| use crate::hir; |
| use crate::hir::def_id::DefId; |
| use crate::infer::InferCtxt; |
| use crate::ty::subst::SubstsRef; |
| use crate::traits; |
| use crate::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable}; |
| use std::iter::once; |
| use syntax_pos::Span; |
| use crate::middle::lang_items; |
| use crate::mir::interpret::ConstValue; |
| |
| /// Returns the set of obligations needed to make `ty` well-formed. |
| /// If `ty` contains unresolved inference variables, this may include |
| /// further WF obligations. However, if `ty` IS an unresolved |
| /// inference variable, returns `None`, because we are not able to |
| /// make any progress at all. This is to prevent "livelock" where we |
| /// say "$0 is WF if $0 is WF". |
| pub fn obligations<'a, 'tcx>( |
| infcx: &InferCtxt<'a, 'tcx>, |
| param_env: ty::ParamEnv<'tcx>, |
| body_id: hir::HirId, |
| ty: Ty<'tcx>, |
| span: Span, |
| ) -> Option<Vec<traits::PredicateObligation<'tcx>>> { |
| let mut wf = WfPredicates { infcx, |
| param_env, |
| body_id, |
| span, |
| out: vec![] }; |
| if wf.compute(ty) { |
| debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out); |
| let result = wf.normalize(); |
| debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result); |
| Some(result) |
| } else { |
| None // no progress made, return None |
| } |
| } |
| |
| /// Returns the obligations that make this trait reference |
| /// well-formed. For example, if there is a trait `Set` defined like |
| /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF |
| /// if `Bar: Eq`. |
| pub fn trait_obligations<'a, 'tcx>( |
| infcx: &InferCtxt<'a, 'tcx>, |
| param_env: ty::ParamEnv<'tcx>, |
| body_id: hir::HirId, |
| trait_ref: &ty::TraitRef<'tcx>, |
| span: Span, |
| ) -> Vec<traits::PredicateObligation<'tcx>> { |
| let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] }; |
| wf.compute_trait_ref(trait_ref, Elaborate::All); |
| wf.normalize() |
| } |
| |
| pub fn predicate_obligations<'a, 'tcx>( |
| infcx: &InferCtxt<'a, 'tcx>, |
| param_env: ty::ParamEnv<'tcx>, |
| body_id: hir::HirId, |
| predicate: &ty::Predicate<'tcx>, |
| span: Span, |
| ) -> Vec<traits::PredicateObligation<'tcx>> { |
| let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] }; |
| |
| // (*) ok to skip binders, because wf code is prepared for it |
| match *predicate { |
| ty::Predicate::Trait(ref t) => { |
| wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*) |
| } |
| ty::Predicate::RegionOutlives(..) => { |
| } |
| ty::Predicate::TypeOutlives(ref t) => { |
| wf.compute(t.skip_binder().0); |
| } |
| ty::Predicate::Projection(ref t) => { |
| let t = t.skip_binder(); // (*) |
| wf.compute_projection(t.projection_ty); |
| wf.compute(t.ty); |
| } |
| ty::Predicate::WellFormed(t) => { |
| wf.compute(t); |
| } |
| ty::Predicate::ObjectSafe(_) => { |
| } |
| ty::Predicate::ClosureKind(..) => { |
| } |
| ty::Predicate::Subtype(ref data) => { |
| wf.compute(data.skip_binder().a); // (*) |
| wf.compute(data.skip_binder().b); // (*) |
| } |
| ty::Predicate::ConstEvaluatable(def_id, substs) => { |
| let obligations = wf.nominal_obligations(def_id, substs); |
| wf.out.extend(obligations); |
| |
| for ty in substs.types() { |
| wf.compute(ty); |
| } |
| } |
| } |
| |
| wf.normalize() |
| } |
| |
| struct WfPredicates<'a, 'tcx> { |
| infcx: &'a InferCtxt<'a, 'tcx>, |
| param_env: ty::ParamEnv<'tcx>, |
| body_id: hir::HirId, |
| span: Span, |
| out: Vec<traits::PredicateObligation<'tcx>>, |
| } |
| |
| /// Controls whether we "elaborate" supertraits and so forth on the WF |
| /// predicates. This is a kind of hack to address #43784. The |
