1 use crate::infer::opaque_types::required_region_bounds;
2 use crate::infer::InferCtxt;
3 use crate::middle::lang_items;
4 use crate::traits::{self, AssocTypeBoundData};
5 use crate::ty::subst::SubstsRef;
6 use crate::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
8 use rustc_hir::def_id::DefId;
9 use rustc_span::symbol::{kw, Ident};
13 /// Returns the set of obligations needed to make `ty` well-formed.
14 /// If `ty` contains unresolved inference variables, this may include
15 /// further WF obligations. However, if `ty` IS an unresolved
16 /// inference variable, returns `None`, because we are not able to
17 /// make any progress at all. This is to prevent "livelock" where we
18 /// say "$0 is WF if $0 is WF".
19 pub fn obligations<'a, 'tcx>(
20 infcx: &InferCtxt<'a, 'tcx>,
21 param_env: ty::ParamEnv<'tcx>,
25 ) -> Option<Vec<traits::PredicateObligation<'tcx>>> {
26 let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
28 debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
29 let result = wf.normalize();
30 debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
33 None // no progress made, return None
37 /// Returns the obligations that make this trait reference
38 /// well-formed. For example, if there is a trait `Set` defined like
39 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
41 pub fn trait_obligations<'a, 'tcx>(
42 infcx: &InferCtxt<'a, 'tcx>,
43 param_env: ty::ParamEnv<'tcx>,
45 trait_ref: &ty::TraitRef<'tcx>,
47 item: Option<&'tcx hir::Item<'tcx>>,
48 ) -> Vec<traits::PredicateObligation<'tcx>> {
49 let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item };
50 wf.compute_trait_ref(trait_ref, Elaborate::All);
54 pub fn predicate_obligations<'a, 'tcx>(
55 infcx: &InferCtxt<'a, 'tcx>,
56 param_env: ty::ParamEnv<'tcx>,
58 predicate: &ty::Predicate<'tcx>,
60 ) -> Vec<traits::PredicateObligation<'tcx>> {
61 let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![], item: None };
63 // (*) ok to skip binders, because wf code is prepared for it
65 ty::Predicate::Trait(ref t) => {
66 wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
68 ty::Predicate::RegionOutlives(..) => {}
69 ty::Predicate::TypeOutlives(ref t) => {
70 wf.compute(t.skip_binder().0);
72 ty::Predicate::Projection(ref t) => {
73 let t = t.skip_binder(); // (*)
74 wf.compute_projection(t.projection_ty);
77 ty::Predicate::WellFormed(t) => {
80 ty::Predicate::ObjectSafe(_) => {}
81 ty::Predicate::ClosureKind(..) => {}
82 ty::Predicate::Subtype(ref data) => {
83 wf.compute(data.skip_binder().a); // (*)
84 wf.compute(data.skip_binder().b); // (*)
86 ty::Predicate::ConstEvaluatable(def_id, substs) => {
87 let obligations = wf.nominal_obligations(def_id, substs);
88 wf.out.extend(obligations);
90 for ty in substs.types() {
99 struct WfPredicates<'a, 'tcx> {
100 infcx: &'a InferCtxt<'a, 'tcx>,
101 param_env: ty::ParamEnv<'tcx>,
104 out: Vec<traits::PredicateObligation<'tcx>>,
105 item: Option<&'tcx hir::Item<'tcx>>,
108 /// Controls whether we "elaborate" supertraits and so forth on the WF
109 /// predicates. This is a kind of hack to address #43784. The
110 /// underlying problem in that issue was a trait structure like:
113 /// trait Foo: Copy { }
114 /// trait Bar: Foo { }
115 /// impl<T: Bar> Foo for T { }
116 /// impl<T> Bar for T { }
119 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
120 /// we decide that this is true because `T: Bar` is in the
121 /// where-clauses (and we can elaborate that to include `T:
122 /// Copy`). This wouldn't be a problem, except that when we check the
123 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
124 /// impl. And so nowhere did we check that `T: Copy` holds!
