_generator(def_id, substs, movability)
786 ty::Param(param) => {
787 // Look it up in the substitution list.
788 match self.map.get(&ty.into()).map(|k| k.unpack()) {
789 // Found it in the substitution list; replace with the parameter from the
791 Some(GenericArgKind::Type(t1)) => t1,
792 Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
794 debug!(?param, ?self.map);
800 "type parameter `{}` is part of concrete type but not \
801 used in parameter list for the `impl Trait` type alias",
807 self.tcx().ty_error()
812 _ => ty.super_fold_with(self),
816 fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
817 trace!("checking const {:?}", ct);
818 // Find a const parameter
820 ty::ConstKind::Param(..) => {
821 // Look it up in the substitution list.
822 match self.map.get(&ct.into()).map(|k| k.unpack()) {
823 // Found it in the substitution list, replace with the parameter from the
825 Some(GenericArgKind::Const(c1)) => c1,
826 Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
833 "const parameter `{}` is part of concrete type but not \
834 used in parameter list for the `impl Trait` type alias",
840 self.tcx().const_error(ct.ty)
850 struct Instantiator<'a, 'tcx> {
851 infcx: &'a InferCtxt<'a, 'tcx>,
853 param_env: ty::ParamEnv<'tcx>,
855 obligations: Vec<PredicateObligation<'tcx>>,
858 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
859 #[instrument(skip(self))]
860 fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
861 let tcx = self.infcx.tcx;
862 value.fold_with(&mut BottomUpFolder {
865 if ty.references_error() {
866 return tcx.ty_error();
867 } else if let ty::Opaque(def_id, substs) = ty.kind() {
868 // Check that this is `impl Trait` type is
869 // declared by `parent_def_id` -- i.e., one whose
870 // value we are inferring. At present, this is
871 // always true during the first phase of
872 // type-check, but not always true later on during
873 // NLL. Once we support named opaque types more fully,
874 // this same scenario will be able to arise during all phases.
876 // Here is an example using type alias `impl Trait`
877 // that indicates the distinction we are checking for:
881 // pub type Foo = impl Iterator;
882 // pub fn make_foo() -> Foo { .. }
886 // fn foo() -> a::Foo { a::make_foo() }
890 // Here, the return type of `foo` references a
891 // `Opaque` indeed, but not one whose value is
892 // presently being inferred. You can get into a
893 // similar situation with closure return types
897 // fn foo() -> impl Iterator { .. }
899 // let x = || foo(); // returns the Opaque assoc with `foo`
902 if let Some(def_id) = def_id.as_local() {
903 let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
904 let parent_def_id = self.infcx.defining_use_anchor;
905 let def_scope_default = || {
906 let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
907 parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
909 let (in_definition_scope, origin) =
910 match tcx.hir().expect_item(opaque_hir_id).kind {
911 // Anonymous `impl Trait`
912 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
913 impl_trait_fn: Some(parent),
916 }) => (parent == parent_def_id.to_def_id(), origin),
917 // Named `type Foo = impl Bar;`
918 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
923 may_define_opaque_type(tcx, parent_def_id, opaque_hir_id),
926 _ => (def_scope_default(), hir::OpaqueTyOrigin::TyAlias),
928 if in_definition_scope {
929 let opaque_type_key =
930 OpaqueTypeKey { def_id: def_id.to_def_id(), substs };
931 return self.fold_opaque_ty(ty, opaque_type_key, origin);
935 "instantiate_opaque_types_in_map: \
936 encountered opaque outside its definition scope \
953 opaque_type_key: OpaqueTypeKey<'tcx>,
954 origin: hir::OpaqueTyOrigin,
956 let infcx = self.infcx;
958 let OpaqueTypeKey { def_id, substs } = opaque_type_key;
960 debug!("instantiate_opaque_types: Opaque(def_id={:?}, substs={:?})", def_id, substs);
962 // Use the same type variable if the exact same opaque type appears more
963 // than once in the return type (e.g., if it's passed to a type alias).
964 if let Some(opaque_defn) = infcx.inner.borrow().opaque_types.get(&opaque_type_key) {
965 debug!("instantiate_opaque_types: returning concrete ty {:?}", opaque_defn.concrete_ty);
966 return opaque_defn.concrete_ty;
968 let span = tcx.def_span(def_id);
969 debug!("fold_opaque_ty {:?} {:?}", self.value_span, span);
971 .next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span });
973 // Make sure that we are in fact defining the *entire* type
974 // (e.g., `type Foo<T: Bound> = impl Bar;` needs to be
975 // defined by a function like `fn foo<T: Bound>() -> Foo<T>`).
