1 use crate::infer::InferCtxtExt as _;
2 use crate::traits::{self, PredicateObligation};
3 use rustc_data_structures::fx::FxHashMap;
4 use rustc_data_structures::sync::Lrc;
6 use rustc_hir::def_id::{DefId, DefIdMap, LocalDefId};
8 use rustc_infer::infer::error_reporting::unexpected_hidden_region_diagnostic;
9 use rustc_infer::infer::free_regions::FreeRegionRelations;
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, SubstsRef};
14 use rustc_middle::ty::{self, Ty, TyCtxt};
17 use std::ops::ControlFlow;
19 pub type OpaqueTypeMap<'tcx> = DefIdMap<OpaqueTypeDecl<'tcx>>;
21 /// Information about the opaque types whose values we
22 /// are inferring in this function (these are the `impl Trait` that
23 /// appear in the return type).
24 #[derive(Copy, Clone, Debug)]
25 pub struct OpaqueTypeDecl<'tcx> {
26 /// The opaque type (`ty::Opaque`) for this declaration.
27 pub opaque_type: Ty<'tcx>,
29 /// The substitutions that we apply to the opaque type that this
30 /// `impl Trait` desugars to. e.g., if:
32 /// fn foo<'a, 'b, T>() -> impl Trait<'a>
34 /// winds up desugared to:
36 /// type Foo<'x, X> = impl Trait<'x>
37 /// fn foo<'a, 'b, T>() -> Foo<'a, T>
39 /// then `substs` would be `['a, T]`.
40 pub substs: SubstsRef<'tcx>,
42 /// The span of this particular definition of the opaque type. So
45 /// ```ignore (incomplete snippet)
46 /// type Foo = impl Baz;
48 /// // ^^^ This is the span we are looking for!
51 /// In cases where the fn returns `(impl Trait, impl Trait)` or
52 /// other such combinations, the result is currently
53 /// over-approximated, but better than nothing.
54 pub definition_span: Span,
56 /// The type variable that represents the value of the opaque type
57 /// that we require. In other words, after we compile this function,
58 /// we will be created a constraint like:
62 /// where `?C` is the value of this type variable. =) It may
63 /// naturally refer to the type and lifetime parameters in scope
64 /// in this function, though ultimately it should only reference
65 /// those that are arguments to `Foo` in the constraint above. (In
66 /// other words, `?C` should not include `'b`, even though it's a
67 /// lifetime parameter on `foo`.)
68 pub concrete_ty: Ty<'tcx>,
70 /// Returns `true` if the `impl Trait` bounds include region bounds.
71 /// For example, this would be true for:
73 /// fn foo<'a, 'b, 'c>() -> impl Trait<'c> + 'a + 'b
77 /// fn foo<'c>() -> impl Trait<'c>
79 /// unless `Trait` was declared like:
81 /// trait Trait<'c>: 'c
83 /// in which case it would be true.
85 /// This is used during regionck to decide whether we need to
86 /// impose any additional constraints to ensure that region
87 /// variables in `concrete_ty` wind up being constrained to
88 /// something from `substs` (or, at minimum, things that outlive
89 /// the fn body). (Ultimately, writeback is responsible for this
91 pub has_required_region_bounds: bool,
93 /// The origin of the opaque type.
94 pub origin: hir::OpaqueTyOrigin,
97 /// Whether member constraints should be generated for all opaque types
98 pub enum GenerateMemberConstraints {
99 /// The default, used by typeck
101 /// The borrow checker needs member constraints in any case where we don't
102 /// have a `'static` bound. This is because the borrow checker has more
103 /// flexibility in the values of regions. For example, given `f<'a, 'b>`
104 /// the borrow checker can have an inference variable outlive `'a` and `'b`,
105 /// but not be equal to `'static`.
109 pub trait InferCtxtExt<'tcx> {
110 fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
112 parent_def_id: LocalDefId,
114 param_env: ty::ParamEnv<'tcx>,
117 ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)>;
119 fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(
121 opaque_types: &OpaqueTypeMap<'tcx>,
122 free_region_relations: &FRR,
125 fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
128 opaque_defn: &OpaqueTypeDecl<'tcx>,
129 mode: GenerateMemberConstraints,
130 free_region_relations: &FRR,
134 fn generate_member_constraint(
136 concrete_ty: Ty<'tcx>,
137 opaque_defn: &OpaqueTypeDecl<'tcx>,
138 opaque_type_def_id: DefId,
139 first_own_region_index: usize,
143 fn member_constraint_feature_gate(
145 opaque_defn: &OpaqueTypeDecl<'tcx>,
146 opaque_type_def_id: DefId,
147 conflict1: ty::Region<'tcx>,
148 conflict2: ty::Region<'tcx>,
151 fn infer_opaque_definition_from_instantiation(
154 substs: SubstsRef<'tcx>,
155 instantiated_ty: Ty<'tcx>,
160 impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
161 /// Replaces all opaque types in `value` with fresh inference variables
162 /// and creates appropriate obligations. For example, given the input:
164 /// impl Iterator<Item = impl Debug>
166 /// this method would create two type variables, `?0` and `?1`. It would
167 /// return the type `?0` but also the obligations:
169 /// ?0: Iterator<Item = ?1>
172 /// Moreover, it returns a `OpaqueTypeMap` that would map `?0` to
173 /// info about the `impl Iterator<..>` type and `?1` to info about
174 /// the `impl Debug` type.
