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 util::common::ErrorReported;
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 components = infcx.outlives_components(ty_a);
182 implied_bounds_from_components(r_b, components)
191 /// When we have an implied bound that `T: 'a`, we can further break
192 /// this down to determine what relationships would have to hold for
193 /// `T: 'a` to hold. We get to assume that the caller has validated
194 /// those relationships.
195 fn implied_bounds_from_components<'tcx>(sub_region: &'tcx ty::Region,
196 sup_components: Vec<Component<'tcx>>)
197 -> Vec<ImpliedBound<'tcx>>
201 .flat_map(|component| {
203 Component::Region(r) =>
204 vec!(ImpliedBound::RegionSubRegion(sub_region, r)),
205 Component::Param(p) =>
206 vec!(ImpliedBound::RegionSubParam(sub_region, p)),
207 Component::Projection(p) =>
208 vec!(ImpliedBound::RegionSubProjection(sub_region, p)),
209 Component::EscapingProjection(_) =>
210 // If the projection has escaping regions, don't
211 // try to infer any implied bounds even for its
212 // free components. This is conservative, because
213 // the caller will still have to prove that those
214 // free components outlive `sub_region`. But the
215 // idea is that the WAY that the caller proves
216 // that may change in the future and we want to
217 // give ourselves room to get smarter here.
219 Component::UnresolvedInferenceVariable(..) =>
226 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
227 infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
228 body_id: ast::NodeId,
230 out: Vec<traits::PredicateObligation<'tcx>>,
233 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
234 fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
235 traits::ObligationCause::new(self.span, self.body_id, code)
238 fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
239 let cause = self.cause(traits::MiscObligation);
240 let infcx = &mut self.infcx;
242 .inspect(|pred| assert!(!pred.has_escaping_regions()))
244 let mut selcx = traits::SelectionContext::new(infcx);
245 let pred = traits::normalize(&mut selcx, cause.clone(), pred);
246 once(pred.value).chain(pred.obligations)
251 /// Pushes the obligations required for `trait_ref` to be WF into
253 fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>) {
254 let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
255 self.out.extend(obligations);
257 let cause = self.cause(traits::MiscObligation);
259 trait_ref.substs.types()
260 .filter(|ty| !ty.has_escaping_regions())
261 .map(|ty| traits::Obligation::new(cause.clone(),
262 ty::Predicate::WellFormed(ty))));
265 /// Pushes the obligations required for `trait_ref::Item` to be WF
267 fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
268 // A projection is well-formed if (a) the trait ref itself is
269 // WF and (b) the trait-ref holds. (It may also be
270 // normalizable and be WF that way.)
272 self.compute_trait_ref(&data.trait_ref);
274 if !data.has_escaping_regions() {
275 let predicate = data.trait_ref.to_predicate();
276 let cause = self.cause(traits::ProjectionWf(data));
277 self.out.push(traits::Obligation::new(cause, predicate));
281 fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
282 if !subty.has_escaping_regions() {
283 let cause = self.cause(cause);
284 match self.infcx.tcx.trait_ref_for_builtin_bound(ty::BoundSized, subty) {
287 traits::Obligation::new(cause,
288 trait_ref.to_predicate()));
290 Err(ErrorReported) => { }
295 /// Push new obligations into `out`. Returns true if it was able
296 /// to generate all the predicates needed to validate that `ty0`
297 /// is WF. Returns false if `ty0` is an unresolved type variable,
298 /// in which case we are not able to simplify at all.
299 fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
300 let tcx = self.infcx.tcx;
301 let mut subtys = ty0.walk();
302 while let Some(ty) = subtys.next() {
313 // WfScalar, WfParameter, etc
317 ty::TyArray(subty, _) => {
318 self.require_sized(subty, traits::SliceOrArrayElem);
321 ty::TyTuple(ref tys) => {
322 if let Some((_last, rest)) = tys.split_last() {
324 self.require_sized(elem, traits::TupleElem);
331 // simple cases that are WF if their type args are WF
334 ty::TyProjection(data) => {
335 subtys.skip_current_subtree(); // subtree handled by compute_projection
336 self.compute_projection(data);
339 ty::TyEnum(def, substs) |
340 ty::TyStruct(def, substs) => {
342 let obligations = self.nominal_obligations(def.did, substs);
343 self.out.extend(obligations);
346 ty::TyRef(r, mt) => {
348 if !r.has_escaping_regions() && !mt.ty.has_escaping_regions() {
349 let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
351 traits::Obligation::new(
353 ty::Predicate::TypeOutlives(
355 ty::OutlivesPredicate(mt.ty, r)))));
359 ty::TyClosure(..) => {
360 // the types in a closure are always the types of
361 // local variables (or possibly references to local
362 // variables), we'll walk those.
364 // (Though, local variables are probably not
365 // needed, as they are separately checked w/r/t
369 ty::TyFnDef(..) | ty::TyFnPtr(_) => {
370 // let the loop iterate into the argument/return
371 // types appearing in the fn signature
375 // all of the requirements on type parameters
376 // should've been checked by the instantiation
377 // of whatever returned this exact `impl Trait`.
