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;
12 use mir::interpret::ConstValue;
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>,
29 param_env: ty::ParamEnv<'tcx>,
33 -> Option<Vec<traits::PredicateObligation<'tcx>>>
35 let mut wf = WfPredicates { infcx,
41 debug!("wf::obligations({:?}, body_id={:?}) = {:?}", ty, body_id, wf.out);
42 let result = wf.normalize();
43 debug!("wf::obligations({:?}, body_id={:?}) ~~> {:?}", ty, body_id, result);
46 None // no progress made, return None
50 /// Returns the obligations that make this trait reference
51 /// well-formed. For example, if there is a trait `Set` defined like
52 /// `trait Set<K:Eq>`, then the trait reference `Foo: Set<Bar>` is WF
54 pub fn trait_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
55 param_env: ty::ParamEnv<'tcx>,
57 trait_ref: &ty::TraitRef<'tcx>,
59 -> Vec<traits::PredicateObligation<'tcx>>
61 let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
62 wf.compute_trait_ref(trait_ref, Elaborate::All);
66 pub fn predicate_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
67 param_env: ty::ParamEnv<'tcx>,
69 predicate: &ty::Predicate<'tcx>,
71 -> Vec<traits::PredicateObligation<'tcx>>
73 let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
75 // (*) ok to skip binders, because wf code is prepared for it
77 ty::Predicate::Trait(ref t) => {
78 wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
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(..) => {
97 ty::Predicate::Subtype(ref data) => {
98 wf.compute(data.skip_binder().a); // (*)
99 wf.compute(data.skip_binder().b); // (*)
101 ty::Predicate::ConstEvaluatable(def_id, substs) => {
102 let obligations = wf.nominal_obligations(def_id, substs);
103 wf.out.extend(obligations);
105 for ty in substs.types() {
114 struct WfPredicates<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
115 infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
116 param_env: ty::ParamEnv<'tcx>,
117 body_id: ast::NodeId,
119 out: Vec<traits::PredicateObligation<'tcx>>,
122 /// Controls whether we "elaborate" supertraits and so forth on the WF
123 /// predicates. This is a kind of hack to address #43784. The
124 /// underlying problem in that issue was a trait structure like:
127 /// trait Foo: Copy { }
128 /// trait Bar: Foo { }
129 /// impl<T: Bar> Foo for T { }
130 /// impl<T> Bar for T { }
133 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
134 /// we decide that this is true because `T: Bar` is in the
135 /// where-clauses (and we can elaborate that to include `T:
136 /// Copy`). This wouldn't be a problem, except that when we check the
137 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
138 /// impl. And so nowhere did we check that `T: Copy` holds!
140 /// To resolve this, we elaborate the WF requirements that must be
141 /// proven when checking impls. This means that (e.g.) the `impl Bar
142 /// for T` will be forced to prove not only that `T: Foo` but also `T:
143 /// Copy` (which it won't be able to do, because there is no `Copy`
145 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
151 impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
152 fn cause(&mut self, code: traits::ObligationCauseCode<'tcx>) -> traits::ObligationCause<'tcx> {
153 traits::ObligationCause::new(self.span, self.body_id, code)
156 fn normalize(&mut self) -> Vec<traits::PredicateObligation<'tcx>> {
157 let cause = self.cause(traits::MiscObligation);
158 let infcx = &mut self.infcx;
159 let param_env = self.param_env;
161 .inspect(|pred| assert!(!pred.has_escaping_regions()))
163 let mut selcx = traits::SelectionContext::new(infcx);
164 let pred = traits::normalize(&mut selcx, param_env, cause.clone(), pred);
165 once(pred.value).chain(pred.obligations)
170 /// Pushes the obligations required for `trait_ref` to be WF into
172 fn compute_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, elaborate: Elaborate) {
173 let obligations = self.nominal_obligations(trait_ref.def_id, trait_ref.substs);
175 let cause = self.cause(traits::MiscObligation);
176 let param_env = self.param_env;
178 if let Elaborate::All = elaborate {
179 let predicates = obligations.iter()
180 .map(|obligation| obligation.predicate.clone())
182 let implied_obligations = traits::elaborate_predicates(self.infcx.tcx, predicates);
183 let implied_obligations = implied_obligations.map(|pred| {
184 traits::Obligation::new(cause.clone(), param_env, pred)
186 self.out.extend(implied_obligations);
189 self.out.extend(obligations);
192 trait_ref.substs.types()
193 .filter(|ty| !ty.has_escaping_regions())
194 .map(|ty| traits::Obligation::new(cause.clone(),
196 ty::Predicate::WellFormed(ty))));
199 /// Pushes the obligations required for `trait_ref::Item` to be WF
201 fn compute_projection(&mut self, data: ty::ProjectionTy<'tcx>) {
202 // A projection is well-formed if (a) the trait ref itself is
203 // WF and (b) the trait-ref holds. (It may also be
204 // normalizable and be WF that way.)