| /// underlying problem in that issue was a trait structure like: |
| /// |
| /// ``` |
| /// trait Foo: Copy { } |
| /// trait Bar: Foo { } |
| /// impl<T: Bar> Foo for T { } |
| /// impl<T> Bar for T { } |
| /// ``` |
| /// |
| /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but |
| /// we decide that this is true because `T: Bar` is in the |
| /// where-clauses (and we can elaborate that to include `T: |
| /// Copy`). This wouldn't be a problem, except that when we check the |
| /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo` |
| /// impl. And so nowhere did we check that `T: Copy` holds! |
| /// |
| /// To resolve this, we elaborate the WF requirements that must be |
| /// proven when checking impls. This means that (e.g.) the `impl Bar |
| /// for T` will be forced to prove not only that `T: Foo` but also `T: |
| /// Copy` (which it won't be able to do, because there is no `Copy` |
| /// impl for `T`). |
| #[derive(Debug, PartialEq, Eq, Copy, Clone)] |
| enum Elaborate { |
| All, |
| None, |
| } |
| |
| impl<'a, 'tcx> WfPredicates<'a, 'tcx> { |
| fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> { |
| traits::ObligationCause::new(self.span, self.body_id, code) |
| } |
| |
| fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> { |
| let cause = self.cause(traits::MiscObligation); |
| let infcx = &mut self.infcx; |
| let param_env = self.param_env; |
| self.out.iter() |
| .inspect(|pred| assert!(!pred.has_escaping_bound_vars())) |
| .flat_map(|pred| { |
| let mut selcx = traits::SelectionContext::new(infcx); |
| let pred = traits::normalize(&mut selcx, param_env, cause.clone(), pred); |
| once(pred.value).chain(pred.obligations) |
| }) |
| .collect() |
| } |
| |
| /// Pushes the obligations required for `trait_ref` to be WF into |
| /// `self.out`. |
| fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) { |
| let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs); |
| |
| let cause = self.cause(traits::MiscObligation); |
| let param_env = self.param_env; |
| |
| if let Elaborate::All = elaborate { |
| let predicates = obligations.iter() |
| .map(|obligation| obligation.predicate.clone()) |
| .collect(); |
| let implied_obligations = traits::elaborate_predicates(self.infcx.tcx, predicates); |
| let implied_obligations = implied_obligations.map(|pred| { |
| traits::Obligation::new(cause.clone(), param_env, pred) |
| }); |
| self.out.extend(implied_obligations); |
| } |
| |
| self.out.extend(obligations); |
| |
| self.out.extend( |
| trait_ref.substs.types() |
| .filter(|ty| !ty.has_escaping_bound_vars()) |
| .map(|ty| traits::Obligation::new(cause.clone(), |
| param_env, |
| ty::Predicate::WellFormed(ty)))); |
| } |
| |
| /// Pushes the obligations required for `trait_ref::Item` to be WF |
| /// into `self.out`. |
| fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) { |
| // A projection is well-formed if (a) the trait ref itself is |
| // WF and (b) the trait-ref holds. (It may also be |
| // normalizable and be WF that way.) |
| let trait_ref = data.trait_ref(self.infcx.tcx); |
| self.compute_trait_ref(&trait_ref, Elaborate::None); |
| |
| if !data.has_escaping_bound_vars() { |
| let predicate = trait_ref.to_predicate(); |
| let cause = self.cause(traits::ProjectionWf(data)); |
| self.out.push(traits::Obligation::new(cause, self.param_env, predicate)); |
| } |
| } |
| |
| /// Pushes the obligations required for an array length to be WF |
| /// into `self.out`. |
| fn compute_array_len(&mut self, constant: ty::Const<'tcx>) { |
| if let ConstValue::Unevaluated(def_id, substs) = constant.val { |