126 /// To resolve this, we elaborate the WF requirements that must be
127 /// proven when checking impls. This means that (e.g.) the `impl Bar
128 /// for T` will be forced to prove not only that `T: Foo` but also `T:
129 /// Copy` (which it won't be able to do, because there is no `Copy`
131 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
137 impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
138 fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
139 traits::ObligationCause::new(self.span, self.body_id, code)
142 fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
143 let cause = self.cause(traits::MiscObligation);
144 let infcx = &mut self.infcx;
145 let param_env = self.param_env;
148 .inspect(|pred| assert!(!pred.has_escaping_bound_vars()))
150 let mut selcx = traits::SelectionContext::new(infcx);
151 let pred = traits::normalize(&mut selcx, param_env, cause.clone(), pred);
152 once(pred.value).chain(pred.obligations)
157 /// Pushes the obligations required for `trait_ref` to be WF into `self.out`.
158 fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
159 let tcx = self.infcx.tcx;
160 let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
162 let cause = self.cause(traits::MiscObligation);
163 let param_env = self.param_env;
165 let item = &self.item;
166 let extend_cause_with_original_assoc_item_obligation =
167 |cause: &mut traits::ObligationCause<'_>,
168 pred: &ty::Predicate<'_>,
169 trait_assoc_items: ty::AssocItemsIterator<'_>| {
172 .as_local_hir_id(trait_ref.def_id)
173 .and_then(|trait_id| tcx.hir().find(trait_id));
174 let (trait_name, trait_generics) = match trait_item {
175 Some(hir::Node::Item(hir::Item {
177 kind: hir::ItemKind::Trait(.., generics, _, _),
180 | Some(hir::Node::Item(hir::Item {
182 kind: hir::ItemKind::TraitAlias(generics, _),
184 })) => (Some(ident), Some(generics)),
188 let item_span = item.map(|i| tcx.sess.source_map().def_span(i.span));
190 ty::Predicate::Projection(proj) => {
191 // The obligation comes not from the current `impl` nor the `trait` being
192 // implemented, but rather from a "second order" obligation, like in
193 // `src/test/ui/associated-types/point-at-type-on-obligation-failure.rs`:
195 // error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
196 // --> $DIR/point-at-type-on-obligation-failure.rs:13:5
199 // | -- associated type defined here
201 // LL | impl Bar for Foo {
202 // | ---------------- in this `impl` item
203 // LL | type Ok = ();
204 // | ^^^^^^^^^^^^^ expected `u32`, found `()`
206 // = note: expected type `u32`
209 // FIXME: we would want to point a span to all places that contributed to this
210 // obligation. In the case above, it should be closer to:
212 // error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
213 // --> $DIR/point-at-type-on-obligation-failure.rs:13:5
216 // | -- associated type defined here
217 // LL | type Sibling: Bar2<Ok=Self::Ok>;
218 // | -------------------------------- obligation set here
220 // LL | impl Bar for Foo {
221 // | ---------------- in this `impl` item
222 // LL | type Ok = ();
223 // | ^^^^^^^^^^^^^ expected `u32`, found `()`
225 // LL | impl Bar2 for Foo2 {
226 // | ---------------- in this `impl` item
227 // LL | type Ok = u32;
228 // | -------------- obligation set here
230 // = note: expected type `u32`
232 if let Some(hir::ItemKind::Impl(.., impl_items)) = item.map(|i| &i.kind) {
233 let trait_assoc_item = tcx.associated_item(proj.projection_def_id());
234 if let Some(impl_item) = impl_items
236 .filter(|item| item.ident == trait_assoc_item.ident)
239 cause.span = impl_item.span;
240 cause.code = traits::AssocTypeBound(Box::new(AssocTypeBoundData {
241 impl_span: item_span,
242 original: trait_assoc_item.ident.span,
248 ty::Predicate::Trait(proj) => {
249 // An associated item obligation born out of the `trait` failed to be met.