178 /// - `parent_def_id` -- the `DefId` of the function in which the opaque type
180 /// - `body_id` -- the body-id with which the resulting obligations should
182 /// - `param_env` -- the in-scope parameter environment to be used for
184 /// - `value` -- the value within which we are instantiating opaque types
185 /// - `value_span` -- the span where the value came from, used in error reporting
186 fn instantiate_opaque_types<T: TypeFoldable<'tcx>>(
188 parent_def_id: LocalDefId,
190 param_env: ty::ParamEnv<'tcx>,
193 ) -> InferOk<'tcx, (T, OpaqueTypeMap<'tcx>)> {
195 "instantiate_opaque_types(value={:?}, parent_def_id={:?}, body_id={:?}, \
196 param_env={:?}, value_span={:?})",
197 value, parent_def_id, body_id, param_env, value_span,
199 let mut instantiator = Instantiator {
205 opaque_types: Default::default(),
208 let value = instantiator.instantiate_opaque_types_in_map(value);
209 InferOk { value: (value, instantiator.opaque_types), obligations: instantiator.obligations }
212 /// Given the map `opaque_types` containing the opaque
213 /// `impl Trait` types whose underlying, hidden types are being
214 /// inferred, this method adds constraints to the regions
215 /// appearing in those underlying hidden types to ensure that they
216 /// at least do not refer to random scopes within the current
217 /// function. These constraints are not (quite) sufficient to
218 /// guarantee that the regions are actually legal values; that
219 /// final condition is imposed after region inference is done.
223 /// Let's work through an example to explain how it works. Assume
224 /// the current function is as follows:
227 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
230 /// Here, we have two `impl Trait` types whose values are being
231 /// inferred (the `impl Bar<'a>` and the `impl
232 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
233 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
234 /// the return type of `foo`, we *reference* those definitions:
237 /// type Foo1<'x> = impl Bar<'x>;
238 /// type Foo2<'x> = impl Bar<'x>;
239 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
246 /// As indicating in the comments above, each of those references
247 /// is (in the compiler) basically a substitution (`substs`)
248 /// applied to the type of a suitable `def_id` (which identifies
249 /// `Foo1` or `Foo2`).
251 /// Now, at this point in compilation, what we have done is to
252 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
253 /// fresh inference variables C1 and C2. We wish to use the values
254 /// of these variables to infer the underlying types of `Foo1` and
255 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
256 /// constraints like:
259 /// for<'a> (Foo1<'a> = C1)
260 /// for<'b> (Foo1<'b> = C2)
263 /// For these equation to be satisfiable, the types `C1` and `C2`
264 /// can only refer to a limited set of regions. For example, `C1`
265 /// can only refer to `'static` and `'a`, and `C2` can only refer
266 /// to `'static` and `'b`. The job of this function is to impose that
269 /// Up to this point, C1 and C2 are basically just random type
270 /// inference variables, and hence they may contain arbitrary
271 /// regions. In fact, it is fairly likely that they do! Consider
272 /// this possible definition of `foo`:
275 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
280 /// Here, the values for the concrete types of the two impl
281 /// traits will include inference variables:
288 /// Ordinarily, the subtyping rules would ensure that these are
289 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
290 /// type per se, we don't get such constraints by default. This
291 /// is where this function comes into play. It adds extra
292 /// constraints to ensure that all the regions which appear in the
293 /// inferred type are regions that could validly appear.
295 /// This is actually a bit of a tricky constraint in general. We
296 /// want to say that each variable (e.g., `'0`) can only take on
297 /// values that were supplied as arguments to the opaque type
298 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
299 /// scope. We don't have a constraint quite of this kind in the current
304 /// We generally prefer to make `<=` constraints, since they
305 /// integrate best into the region solver. To do that, we find the
306 /// "minimum" of all the arguments that appear in the substs: that
307 /// is, some region which is less than all the others. In the case
308 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
309 /// all). Then we apply that as a least bound to the variables
310 /// (e.g., `'a <= '0`).
312 /// In some cases, there is no minimum. Consider this example:
315 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
318 /// Here we would report a more complex "in constraint", like `'r
319 /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
320 /// the hidden type).
322 /// # Constrain regions, not the hidden concrete type
324 /// Note that generating constraints on each region `Rc` is *not*
325 /// the same as generating an outlives constraint on `Tc` iself.
