1 use crate::infer::InferCtxt;
2 use crate::opaque_types::required_region_bounds;
5 use rustc_hir::def_id::DefId;
6 use rustc_hir::lang_items;
7 use rustc_middle::ty::subst::{GenericArg, GenericArgKind, SubstsRef};
8 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable, WithConstness};
12 /// Returns the set of obligations needed to make `arg` well-formed.
13 /// If `arg` contains unresolved inference variables, this may include
14 /// further WF obligations. However, if `arg` IS an unresolved
15 /// inference variable, returns `None`, because we are not able to
16 /// make any progress at all. This is to prevent "livelock" where we
17 /// say "$0 is WF if $0 is WF".
18 pub fn obligations<'a, 'tcx>(
19 infcx: &InferCtxt<'a, 'tcx>,
20 param_env: ty::ParamEnv<'tcx>,
22 arg: GenericArg<'tcx>,
24 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
25 // Handle the "livelock" case (see comment above) by bailing out if necessary.
26 let arg = match arg.unpack() {
27 GenericArgKind::Type(ty) => {
29 ty::Infer(ty::TyVar(_)) => {
30 let resolved_ty = infcx.shallow_resolve(ty);
31 if resolved_ty == ty {
32 // No progress, bail out to prevent "livelock".
42 GenericArgKind::Const(ct) => {
44 ty::ConstKind::Infer(infer) => {
45 let resolved = infcx.shallow_resolve(infer);
46 if resolved == infer {
51 infcx.tcx.mk_const(ty::Const { val: ty::ConstKind::Infer(resolved), ty: ct.ty })
57 // There is nothing we have to do for lifetimes.
58 GenericArgKind::Lifetime(..) => return Some(Vec::new()),
61 let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
63 debug!("wf::obligations({:?}, body_id={:?}) = {:?}", arg, body_id, wf.out);
65 let result = wf.normalize();
66 debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", arg, body_id, result);
70 /// Returns the obligations that make this trait reference
71 /// well-formed. For example, if there is a trait `Set` defined like
72 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
74 pub fn trait_obligations<'a, 'tcx>(
75 infcx: &InferCtxt<'a, 'tcx>,
76 param_env: ty::ParamEnv<'tcx>,
78 trait_ref: &ty::TraitRef<'tcx>,
80 item: Option<&'tcx hir::Item<'tcx>>,
81 ) -> Vec<traits::PredicateObligation<'tcx>> {
82 let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item };
83 wf.compute_trait_ref(trait_ref, Elaborate::All);
87 pub fn predicate_obligations<'a, 'tcx>(
88 infcx: &InferCtxt<'a, 'tcx>,
89 param_env: ty::ParamEnv<'tcx>,
91 predicate: ty::Predicate<'tcx>,
93 ) -> Vec<traits::PredicateObligation<'tcx>> {
94 let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
96 // (*) ok to skip binders, because wf code is prepared for it
97 match predicate.kind() {
98 ty::PredicateKind::Trait(t, _) => {
99 wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
101 ty::PredicateKind::RegionOutlives(..) => {}
102 ty::PredicateKind::TypeOutlives(t) => {
103 wf.compute(t.skip_binder().0.into());
105 ty::PredicateKind::Projection(t) => {
106 let t = t.skip_binder(); // (*)
107 wf.compute_projection(t.projection_ty);
108 wf.compute(t.ty.into());
110 &ty::PredicateKind::WellFormed(arg) => {
113 ty::PredicateKind::ObjectSafe(_) => {}
114 ty::PredicateKind::ClosureKind(..) => {}
115 ty::PredicateKind::Subtype(data) => {
116 wf.compute(data.skip_binder().a.into()); // (*)
117 wf.compute(data.skip_binder().b.into()); // (*)
119 &ty::PredicateKind::ConstEvaluatable(def_id, substs) => {
120 let obligations = wf.nominal_obligations(def_id, substs);
121 wf.out.extend(obligations);
123 for arg in substs.iter() {
127 &ty::PredicateKind::ConstEquate(c1, c2) => {
128 wf.compute(c1.into());
129 wf.compute(c2.into());
136 struct WfPredicates<'a, 'tcx> {
137 infcx: &'a InferCtxt<'a, 'tcx>,
138 param_env: ty::ParamEnv<'tcx>,
141 out: Vec<traits::PredicateObligation<'tcx>>,
142 item: Option<&'tcx hir::Item<'tcx>>,
145 /// Controls whether we "elaborate" supertraits and so forth on the WF
146 /// predicates. This is a kind of hack to address #43784. The
147 /// underlying problem in that issue was a trait structure like:
150 /// trait Foo: Copy { }
151 /// trait Bar: Foo { }
152 /// impl<T: Bar> Foo for T { }
153 /// impl<T> Bar for T { }
156 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
157 /// we decide that this is true because `T: Bar` is in the
158 /// where-clauses (and we can elaborate that to include `T:
159 /// Copy`). This wouldn't be a problem, except that when we check the
160 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
161 /// impl. And so nowhere did we check that `T: Copy` holds!