| let obligations = self.nominal_obligations(def_id, substs); |
| self.out.extend(obligations); |
| |
| let predicate = ty::Predicate::ConstEvaluatable(def_id, substs); |
| let cause = self.cause(traits::MiscObligation); |
| self.out.push(traits::Obligation::new(cause, |
| self.param_env, |
| predicate)); |
| } |
| } |
| |
| fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) { |
| if !subty.has_escaping_bound_vars() { |
| let cause = self.cause(cause); |
| let trait_ref = ty::TraitRef { |
| def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None), |
| substs: self.infcx.tcx.mk_substs_trait(subty, &[]), |
| }; |
| self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate())); |
| } |
| } |
| |
| /// Pushes new obligations into `out`. Returns `true` if it was able |
| /// to generate all the predicates needed to validate that `ty0` |
| /// is WF. Returns false if `ty0` is an unresolved type variable, |
| /// in which case we are not able to simplify at all. |
| fn compute(&mut self, ty0: Ty<'tcx>) -> bool { |
| let mut subtys = ty0.walk(); |
| let param_env = self.param_env; |
| while let Some(ty) = subtys.next() { |
| match ty.sty { |
| ty::Bool | |
| ty::Char | |
| ty::Int(..) | |
| ty::Uint(..) | |
| ty::Float(..) | |
| ty::Error | |
| ty::Str | |
| ty::GeneratorWitness(..) | |
| ty::Never | |
| ty::Param(_) | |
| ty::Bound(..) | |
| ty::Placeholder(..) | |
| ty::Foreign(..) => { |
| // WfScalar, WfParameter, etc |
| } |
| |
| ty::Slice(subty) => { |
| self.require_sized(subty, traits::SliceOrArrayElem); |
| } |
| |
| ty::Array(subty, len) => { |
| self.require_sized(subty, traits::SliceOrArrayElem); |
| self.compute_array_len(*len); |
| } |
| |
| ty::Tuple(ref tys) => { |
| if let Some((_last, rest)) = tys.split_last() { |
| for elem in rest { |
| self.require_sized(elem.expect_ty(), traits::TupleElem); |
| } |
| } |
| } |
| |
| ty::RawPtr(_) => { |
| // simple cases that are WF if their type args are WF |
| } |
| |
| ty::Projection(data) => { |
| subtys.skip_current_subtree(); // subtree handled by compute_projection |
| self.compute_projection(data); |
| } |
| |
| ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"), |
| |
| ty::Adt(def, substs) => { |
| // WfNominalType |
| let obligations = self.nominal_obligations(def.did, substs); |
| self.out.extend(obligations); |
| } |
| |
| ty::FnDef(did, substs) => { |
| let obligations = self.nominal_obligations(did, substs); |
| self.out.extend(obligations); |
| } |
| |
| ty::Ref(r, rty, _) => { |
| // WfReference |
| if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() { |
| let cause = self.cause(traits::ReferenceOutlivesReferent(ty)); |
| self.out.push( |
| traits::Obligation::new( |
| cause, |
| param_env, |
| ty::Predicate::TypeOutlives( |
| ty::Binder::dummy( |
| ty::OutlivesPredicate(rty, r))))); |
| } |
| } |
| |
| ty::Generator(..) => { |
| // Walk ALL the types in the generator: this will |
| // include the upvar types as well as the yield |
| // type. Note that this is mildly distinct from |
| // the closure case, where we have to be careful |
| // about the signature of the closure. We don't |
| // have the problem of implied bounds here since |
| // generators don't take arguments. |
| } |
| |
| ty::Closure(def_id, substs) => { |
| // Only check the upvar types for WF, not the rest |
| // of the types within. This is needed because we |
| // capture the signature and it may not be WF |
| // without the implied bounds. Consider a closure |
| // like `|x: &'a T|` -- it may be that `T: 'a` is |
| // not known to hold in the creator's context (and |
| // indeed the closure may not be invoked by its |