326 /// For example, if we had a function like this:
329 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
333 /// // Equivalent to:
334 /// type FooReturn<'a, T> = impl Foo<'a>;
335 /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
338 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
339 /// is an inference variable). If we generated a constraint that
340 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
341 /// but this is not necessary, because the opaque type we
342 /// create will be allowed to reference `T`. So we only generate a
343 /// constraint that `'0: 'a`.
345 /// # The `free_region_relations` parameter
347 /// The `free_region_relations` argument is used to find the
348 /// "minimum" of the regions supplied to a given opaque type.
349 /// It must be a relation that can answer whether `'a <= 'b`,
350 /// where `'a` and `'b` are regions that appear in the "substs"
351 /// for the opaque type references (the `<'a>` in `Foo1<'a>`).
353 /// Note that we do not impose the constraints based on the
354 /// generic regions from the `Foo1` definition (e.g., `'x`). This
355 /// is because the constraints we are imposing here is basically
356 /// the concern of the one generating the constraining type C1,
357 /// which is the current function. It also means that we can
358 /// take "implied bounds" into account in some cases:
361 /// trait SomeTrait<'a, 'b> { }
362 /// fn foo<'a, 'b>(_: &'a &'b u32) -> impl SomeTrait<'a, 'b> { .. }
365 /// Here, the fact that `'b: 'a` is known only because of the
366 /// implied bounds from the `&'a &'b u32` parameter, and is not
367 /// "inherent" to the opaque type definition.
371 /// - `opaque_types` -- the map produced by `instantiate_opaque_types`
372 /// - `free_region_relations` -- something that can be used to relate
373 /// the free regions (`'a`) that appear in the impl trait.
374 fn constrain_opaque_types<FRR: FreeRegionRelations<'tcx>>(
376 opaque_types: &OpaqueTypeMap<'tcx>,
377 free_region_relations: &FRR,
379 debug!("constrain_opaque_types()");
381 for (&def_id, opaque_defn) in opaque_types {
382 self.constrain_opaque_type(
385 GenerateMemberConstraints::WhenRequired,
386 free_region_relations,
391 /// See `constrain_opaque_types` for documentation.
392 fn constrain_opaque_type<FRR: FreeRegionRelations<'tcx>>(
395 opaque_defn: &OpaqueTypeDecl<'tcx>,
396 mode: GenerateMemberConstraints,
397 free_region_relations: &FRR,
399 debug!("constrain_opaque_type()");
400 debug!("constrain_opaque_type: def_id={:?}", def_id);
401 debug!("constrain_opaque_type: opaque_defn={:#?}", opaque_defn);
405 let concrete_ty = self.resolve_vars_if_possible(opaque_defn.concrete_ty);
407 debug!("constrain_opaque_type: concrete_ty={:?}", concrete_ty);
409 let first_own_region = match opaque_defn.origin {
410 hir::OpaqueTyOrigin::FnReturn | hir::OpaqueTyOrigin::AsyncFn => {
413 // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
417 // type foo::<'p0..'pn>::Foo<'q0..'qm>
418 // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
420 // For these types we onlt iterate over `'l0..lm` below.
421 tcx.generics_of(def_id).parent_count
423 // These opaque type inherit all lifetime parameters from their
424 // parent, so we have to check them all.
425 hir::OpaqueTyOrigin::Binding | hir::OpaqueTyOrigin::Misc => 0,
428 let span = tcx.def_span(def_id);
430 // If there are required region bounds, we can use them.
431 if opaque_defn.has_required_region_bounds {
432 let bounds = tcx.explicit_item_bounds(def_id);
433 debug!("constrain_opaque_type: predicates: {:#?}", bounds);
435 bounds.iter().map(|(bound, _)| bound.subst(tcx, opaque_defn.substs)).collect();
436 debug!("constrain_opaque_type: bounds={:#?}", bounds);
437 let opaque_type = tcx.mk_opaque(def_id, opaque_defn.substs);
439 let required_region_bounds =
440 required_region_bounds(tcx, opaque_type, bounds.into_iter());
441 debug_assert!(!required_region_bounds.is_empty());
443 for required_region in required_region_bounds {
444 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
445 op: |r| self.sub_regions(infer::CallReturn(span), required_region, r),
448 if let GenerateMemberConstraints::IfNoStaticBound = mode {
449 self.generate_member_constraint(concrete_ty, opaque_defn, def_id, first_own_region);
454 // There were no `required_region_bounds`,
455 // so we have to search for a `least_region`.
456 // Go through all the regions used as arguments to the
457 // opaque type. These are the parameters to the opaque
458 // type; so in our example above, `substs` would contain
459 // `['a]` for the first impl trait and `'b` for the
461 let mut least_region = None;
463 for subst_arg in &opaque_defn.substs[first_own_region..] {
464 let subst_region = match subst_arg.unpack() {
465 GenericArgKind::Lifetime(r) => r,
466 GenericArgKind::Type(_) | GenericArgKind::Const(_) => continue,
469 // Compute the least upper bound of it with the other regions.