163 /// To resolve this, we elaborate the WF requirements that must be
164 /// proven when checking impls. This means that (e.g.) the `impl Bar
165 /// for T` will be forced to prove not only that `T: Foo` but also `T:
166 /// Copy` (which it won't be able to do, because there is no `Copy`
168 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
174 fn extend_cause_with_original_assoc_item_obligation<'tcx>(
176 trait_ref: &ty::TraitRef<'tcx>,
177 item: Option<&hir::Item<'tcx>>,
178 cause: &mut traits::ObligationCause<'tcx>,
179 pred: &ty::Predicate<'_>,
180 mut trait_assoc_items: impl Iterator<Item = &'tcx ty::AssocItem>,
183 "extended_cause_with_original_assoc_item_obligation {:?} {:?} {:?} {:?}",
184 trait_ref, item, cause, pred
186 let items = match item {
187 Some(hir::Item { kind: hir::ItemKind::Impl { items, .. }, .. }) => items,
191 |impl_item_ref: &hir::ImplItemRef<'_>| match tcx.hir().impl_item(impl_item_ref.id).kind {
192 hir::ImplItemKind::Const(ty, _) | hir::ImplItemKind::TyAlias(ty) => ty.span,
193 _ => impl_item_ref.span,
196 ty::PredicateKind::Projection(proj) => {
197 // The obligation comes not from the current `impl` nor the `trait` being implemented,
198 // but rather from a "second order" obligation, where an associated type has a
199 // projection coming from another associated type. See
200 // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs` and
201 // `traits-assoc-type-in-supertrait-bad.rs`.
202 let kind = &proj.ty().skip_binder().kind;
203 if let ty::Projection(projection_ty) = kind {
204 let trait_assoc_item = tcx.associated_item(projection_ty.item_def_id);
205 if let Some(impl_item_span) =
206 items.iter().find(|item| item.ident == trait_assoc_item.ident).map(fix_span)
208 cause.make_mut().span = impl_item_span;
212 ty::PredicateKind::Trait(pred, _) => {
213 // An associated item obligation born out of the `trait` failed to be met. An example
214 // can be seen in `ui/associated-types/point-at-type-on-obligation-failure-2.rs`.
