1 use crate::hir::def_id::DefId;
2 use crate::infer::InferCtxt;
3 use crate::ty::subst::Substs;
5 use crate::ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
9 use crate::middle::lang_items;
11 /// Returns the set of obligations needed to make `ty` well-formed.
12 /// If `ty` contains unresolved inference variables, this may include
13 /// further WF obligations. However, if `ty` IS an unresolved
14 /// inference variable, returns `None`, because we are not able to
15 /// make any progress at all. This is to prevent "livelock" where we
16 /// say "$0 is WF if $0 is WF".
17 pub fn obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
18 param_env: ty::ParamEnv<'tcx>,
22 -> Option<Vec<traits::PredicateObligation<'tcx>>>
24 let mut wf = WfPredicates { infcx,
30 debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
31 let result = wf.normalize();
32 debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
35 None // no progress made, return None
39 /// Returns the obligations that make this trait reference
40 /// well-formed. For example, if there is a trait `Set` defined like
41 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
43 pub fn trait_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
44 param_env: ty::ParamEnv<'tcx>,
46 trait_ref: &ty::TraitRef<'tcx>,
48 -> Vec<traits::PredicateObligation<'tcx>>
50 let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
51 wf.compute_trait_ref(trait_ref, Elaborate::All);
55 pub fn predicate_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
56 param_env: ty::ParamEnv<'tcx>,
58 predicate: &ty::Predicate<'tcx>,
60 -> Vec<traits::PredicateObligation<'tcx>>
62 let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
64 // (*) ok to skip binders, because wf code is prepared for it
66 ty::Predicate::Trait(ref t) => {
67 wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
69 ty::Predicate::RegionOutlives(..) => {
71 ty::Predicate::TypeOutlives(ref t) => {
72 wf.compute(t.skip_binder().0);
74 ty::Predicate::Projection(ref t) => {
75 let t = t.skip_binder(); // (*)
76 wf.compute_projection(t.projection_ty);
79 ty::Predicate::WellFormed(t) => {
82 ty::Predicate::ObjectSafe(_) => {
84 ty::Predicate::ClosureKind(..) => {
86 ty::Predicate::Subtype(ref data) => {
87 wf.compute(data.skip_binder().a); // (*)
88 wf.compute(data.skip_binder().b); // (*)
90 ty::Predicate::ConstEvaluatable(def_id, substs) => {
91 let obligations = wf.nominal_obligations(def_id, substs);
92 wf.out.extend(obligations);
94 for ty in substs.types() {
103 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
104 infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
105 param_env: ty::ParamEnv<'tcx>,
106 body_id: ast::NodeId,
108 out: Vec<traits::PredicateObligation<'tcx>>,
111 /// Controls whether we "elaborate" supertraits and so forth on the WF
112 /// predicates. This is a kind of hack to address #43784. The
113 /// underlying problem in that issue was a trait structure like:
116 /// trait Foo: Copy { }
117 /// trait Bar: Foo { }
118 /// impl<T: Bar> Foo for T { }
119 /// impl<T> Bar for T { }
122 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
123 /// we decide that this is true because `T: Bar` is in the
124 /// where-clauses (and we can elaborate that to include `T:
125 /// Copy`). This wouldn't be a problem, except that when we check the
126 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
127 /// impl. And so nowhere did we check that `T: Copy` holds!
129 /// To resolve this, we elaborate the WF requirements that must be
130 /// proven when checking impls. This means that (e.g.) the `impl Bar
131 /// for T` will be forced to prove not only that `T: Foo` but also `T:
132 /// Copy` (which it won't be able to do, because there is no `Copy`
134 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
140 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
141 fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
142 traits::ObligationCause::new(self.span, self.body_id, code)
145 fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
146 let cause = self.cause(traits::MiscObligation);
147 let infcx = &mut self.infcx;
148 let param_env = self.param_env;
150 .inspect(|pred| assert!(!pred.has_escaping_bound_vars()))
152 let mut selcx = traits::SelectionContext::new(infcx);
153 let pred = traits::normalize(&mut selcx, param_env, cause.clone(), pred);
154 once(pred.value).chain(pred.obligations)
159 /// Pushes the obligations required for `trait_ref` to be WF into
161 fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
162 let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
164 let cause = self.cause(traits::MiscObligation);
165 let param_env = self.param_env;
167 if let Elaborate::All = elaborate {
168 let predicates = obligations.iter()
169 .map(|obligation| obligation.predicate.clone())
171 let implied_obligations = traits::elaborate_predicates(self.infcx.tcx, predicates);
172 let implied_obligations = implied_obligations.map(|pred| {
173 traits::Obligation::new(cause.clone(), param_env, pred)
175 self.out.extend(implied_obligations);
178 self.out.extend(obligations);
181 trait_ref.substs.types()
182 .filter(|ty| !ty.has_escaping_bound_vars())
183 .map(|ty| traits::Obligation::new(cause.clone(),
185 ty::Predicate::WellFormed(ty))));
188 /// Pushes the obligations required for `trait_ref::Item` to be WF
190 fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
191 // A projection is well-formed if (a) the trait ref itself is
192 // WF and (b) the trait-ref holds. (It may also be
193 // normalizable and be WF that way.)