470 debug!("constrain_opaque_types: least_region={:?}", least_region);
471 debug!("constrain_opaque_types: subst_region={:?}", subst_region);
473 None => least_region = Some(subst_region),
475 if free_region_relations.sub_free_regions(self.tcx, lr, subst_region) {
476 // keep the current least region
477 } else if free_region_relations.sub_free_regions(self.tcx, subst_region, lr) {
478 // switch to `subst_region`
479 least_region = Some(subst_region);
481 // There are two regions (`lr` and
482 // `subst_region`) which are not relatable. We
483 // can't find a best choice. Therefore,
484 // instead of creating a single bound like
485 // `'r: 'a` (which is our preferred choice),
486 // we will create a "in bound" like `'r in
487 // ['a, 'b, 'c]`, where `'a..'c` are the
488 // regions that appear in the impl trait.
490 // For now, enforce a feature gate outside of async functions.
491 self.member_constraint_feature_gate(opaque_defn, def_id, lr, subst_region);
493 return self.generate_member_constraint(
504 let least_region = least_region.unwrap_or(tcx.lifetimes.re_static);
505 debug!("constrain_opaque_types: least_region={:?}", least_region);
507 if let GenerateMemberConstraints::IfNoStaticBound = mode {
508 if least_region != tcx.lifetimes.re_static {
509 self.generate_member_constraint(concrete_ty, opaque_defn, def_id, first_own_region);
512 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
513 op: |r| self.sub_regions(infer::CallReturn(span), least_region, r),
517 /// As a fallback, we sometimes generate an "in constraint". For
518 /// a case like `impl Foo<'a, 'b>`, where `'a` and `'b` cannot be
519 /// related, we would generate a constraint `'r in ['a, 'b,
520 /// 'static]` for each region `'r` that appears in the hidden type
521 /// (i.e., it must be equal to `'a`, `'b`, or `'static`).
523 /// `conflict1` and `conflict2` are the two region bounds that we
524 /// detected which were unrelated. They are used for diagnostics.
525 fn generate_member_constraint(
527 concrete_ty: Ty<'tcx>,
528 opaque_defn: &OpaqueTypeDecl<'tcx>,
529 opaque_type_def_id: DefId,
530 first_own_region: usize,
532 // Create the set of choice regions: each region in the hidden
533 // type can be equal to any of the region parameters of the
534 // opaque type definition.
535 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
536 opaque_defn.substs[first_own_region..]
538 .filter_map(|arg| match arg.unpack() {
539 GenericArgKind::Lifetime(r) => Some(r),
540 GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
542 .chain(std::iter::once(self.tcx.lifetimes.re_static))
546 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
548 self.member_constraint(
550 opaque_defn.definition_span,
559 /// Member constraints are presently feature-gated except for
560 /// async-await. We expect to lift this once we've had a bit more
562 fn member_constraint_feature_gate(
564 opaque_defn: &OpaqueTypeDecl<'tcx>,
565 opaque_type_def_id: DefId,
566 conflict1: ty::Region<'tcx>,
567 conflict2: ty::Region<'tcx>,
569 // If we have `#![feature(member_constraints)]`, no problems.
570 if self.tcx.features().member_constraints {
574 let span = self.tcx.def_span(opaque_type_def_id);
576 // Without a feature-gate, we only generate member-constraints for async-await.
577 let context_name = match opaque_defn.origin {
578 // No feature-gate required for `async fn`.
579 hir::OpaqueTyOrigin::AsyncFn => return false,
581 // Otherwise, generate the label we'll use in the error message.
582 hir::OpaqueTyOrigin::Binding
583 | hir::OpaqueTyOrigin::FnReturn
584 | hir::OpaqueTyOrigin::Misc => "impl Trait",
586 let msg = format!("ambiguous lifetime bound in `{}`", context_name);
587 let mut err = self.tcx.sess.struct_span_err(span, &msg);
589 let conflict1_name = conflict1.to_string();
590 let conflict2_name = conflict2.to_string();
592 let label = match (&*conflict1_name, &*conflict2_name) {
593 ("'_", "'_") => "the elided lifetimes here do not outlive one another",
595 label_owned = format!(
596 "neither `{}` nor `{}` outlives the other",
597 conflict1_name, conflict2_name,
602 err.span_label(span, label);
604 if self.tcx.sess.is_nightly_build() {
605 err.help("add #![feature(member_constraints)] to the crate attributes to enable");
612 /// Given the fully resolved, instantiated type for an opaque
613 /// type, i.e., the value of an inference variable like C1 or C2
614 /// (*), computes the "definition type" for an opaque type
615 /// definition -- that is, the inferred value of `Foo1<'x>` or
616 /// `Foo2<'x>` that we would conceptually use in its definition:
618 /// type Foo1<'x> = impl Bar<'x> = AAA; <-- this type AAA
619 /// type Foo2<'x> = impl Bar<'x> = BBB; <-- or this type BBB
620 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
622 /// Note that these values are defined in terms of a distinct set of
623 /// generic parameters (`'x` instead of `'a`) from C1 or C2. The main
624 /// purpose of this function is to do that translation.