215 debug!("extended_cause_with_original_assoc_item_obligation trait proj {:?}", pred);
216 if let ty::Projection(ty::ProjectionTy { item_def_id, .. }) =
217 &pred.skip_binder().self_ty().kind
219 if let Some(impl_item_span) = trait_assoc_items
220 .find(|i| i.def_id == *item_def_id)
221 .and_then(|trait_assoc_item| {
222 items.iter().find(|i| i.ident == trait_assoc_item.ident).map(fix_span)
225 cause.make_mut().span = impl_item_span;
233 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
234 fn tcx(&self) -> TyCtxt<'tcx> {
238 fn cause(&self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
239 traits::ObligationCause::new(self.span, self.body_id, code)
242 fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
243 let cause = self.cause(traits::MiscObligation);
244 let infcx = &mut self.infcx;
245 let param_env = self.param_env;
246 let mut obligations = Vec::with_capacity(self.out.len());
247 for pred in &self.out {
248 assert!(!pred.has_escaping_bound_vars());
249 let mut selcx = traits::SelectionContext::new(infcx);
250 let i = obligations.len();
252 traits::normalize_to(&mut selcx, param_env, cause.clone(), pred, &mut obligations);
253 obligations.insert(i, value);
258 /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
259 fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
260 let tcx = self.infcx.tcx;
261 let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
263 debug!("compute_trait_ref obligations {:?}", obligations);
264 let cause = self.cause(traits::MiscObligation);
265 let param_env = self.param_env;
267 let item = self.item;
269 let extend = |obligation: traits::PredicateObligation<'tcx>| {
270 let mut cause = cause.clone();
271 if let Some(parent_trait_ref) = obligation.predicate.to_opt_poly_trait_ref() {
272 let derived_cause = traits::DerivedObligationCause {
274 parent_code: Rc::new(obligation.cause.code.clone()),
276 cause.make_mut().code =
277 traits::ObligationCauseCode::DerivedObligation(derived_cause);
279 extend_cause_with_original_assoc_item_obligation(
284 &obligation.predicate,
285 tcx.associated_items(trait_ref.def_id).in_definition_order(),
287 traits::Obligation::new(cause, param_env, obligation.predicate)
290 if let Elaborate::All = elaborate {
291 let implied_obligations = traits::util::elaborate_obligations(tcx, obligations);
292 let implied_obligations = implied_obligations.map(extend);
293 self.out.extend(implied_obligations);
295 self.out.extend(obligations);
298 let tcx = self.tcx();
304 matches!(arg.unpack(), GenericArgKind::Type(..) | GenericArgKind::Const(..))
306 .filter(|arg| !arg.has_escaping_bound_vars())
308 traits::Obligation::new(
311 ty::PredicateKind::WellFormed(arg).to_predicate(tcx),
317 /// Pushes the obligations required for `trait_ref::Item` to be WF
319 fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
320 // A projection is well-formed if (a) the trait ref itself is
321 // WF and (b) the trait-ref holds. (It may also be
322 // normalizable and be WF that way.)
323 let trait_ref = data.trait_ref(self.infcx.tcx);
324 self.compute_trait_ref(&trait_ref, Elaborate::None);
326 if !data.has_escaping_bound_vars() {
327 let predicate = trait_ref.without_const().to_predicate(self.infcx.tcx);
328 let cause = self.cause(traits::ProjectionWf(data));
329 self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
333 fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
334 if !subty.has_escaping_bound_vars() {
335 let cause = self.cause(cause);
336 let trait_ref = ty::TraitRef {
337 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
338 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
340 self.out.push(traits::Obligation::new(
343 trait_ref.without_const().to_predicate(self.infcx.tcx),
348 /// Pushes all the predicates needed to validate that `ty` is WF into `out`.
349 fn compute(&mut self, arg: GenericArg<'tcx>) {
350 let mut walker = arg.walk();
351 let param_env = self.param_env;
352 while let Some(arg) = walker.next() {
353 let ty = match arg.unpack() {
354 GenericArgKind::Type(ty) => ty,
356 // No WF constraints for lifetimes being present, any outlives
357 // obligations are handled by the parent (e.g. `ty::Ref`).
358 GenericArgKind::Lifetime(_) => continue,
360 GenericArgKind::Const(constant) => {
362 ty::ConstKind::Unevaluated(def_id, substs, promoted) => {
363 assert!(promoted.is_none());
365 let obligations = self.nominal_obligations(def_id, substs);
366 self.out.extend(obligations);
368 let predicate = ty::PredicateKind::ConstEvaluatable(def_id, substs)
369 .to_predicate(self.tcx());
370 let cause = self.cause(traits::MiscObligation);
371 self.out.push(traits::Obligation::new(
377 ty::ConstKind::Infer(infer) => {
378 let resolved = self.infcx.shallow_resolve(infer);
379 // the `InferConst` changed, meaning that we made progress.
380 if resolved != infer {
381 let cause = self.cause(traits::MiscObligation);
383 let resolved_constant = self.infcx.tcx.mk_const(ty::Const {
384 val: ty::ConstKind::Infer(resolved),
387 self.out.push(traits::Obligation::new(
390 ty::PredicateKind::WellFormed(resolved_constant.into())
391 .to_predicate(self.tcx()),
395 ty::ConstKind::Error(_)
396 | ty::ConstKind::Param(_)
397 | ty::ConstKind::Bound(..)
398 | ty::ConstKind::Placeholder(..) => {
399 // These variants are trivially WF, so nothing to do here.