976 debug!("instantiate_opaque_types: param_env={:#?}", self.param_env,);
977 debug!("instantiate_opaque_types: generics={:#?}", tcx.generics_of(def_id),);
979 // Ideally, we'd get the span where *this specific `ty` came
980 // from*, but right now we just use the span from the overall
981 // value being folded. In simple cases like `-> impl Foo`,
982 // these are the same span, but not in cases like `-> (impl
984 let definition_span = self.value_span;
987 let mut infcx = self.infcx.inner.borrow_mut();
988 infcx.opaque_types.insert(
989 OpaqueTypeKey { def_id, substs },
990 OpaqueTypeDecl { opaque_type: ty, definition_span, concrete_ty: ty_var, origin },
992 infcx.opaque_types_vars.insert(ty_var, ty);
995 debug!("instantiate_opaque_types: ty_var={:?}", ty_var);
996 self.compute_opaque_type_obligations(opaque_type_key, span);
1001 fn compute_opaque_type_obligations(
1003 opaque_type_key: OpaqueTypeKey<'tcx>,
1006 let infcx = self.infcx;
1007 let tcx = infcx.tcx;
1008 let OpaqueTypeKey { def_id, substs } = opaque_type_key;
1010 let item_bounds = tcx.explicit_item_bounds(def_id);
1011 debug!("instantiate_opaque_types: bounds={:#?}", item_bounds);
1012 let bounds: Vec<_> =
1013 item_bounds.iter().map(|(bound, _)| bound.subst(tcx, substs)).collect();
1015 let param_env = tcx.param_env(def_id);
1016 let InferOk { value: bounds, obligations } = infcx.partially_normalize_associated_types_in(
1017 ObligationCause::misc(span, self.body_id),
1021 self.obligations.extend(obligations);
1023 debug!("instantiate_opaque_types: bounds={:?}", bounds);
1025 for predicate in &bounds {
1026 if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
1027 if projection.ty.references_error() {
1028 // No point on adding these obligations since there's a type error involved.
1034 self.obligations.reserve(bounds.len());
1035 for predicate in bounds {
1036 // Change the predicate to refer to the type variable,
1037 // which will be the concrete type instead of the opaque type.
1038 // This also instantiates nested instances of `impl Trait`.
1039 let predicate = self.instantiate_opaque_types_in_map(predicate);
1041 let cause = traits::ObligationCause::new(span, self.body_id, traits::OpaqueType);
1043 // Require that the predicate holds for the concrete type.
1044 debug!("instantiate_opaque_types: predicate={:?}", predicate);
1045 self.obligations.push(traits::Obligation::new(cause, self.param_env, predicate));
1050 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
1056 /// pub trait Bar { .. }
1058 /// pub type Baz = impl Bar;
1060 /// fn f1() -> Baz { .. }
1063 /// fn f2() -> bar::Baz { .. }
1067 /// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
1068 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
1069 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
1070 pub fn may_define_opaque_type(
1073 opaque_hir_id: hir::HirId,
1075 let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
1077 // Named opaque types can be defined by any siblings or children of siblings.
1078 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
1079 // We walk up the node tree until we hit the root or the scope of the opaque type.
1080 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
1081 hir_id = tcx.hir().get_parent_item(hir_id);
1083 // Syntactically, we are allowed to define the concrete type if:
1084 let res = hir_id == scope;
1086 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
1087 tcx.hir().find(hir_id),
1088 tcx.hir().get(opaque_hir_id),
1094 /// Given a set of predicates that apply to an object type, returns
1095 /// the region bounds that the (erased) `Self` type must
1096 /// outlive. Precisely *because* the `Self` type is erased, the
1097 /// parameter `erased_self_ty` must be supplied to indicate what type
1098 /// has been used to represent `Self` in the predicates
1099 /// themselves. This should really be a unique type; `FreshTy(0)` is a
1102 /// N.B., in some cases, particularly around higher-ranked bounds,
1103 /// this function returns a kind of conservative approximation.
1104 /// That is, all regions returned by this function are definitely
1105 /// required, but there may be other region bounds that are not
1106 /// returned, as well as requirements like `for<'a> T: 'a`.
1108 /// Requires that trait definitions have been processed so that we can
1109 /// elaborate predicates and walk supertraits.
1110 crate fn required_region_bounds(
1112 erased_self_ty: Ty<'tcx>,
1113 predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
1114 ) -> Vec<ty::Region<'tcx>> {
1115 debug!("required_region_bounds(erased_self_ty={:?})", erased_self_ty);
1117 assert!(!erased_self_ty.has_escaping_bound_vars());
1119 traits::elaborate_predicates(tcx, predicates)
1120 .filter_map(|obligation| {
1121 debug!("required_region_bounds(obligation={:?})", obligation);
1122 match obligation.predicate.kind().skip_binder() {
1123 ty::PredicateKind::Projection(..)
1124 | ty::PredicateKind::Trait(..)
1125 | ty::PredicateKind::Subtype(..)
1126 | ty::PredicateKind::WellFormed(..)
1127 | ty::PredicateKind::ObjectSafe(..)
1128 | ty::PredicateKind::ClosureKind(..)
1129 | ty::PredicateKind::RegionOutlives(..)
1130 | ty::PredicateKind::ConstEvaluatable(..)
1131 | ty::PredicateKind::ConstEquate(..)
1132 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
1133 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
1134 // Search for a bound of the form `erased_self_ty
1135 // : 'a`, but be wary of something like `for<'a>
1136 // erased_self_ty : 'a` (we interpret a
1137 // higher-ranked bound like that as 'static,
1138 // though at present the code in `fulfill.rs`
1139 // considers such bounds to be unsatisfiable, so
1140 // it's kind of a moot point since you could never
1141 // construct such an object, but this seems
1142 // correct even if that code changes).
1143 if t == &erased_self_ty && !r.has_escaping_bound_vars() {