626 /// (*) C1 and C2 were introduced in the comments on
627 /// `constrain_opaque_types`. Read that comment for more context.
631 /// - `def_id`, the `impl Trait` type
632 /// - `substs`, the substs used to instantiate this opaque type
633 /// - `instantiated_ty`, the inferred type C1 -- fully resolved, lifted version of
634 /// `opaque_defn.concrete_ty`
635 fn infer_opaque_definition_from_instantiation(
638 substs: SubstsRef<'tcx>,
639 instantiated_ty: Ty<'tcx>,
643 "infer_opaque_definition_from_instantiation(def_id={:?}, instantiated_ty={:?})",
644 def_id, instantiated_ty
647 // Use substs to build up a reverse map from regions to their
648 // identity mappings. This is necessary because of `impl
649 // Trait` lifetimes are computed by replacing existing
650 // lifetimes with 'static and remapping only those used in the
651 // `impl Trait` return type, resulting in the parameters
653 let id_substs = InternalSubsts::identity_for_item(self.tcx, def_id);
654 let map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>> =
655 substs.iter().enumerate().map(|(index, subst)| (subst, id_substs[index])).collect();
657 // Convert the type from the function into a type valid outside
658 // the function, by replacing invalid regions with 'static,
659 // after producing an error for each of them.
660 let definition_ty = instantiated_ty.fold_with(&mut ReverseMapper::new(
662 self.is_tainted_by_errors(),
668 debug!("infer_opaque_definition_from_instantiation: definition_ty={:?}", definition_ty);
674 // Visitor that requires that (almost) all regions in the type visited outlive
675 // `least_region`. We cannot use `push_outlives_components` because regions in
676 // closure signatures are not included in their outlives components. We need to
677 // ensure all regions outlive the given bound so that we don't end up with,
678 // say, `ReVar` appearing in a return type and causing ICEs when other
679 // functions end up with region constraints involving regions from other
682 // We also cannot use `for_each_free_region` because for closures it includes
683 // the regions parameters from the enclosing item.
685 // We ignore any type parameters because impl trait values are assumed to
686 // capture all the in-scope type parameters.
687 struct ConstrainOpaqueTypeRegionVisitor<OP> {
691 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
693 OP: FnMut(ty::Region<'tcx>),
695 fn visit_binder<T: TypeFoldable<'tcx>>(
698 ) -> ControlFlow<Self::BreakTy> {
699 t.as_ref().skip_binder().visit_with(self);
700 ControlFlow::CONTINUE
703 fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
705 // ignore bound regions, keep visiting
706 ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
709 ControlFlow::CONTINUE
714 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
715 // We're only interested in types involving regions
716 if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
717 return ControlFlow::CONTINUE;
721 ty::Closure(_, ref substs) => {
722 // Skip lifetime parameters of the enclosing item(s)
724 substs.as_closure().tupled_upvars_ty().visit_with(self);
725 substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
728 ty::Generator(_, ref substs, _) => {
729 // Skip lifetime parameters of the enclosing item(s)
730 // Also skip the witness type, because that has no free regions.
732 substs.as_generator().tupled_upvars_ty().visit_with(self);
733 substs.as_generator().return_ty().visit_with(self);
734 substs.as_generator().yield_ty().visit_with(self);
735 substs.as_generator().resume_ty().visit_with(self);
738 ty.super_visit_with(self);
742 ControlFlow::CONTINUE
746 struct ReverseMapper<'tcx> {
749 /// If errors have already been reported in this fn, we suppress
750 /// our own errors because they are sometimes derivative.
751 tainted_by_errors: bool,
753 opaque_type_def_id: DefId,
754 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
755 map_missing_regions_to_empty: bool,
757 /// initially `Some`, set to `None` once error has been reported
758 hidden_ty: Option<Ty<'tcx>>,
760 /// Span of function being checked.
764 impl ReverseMapper<'tcx> {
767 tainted_by_errors: bool,
768 opaque_type_def_id: DefId,
769 map: FxHashMap<GenericArg<'tcx>, GenericArg<'tcx>>,
778 map_missing_regions_to_empty: false,
779 hidden_ty: Some(hidden_ty),
784 fn fold_kind_mapping_missing_regions_to_empty(
786 kind: GenericArg<'tcx>,
787 ) -> GenericArg<'tcx> {
788 assert!(!self.map_missing_regions_to_empty);
789 self.map_missing_regions_to_empty = true;
790 let kind = kind.fold_with(self);
791 self.map_missing_regions_to_empty = false;
795 fn fold_kind_normally(&mut self, kind: GenericArg<'tcx>) -> GenericArg<'tcx> {
796 assert!(!self.map_missing_regions_to_empty);
801 impl TypeFolder<'tcx> for ReverseMapper<'tcx> {
802 fn tcx(&self) -> TyCtxt<'tcx> {
806 fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
808 // Ignore bound regions and `'static` regions that appear in the
809 // type, we only need to remap regions that reference lifetimes
810 // from the function declaraion.