401 ty::ConstKind::Value(..) => {
402 // FIXME: Enforce that values are structurally-matchable.
417 | ty::GeneratorWitness(..)
421 | ty::Placeholder(..)
422 | ty::Foreign(..) => {
423 // WfScalar, WfParameter, etc
426 // Can only infer to `ty::Int(_) | ty::Uint(_)`.
427 ty::Infer(ty::IntVar(_)) => {}
429 // Can only infer to `ty::Float(_)`.
430 ty::Infer(ty::FloatVar(_)) => {}
432 ty::Slice(subty) => {
433 self.require_sized(subty, traits::SliceOrArrayElem);
436 ty::Array(subty, _) => {
437 self.require_sized(subty, traits::SliceOrArrayElem);
438 // Note that we handle the len is implicitly checked while walking `arg`.
441 ty::Tuple(ref tys) => {
442 if let Some((_last, rest)) = tys.split_last() {
444 self.require_sized(elem.expect_ty(), traits::TupleElem);
450 // Simple cases that are WF if their type args are WF.
453 ty::Projection(data) => {
454 walker.skip_current_subtree(); // Subtree handled by compute_projection.
455 self.compute_projection(data);
458 ty::Adt(def, substs) => {
460 let obligations = self.nominal_obligations(def.did, substs);
461 self.out.extend(obligations);
464 ty::FnDef(did, substs) => {
465 let obligations = self.nominal_obligations(did, substs);
466 self.out.extend(obligations);
469 ty::Ref(r, rty, _) => {
471 if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
472 let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
473 self.out.push(traits::Obligation::new(
476 ty::PredicateKind::TypeOutlives(ty::Binder::dummy(
477 ty::OutlivesPredicate(rty, r),
479 .to_predicate(self.tcx()),
484 ty::Generator(..) => {
485 // Walk ALL the types in the generator: this will
486 // include the upvar types as well as the yield
487 // type. Note that this is mildly distinct from
488 // the closure case, where we have to be careful
489 // about the signature of the closure. We don't
490 // have the problem of implied bounds here since
491 // generators don't take arguments.
494 ty::Closure(_, substs) => {
495 // Only check the upvar types for WF, not the rest
496 // of the types within. This is needed because we
497 // capture the signature and it may not be WF
498 // without the implied bounds. Consider a closure
499 // like `|x: &'a T|` -- it may be that `T: 'a` is
500 // not known to hold in the creator's context (and
501 // indeed the closure may not be invoked by its
502 // creator, but rather turned to someone who *can*
505 // The special treatment of closures here really
506 // ought not to be necessary either; the problem
507 // is related to #25860 -- there is no way for us
508 // to express a fn type complete with the implied
509 // bounds that it is assuming. I think in reality
510 // the WF rules around fn are a bit messed up, and
511 // that is the rot problem: `fn(&'a T)` should
512 // probably always be WF, because it should be
513 // shorthand for something like `where(T: 'a) {
514 // fn(&'a T) }`, as discussed in #25860.
516 // Note that we are also skipping the generic
517 // types. This is consistent with the `outlives`
518 // code, but anyway doesn't matter: within the fn
519 // body where they are created, the generics will
520 // always be WF, and outside of that fn body we
521 // are not directly inspecting closure types
522 // anyway, except via auto trait matching (which
523 // only inspects the upvar types).
524 walker.skip_current_subtree(); // subtree handled below
525 for upvar_ty in substs.as_closure().upvar_tys() {
526 // FIXME(eddyb) add the type to `walker` instead of recursing.
527 self.compute(upvar_ty.into());
532 // let the loop iterate into the argument/return
533 // types appearing in the fn signature
536 ty::Opaque(did, substs) => {
537 // all of the requirements on type parameters
538 // should've been checked by the instantiation
539 // of whatever returned this exact `impl Trait`.
541 // for named opaque `impl Trait` types we still need to check them
542 if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
543 let obligations = self.nominal_obligations(did, substs);
544 self.out.extend(obligations);
548 ty::Dynamic(data, r) => {
551 // Here, we defer WF checking due to higher-ranked
552 // regions. This is perhaps not ideal.
