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Move `BoundTy` to `ty::TyKind`
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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.
4 //
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.
10
11 use hir::def_id::DefId;
12 use mir::interpret::ConstValue;
13 use infer::InferCtxt;
14 use ty::subst::Substs;
15 use traits;
16 use ty::{self, ToPredicate, Ty, TyCtxt, TypeFoldable};
17 use std::iter::once;
18 use syntax::ast;
19 use syntax_pos::Span;
20 use middle::lang_items;
21
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>,
30                                    body_id: ast::NodeId,
31                                    ty: Ty<'tcx>,
32                                    span: Span)
33                                    -> Option<Vec<traits::PredicateObligation<'tcx>>>
34 {
35     let mut wf = WfPredicates { infcx,
36                                 param_env,
37                                 body_id,
38                                 span,
39                                 out: vec![] };
40     if wf.compute(ty) {
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);
44         Some(result)
45     } else {
46         None // no progress made, return None
47     }
48 }
49
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
53 /// if `Bar: Eq`.
54 pub fn trait_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
55                                          param_env: ty::ParamEnv<'tcx>,
56                                          body_id: ast::NodeId,
57                                          trait_ref: &ty::TraitRef<'tcx>,
58                                          span: Span)
59                                          -> Vec<traits::PredicateObligation<'tcx>>
60 {
61     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
62     wf.compute_trait_ref(trait_ref, Elaborate::All);
63     wf.normalize()
64 }
65
66 pub fn predicate_obligations<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
67                                              param_env: ty::ParamEnv<'tcx>,
68                                              body_id: ast::NodeId,
69                                              predicate: &ty::Predicate<'tcx>,
70                                              span: Span)
71                                              -> Vec<traits::PredicateObligation<'tcx>>
72 {
73     let mut wf = WfPredicates { infcx, param_env, body_id, span, out: vec![] };
74
75     // (*) ok to skip binders, because wf code is prepared for it
76     match *predicate {
77         ty::Predicate::Trait(ref t) => {
78             wf.compute_trait_ref(&t.skip_binder().trait_ref, Elaborate::None); // (*)
79         }
80         ty::Predicate::RegionOutlives(..) => {
81         }
82         ty::Predicate::TypeOutlives(ref t) => {
83             wf.compute(t.skip_binder().0);
84         }
85         ty::Predicate::Projection(ref t) => {
86             let t = t.skip_binder(); // (*)
87             wf.compute_projection(t.projection_ty);
88             wf.compute(t.ty);
89         }
90         ty::Predicate::WellFormed(t) => {
91             wf.compute(t);
92         }
93         ty::Predicate::ObjectSafe(_) => {
94         }
95         ty::Predicate::ClosureKind(..) => {
96         }
97         ty::Predicate::Subtype(ref data) => {
98             wf.compute(data.skip_binder().a); // (*)
99             wf.compute(data.skip_binder().b); // (*)
100         }
101         ty::Predicate::ConstEvaluatable(def_id, substs) => {
102             let obligations = wf.nominal_obligations(def_id, substs);
103             wf.out.extend(obligations);
104
105             for ty in substs.types() {
106                 wf.compute(ty);
107             }
108         }
109     }
110
111     wf.normalize()
112 }
113
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,
118     span: Span,
119     out: Vec<traits::PredicateObligation<'tcx>>,
120 }
121
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:
125 ///
126 /// ```
127 /// trait Foo: Copy { }
128 /// trait Bar: Foo { }
129 /// impl<T: Bar> Foo for T { }
130 /// impl<T> Bar for T { }
131 /// ```
132 ///
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!
139 ///
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`
144 /// impl for `T`).
145 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
146 enum Elaborate {
147     All,
148     None,
149 }
150
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)
154     }
155
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;
160         self.out.iter()
161                 .inspect(|pred| assert!(!pred.has_escaping_regions()))
162                 .flat_map(|pred| {
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)
166                 })
167                 .collect()
168     }
169
170     /// Pushes the obligations required for `trait_ref` to be WF into
171     /// `self.out`.
