1 // Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 use hir::def_id::DefId;
13 use ty::outlives::Component;
14 use ty::subst::Substs;
16 use ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
20 use middle::lang_items;
22 /// Returns the set of obligations needed to make `ty` well-formed.
23 /// If `ty` contains unresolved inference variables, this may include
24 /// further WF obligations. However, if `ty` IS an unresolved
25 /// inference variable, returns `None`, because we are not able to
26 /// make any progress at all. This is to prevent "livelock" where we
27 /// say "$0 is WF if $0 is WF".
28 pub fn obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
32 -> Option<Vec<traits::PredicateObligation<'tcx>>>
34 let mut wf = WfPredicates { infcx: infcx,
39 debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
40 let result = wf.normalize();
41 debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
44 None // no progress made, return None
48 /// Returns the obligations that make this trait reference
49 /// well-formed. For example, if there is a trait `Set` defined like
50 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
52 pub fn trait_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
54 trait_ref: &ty::TraitRef<'tcx>,
56 -> Vec<traits::PredicateObligation<'tcx>>
58 let mut wf = WfPredicates { infcx: infcx, body_id: body_id, span: span, out: vec![] };
59 wf.compute_trait_ref(trait_ref);
63 pub fn predicate_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
65 predicate: &ty::Predicate<'tcx>,
67 -> Vec<traits::PredicateObligation<'tcx>>
69 let mut wf = WfPredicates { infcx: infcx, body_id: body_id, span: span, out: vec![] };
71 // (*) ok to skip binders, because wf code is prepared for it
73 ty::Predicate::Trait(ref t) => {
74 wf.compute_trait_ref(&t.skip_binder().trait_ref); // (*)
76 ty::Predicate::Equate(ref t) => {
77 wf.compute(t.skip_binder().0);
78 wf.compute(t.skip_binder().1);
80 ty::Predicate::RegionOutlives(..) => {
82 ty::Predicate::TypeOutlives(ref t) => {
83 wf.compute(t.skip_binder().0);
85 ty::Predicate::Projection(ref t) => {
86 let t = t.skip_binder(); // (*)
87 wf.compute_projection(t.projection_ty);
90 ty::Predicate::WellFormed(t) => {
93 ty::Predicate::ObjectSafe(_) => {
95 ty::Predicate::ClosureKind(..) => {
102 /// Implied bounds are region relationships that we deduce
103 /// automatically. The idea is that (e.g.) a caller must check that a
104 /// function's argument types are well-formed immediately before
105 /// calling that fn, and hence the *callee* can assume that its
106 /// argument types are well-formed. This may imply certain relationships
107 /// between generic parameters. For example:
109 /// fn foo<'a,T>(x: &'a T)
111 /// can only be called with a `'a` and `T` such that `&'a T` is WF.
112 /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
114 pub enum ImpliedBound<'tcx> {
115 RegionSubRegion(&'tcx ty::Region, &'tcx ty::Region),
116 RegionSubParam(&'tcx ty::Region, ty::ParamTy),
117 RegionSubProjection(&'tcx ty::Region, ty::ProjectionTy<'tcx>),
120 /// Compute the implied bounds that a callee/impl can assume based on
121 /// the fact that caller/projector has ensured that `ty` is WF. See
122 /// the `ImpliedBound` type for more details.
123 pub fn implied_bounds<'a, 'gcx, 'tcx>(
124 infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
125 body_id: ast::NodeId,
128 -> Vec<ImpliedBound<'tcx>>
130 // Sometimes when we ask what it takes for T: WF, we get back that
131 // U: WF is required; in that case, we push U onto this stack and
132 // process it next. Currently (at least) these resulting
133 // predicates are always guaranteed to be a subset of the original
134 // type, so we need not fear non-termination.
135 let mut wf_types = vec![ty];
137 let mut implied_bounds = vec![];
139 while let Some(ty) = wf_types.pop() {
140 // Compute the obligations for `ty` to be well-formed. If `ty` is
141 // an unresolved inference variable, just substituted an empty set
142 // -- because the return type here is going to be things we *add*
143 // to the environment, it's always ok for this set to be smaller
144 // than the ultimate set. (Note: normally there won't be
145 // unresolved inference variables here anyway, but there might be
146 // during typeck under some circumstances.)
147 let obligations = obligations(infcx, body_id, ty, span).unwrap_or(vec![]);
149 // From the full set of obligations, just filter down to the
150 // region relationships.