250 // Point at the `impl` that failed the obligation, the associated item that
251 // needed to meet the obligation, and the definition of that associated item,
252 // which should hold the obligation in most cases. An example can be seen in
253 // `src/test/ui/associated-types/point-at-type-on-obligation-failure-2.rs`:
255 // error[E0277]: the trait bound `bool: Bar` is not satisfied
256 // --> $DIR/point-at-type-on-obligation-failure-2.rs:8:5
258 // LL | type Assoc: Bar;
259 // | ----- associated type defined here
261 // LL | impl Foo for () {
262 // | --------------- in this `impl` item
263 // LL | type Assoc = bool;
264 // | ^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `bool`
266 // If the obligation comes from the where clause in the `trait`, we point at it:
268 // error[E0277]: the trait bound `bool: Bar` is not satisfied
269 // --> $DIR/point-at-type-on-obligation-failure-2.rs:8:5
271 // | trait Foo where <Self as Foo>>::Assoc: Bar {
272 // | -------------------------- restricted in this bound
274 // | ----- associated type defined here
276 // LL | impl Foo for () {
277 // | --------------- in this `impl` item
278 // LL | type Assoc = bool;
279 // | ^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `bool`
281 ty::Projection(ty::ProjectionTy { item_def_id, .. }),
282 Some(hir::ItemKind::Impl(.., impl_items)),
283 ) = (&proj.skip_binder().self_ty().kind, item.map(|i| &i.kind))
285 if let Some((impl_item, trait_assoc_item)) = trait_assoc_items
286 .filter(|i| i.def_id == *item_def_id)
288 .and_then(|trait_assoc_item| {
291 .filter(|i| i.ident == trait_assoc_item.ident)
293 .map(|impl_item| (impl_item, trait_assoc_item))
296 let bounds = trait_generics
298 get_generic_bound_spans(
301 trait_assoc_item.ident,
304 .unwrap_or_else(Vec::new);
305 cause.span = impl_item.span;
306 cause.code = traits::AssocTypeBound(Box::new(AssocTypeBoundData {
307 impl_span: item_span,
308 original: trait_assoc_item.ident.span,
318 if let Elaborate::All = elaborate {
319 let trait_assoc_items = tcx.associated_items(trait_ref.def_id);
322 obligations.iter().map(|obligation| obligation.predicate.clone()).collect();
323 let implied_obligations = traits::elaborate_predicates(tcx, predicates);
324 let implied_obligations = implied_obligations.map(|pred| {
325 let mut cause = cause.clone();
326 extend_cause_with_original_assoc_item_obligation(
329 trait_assoc_items.clone(),
331 traits::Obligation::new(cause, param_env, pred)
333 self.out.extend(implied_obligations);
336 self.out.extend(obligations);
338 self.out.extend(trait_ref.substs.types().filter(|ty| !ty.has_escaping_bound_vars()).map(
339 |ty| traits::Obligation::new(cause.clone(), param_env, ty::Predicate::WellFormed(ty)),
343 /// Pushes the obligations required for `trait_ref::Item` to be WF
345 fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
346 // A projection is well-formed if (a) the trait ref itself is
347 // WF and (b) the trait-ref holds. (It may also be
348 // normalizable and be WF that way.)
349 let trait_ref = data.trait_ref(self.infcx.tcx);
350 self.compute_trait_ref(&trait_ref, Elaborate::None);
352 if !data.has_escaping_bound_vars() {
353 let predicate = trait_ref.to_predicate();
354 let cause = self.cause(traits::ProjectionWf(data));
355 self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
359 /// Pushes the obligations required for an array length to be WF
361 fn compute_array_len(&mut self, constant: ty::Const<'tcx>) {
362 if let ty::ConstKind::Unevaluated(def_id, substs, promoted) = constant.val {
363 assert!(promoted.is_none());
365 let obligations = self.nominal_obligations(def_id, substs);
366 self.out.extend(obligations);
368 let predicate = ty::Predicate::ConstEvaluatable(def_id, substs);
369 let cause = self.cause(traits::MiscObligation);
370 self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
374 fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
375 if !subty.has_escaping_bound_vars() {
376 let cause = self.cause(cause);
377 let trait_ref = ty::TraitRef {
378 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None),
379 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
381 self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
385 /// Pushes new obligations into `out`. Returns `true` if it was able
386 /// to generate all the predicates needed to validate that `ty0`
387 /// is WF. Returns false if `ty0` is an unresolved type variable,
388 /// in which case we are not able to simplify at all.
389 fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
390 let mut subtys = ty0.walk();
391 let param_env = self.param_env;
392 while let Some(ty) = subtys.next() {
401 | ty::GeneratorWitness(..)
405 | ty::Placeholder(..)