194 let trait_ref = data.trait_ref(self.infcx.tcx);
195 self.compute_trait_ref(&trait_ref, Elaborate::None);
197 if !data.has_escaping_bound_vars() {
198 let predicate = trait_ref.to_predicate();
199 let cause = self.cause(traits::ProjectionWf(data));
200 self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
204 /// Pushes the obligations required for an array length to be WF
206 fn compute_array_len(&mut self, constant: ty::LazyConst<'tcx>) {
207 if let ty::LazyConst::Unevaluated(def_id, substs) = constant {
208 let obligations = self.nominal_obligations(def_id, substs);
209 self.out.extend(obligations);
211 let predicate = ty::Predicate::ConstEvaluatable(def_id, substs);
212 let cause = self.cause(traits::MiscObligation);
213 self.out.push(traits::Obligation::new(cause,
219 fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
220 if !subty.has_escaping_bound_vars() {
221 let cause = self.cause(cause);
222 let trait_ref = ty::TraitRef {
223 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
224 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
226 self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
230 /// Pushes new obligations into `out`. Returns `true` if it was able
231 /// to generate all the predicates needed to validate that `ty0`
232 /// is WF. Returns false if `ty0` is an unresolved type variable,
233 /// in which case we are not able to simplify at all.
234 fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
235 let mut subtys = ty0.walk();
236 let param_env = self.param_env;
237 while let Some(ty) = subtys.next() {
246 ty::GeneratorWitness(..) |
250 ty::Placeholder(..) |
252 // WfScalar, WfParameter, etc
255 ty::Slice(subty) => {
256 self.require_sized(subty, traits::SliceOrArrayElem);
259 ty::Array(subty, len) => {
260 self.require_sized(subty, traits::SliceOrArrayElem);
261 self.compute_array_len(*len);
264 ty::Tuple(ref tys) => {
265 if let Some((_last, rest)) = tys.split_last() {
267 self.require_sized(elem, traits::TupleElem);
273 // simple cases that are WF if their type args are WF
276 ty::Projection(data) => {
277 subtys.skip_current_subtree(); // subtree handled by compute_projection
278 self.compute_projection(data);
281 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
283 ty::Adt(def, substs) => {
285 let obligations = self.nominal_obligations(def.did, substs);
286 self.out.extend(obligations);
289 ty::FnDef(did, substs) => {
290 let obligations = self.nominal_obligations(did, substs);
291 self.out.extend(obligations);
294 ty::Ref(r, rty, _) => {
296 if !r.has_escaping_bound_vars() && !rty.has_escaping_bound_vars() {
297 let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
299 traits::Obligation::new(
302 ty::Predicate::TypeOutlives(
304 ty::OutlivesPredicate(rty, r)))));
308 ty::Generator(..) => {
309 // Walk ALL the types in the generator: this will
310 // include the upvar types as well as the yield
311 // type. Note that this is mildly distinct from
312 // the closure case, where we have to be careful
313 // about the signature of the closure. We don't
314 // have the problem of implied bounds here since
315 // generators don't take arguments.
318 ty::Closure(def_id, substs) => {
319 // Only check the upvar types for WF, not the rest
320 // of the types within. This is needed because we
321 // capture the signature and it may not be WF
322 // without the implied bounds. Consider a closure
323 // like `|x: &'a T|` -- it may be that `T: 'a` is
324 // not known to hold in the creator's context (and
325 // indeed the closure may not be invoked by its
326 // creator, but rather turned to someone who *can*
329 // The special treatment of closures here really
330 // ought not to be necessary either; the problem
331 // is related to #25860 -- there is no way for us
332 // to express a fn type complete with the implied
333 // bounds that it is assuming. I think in reality
334 // the WF rules around fn are a bit messed up, and
335 // that is the rot problem: `fn(&'a T)` should
336 // probably always be WF, because it should be
337 // shorthand for something like `where(T: 'a) {
338 // fn(&'a T) }`, as discussed in #25860.
340 // Note that we are also skipping the generic
341 // types. This is consistent with the `outlives`
342 // code, but anyway doesn't matter: within the fn
343 // body where they are created, the generics will
344 // always be WF, and outside of that fn body we
345 // are not directly inspecting closure types
346 // anyway, except via auto trait matching (which
347 // only inspects the upvar types).
348 subtys.skip_current_subtree(); // subtree handled by compute_projection
349 for upvar_ty in substs.upvar_tys(def_id, self.infcx.tcx) {
350 self.compute(upvar_ty);
355 // let the loop iterate into the argument/return
356 // types appearing in the fn signature
359 ty::Opaque(did, substs) => {
360 // all of the requirements on type parameters
361 // should've been checked by the instantiation
362 // of whatever returned this exact `impl Trait`.