151 implied_bounds.extend(
154 .flat_map(|obligation| {
155 assert!(!obligation.has_escaping_regions());
156 match obligation.predicate {
157 ty::Predicate::Trait(..) |
158 ty::Predicate::Equate(..) |
159 ty::Predicate::Projection(..) |
160 ty::Predicate::ClosureKind(..) |
161 ty::Predicate::ObjectSafe(..) =>
164 ty::Predicate::WellFormed(subty) => {
165 wf_types.push(subty);
169 ty::Predicate::RegionOutlives(ref data) =>
170 match infcx.tcx.no_late_bound_regions(data) {
173 Some(ty::OutlivesPredicate(r_a, r_b)) =>
174 vec![ImpliedBound::RegionSubRegion(r_b, r_a)],
177 ty::Predicate::TypeOutlives(ref data) =>
178 match infcx.tcx.no_late_bound_regions(data) {
180 Some(ty::OutlivesPredicate(ty_a, r_b)) => {
181 let ty_a = infcx.resolve_type_vars_if_possible(&ty_a);
182 let components = infcx.tcx.outlives_components(ty_a);
183 implied_bounds_from_components(r_b, components)
192 /// When we have an implied bound that `T: 'a`, we can further break
193 /// this down to determine what relationships would have to hold for
194 /// `T: 'a` to hold. We get to assume that the caller has validated
195 /// those relationships.
196 fn implied_bounds_from_components<'tcx>(sub_region: &'tcx ty::Region,
197 sup_components: Vec<Component<'tcx>>)
198 -> Vec<ImpliedBound<'tcx>>
202 .flat_map(|component| {
204 Component::Region(r) =>
205 vec![ImpliedBound::RegionSubRegion(sub_region, r)],
206 Component::Param(p) =>
207 vec![ImpliedBound::RegionSubParam(sub_region, p)],
208 Component::Projection(p) =>
209 vec![ImpliedBound::RegionSubProjection(sub_region, p)],
210 Component::EscapingProjection(_) =>
211 // If the projection has escaping regions, don't
212 // try to infer any implied bounds even for its
213 // free components. This is conservative, because
214 // the caller will still have to prove that those
215 // free components outlive `sub_region`. But the
216 // idea is that the WAY that the caller proves
217 // that may change in the future and we want to
218 // give ourselves room to get smarter here.
220 Component::UnresolvedInferenceVariable(..) =>
227 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
228 infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
229 body_id: ast::NodeId,
231 out: Vec<traits::PredicateObligation<'tcx>>,
234 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
235 fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
236 traits::ObligationCause::new(self.span, self.body_id, code)
239 fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
240 let cause = self.cause(traits::MiscObligation);
241 let infcx = &mut self.infcx;
243 .inspect(|pred| assert!(!pred.has_escaping_regions()))
245 let mut selcx = traits::SelectionContext::new(infcx);
246 let pred = traits::normalize(&mut selcx, cause.clone(), pred);
247 once(pred.value).chain(pred.obligations)
252 /// Pushes the obligations required for `trait_ref` to be WF into
254 fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>) {
255 let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
256 self.out.extend(obligations);
258 let cause = self.cause(traits::MiscObligation);
260 trait_ref.substs.types()
261 .filter(|ty| !ty.has_escaping_regions())
262 .map(|ty| traits::Obligation::new(cause.clone(),
263 ty::Predicate::WellFormed(ty))));
266 /// Pushes the obligations required for `trait_ref::Item` to be WF
268 fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
269 // A projection is well-formed if (a) the trait ref itself is
270 // WF and (b) the trait-ref holds. (It may also be
271 // normalizable and be WF that way.)
273 self.compute_trait_ref(&data.trait_ref);
275 if !data.has_escaping_regions() {
276 let predicate = data.trait_ref.to_predicate();
277 let cause = self.cause(traits::ProjectionWf(data));
278 self.out.push(traits::Obligation::new(cause, predicate));
282 fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
283 if !subty.has_escaping_regions() {
284 let cause = self.cause(cause);
285 let trait_ref = ty::TraitRef {
286 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
287 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
289 self.out.push(traits::Obligation::new(cause, trait_ref.to_predicate()));
293 /// Push new obligations into `out`. Returns true if it was able
294 /// to generate all the predicates needed to validate that `ty0`
295 /// is WF. Returns false if `ty0` is an unresolved type variable,
296 /// in which case we are not able to simplify at all.
