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 middle
::const_val
::{ConstVal, ConstAggregate}
;
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
::Equate(ref t
) => {
81 wf
.compute(t
.skip_binder().0);
82 wf
.compute(t
.skip_binder().1);
84 ty
::Predicate
::RegionOutlives(..) => {
86 ty
::Predicate
::TypeOutlives(ref t
) => {
87 wf
.compute(t
.skip_binder().0);
89 ty
::Predicate
::Projection(ref t
) => {
90 let t
= t
.skip_binder(); // (*)
91 wf
.compute_projection(t
.projection_ty
);
94 ty
::Predicate
::WellFormed(t
) => {
97 ty
::Predicate
::ObjectSafe(_
) => {
99 ty
::Predicate
::ClosureKind(..) => {
101 ty
::Predicate
::Subtype(ref data
) => {
102 wf
.compute(data
.skip_binder().a
); // (*)
103 wf
.compute(data
.skip_binder().b
); // (*)
105 ty
::Predicate
::ConstEvaluatable(def_id
, substs
) => {
106 let obligations
= wf
.nominal_obligations(def_id
, substs
);
107 wf
.out
.extend(obligations
);
109 for ty
in substs
.types() {
118 struct WfPredicates
<'a
, 'gcx
: 'a
+'tcx
, 'tcx
: 'a
> {
119 infcx
: &'a InferCtxt
<'a
, 'gcx
, 'tcx
>,
120 param_env
: ty
::ParamEnv
<'tcx
>,
121 body_id
: ast
::NodeId
,
123 out
: Vec
<traits
::PredicateObligation
<'tcx
>>,
126 /// Controls whether we "elaborate" supertraits and so forth on the WF
127 /// predicates. This is a kind of hack to address #43784. The
128 /// underlying problem in that issue was a trait structure like:
131 /// trait Foo: Copy { }
132 /// trait Bar: Foo { }
133 /// impl<T: Bar> Foo for T { }
134 /// impl<T> Bar for T { }
137 /// Here, in the `Foo` impl, we will check that `T: Copy` holds -- but
138 /// we decide that this is true because `T: Bar` is in the
139 /// where-clauses (and we can elaborate that to include `T:
140 /// Copy`). This wouldn't be a problem, except that when we check the
141 /// `Bar` impl, we decide that `T: Foo` must hold because of the `Foo`
142 /// impl. And so nowhere did we check that `T: Copy` holds!
144 /// To resolve this, we elaborate the WF requirements that must be
145 /// proven when checking impls. This means that (e.g.) the `impl Bar
146 /// for T` will be forced to prove not only that `T: Foo` but also `T:
147 /// Copy` (which it won't be able to do, because there is no `Copy`
149 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
155 impl<'a
, 'gcx
, 'tcx
> WfPredicates
<'a
, 'gcx
, 'tcx
> {
156 fn cause(&mut self, code
: traits
::ObligationCauseCode
<'tcx
>) -> traits
::ObligationCause
<'tcx
> {
157 traits
::ObligationCause
::new(self.span
, self.body_id
, code
)
160 fn normalize(&mut self) -> Vec
<traits
::PredicateObligation
<'tcx
>> {
161 let cause
= self.cause(traits
::MiscObligation
);
162 let infcx
= &mut self.infcx
;
163 let param_env
= self.param_env
;
165 .inspect(|pred
| assert
!(!pred
.has_escaping_regions()))
167 let mut selcx
= traits
::SelectionContext
::new(infcx
);
168 let pred
= traits
::normalize(&mut selcx
, param_env
, cause
.clone(), pred
);
169 once(pred
.value
).chain(pred
.obligations
)
174 /// Pushes the obligations required for `trait_ref` to be WF into
176 fn compute_trait_ref(&mut self, trait_ref
: &ty
::TraitRef
<'tcx
>, elaborate
: Elaborate
) {
177 let obligations
= self.nominal_obligations(trait_ref
.def_id
, trait_ref
.substs
);
179 let cause
= self.cause(traits
::MiscObligation
);
180 let param_env
= self.param_env
;
182 if let Elaborate
::All
= elaborate
{
183 let predicates
= obligations
.iter()
184 .map(|obligation
| obligation
.predicate
.clone())
186 let implied_obligations
= traits
::elaborate_predicates(self.infcx
.tcx
, predicates
);
187 let implied_obligations
= implied_obligations
.map(|pred
| {
188 traits
::Obligation
::new(cause
.clone(), param_env
, pred
)
190 self.out
.extend(implied_obligations
);
193 self.out
.extend(obligations
);
196 trait_ref
.substs
.types()
197 .filter(|ty
| !ty
.has_escaping_regions())
198 .map(|ty
| traits
::Obligation
::new(cause
.clone(),
200 ty
::Predicate
::WellFormed(ty
))));
203 /// Pushes the obligations required for `trait_ref::Item` to be WF
205 fn compute_projection(&mut self, data
: ty
::ProjectionTy
<'tcx
>) {
206 // A projection is well-formed if (a) the trait ref itself is
207 // WF and (b) the trait-ref holds. (It may also be
208 // normalizable and be WF that way.)