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);
174
175         let cause = self.cause(traits::MiscObligation);
176         let param_env = self.param_env;
177
178         if let Elaborate::All = elaborate {
179             let predicates = obligations.iter()
180                                         .map(|obligation| obligation.predicate.clone())
181                                         .collect();
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)
185             });
186             self.out.extend(implied_obligations);
187         }
188
189         self.out.extend(obligations);
190
191         self.out.extend(
192             trait_ref.substs.types()
193                             .filter(|ty| !ty.has_escaping_regions())
194                             .map(|ty| traits::Obligation::new(cause.clone(),
195                                                               param_env,
196                                                               ty::Predicate::WellFormed(ty))));
197     }
198
199     /// Pushes the obligations required for `trait_ref::Item` to be WF
200     /// into `self.out`.
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);
207
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));
212         }
213     }
214
215     /// Pushes the obligations required for a constant value to be WF
216     /// into `self.out`.
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);
222
223             let predicate = ty::Predicate::ConstEvaluatable(def_id, substs);
224             let cause = self.cause(traits::MiscObligation);
225             self.out.push(traits::Obligation::new(cause,
226                                                   self.param_env,
227                                                   predicate));
228         }
229     }
230
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, &[]),
237             };
238             self.out.push(traits::Obligation::new(cause, self.param_env, trait_ref.to_predicate()));
239         }
240     }
241
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() {
250             match ty.sty {
251                 ty::Bool |
252                 ty::Char |
253                 ty::Int(..) |
254                 ty::Uint(..) |
255                 ty::Float(..) |
256                 ty::Error |
257                 ty::Str |
258                 ty::GeneratorWitness(..) |
259                 ty::Never |
260                 ty::Param(_) |
261                 ty::Bound(..) |
262                 ty::Foreign(..) => {
263                     // WfScalar, WfParameter, etc
264                 }
265
266                 ty::Slice(subty) => {
267                     self.require_sized(subty, traits::SliceOrArrayElem);
268                 }
269
270                 ty::Array(subty, len) => {
271                     self.require_sized(subty, traits::SliceOrArrayElem);
272                     assert_eq!(len.ty, self.infcx.tcx.types.usize);
273                     self.compute_const(len);
274                 }
275
276                 ty::Tuple(ref tys) => {
277                     if let Some((_last, rest)) = tys.split_last() {
278                         for elem in rest {
279                             self.require_sized(elem, traits::TupleElem);
280                         }
281                     }
282                 }
283
284                 ty::RawPtr(_) => {
285                     // simple cases that are WF if their type args are WF
286                 }
287
288                 ty::Projection(data) => {
289                     subtys.skip_current_subtree(); // subtree handled by compute_projection
290                     self.compute_projection(data);
291                 }
292
293                 ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
294
295                 ty::Adt(def, substs) => {
296                     // WfNominalType
297                     let obligations = self.nominal_obligations(def.did, substs);
298                     self.out.extend(obligations);
299                 }
300
301                 ty::Ref(r, rty, _) => {
302                     // WfReference
303                     if !r.has_escaping_regions() && !rty.has_escaping_regions() {
304                         let cause = self.cause(traits::ReferenceOutlivesReferent(ty));
305                         self.out.push(
306                             traits::Obligation::new(
307                                 cause,
308                                 param_env,
309                                 ty::Predicate::TypeOutlives(
310                                     ty::Binder::dummy(
311                                         ty::OutlivesPredicate(rty, r)))));
312                     }
313                 }
314
315                 ty::Generator(..) => {
316                     // Walk ALL the types in the generator: this will
317                     // include the upvar types as well as the yield
318                     // type. Note that this is mildly distinct from
319                     // the closure case, where we have to be careful
320                     // about the signature of the closure. We don't
321                     // have the problem of implied bounds here since
322                     // generators don't take arguments.