| // creator, but rather turned to someone who *can* |
| // verify that). |
| // |
| // The special treatment of closures here really |
| // ought not to be necessary either; the problem |
| // is related to #25860 -- there is no way for us |
| // to express a fn type complete with the implied |
| // bounds that it is assuming. I think in reality |
| // the WF rules around fn are a bit messed up, and |
| // that is the rot problem: `fn(&'a T)` should |
| // probably always be WF, because it should be |
| // shorthand for something like `where(T: 'a) { |
| // fn(&'a T) }`, as discussed in #25860. |
| // |
| // Note that we are also skipping the generic |
| // types. This is consistent with the `outlives` |
| // code, but anyway doesn't matter: within the fn |
| // body where they are created, the generics will |
| // always be WF, and outside of that fn body we |
| // are not directly inspecting closure types |
| // anyway, except via auto trait matching (which |
| // only inspects the upvar types). |
| subtys.skip_current_subtree(); // subtree handled by compute_projection |
| for upvar_ty in substs.upvar_tys(def_id, self.infcx.tcx) { |
| self.compute(upvar_ty); |
| } |
| } |
| |
| ty::FnPtr(_) => { |
| // let the loop iterate into the argument/return |
| // types appearing in the fn signature |
| } |
| |
| ty::Opaque(did, substs) => { |
| // all of the requirements on type parameters |
| // should've been checked by the instantiation |
| // of whatever returned this exact `impl Trait`. |
| |
| // for named opaque `impl Trait` types we still need to check them |
| if super::is_impl_trait_defn(self.infcx.tcx, did).is_none() { |
| let obligations = self.nominal_obligations(did, substs); |
| self.out.extend(obligations); |
| } |
| } |
| |
| ty::Dynamic(data, r) => { |
| // WfObject |
| // |
| // Here, we defer WF checking due to higher-ranked |
| // regions. This is perhaps not ideal. |
| self.from_object_ty(ty, data, r); |
| |
| // FIXME(#27579) RFC also considers adding trait |
| // obligations that don't refer to Self and |
| // checking those |
| |
| let cause = self.cause(traits::MiscObligation); |
| let component_traits = |
| data.auto_traits().chain(data.principal_def_id()); |
| self.out.extend( |
| component_traits.map(|did| traits::Obligation::new( |
| cause.clone(), |
| param_env, |
| ty::Predicate::ObjectSafe(did) |
| )) |
| ); |
| } |
| |
| // Inference variables are the complicated case, since we don't |
| // know what type they are. We do two things: |
| // |
| // 1. Check if they have been resolved, and if so proceed with |
| // THAT type. |
| // 2. If not, check whether this is the type that we |
| // started with (ty0). In that case, we've made no |
| // progress at all, so return false. Otherwise, |
| // we've at least simplified things (i.e., we went |
| // from `Vec<$0>: WF` to `$0: WF`, so we can |
| // register a pending obligation and keep |
| // moving. (Goal is that an "inductive hypothesis" |
| // is satisfied to ensure termination.) |
| ty::Infer(_) => { |
| let ty = self.infcx.shallow_resolve(ty); |
| if let ty::Infer(_) = ty.sty { // not yet resolved... |
| if ty == ty0 { // ...this is the type we started from! no progress. |
| return false; |
| } |
| |
| let cause = self.cause(traits::MiscObligation); |
| self.out.push( // ...not the type we started from, so we made progress. |
| traits::Obligation::new(cause, |
| self.param_env, |
| ty::Predicate::WellFormed(ty))); |
| } else { |
| // Yes, resolved, proceed with the |
| // result. Should never return false because |
| // `ty` is not a Infer. |
| assert!(self.compute(ty)); |
| } |