209 let trait_ref
= data
.trait_ref(self.infcx
.tcx
);
210 self.compute_trait_ref(&trait_ref
, Elaborate
::None
);
212 if !data
.has_escaping_regions() {
213 let predicate
= trait_ref
.to_predicate();
214 let cause
= self.cause(traits
::ProjectionWf(data
));
215 self.out
.push(traits
::Obligation
::new(cause
, self.param_env
, predicate
));
219 /// Pushes the obligations required for a constant value to be WF
221 fn compute_const(&mut self, constant
: &'tcx ty
::Const
<'tcx
>) {
222 self.require_sized(constant
.ty
, traits
::ConstSized
);
224 ConstVal
::Integral(_
) |
227 ConstVal
::ByteStr(_
) |
230 ConstVal
::Variant(_
) |
231 ConstVal
::Function(..) => {}
232 ConstVal
::Aggregate(ConstAggregate
::Struct(fields
)) => {
233 for &(_
, v
) in fields
{
234 self.compute_const(v
);
237 ConstVal
::Aggregate(ConstAggregate
::Tuple(fields
)) |
238 ConstVal
::Aggregate(ConstAggregate
::Array(fields
)) => {
240 self.compute_const(v
);
243 ConstVal
::Aggregate(ConstAggregate
::Repeat(v
, _
)) => {
244 self.compute_const(v
);
246 ConstVal
::Unevaluated(def_id
, substs
) => {
247 let obligations
= self.nominal_obligations(def_id
, substs
);
248 self.out
.extend(obligations
);
250 let predicate
= ty
::Predicate
::ConstEvaluatable(def_id
, substs
);
251 let cause
= self.cause(traits
::MiscObligation
);
252 self.out
.push(traits
::Obligation
::new(cause
,
259 fn require_sized(&mut self, subty
: Ty
<'tcx
>, cause
: traits
::ObligationCauseCode
<'tcx
>) {
260 if !subty
.has_escaping_regions() {
261 let cause
= self.cause(cause
);
262 let trait_ref
= ty
::TraitRef
{
263 def_id
: self.infcx
.tcx
.require_lang_item(lang_items
::SizedTraitLangItem
),
264 substs
: self.infcx
.tcx
.mk_substs_trait(subty
, &[]),
266 self.out
.push(traits
::Obligation
::new(cause
, self.param_env
, trait_ref
.to_predicate()));
270 /// Push new obligations into `out`. Returns true if it was able
271 /// to generate all the predicates needed to validate that `ty0`
272 /// is WF. Returns false if `ty0` is an unresolved type variable,
273 /// in which case we are not able to simplify at all.
274 fn compute(&mut self, ty0
: Ty
<'tcx
>) -> bool
{
275 let mut subtys
= ty0
.walk();
276 let param_env
= self.param_env
;
277 while let Some(ty
) = subtys
.next() {
288 ty
::TyForeign(..) => {
289 // WfScalar, WfParameter, etc
292 ty
::TySlice(subty
) => {
293 self.require_sized(subty
, traits
::SliceOrArrayElem
);
296 ty
::TyArray(subty
, len
) => {
297 self.require_sized(subty
, traits
::SliceOrArrayElem
);
298 assert_eq
!(len
.ty
, self.infcx
.tcx
.types
.usize);
299 self.compute_const(len
);
302 ty
::TyTuple(ref tys
, _
) => {
303 if let Some((_last
, rest
)) = tys
.split_last() {
305 self.require_sized(elem
, traits
::TupleElem
);
311 // simple cases that are WF if their type args are WF
314 ty
::TyProjection(data
) => {
315 subtys
.skip_current_subtree(); // subtree handled by compute_projection
316 self.compute_projection(data
);
319 ty
::TyAdt(def
, substs
) => {
321 let obligations
= self.nominal_obligations(def
.did
, substs
);
322 self.out
.extend(obligations
);
325 ty
::TyRef(r
, mt
) => {
327 if !r
.has_escaping_regions() && !mt
.ty
.has_escaping_regions() {
328 let cause
= self.cause(traits
::ReferenceOutlivesReferent(ty
));
330 traits
::Obligation
::new(
333 ty
::Predicate
::TypeOutlives(
335 ty
::OutlivesPredicate(mt
.ty
, r
)))));
339 ty
::TyGenerator(..) | ty
::TyClosure(..) => {
340 // the types in a closure or generator are always the types of
341 // local variables (or possibly references to local
342 // variables), we'll walk those.
344 // (Though, local variables are probably not
345 // needed, as they are separately checked w/r/t
349 ty
::TyFnDef(..) | ty
::TyFnPtr(_
) => {
350 // let the loop iterate into the argument/return
351 // types appearing in the fn signature
355 // all of the requirements on type parameters
356 // should've been checked by the instantiation
357 // of whatever returned this exact `impl Trait`.
360 ty
::TyDynamic(data
, r
) => {
363 // Here, we defer WF checking due to higher-ranked
364 // regions. This is perhaps not ideal.