811 // This would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
812 ty::ReLateBound(..) | ty::ReStatic => return r,
814 // If regions have been erased (by writeback), don't try to unerase
816 ty::ReErased => return r,
818 // The regions that we expect from borrow checking.
819 ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReEmpty(ty::UniverseIndex::ROOT) => {}
821 ty::ReEmpty(_) | ty::RePlaceholder(_) | ty::ReVar(_) => {
822 // All of the regions in the type should either have been
823 // erased by writeback, or mapped back to named regions by
825 bug!("unexpected region kind in opaque type: {:?}", r);
829 let generics = self.tcx().generics_of(self.opaque_type_def_id);
830 match self.map.get(&r.into()).map(|k| k.unpack()) {
831 Some(GenericArgKind::Lifetime(r1)) => r1,
832 Some(u) => panic!("region mapped to unexpected kind: {:?}", u),
833 None if self.map_missing_regions_to_empty || self.tainted_by_errors => {
834 self.tcx.lifetimes.re_root_empty
836 None if generics.parent.is_some() => {
837 if let Some(hidden_ty) = self.hidden_ty.take() {
838 unexpected_hidden_region_diagnostic(
840 self.tcx.def_span(self.opaque_type_def_id),
846 self.tcx.lifetimes.re_root_empty
851 .struct_span_err(self.span, "non-defining opaque type use in defining scope")
855 "lifetime `{}` is part of concrete type but not used in \
856 parameter list of the `impl Trait` type alias",
862 self.tcx().lifetimes.re_static
867 fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
869 ty::Closure(def_id, substs) => {
870 // I am a horrible monster and I pray for death. When
871 // we encounter a closure here, it is always a closure
872 // from within the function that we are currently
873 // type-checking -- one that is now being encapsulated
874 // in an opaque type. Ideally, we would
875 // go through the types/lifetimes that it references
876 // and treat them just like we would any other type,
877 // which means we would error out if we find any
878 // reference to a type/region that is not in the
881 // **However,** in the case of closures, there is a
882 // somewhat subtle (read: hacky) consideration. The
883 // problem is that our closure types currently include
884 // all the lifetime parameters declared on the
885 // enclosing function, even if they are unused by the
886 // closure itself. We can't readily filter them out,
887 // so here we replace those values with `'empty`. This
888 // can't really make a difference to the rest of the
889 // compiler; those regions are ignored for the
890 // outlives relation, and hence don't affect trait
891 // selection or auto traits, and they are erased
894 let generics = self.tcx.generics_of(def_id);
895 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
896 if index < generics.parent_count {
897 // Accommodate missing regions in the parent kinds...
898 self.fold_kind_mapping_missing_regions_to_empty(kind)
900 // ...but not elsewhere.
901 self.fold_kind_normally(kind)
905 self.tcx.mk_closure(def_id, substs)
908 ty::Generator(def_id, substs, movability) => {
909 let generics = self.tcx.generics_of(def_id);
910 let substs = self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
911 if index < generics.parent_count {
912 // Accommodate missing regions in the parent kinds...
913 self.fold_kind_mapping_missing_regions_to_empty(kind)
915 // ...but not elsewhere.
916 self.fold_kind_normally(kind)
920 self.tcx.mk_generator(def_id, substs, movability)
924 // Look it up in the substitution list.
925 match self.map.get(&ty.into()).map(|k| k.unpack()) {
926 // Found it in the substitution list; replace with the parameter from the
928 Some(GenericArgKind::Type(t1)) => t1,
929 Some(u) => panic!("type mapped to unexpected kind: {:?}", u),
936 "type parameter `{}` is part of concrete type but not \
937 used in parameter list for the `impl Trait` type alias",
943 self.tcx().ty_error()
948 _ => ty.super_fold_with(self),
952 fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
953 trace!("checking const {:?}", ct);
954 // Find a const parameter
956 ty::ConstKind::Param(..) => {
957 // Look it up in the substitution list.