323                 }
324
325                 ty::Closure(def_id, substs) => {
326                     // Only check the upvar types for WF, not the rest
327                     // of the types within. This is needed because we
328                     // capture the signature and it may not be WF
329                     // without the implied bounds. Consider a closure
330                     // like `|x: &'a T|` -- it may be that `T: 'a` is
331                     // not known to hold in the creator's context (and
332                     // indeed the closure may not be invoked by its
333                     // creator, but rather turned to someone who *can*
334                     // verify that).
335                     //
336                     // The special treatment of closures here really
337                     // ought not to be necessary either; the problem
338                     // is related to #25860 -- there is no way for us
339                     // to express a fn type complete with the implied
340                     // bounds that it is assuming. I think in reality
341                     // the WF rules around fn are a bit messed up, and
342                     // that is the rot problem: `fn(&'a T)` should
343                     // probably always be WF, because it should be
344                     // shorthand for something like `where(T: 'a) {
345                     // fn(&'a T) }`, as discussed in #25860.
346                     //
347                     // Note that we are also skipping the generic
348                     // types. This is consistent with the `outlives`
349                     // code, but anyway doesn't matter: within the fn
350                     // body where they are created, the generics will
351                     // always be WF, and outside of that fn body we
352                     // are not directly inspecting closure types
353                     // anyway, except via auto trait matching (which
354                     // only inspects the upvar types).
355                     subtys.skip_current_subtree(); // subtree handled by compute_projection
356                     for upvar_ty in substs.upvar_tys(def_id, self.infcx.tcx) {
357                         self.compute(upvar_ty);
358                     }
359                 }
360
361                 ty::FnDef(..) | ty::FnPtr(_) => {
362                     // let the loop iterate into the argument/return
363                     // types appearing in the fn signature
364                 }
365
366                 ty::Opaque(did, substs) => {
367                     // all of the requirements on type parameters
368                     // should've been checked by the instantiation
369                     // of whatever returned this exact `impl Trait`.
370
371                     // for named existential types we still need to check them
372                     if super::is_impl_trait_defn(self.infcx.tcx, did).is_none() {
373                         let obligations = self.nominal_obligations(did, substs);
374                         self.out.extend(obligations);
375                     }
376                 }
377
378                 ty::Dynamic(data, r) => {
379                     // WfObject
380                     //
381                     // Here, we defer WF checking due to higher-ranked
382                     // regions. This is perhaps not ideal.
383                     self.from_object_ty(ty, data, r);
384
385                     // FIXME(#27579) RFC also considers adding trait
386                     // obligations that don't refer to Self and
387                     // checking those
388
389                     let cause = self.cause(traits::MiscObligation);
390                     let component_traits =
391                         data.auto_traits().chain(once(data.principal().def_id()));
392                     self.out.extend(
393                         component_traits.map(|did| traits::Obligation::new(
394                             cause.clone(),
395                             param_env,
396                             ty::Predicate::ObjectSafe(did)
397                         ))
398                     );
399                 }
400
401                 // Inference variables are the complicated case, since we don't
402                 // know what type they are. We do two things:
403                 //
404                 // 1. Check if they have been resolved, and if so proceed with
405                 //    THAT type.
406                 // 2. If not, check whether this is the type that we
407                 //    started with (ty0). In that case, we've made no
408                 //    progress at all, so return false. Otherwise,
409                 //    we've at least simplified things (i.e., we went
410                 //    from `Vec<$0>: WF` to `$0: WF`, so we can
411                 //    register a pending obligation and keep
412                 //    moving. (Goal is that an "inductive hypothesis"
413                 //    is satisfied to ensure termination.)
414                 ty::Infer(_) => {
415                     let ty = self.infcx.shallow_resolve(ty);
416                     if let ty::Infer(_) = ty.sty { // not yet resolved...
417                         if ty == ty0 { // ...this is the type we started from! no progress.
418                             return false;
419                         }
420
421                         let cause = self.cause(traits::MiscObligation);
422                         self.out.push( // ...not the type we started from, so we made progress.
423                             traits::Obligation::new(cause,
424                                                     self.param_env,
425                                                     ty::Predicate::WellFormed(ty)));
426                     } else {
427                         // Yes, resolved, proceed with the
428                         // result. Should never return false because
429                         // `ty` is not a Infer.