365 self.from_object_ty(ty
, data
, r
);
367 // FIXME(#27579) RFC also considers adding trait
368 // obligations that don't refer to Self and
371 let cause
= self.cause(traits
::MiscObligation
);
372 let component_traits
=
373 data
.auto_traits().chain(data
.principal().map(|p
| p
.def_id()));
375 component_traits
.map(|did
| traits
::Obligation
::new(
378 ty
::Predicate
::ObjectSafe(did
)
383 // Inference variables are the complicated case, since we don't
384 // know what type they are. We do two things:
386 // 1. Check if they have been resolved, and if so proceed with
388 // 2. If not, check whether this is the type that we
389 // started with (ty0). In that case, we've made no
390 // progress at all, so return false. Otherwise,
391 // we've at least simplified things (i.e., we went
392 // from `Vec<$0>: WF` to `$0: WF`, so we can
393 // register a pending obligation and keep
394 // moving. (Goal is that an "inductive hypothesis"
395 // is satisfied to ensure termination.)
397 let ty
= self.infcx
.shallow_resolve(ty
);
398 if let ty
::TyInfer(_
) = ty
.sty
{ // not yet resolved...
399 if ty
== ty0
{ // ...this is the type we started from! no progress.
403 let cause
= self.cause(traits
::MiscObligation
);
404 self.out
.push( // ...not the type we started from, so we made progress.
405 traits
::Obligation
::new(cause
,
407 ty
::Predicate
::WellFormed(ty
)));
409 // Yes, resolved, proceed with the
410 // result. Should never return false because
411 // `ty` is not a TyInfer.
412 assert
!(self.compute(ty
));
418 // if we made it through that loop above, we made progress!
422 fn nominal_obligations(&mut self,
424 substs
: &Substs
<'tcx
>)
425 -> Vec
<traits
::PredicateObligation
<'tcx
>>
428 self.infcx
.tcx
.predicates_of(def_id
)
429 .instantiate(self.infcx
.tcx
, substs
);
430 let cause
= self.cause(traits
::ItemObligation(def_id
));
431 predicates
.predicates
433 .map(|pred
| traits
::Obligation
::new(cause
.clone(),
436 .filter(|pred
| !pred
.has_escaping_regions())
440 fn from_object_ty(&mut self, ty
: Ty
<'tcx
>,
441 data
: ty
::Binder
<&'tcx ty
::Slice
<ty
::ExistentialPredicate
<'tcx
>>>,
442 region
: ty
::Region
<'tcx
>) {
443 // Imagine a type like this:
446 // trait Bar<'c> : 'c { }
448 // &'b (Foo+'c+Bar<'d>)
451 // In this case, the following relationships must hold:
456 // The first conditions is due to the normal region pointer
457 // rules, which say that a reference cannot outlive its
460 // The final condition may be a bit surprising. In particular,
461 // you may expect that it would have been `'c <= 'd`, since
462 // usually lifetimes of outer things are conservative
463 // approximations for inner things. However, it works somewhat
464 // differently with trait objects: here the idea is that if the
465 // user specifies a region bound (`'c`, in this case) it is the
466 // "master bound" that *implies* that bounds from other traits are
467 // all met. (Remember that *all bounds* in a type like
468 // `Foo+Bar+Zed` must be met, not just one, hence if we write
469 // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
472 // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
473 // am looking forward to the future here.
475 if !data
.has_escaping_regions() {
476 let implicit_bounds
=
477 object_region_bounds(self.infcx
.tcx
, data
);
479 let explicit_bound
= region
;
481 for implicit_bound
in implicit_bounds
{
482 let cause
= self.cause(traits
::ObjectTypeBound(ty
, explicit_bound
));
483 let outlives
= ty
::Binder(ty
::OutlivesPredicate(explicit_bound
, implicit_bound
));
484 self.out
.push(traits
::Obligation
::new(cause
,
486 outlives
.to_predicate()));
492 /// Given an object type like `SomeTrait+Send`, computes the lifetime
493 /// bounds that must hold on the elided self type. These are derived
494 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
495 /// they declare `trait SomeTrait : 'static`, for example, then
496 /// `'static` would appear in the list. The hard work is done by
497 /// `ty::required_region_bounds`, see that for more information.
498 pub fn object_region_bounds
<'a
, 'gcx
, 'tcx
>(
499 tcx
: TyCtxt
<'a
, 'gcx
, 'tcx
>,
500 existential_predicates
: ty
::Binder
<&'tcx ty
::Slice
<ty
::ExistentialPredicate
<'tcx
>>>)
501 -> Vec
<ty
::Region
<'tcx
>>
503 // Since we don't actually *know* the self type for an object,
504 // this "open(err)" serves as a kind of dummy standin -- basically
505 // a skolemized type.
506 let open_ty
= tcx
.mk_infer(ty
::FreshTy(0));
508 let predicates
= existential_predicates
.iter().filter_map(|predicate
| {
509 if let ty
::ExistentialPredicate
::Projection(_
) = *predicate
.skip_binder() {
512 Some(predicate
.with_self_ty(tcx
, open_ty
))
516 tcx
.required_region_bounds(open_ty
, predicates
)