958 match self.map.get(&ct.into()).map(|k| k.unpack()) {
959 // Found it in the substitution list, replace with the parameter from the
961 Some(GenericArgKind::Const(c1)) => c1,
962 Some(u) => panic!("const mapped to unexpected kind: {:?}", u),
969 "const parameter `{}` is part of concrete type but not \
970 used in parameter list for the `impl Trait` type alias",
976 self.tcx().const_error(ct.ty)
986 struct Instantiator<'a, 'tcx> {
987 infcx: &'a InferCtxt<'a, 'tcx>,
988 parent_def_id: LocalDefId,
990 param_env: ty::ParamEnv<'tcx>,
992 opaque_types: OpaqueTypeMap<'tcx>,
993 obligations: Vec<PredicateObligation<'tcx>>,
996 impl<'a, 'tcx> Instantiator<'a, 'tcx> {
997 fn instantiate_opaque_types_in_map<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
998 debug!("instantiate_opaque_types_in_map(value={:?})", value);
999 let tcx = self.infcx.tcx;
1000 value.fold_with(&mut BottomUpFolder {
1003 if ty.references_error() {
1004 return tcx.ty_error();
1005 } else if let ty::Opaque(def_id, substs) = ty.kind() {
1006 // Check that this is `impl Trait` type is
1007 // declared by `parent_def_id` -- i.e., one whose
1008 // value we are inferring. At present, this is
1009 // always true during the first phase of
1010 // type-check, but not always true later on during
1011 // NLL. Once we support named opaque types more fully,
1012 // this same scenario will be able to arise during all phases.
1014 // Here is an example using type alias `impl Trait`
1015 // that indicates the distinction we are checking for:
1019 // pub type Foo = impl Iterator;
1020 // pub fn make_foo() -> Foo { .. }
1024 // fn foo() -> a::Foo { a::make_foo() }
1028 // Here, the return type of `foo` references a
1029 // `Opaque` indeed, but not one whose value is
1030 // presently being inferred. You can get into a
1031 // similar situation with closure return types
1035 // fn foo() -> impl Iterator { .. }
1037 // let x = || foo(); // returns the Opaque assoc with `foo`
1040 if let Some(def_id) = def_id.as_local() {
1041 let opaque_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
1042 let parent_def_id = self.parent_def_id;
1043 let def_scope_default = || {
1044 let opaque_parent_hir_id = tcx.hir().get_parent_item(opaque_hir_id);
1045 parent_def_id == tcx.hir().local_def_id(opaque_parent_hir_id)
1047 let (in_definition_scope, origin) = match tcx.hir().find(opaque_hir_id) {
1048 Some(Node::Item(item)) => match item.kind {
1049 // Anonymous `impl Trait`
1050 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1051 impl_trait_fn: Some(parent),
1054 }) => (parent == self.parent_def_id.to_def_id(), origin),
1055 // Named `type Foo = impl Bar;`
1056 hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1057 impl_trait_fn: None,
1061 may_define_opaque_type(tcx, self.parent_def_id, opaque_hir_id),
1064 _ => (def_scope_default(), hir::OpaqueTyOrigin::Misc),
1067 "expected item, found {}",
1068 tcx.hir().node_to_string(opaque_hir_id),
1071 if in_definition_scope {
1072 return self.fold_opaque_ty(ty, def_id.to_def_id(), substs, origin);
1076 "instantiate_opaque_types_in_map: \
1077 encountered opaque outside its definition scope \
1095 substs: SubstsRef<'tcx>,
1096 origin: hir::OpaqueTyOrigin,
1098 let infcx = self.infcx;
1099 let tcx = infcx.tcx;
1101 debug!("instantiate_opaque_types: Opaque(def_id={:?}, substs={:?})", def_id, substs);
1103 // Use the same type variable if the exact same opaque type appears more
1104 // than once in the return type (e.g., if it's passed to a type alias).
1105 if let Some(opaque_defn) = self.opaque_types.get(&def_id) {
1106 debug!("instantiate_opaque_types: returning concrete ty {:?}", opaque_defn.concrete_ty);
1107 return opaque_defn.concrete_ty;
1109 let span = tcx.def_span(def_id);
1110 debug!("fold_opaque_ty {:?} {:?}", self.value_span, span);
1112 .next_ty_var(TypeVariableOrigin { kind: TypeVariableOriginKind::TypeInference, span });
1114 let item_bounds = tcx.explicit_item_bounds(def_id);
1115 debug!("instantiate_opaque_types: bounds={:#?}", item_bounds);
1116 let bounds: Vec<_> =
1117 item_bounds.iter().map(|(bound, _)| bound.subst(tcx, substs)).collect();
1119 let param_env = tcx.param_env(def_id);
1120 let InferOk { value: bounds, obligations } =
1121 infcx.partially_normalize_associated_types_in(span, self.body_id, param_env, bounds);
1122 self.obligations.extend(obligations);
1124 debug!("instantiate_opaque_types: bounds={:?}", bounds);
1126 let required_region_bounds = required_region_bounds(tcx, ty, bounds.iter().copied());
1127 debug!("instantiate_opaque_types: required_region_bounds={:?}", required_region_bounds);
1129 // Make sure that we are in fact defining the *entire* type
1130 // (e.g., `type Foo<T: Bound> = impl Bar;` needs to be
1131 // defined by a function like `fn foo<T: Bound>() -> Foo<T>`).