297 fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
298 let mut subtys = ty0.walk();
299 while let Some(ty) = subtys.next() {
310 // WfScalar, WfParameter, etc
314 ty::TyArray(subty, _) => {
315 self.require_sized(subty, traits::SliceOrArrayElem);
318 ty::TyTuple(ref tys) => {
319 if let Some((_last, rest)) = tys.split_last() {
321 self.require_sized(elem, traits::TupleElem);
327 // simple cases that are WF if their type args are WF
330 ty::TyProjection(data) => {
331 subtys.skip_current_subtree(); // subtree handled by compute_projection
332 self.compute_projection(data);
335 ty::TyAdt(def, substs) => {
337 let obligations = self.nominal_obligations(def.did, substs);
338 self.out.extend(obligations);
341 ty::TyRef(r, mt) => {
343 if !r.has_escaping_regions() && !mt.ty.has_escaping_regions() {
344 let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
346 traits::Obligation::new(
348 ty::Predicate::TypeOutlives(
350 ty::OutlivesPredicate(mt.ty, r)))));
354 ty::TyClosure(..) => {
355 // the types in a closure are always the types of
356 // local variables (or possibly references to local
357 // variables), we'll walk those.
359 // (Though, local variables are probably not
360 // needed, as they are separately checked w/r/t
364 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
365 // let the loop iterate into the argument/return
366 // types appearing in the fn signature
370 // all of the requirements on type parameters
371 // should've been checked by the instantiation
372 // of whatever returned this exact `impl Trait`.
375 ty::TyDynamic(data, r) => {
378 // Here, we defer WF checking due to higher-ranked
379 // regions. This is perhaps not ideal.
380 self.from_object_ty(ty, data, r);
382 // FIXME(#27579) RFC also considers adding trait
383 // obligations that don't refer to Self and
386 let cause = self.cause(traits::MiscObligation);
388 let component_traits =
389 data.auto_traits().chain(data.principal().map(|p| p.def_id()));
391 component_traits.map(|did| traits::Obligation::new(
393 ty::Predicate::ObjectSafe(did)
398 // Inference variables are the complicated case, since we don't
399 // know what type they are. We do two things:
401 // 1. Check if they have been resolved, and if so proceed with
403 // 2. If not, check whether this is the type that we
404 // started with (ty0). In that case, we've made no
405 // progress at all, so return false. Otherwise,
406 // we've at least simplified things (i.e., we went
407 // from `Vec<$0>: WF` to `$0: WF`, so we can
408 // register a pending obligation and keep
409 // moving. (Goal is that an "inductive hypothesis"
410 // is satisfied to ensure termination.)
412 let ty = self.infcx.shallow_resolve(ty);
413 if let ty::TyInfer(_) = ty.sty { // not yet resolved...
414 if ty == ty0 { // ...this is the type we started from! no progress.
418 let cause = self.cause(traits::MiscObligation);
419 self.out.push( // ...not the type we started from, so we made progress.
420 traits::Obligation::new(cause, ty::Predicate::WellFormed(ty)));
422 // Yes, resolved, proceed with the
423 // result. Should never return false because
424 // `ty` is not a TyInfer.
425 assert!(self.compute(ty));
431 // if we made it through that loop above, we made progress!
435 fn nominal_obligations(&mut self,
437 substs: &Substs<'tcx>)
438 -> Vec<traits::PredicateObligation<'tcx>>
441 self.infcx.tcx.item_predicates(def_id)
442 .instantiate(self.infcx.tcx, substs);
443 let cause = self.cause(traits::ItemObligation(def_id));
444 predicates.predicates
446 .map(|pred| traits::Obligation::new(cause.clone(), pred))
447 .filter(|pred| !pred.has_escaping_regions())
451 fn from_object_ty(&mut self, ty: Ty<'tcx>,
452 data: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>,
453 region: &'tcx ty::Region) {
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_regions() {
487 let implicit_bounds =
488 object_region_bounds(self.infcx.tcx, data);
490 let explicit_bound = region;
492 for implicit_bound in implicit_bounds {
493 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
494 let outlives = ty::Binder(ty::OutlivesPredicate(explicit_bound, implicit_bound));
495 self.out.push(traits::Obligation::new(cause, outlives.to_predicate()));
501 /// Given an object type like `SomeTrait+Send`, computes the lifetime
502 /// bounds that must hold on the elided self type. These are derived
503 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
504 /// they declare `trait SomeTrait : 'static`, for example, then
505 /// `'static` would appear in the list. The hard work is done by
506 /// `ty::required_region_bounds`, see that for more information.
507 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
508 tcx: TyCtxt<'a, 'gcx, 'tcx>,
509 existential_predicates: ty::Binder<&'tcx ty::Slice<ty::ExistentialPredicate<'tcx>>>)
510 -> Vec<&'tcx ty::Region>
512 // Since we don't actually *know* the self type for an object,
513 // this "open(err)" serves as a kind of dummy standin -- basically
514 // a skolemized type.
515 let open_ty = tcx.mk_infer(ty::FreshTy(0));
517 let predicates = existential_predicates.iter().filter_map(|predicate| {
518 if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
521 Some(predicate.with_self_ty(tcx, open_ty))
525 tcx.required_region_bounds(open_ty, predicates)