406 | ty::Foreign(..) => {
407 // WfScalar, WfParameter, etc
410 ty::Slice(subty) => {
411 self.require_sized(subty, traits::SliceOrArrayElem);
414 ty::Array(subty, len) => {
415 self.require_sized(subty, traits::SliceOrArrayElem);
416 self.compute_array_len(*len);
419 ty::Tuple(ref tys) => {
420 if let Some((_last, rest)) = tys.split_last() {
422 self.require_sized(elem.expect_ty(), traits::TupleElem);
428 // simple cases that are WF if their type args are WF
431 ty::Projection(data) => {
432 subtys.skip_current_subtree(); // subtree handled by compute_projection
433 self.compute_projection(data);
436 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
438 ty::Adt(def, substs) => {
440 let obligations = self.nominal_obligations(def.did, substs);
441 self.out.extend(obligations);
444 ty::FnDef(did, substs) => {
445 let obligations = self.nominal_obligations(did, substs);
446 self.out.extend(obligations);
449 ty::Ref(r, rty, _) => {
451 if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
452 let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
453 self.out.push(traits::Obligation::new(
456 ty::Predicate::TypeOutlives(ty::Binder::dummy(ty::OutlivesPredicate(
463 ty::Generator(..) => {
464 // Walk ALL the types in the generator: this will
465 // include the upvar types as well as the yield
466 // type. Note that this is mildly distinct from
467 // the closure case, where we have to be careful
468 // about the signature of the closure. We don't
469 // have the problem of implied bounds here since
470 // generators don't take arguments.
473 ty::Closure(def_id, substs) => {
474 // Only check the upvar types for WF, not the rest
475 // of the types within. This is needed because we
476 // capture the signature and it may not be WF
477 // without the implied bounds. Consider a closure
478 // like `|x: &'a T|` -- it may be that `T: 'a` is
479 // not known to hold in the creator's context (and
480 // indeed the closure may not be invoked by its
481 // creator, but rather turned to someone who *can*
484 // The special treatment of closures here really
485 // ought not to be necessary either; the problem
486 // is related to #25860 -- there is no way for us
487 // to express a fn type complete with the implied
488 // bounds that it is assuming. I think in reality
489 // the WF rules around fn are a bit messed up, and
490 // that is the rot problem: `fn(&'a T)` should
491 // probably always be WF, because it should be
492 // shorthand for something like `where(T: 'a) {
493 // fn(&'a T) }`, as discussed in #25860.
495 // Note that we are also skipping the generic
496 // types. This is consistent with the `outlives`
497 // code, but anyway doesn't matter: within the fn
498 // body where they are created, the generics will
499 // always be WF, and outside of that fn body we
500 // are not directly inspecting closure types
501 // anyway, except via auto trait matching (which
502 // only inspects the upvar types).
503 subtys.skip_current_subtree(); // subtree handled by compute_projection
504 for upvar_ty in substs.as_closure().upvar_tys(def_id, self.infcx.tcx) {
505 self.compute(upvar_ty);
510 // let the loop iterate into the argument/return
511 // types appearing in the fn signature
514 ty::Opaque(did, substs) => {
515 // all of the requirements on type parameters
516 // should've been checked by the instantiation
517 // of whatever returned this exact `impl Trait`.
519 // for named opaque `impl Trait` types we still need to check them
520 if ty::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
521 let obligations = self.nominal_obligations(did, substs);
522 self.out.extend(obligations);
526 ty::Dynamic(data, r) => {
529 // Here, we defer WF checking due to higher-ranked
530 // regions. This is perhaps not ideal.
531 self.from_object_ty(ty, data, r);
533 // FIXME(#27579) RFC also considers adding trait
534 // obligations that don't refer to Self and
537 let defer_to_coercion = self.infcx.tcx.features().object_safe_for_dispatch;
539 if !defer_to_coercion {
540 let cause = self.cause(traits::MiscObligation);
541 let component_traits = data.auto_traits().chain(data.principal_def_id());
542 self.out.extend(component_traits.map(|did| {
543 traits::Obligation::new(
546 ty::Predicate::ObjectSafe(did),
552 // Inference variables are the complicated case, since we don't
553 // know what type they are. We do two things:
555 // 1. Check if they have been resolved, and if so proceed with
557 // 2. If not, check whether this is the type that we
558 // started with (ty0). In that case, we've made no
559 // progress at all, so return false. Otherwise,
560 // we've at least simplified things (i.e., we went
561 // from `Vec<$0>: WF` to `$0: WF`, so we can
562 // register a pending obligation and keep
563 // moving. (Goal is that an "inductive hypothesis"
564 // is satisfied to ensure termination.)
566 let ty = self.infcx.shallow_resolve(ty);
567 if let ty::Infer(_) = ty.kind {
568 // not yet resolved...
570 // ...this is the type we started from! no progress.
574 let cause = self.cause(traits::MiscObligation);
576 // ...not the type we started from, so we made progress.