205 let trait_ref = data.trait_ref(self.infcx.tcx);
206 self.compute_trait_ref(&trait_ref, Elaborate::None);
208 if !data.has_escaping_regions() {
209 let predicate = trait_ref.to_predicate();
210 let cause = self.cause(traits::ProjectionWf(data));
211 self.out.push(traits::Obligation::new(cause, self.param_env, predicate));
215 /// Pushes the obligations required for a constant value to be WF
217 fn compute_const(&mut self, constant: &'tcx ty::Const<'tcx>) {
218 self.require_sized(constant.ty, traits::ConstSized);
219 if let ConstValue::Unevaluated(def_id, substs) = constant.val {
220 let obligations = self.nominal_obligations(def_id, substs);
221 self.out.extend(obligations);
223 let predicate = ty::Predicate::ConstEvaluatable(def_id, substs);
224 let cause = self.cause(traits::MiscObligation);
225 self.out.push(traits::Obligation::new(cause,
231 fn require_sized(&mut self, subty: Ty<'tcx>, cause: traits::ObligationCauseCode<'tcx>) {
232 if !subty.has_escaping_regions() {
233 let cause = self.cause(cause);
234 let trait_ref = ty::TraitRef {
235 def_id: self.infcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
236 substs: self.infcx.tcx.mk_substs_trait(subty, &[]),
238 self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
242 /// Push new obligations into `out`. Returns true if it was able
243 /// to generate all the predicates needed to validate that `ty0`
244 /// is WF. Returns false if `ty0` is an unresolved type variable,
245 /// in which case we are not able to simplify at all.
246 fn compute(&mut self, ty0: Ty<'tcx>) -> bool {
247 let mut subtys = ty0.walk();
248 let param_env = self.param_env;
249 while let Some(ty) = subtys.next() {
258 ty::GeneratorWitness(..) |
262 // WfScalar, WfParameter, etc
265 ty::Slice(subty) => {
266 self.require_sized(subty, traits::SliceOrArrayElem);
269 ty::Array(subty, len) => {
270 self.require_sized(subty, traits::SliceOrArrayElem);
271 assert_eq!(len.ty, self.infcx.tcx.types.usize);
272 self.compute_const(len);
275 ty::Tuple(ref tys) => {
276 if let Some((_last, rest)) = tys.split_last() {
278 self.require_sized(elem, traits::TupleElem);
284 // simple cases that are WF if their type args are WF
287 ty::Projection(data) => {
288 subtys.skip_current_subtree(); // subtree handled by compute_projection
289 self.compute_projection(data);
292 ty::Adt(def, substs) => {
294 let obligations = self.nominal_obligations(def.did, substs);
295 self.out.extend(obligations);
298 ty::Ref(r, rty, _) => {
300 if !r.has_escaping_regions() && !rty.has_escaping_regions() {
301 let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
303 traits::Obligation::new(
306 ty::Predicate::TypeOutlives(
308 ty::OutlivesPredicate(rty, r)))));
312 ty::Generator(..) => {
313 // Walk ALL the types in the generator: this will
314 // include the upvar types as well as the yield
315 // type. Note that this is mildly distinct from
316 // the closure case, where we have to be careful
317 // about the signature of the closure. We don't
318 // have the problem of implied bounds here since
319 // generators don't take arguments.
322 ty::Closure(def_id, substs) => {
323 // Only check the upvar types for WF, not the rest
324 // of the types within. This is needed because we
325 // capture the signature and it may not be WF
326 // without the implied bounds. Consider a closure
327 // like `|x: &'a T|` -- it may be that `T: 'a` is
328 // not known to hold in the creator's context (and
329 // indeed the closure may not be invoked by its
330 // creator, but rather turned to someone who *can*
333 // The special treatment of closures here really
334 // ought not to be necessary either; the problem
335 // is related to #25860 -- there is no way for us
336 // to express a fn type complete with the implied
337 // bounds that it is assuming. I think in reality
338 // the WF rules around fn are a bit messed up, and
339 // that is the rot problem: `fn(&'a T)` should
340 // probably always be WF, because it should be
341 // shorthand for something like `where(T: 'a) {
342 // fn(&'a T) }`, as discussed in #25860.