364 // for named existential types we still need to check them
365 if super::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
366 let obligations = self.nominal_obligations(did, substs);
367 self.out.extend(obligations);
371 ty::Dynamic(data, r) => {
374 // Here, we defer WF checking due to higher-ranked
375 // regions. This is perhaps not ideal.
376 self.from_object_ty(ty, data, r);
378 // FIXME(#27579) RFC also considers adding trait
379 // obligations that don't refer to Self and
382 let cause = self.cause(traits::MiscObligation);
383 let component_traits =
384 data.auto_traits().chain(data.principal_def_id());
386 component_traits.map(|did| traits::Obligation::new(
389 ty::Predicate::ObjectSafe(did)
394 // Inference variables are the complicated case, since we don't
395 // know what type they are. We do two things:
397 // 1. Check if they have been resolved, and if so proceed with
399 // 2. If not, check whether this is the type that we
400 // started with (ty0). In that case, we've made no
401 // progress at all, so return false. Otherwise,
402 // we've at least simplified things (i.e., we went
403 // from `Vec<$0>: WF` to `$0: WF`, so we can
404 // register a pending obligation and keep
405 // moving. (Goal is that an "inductive hypothesis"
406 // is satisfied to ensure termination.)
408 let ty = self.infcx.shallow_resolve(ty);
409 if let ty::Infer(_) = ty.sty { // not yet resolved...
410 if ty == ty0 { // ...this is the type we started from! no progress.
414 let cause = self.cause(traits::MiscObligation);
415 self.out.push( // ...not the type we started from, so we made progress.
416 traits::Obligation::new(cause,
418 ty::Predicate::WellFormed(ty)));
420 // Yes, resolved, proceed with the
421 // result. Should never return false because
422 // `ty` is not a Infer.
423 assert!(self.compute(ty));
429 // if we made it through that loop above, we made progress!
433 fn nominal_obligations(&mut self,
435 substs: &Substs<'tcx>)
436 -> Vec<traits::PredicateObligation<'tcx>>
439 self.infcx.tcx.predicates_of(def_id)
440 .instantiate(self.infcx.tcx, substs);
441 let cause = self.cause(traits::ItemObligation(def_id));
442 predicates.predicates
444 .map(|pred| traits::Obligation::new(cause.clone(),
447 .filter(|pred| !pred.has_escaping_bound_vars())
451 fn from_object_ty(&mut self, ty: Ty<'tcx>,
452 data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
453 region: ty::Region<'tcx>) {
454 // Imagine a type like this:
457 // trait Bar<'c> : 'c { }
459 // &'b (Foo+'c+Bar<'d>)
462 // In this case, the following relationships must hold:
467 // The first conditions is due to the normal region pointer
468 // rules, which say that a reference cannot outlive its
471 // The final condition may be a bit surprising. In particular,
472 // you may expect that it would have been `'c <= 'd`, since
473 // usually lifetimes of outer things are conservative
474 // approximations for inner things. However, it works somewhat
475 // differently with trait objects: here the idea is that if the
476 // user specifies a region bound (`'c`, in this case) it is the
477 // "master bound" that *implies* that bounds from other traits are
478 // all met. (Remember that *all bounds* in a type like
479 // `Foo+Bar+Zed` must be met, not just one, hence if we write
480 // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
483 // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
484 // am looking forward to the future here.
486 if !data.has_escaping_bound_vars() {
487 let implicit_bounds =
488 object_region_bounds(self.infcx.tcx, data);
490 let explicit_bound = region;
492 self.out.reserve(implicit_bounds.len());
493 for implicit_bound in implicit_bounds {
494 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
495 let outlives = ty::Binder::dummy(
496 ty::OutlivesPredicate(explicit_bound, implicit_bound));
497 self.out.push(traits::Obligation::new(cause,
499 outlives.to_predicate()));
505 /// Given an object type like `SomeTrait + Send`, computes the lifetime
506 /// bounds that must hold on the elided self type. These are derived
507 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
508 /// they declare `trait SomeTrait : 'static`, for example, then
509 /// `'static` would appear in the list. The hard work is done by
510 /// `ty::required_region_bounds`, see that for more information.
511 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
512 tcx: TyCtxt<'a, 'gcx, 'tcx>,
513 existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>)
514 -> Vec<ty::Region<'tcx>>
516 // Since we don't actually *know* the self type for an object,
517 // this "open(err)" serves as a kind of dummy standin -- basically
518 // a placeholder type.
519 let open_ty = tcx.mk_infer(ty::FreshTy(0));
521 let predicates = existential_predicates.iter().filter_map(|predicate| {
522 if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
525 Some(predicate.with_self_ty(tcx, open_ty))
529 tcx.required_region_bounds(open_ty, predicates)