553 self.from_object_ty(ty, data, r);
555 // FIXME(#27579) RFC also considers adding trait
556 // obligations that don't refer to Self and
559 let defer_to_coercion = self.tcx().features().object_safe_for_dispatch;
561 if !defer_to_coercion {
562 let cause = self.cause(traits::MiscObligation);
563 let component_traits = data.auto_traits().chain(data.principal_def_id());
564 let tcx = self.tcx();
565 self.out.extend(component_traits.map(|did| {
566 traits::Obligation::new(
569 ty::PredicateKind::ObjectSafe(did).to_predicate(tcx),
575 // Inference variables are the complicated case, since we don't
576 // know what type they are. We do two things:
578 // 1. Check if they have been resolved, and if so proceed with
580 // 2. If not, we've at least simplified things (e.g., we went
581 // from `Vec<$0>: WF` to `$0: WF`), so we can
582 // register a pending obligation and keep
583 // moving. (Goal is that an "inductive hypothesis"
584 // is satisfied to ensure termination.)
585 // See also the comment on `fn obligations`, describing "livelock"
586 // prevention, which happens before this can be reached.
588 let ty = self.infcx.shallow_resolve(ty);
589 if let ty::Infer(ty::TyVar(_)) = ty.kind {
590 // Not yet resolved, but we've made progress.
591 let cause = self.cause(traits::MiscObligation);
592 self.out.push(traits::Obligation::new(
595 ty::PredicateKind::WellFormed(ty.into()).to_predicate(self.tcx()),
598 // Yes, resolved, proceed with the result.
599 // FIXME(eddyb) add the type to `walker` instead of recursing.
600 self.compute(ty.into());
607 fn nominal_obligations(
610 substs: SubstsRef<'tcx>,
611 ) -> Vec<traits::PredicateObligation<'tcx>> {
612 let predicates = self.infcx.tcx.predicates_of(def_id).instantiate(self.infcx.tcx, substs);
616 .zip(predicates.spans.into_iter())
617 .map(|(pred, span)| {
618 let cause = self.cause(traits::BindingObligation(def_id, span));
619 traits::Obligation::new(cause, self.param_env, pred)
621 .filter(|pred| !pred.has_escaping_bound_vars())
628 data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
629 region: ty::Region<'tcx>,
631 // Imagine a type like this:
634 // trait Bar<'c> : 'c { }
636 // &'b (Foo+'c+Bar<'d>)
639 // In this case, the following relationships must hold:
644 // The first conditions is due to the normal region pointer
645 // rules, which say that a reference cannot outlive its
648 // The final condition may be a bit surprising. In particular,
649 // you may expect that it would have been `'c <= 'd`, since
650 // usually lifetimes of outer things are conservative
651 // approximations for inner things. However, it works somewhat
652 // differently with trait objects: here the idea is that if the
653 // user specifies a region bound (`'c`, in this case) it is the
654 // "master bound" that *implies* that bounds from other traits are
655 // all met. (Remember that *all bounds* in a type like
656 // `Foo+Bar+Zed` must be met, not just one, hence if we write
657 // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
660 // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
661 // am looking forward to the future here.
662 if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
663 let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
665 let explicit_bound = region;
667 self.out.reserve(implicit_bounds.len());
668 for implicit_bound in implicit_bounds {
669 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
671 ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
672 self.out.push(traits::Obligation::new(
675 outlives.to_predicate(self.infcx.tcx),
682 /// Given an object type like `SomeTrait + Send`, computes the lifetime
683 /// bounds that must hold on the elided self type. These are derived
684 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
685 /// they declare `trait SomeTrait : 'static`, for example, then
686 /// `'static` would appear in the list. The hard work is done by
687 /// `infer::required_region_bounds`, see that for more information.
688 pub fn object_region_bounds<'tcx>(
690 existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
691 ) -> Vec<ty::Region<'tcx>> {
692 // Since we don't actually *know* the self type for an object,
693 // this "open(err)" serves as a kind of dummy standin -- basically
694 // a placeholder type.
695 let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
697 let predicates = existential_predicates.iter().filter_map(|predicate| {
698 if let ty::ExistentialPredicate::Projection(_) = predicate.skip_binder() {
701 Some(predicate.with_self_ty(tcx, open_ty))
705 required_region_bounds(tcx, open_ty, predicates)