344 // Note that we are also skipping the generic
345 // types. This is consistent with the `outlives`
346 // code, but anyway doesn't matter: within the fn
347 // body where they are created, the generics will
348 // always be WF, and outside of that fn body we
349 // are not directly inspecting closure types
350 // anyway, except via auto trait matching (which
351 // only inspects the upvar types).
352 subtys.skip_current_subtree(); // subtree handled by compute_projection
353 for upvar_ty in substs.upvar_tys(def_id, self.infcx.tcx) {
354 self.compute(upvar_ty);
358 ty::FnDef(..) | ty::FnPtr(_) => {
359 // let the loop iterate into the argument/return
360 // types appearing in the fn signature
363 ty::Opaque(did, substs) => {
364 // all of the requirements on type parameters
365 // should've been checked by the instantiation
366 // of whatever returned this exact `impl Trait`.
368 // for named existential types we still need to check them
369 if super::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
370 let obligations = self.nominal_obligations(did, substs);
371 self.out.extend(obligations);
375 ty::Dynamic(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);
387 let component_traits =
388 data.auto_traits().chain(data.principal().map(|p| p.def_id()));
390 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::Infer(_) = 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,
422 ty::Predicate::WellFormed(ty)));
424 // Yes, resolved, proceed with the
425 // result. Should never return false because
426 // `ty` is not a Infer.
427 assert!(self.compute(ty));
433 // if we made it through that loop above, we made progress!
437 fn nominal_obligations(&mut self,
439 substs: &Substs<'tcx>)
440 -> Vec<traits::PredicateObligation<'tcx>>
443 self.infcx.tcx.predicates_of(def_id)
444 .instantiate(self.infcx.tcx, substs);
445 let cause = self.cause(traits::ItemObligation(def_id));
446 predicates.predicates
448 .map(|pred| traits::Obligation::new(cause.clone(),
451 .filter(|pred| !pred.has_escaping_regions())
455 fn from_object_ty(&mut self, ty: Ty<'tcx>,
456 data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
457 region: ty::Region<'tcx>) {
458 // Imagine a type like this:
461 // trait Bar<'c> : 'c { }
463 // &'b (Foo+'c+Bar<'d>)
466 // In this case, the following relationships must hold:
471 // The first conditions is due to the normal region pointer
472 // rules, which say that a reference cannot outlive its
475 // The final condition may be a bit surprising. In particular,
476 // you may expect that it would have been `'c <= 'd`, since
477 // usually lifetimes of outer things are conservative
478 // approximations for inner things. However, it works somewhat
479 // differently with trait objects: here the idea is that if the
480 // user specifies a region bound (`'c`, in this case) it is the
481 // "master bound" that *implies* that bounds from other traits are
482 // all met. (Remember that *all bounds* in a type like
483 // `Foo+Bar+Zed` must be met, not just one, hence if we write
484 // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
487 // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
488 // am looking forward to the future here.
490 if !data.has_escaping_regions() {
491 let implicit_bounds =
492 object_region_bounds(self.infcx.tcx, data);
494 let explicit_bound = region;
496 for implicit_bound in implicit_bounds {
497 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
498 let outlives = ty::Binder::dummy(
499 ty::OutlivesPredicate(explicit_bound, implicit_bound));
500 self.out.push(traits::Obligation::new(cause,
502 outlives.to_predicate()));
508 /// Given an object type like `SomeTrait+Send`, computes the lifetime
509 /// bounds that must hold on the elided self type. These are derived
510 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
511 /// they declare `trait SomeTrait : 'static`, for example, then
512 /// `'static` would appear in the list. The hard work is done by
513 /// `ty::required_region_bounds`, see that for more information.
514 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
515 tcx: TyCtxt<'a, 'gcx, 'tcx>,
516 existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>)
517 -> Vec<ty::Region<'tcx>>
519 // Since we don't actually *know* the self type for an object,
520 // this "open(err)" serves as a kind of dummy standin -- basically
521 // a skolemized type.
522 let open_ty = tcx.mk_infer(ty::FreshTy(0));
524 let predicates = existential_predicates.iter().filter_map(|predicate| {
525 if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
528 Some(predicate.with_self_ty(tcx, open_ty))
532 tcx.required_region_bounds(open_ty, predicates)