430                         assert!(self.compute(ty));
431                     }
432                 }
433             }
434         }
435
436         // if we made it through that loop above, we made progress!
437         return true;
438     }
439
440     fn nominal_obligations(&mut self,
441                            def_id: DefId,
442                            substs: &Substs<'tcx>)
443                            -> Vec<traits::PredicateObligation<'tcx>>
444     {
445         let predicates =
446             self.infcx.tcx.predicates_of(def_id)
447                           .instantiate(self.infcx.tcx, substs);
448         let cause = self.cause(traits::ItemObligation(def_id));
449         predicates.predicates
450                   .into_iter()
451                   .map(|pred| traits::Obligation::new(cause.clone(),
452                                                       self.param_env,
453                                                       pred))
454                   .filter(|pred| !pred.has_escaping_regions())
455                   .collect()
456     }
457
458     fn from_object_ty(&mut self, ty: Ty<'tcx>,
459                       data: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>,
460                       region: ty::Region<'tcx>) {
461         // Imagine a type like this:
462         //
463         //     trait Foo { }
464         //     trait Bar<'c> : 'c { }
465         //
466         //     &'b (Foo+'c+Bar<'d>)
467         //         ^
468         //
469         // In this case, the following relationships must hold:
470         //
471         //     'b <= 'c
472         //     'd <= 'c
473         //
474         // The first conditions is due to the normal region pointer
475         // rules, which say that a reference cannot outlive its
476         // referent.
477         //
478         // The final condition may be a bit surprising. In particular,
479         // you may expect that it would have been `'c <= 'd`, since
480         // usually lifetimes of outer things are conservative
481         // approximations for inner things. However, it works somewhat
482         // differently with trait objects: here the idea is that if the
483         // user specifies a region bound (`'c`, in this case) it is the
484         // "master bound" that *implies* that bounds from other traits are
485         // all met. (Remember that *all bounds* in a type like
486         // `Foo+Bar+Zed` must be met, not just one, hence if we write
487         // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
488         // 'y.)
489         //
490         // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
491         // am looking forward to the future here.
492
493         if !data.has_escaping_regions() {
494             let implicit_bounds =
495                 object_region_bounds(self.infcx.tcx, data);
496
497             let explicit_bound = region;
498
499             self.out.reserve(implicit_bounds.len());
500             for implicit_bound in implicit_bounds {
501                 let cause = self.cause(traits::ObjectTypeBound(ty, explicit_bound));
502                 let outlives = ty::Binder::dummy(
503                     ty::OutlivesPredicate(explicit_bound, implicit_bound));
504                 self.out.push(traits::Obligation::new(cause,
505                                                       self.param_env,
506                                                       outlives.to_predicate()));
507             }
508         }
509     }
510 }
511
512 /// Given an object type like `SomeTrait+Send`, computes the lifetime
513 /// bounds that must hold on the elided self type. These are derived
514 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
515 /// they declare `trait SomeTrait : 'static`, for example, then
516 /// `'static` would appear in the list. The hard work is done by
517 /// `ty::required_region_bounds`, see that for more information.
518 pub fn object_region_bounds<'a, 'gcx, 'tcx>(
519     tcx: TyCtxt<'a, 'gcx, 'tcx>,
520     existential_predicates: ty::Binder<&'tcx ty::List<ty::ExistentialPredicate<'tcx>>>)
521     -> Vec<ty::Region<'tcx>>
522 {
523     // Since we don't actually *know* the self type for an object,
524     // this "open(err)" serves as a kind of dummy standin -- basically
525     // a placeholder type.
526     let open_ty = tcx.mk_infer(ty::FreshTy(0));
527
528     let predicates = existential_predicates.iter().filter_map(|predicate| {
529         if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
530             None
531         } else {
532             Some(predicate.with_self_ty(tcx, open_ty))
533         }
534     }).collect();
535
536     tcx.required_region_bounds(open_ty, predicates)
537 }