1132 debug!("instantiate_opaque_types: param_env={:#?}", self.param_env,);
1133 debug!("instantiate_opaque_types: generics={:#?}", tcx.generics_of(def_id),);
1135 // Ideally, we'd get the span where *this specific `ty` came
1136 // from*, but right now we just use the span from the overall
1137 // value being folded. In simple cases like `-> impl Foo`,
1138 // these are the same span, but not in cases like `-> (impl
1140 let definition_span = self.value_span;
1142 self.opaque_types.insert(
1148 concrete_ty: ty_var,
1149 has_required_region_bounds: !required_region_bounds.is_empty(),
1153 debug!("instantiate_opaque_types: ty_var={:?}", ty_var);
1155 for predicate in &bounds {
1156 if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
1157 if projection.ty.references_error() {
1158 // No point on adding these obligations since there's a type error involved.
1164 self.obligations.reserve(bounds.len());
1165 for predicate in bounds {
1166 // Change the predicate to refer to the type variable,
1167 // which will be the concrete type instead of the opaque type.
1168 // This also instantiates nested instances of `impl Trait`.
1169 let predicate = self.instantiate_opaque_types_in_map(predicate);
1171 let cause = traits::ObligationCause::new(span, self.body_id, traits::MiscObligation);
1173 // Require that the predicate holds for the concrete type.
1174 debug!("instantiate_opaque_types: predicate={:?}", predicate);
1175 self.obligations.push(traits::Obligation::new(cause, self.param_env, predicate));
1182 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
1188 /// pub trait Bar { .. }
1190 /// pub type Baz = impl Bar;
1192 /// fn f1() -> Baz { .. }
1195 /// fn f2() -> bar::Baz { .. }
1199 /// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
1200 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
1201 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
1202 pub fn may_define_opaque_type(
1205 opaque_hir_id: hir::HirId,
1207 let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
1209 // Named opaque types can be defined by any siblings or children of siblings.
1210 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
1211 // We walk up the node tree until we hit the root or the scope of the opaque type.
1212 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
1213 hir_id = tcx.hir().get_parent_item(hir_id);
1215 // Syntactically, we are allowed to define the concrete type if:
1216 let res = hir_id == scope;
1218 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
1219 tcx.hir().find(hir_id),
1220 tcx.hir().get(opaque_hir_id),
1226 /// Given a set of predicates that apply to an object type, returns
1227 /// the region bounds that the (erased) `Self` type must
1228 /// outlive. Precisely *because* the `Self` type is erased, the
1229 /// parameter `erased_self_ty` must be supplied to indicate what type
1230 /// has been used to represent `Self` in the predicates
1231 /// themselves. This should really be a unique type; `FreshTy(0)` is a
1234 /// N.B., in some cases, particularly around higher-ranked bounds,
1235 /// this function returns a kind of conservative approximation.
1236 /// That is, all regions returned by this function are definitely
1237 /// required, but there may be other region bounds that are not
1238 /// returned, as well as requirements like `for<'a> T: 'a`.
1240 /// Requires that trait definitions have been processed so that we can
1241 /// elaborate predicates and walk supertraits.
1242 crate fn required_region_bounds(
1244 erased_self_ty: Ty<'tcx>,
1245 predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
1246 ) -> Vec<ty::Region<'tcx>> {
1247 debug!("required_region_bounds(erased_self_ty={:?})", erased_self_ty);
1249 assert!(!erased_self_ty.has_escaping_bound_vars());
1251 traits::elaborate_predicates(tcx, predicates)
1252 .filter_map(|obligation| {
1253 debug!("required_region_bounds(obligation={:?})", obligation);
1254 match obligation.predicate.kind().skip_binder() {
1255 ty::PredicateKind::Projection(..)
1256 | ty::PredicateKind::Trait(..)
1257 | ty::PredicateKind::Subtype(..)
1258 | ty::PredicateKind::WellFormed(..)
1259 | ty::PredicateKind::ObjectSafe(..)
1260 | ty::PredicateKind::ClosureKind(..)
1261 | ty::PredicateKind::RegionOutlives(..)
1262 | ty::PredicateKind::ConstEvaluatable(..)
1263 | ty::PredicateKind::ConstEquate(..)
1264 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
1265 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ref t, ref r)) => {
1266 // Search for a bound of the form `erased_self_ty
1267 // : 'a`, but be wary of something like `for<'a>
1268 // erased_self_ty : 'a` (we interpret a
1269 // higher-ranked bound like that as 'static,
1270 // though at present the code in `fulfill.rs`
1271 // considers such bounds to be unsatisfiable, so
1272 // it's kind of a moot point since you could never
1273 // construct such an object, but this seems
1274 // correct even if that code changes).
1275 if t == &erased_self_ty && !r.has_escaping_bound_vars() {