| } |
| } |
| } |
| |
| // if we made it through that loop above, we made progress! |
| return true; |
| } |
| |
| fn nominal_obligations(&mut self, |
| def_id: DefId, |
| substs: SubstsRef<'tcx>) |
| -> Vec<traits::PredicateObligation<'tcx>> |
| { |
| let predicates = |
| self.infcx.tcx.predicates_of(def_id) |
| .instantiate(self.infcx.tcx, substs); |
| let cause = self.cause(traits::ItemObligation(def_id)); |
| predicates.predicates |
| .into_iter() |
| .map(|pred| traits::Obligation::new(cause.clone(), |
| self.param_env, |
| pred)) |
| .filter(|pred| !pred.has_escaping_bound_vars()) |
| .collect() |
| } |
| |
| fn from_object_ty(&mut self, ty: Ty<'tcx>, |
| data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>, |
| region: ty::Region<'tcx>) { |
| // Imagine a type like this: |
| // |
| // trait Foo { } |
| // trait Bar<'c> : 'c { } |
| // |
| // &'b (Foo+'c+Bar<'d>) |
| // ^ |
| // |
| // In this case, the following relationships must hold: |
| // |
| // 'b <= 'c |
| // 'd <= 'c |
| // |
| // The first conditions is due to the normal region pointer |
| // rules, which say that a reference cannot outlive its |
| // referent. |
| // |
| // The final condition may be a bit surprising. In particular, |
| // you may expect that it would have been `'c <= 'd`, since |
| // usually lifetimes of outer things are conservative |
| // approximations for inner things. However, it works somewhat |
| // differently with trait objects: here the idea is that if the |
| // user specifies a region bound (`'c`, in this case) it is the |
| // "master bound" that *implies* that bounds from other traits are |
| // all met. (Remember that *all bounds* in a type like |
| // `Foo+Bar+Zed` must be met, not just one, hence if we write |
| // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and |
| // 'y.) |
| // |
| // Note: in fact we only permit builtin traits, not `Bar<'d>`, I |
| // am looking forward to the future here. |
| if !data.has_escaping_bound_vars() && !region.has_escaping_bound_vars() { |
| let implicit_bounds = |
| object_region_bounds(self.infcx.tcx, data); |
| |
| let explicit_bound = region; |
| |
| self.out.reserve(implicit_bounds.len()); |
| for implicit_bound in implicit_bounds { |
| let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound)); |
| let outlives = ty::Binder::dummy( |
| ty::OutlivesPredicate(explicit_bound, implicit_bound)); |
| self.out.push(traits::Obligation::new(cause, |
| self.param_env, |
| outlives.to_predicate())); |
| } |
| } |
| } |
| } |
| |
| /// Given an object type like `SomeTrait + Send`, computes the lifetime |
| /// bounds that must hold on the elided self type. These are derived |
| /// from the declarations of `SomeTrait`, `Send`, and friends -- if |
| /// they declare `trait SomeTrait : 'static`, for example, then |
| /// `'static` would appear in the list. The hard work is done by |
| /// `ty::required_region_bounds`, see that for more information. |
| pub fn object_region_bounds<'tcx>( |
| tcx: TyCtxt<'tcx>, |
| existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>, |
| ) -> Vec<ty::Region<'tcx>> { |
| // Since we don't actually *know* the self type for an object, |
| // this "open(err)" serves as a kind of dummy standin -- basically |
| // a placeholder type. |
| let open_ty = tcx.mk_ty_infer(ty::FreshTy(0)); |
| |
| let predicates = existential_predicates.iter().filter_map(|predicate| { |
| if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() { |
| None |
| } else { |
| Some(predicate.with_self_ty(tcx, open_ty)) |
| } |
| }).collect(); |
| |
| tcx.required_region_bounds(open_ty, predicates) |
| } |