577 traits::Obligation::new(
580 ty::Predicate::WellFormed(ty),
584 // Yes, resolved, proceed with the
585 // result. Should never return false because
586 // `ty` is not a Infer.
587 assert!(self.compute(ty));
593 // if we made it through that loop above, we made progress!
597 fn nominal_obligations(
600 substs: SubstsRef<'tcx>,
601 ) -> Vec<traits::PredicateObligation<'tcx>> {
602 let predicates = self.infcx.tcx.predicates_of(def_id).instantiate(self.infcx.tcx, substs);
603 let cause = self.cause(traits::ItemObligation(def_id));
607 .map(|pred| traits::Obligation::new(cause.clone(), self.param_env, pred))
608 .filter(|pred| !pred.has_escaping_bound_vars())
615 data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
616 region: ty::Region<'tcx>,
618 // Imagine a type like this:
621 // trait Bar<'c> : 'c { }
623 // &'b (Foo+'c+Bar<'d>)
626 // In this case, the following relationships must hold:
631 // The first conditions is due to the normal region pointer
632 // rules, which say that a reference cannot outlive its
635 // The final condition may be a bit surprising. In particular,
636 // you may expect that it would have been `'c <= 'd`, since
637 // usually lifetimes of outer things are conservative
638 // approximations for inner things. However, it works somewhat
639 // differently with trait objects: here the idea is that if the
640 // user specifies a region bound (`'c`, in this case) it is the
641 // "master bound" that *implies* that bounds from other traits are
642 // all met. (Remember that *all bounds* in a type like
643 // `Foo+Bar+Zed` must be met, not just one, hence if we write
644 // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
647 // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
648 // am looking forward to the future here.
649 if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() {
650 let implicit_bounds = object_region_bounds(self.infcx.tcx, data);
652 let explicit_bound = region;
654 self.out.reserve(implicit_bounds.len());
655 for implicit_bound in implicit_bounds {
656 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
658 ty::Binder::dummy(ty::OutlivesPredicate(explicit_bound, implicit_bound));
659 self.out.push(traits::Obligation::new(
662 outlives.to_predicate(),
669 /// Given an object type like `SomeTrait + Send`, computes the lifetime
670 /// bounds that must hold on the elided self type. These are derived
671 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
672 /// they declare `trait SomeTrait : 'static`, for example, then
673 /// `'static` would appear in the list. The hard work is done by
674 /// `infer::required_region_bounds`, see that for more information.
675 pub fn object_region_bounds<'tcx>(
677 existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
678 ) -> Vec<ty::Region<'tcx>> {
679 // Since we don't actually *know* the self type for an object,
680 // this "open(err)" serves as a kind of dummy standin -- basically
681 // a placeholder type.
682 let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
684 let predicates = existential_predicates
686 .filter_map(|predicate| {
687 if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
690 Some(predicate.with_self_ty(tcx, open_ty))
695 required_region_bounds(tcx, open_ty, predicates)
698 /// Find the span of a generic bound affecting an associated type.
699 fn get_generic_bound_spans(
700 generics: &hir::Generics<'_>,
701 trait_name: Option<&Ident>,
702 assoc_item_name: Ident,
704 let mut bounds = vec![];
705 for clause in generics.where_clause.predicates.iter() {
706 if let hir::WherePredicate::BoundPredicate(pred) = clause {
707 match &pred.bounded_ty.kind {
708 hir::TyKind::Path(hir::QPath::Resolved(Some(ty), path)) => {
709 let mut s = path.segments.iter();
710 if let (a, Some(b), None) = (s.next(), s.next(), s.next()) {
711 if a.map(|s| &s.ident) == trait_name
712 && b.ident == assoc_item_name
713 && is_self_path(&ty.kind)
715 // `<Self as Foo>::Bar`
716 bounds.push(pred.span);
720 hir::TyKind::Path(hir::QPath::TypeRelative(ty, segment)) => {
721 if segment.ident == assoc_item_name {
722 if is_self_path(&ty.kind) {
724 bounds.push(pred.span);
735 fn is_self_path(kind: &hir::TyKind<'_>) -> bool {
737 hir::TyKind::Path(hir::QPath::Resolved(None, path)) => {
738 let mut s = path.segments.iter();
739 if let (Some(segment), None) = (s.next(), s.next()) {
740 if segment.ident.name == kw::SelfUpper {