380 ty::TyTrait(ref data) => {
383 // Here, we defer WF checking due to higher-ranked
384 // regions. This is perhaps not ideal.
385 self.from_object_ty(ty, data);
387 // FIXME(#27579) RFC also considers adding trait
388 // obligations that don't refer to Self and
391 let cause = self.cause(traits::MiscObligation);
393 let component_traits =
394 data.builtin_bounds.iter().flat_map(|bound| {
395 tcx.lang_items.from_builtin_kind(bound).ok()
397 .chain(Some(data.principal.def_id()));
399 component_traits.map(|did| { traits::Obligation::new(
401 ty::Predicate::ObjectSafe(did)
406 // Inference variables are the complicated case, since we don't
407 // know what type they are. We do two things:
409 // 1. Check if they have been resolved, and if so proceed with
411 // 2. If not, check whether this is the type that we
412 // started with (ty0). In that case, we've made no
413 // progress at all, so return false. Otherwise,
414 // we've at least simplified things (i.e., we went
415 // from `Vec<$0>: WF` to `$0: WF`, so we can
416 // register a pending obligation and keep
417 // moving. (Goal is that an "inductive hypothesis"
418 // is satisfied to ensure termination.)
420 let ty = self.infcx.shallow_resolve(ty);
421 if let ty::TyInfer(_) = ty.sty { // not yet resolved...
422 if ty == ty0 { // ...this is the type we started from! no progress.
426 let cause = self.cause(traits::MiscObligation);
427 self.out.push( // ...not the type we started from, so we made progress.
428 traits::Obligation::new(cause, ty::Predicate::WellFormed(ty)));
430 // Yes, resolved, proceed with the
431 // result. Should never return false because
432 // `ty` is not a TyInfer.
433 assert!(self.compute(ty));
439 // if we made it through that loop above, we made progress!
443 fn nominal_obligations(&mut self,
445 substs: &Substs<'tcx>)
446 -> Vec<traits::PredicateObligation<'tcx>>
449 self.infcx.tcx.lookup_predicates(def_id)
450 .instantiate(self.infcx.tcx, substs);
451 let cause = self.cause(traits::ItemObligation(def_id));
452 predicates.predicates
454 .map(|pred| traits::Obligation::new(cause.clone(), pred))
455 .filter(|pred| !pred.has_escaping_regions())
459 fn from_object_ty(&mut self, ty: Ty<'tcx>, data: &ty::TraitObject<'tcx>) {
460 // Imagine a type like this:
463 // trait Bar<'c> : 'c { }
465 // &'b (Foo+'c+Bar<'d>)
468 // In this case, the following relationships must hold:
473 // The first conditions is due to the normal region pointer
474 // rules, which say that a reference cannot outlive its
477 // The final condition may be a bit surprising. In particular,
478 // you may expect that it would have been `'c <= 'd`, since
479 // usually lifetimes of outer things are conservative
480 // approximations for inner things. However, it works somewhat
481 // differently with trait objects: here the idea is that if the
482 // user specifies a region bound (`'c`, in this case) it is the
483 // "master bound" that *implies* that bounds from other traits are
484 // all met. (Remember that *all bounds* in a type like
485 // `Foo+Bar+Zed` must be met, not just one, hence if we write
486 // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
489 // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
490 // am looking forward to the future here.
492 if !data.has_escaping_regions() {
493 let implicit_bounds =
494 object_region_bounds(self.infcx.tcx,
496 data.builtin_bounds);
498 let explicit_bound = data.region_bound;
500 for implicit_bound in implicit_bounds {
501 let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
502 let outlives = ty::Binder(ty::OutlivesPredicate(explicit_bound, implicit_bound));
503 self.out.push(traits::Obligation::new(cause, outlives.to_predicate()));
509 /// Given an object type like `SomeTrait+Send`, computes the lifetime
510 /// bounds that must hold on the elided self type. These are derived
511 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
512 /// they declare `trait SomeTrait : 'static`, for example, then
513 /// `'static` would appear in the list. The hard work is done by
514 /// `ty::required_region_bounds`, see that for more information.
515 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
516 tcx: TyCtxt<'a, 'gcx, 'tcx>,
517 principal: ty::PolyExistentialTraitRef<'tcx>,
518 others: ty::BuiltinBounds)
519 -> Vec<&'tcx ty::Region>
521 // Since we don't actually *know* the self type for an object,
522 // this "open(err)" serves as a kind of dummy standin -- basically
523 // a skolemized type.
524 let open_ty = tcx.mk_infer(ty::FreshTy(0));
526 let mut predicates = others.to_predicates(tcx, open_ty);
527 predicates.push(principal.with_self_ty(tcx, open_ty).to_predicate());
529 tcx.required_region_bounds(open_ty, predicates)