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(..) => {
97 ty
::Predicate
::Rfc1592(ref data
) => {
98 bug
!("RFC1592 predicate `{:?}` in predicate_obligations", data
);
105 /// Implied bounds are region relationships that we deduce
106 /// automatically. The idea is that (e.g.) a caller must check that a
107 /// function's argument types are well-formed immediately before
108 /// calling that fn, and hence the *callee* can assume that its
109 /// argument types are well-formed. This may imply certain relationships
110 /// between generic parameters. For example:
112 /// fn foo<'a,T>(x: &'a T)
114 /// can only be called with a `'a` and `T` such that `&'a T` is WF.
115 /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
117 pub enum ImpliedBound
<'tcx
> {
118 RegionSubRegion(ty
::Region
, ty
::Region
),
119 RegionSubParam(ty
::Region
, ty
::ParamTy
),
120 RegionSubProjection(ty
::Region
, ty
::ProjectionTy
<'tcx
>),
123 /// Compute the implied bounds that a callee/impl can assume based on
124 /// the fact that caller/projector has ensured that `ty` is WF. See
125 /// the `ImpliedBound` type for more details.
126 pub fn implied_bounds
<'a
, 'gcx
, 'tcx
>(
127 infcx
: &'a InferCtxt
<'a
, 'gcx
, 'tcx
>,
128 body_id
: ast
::NodeId
,
131 -> Vec
<ImpliedBound
<'tcx
>>
133 // Sometimes when we ask what it takes for T: WF, we get back that
134 // U: WF is required; in that case, we push U onto this stack and
135 // process it next. Currently (at least) these resulting
136 // predicates are always guaranteed to be a subset of the original
137 // type, so we need not fear non-termination.
138 let mut wf_types
= vec
![ty
];
140 let mut implied_bounds
= vec
![];
142 while let Some(ty
) = wf_types
.pop() {
143 // Compute the obligations for `ty` to be well-formed. If `ty` is
144 // an unresolved inference variable, just substituted an empty set
145 // -- because the return type here is going to be things we *add*
146 // to the environment, it's always ok for this set to be smaller
147 // than the ultimate set. (Note: normally there won't be
148 // unresolved inference variables here anyway, but there might be
149 // during typeck under some circumstances.)
150 let obligations
= obligations(infcx
, body_id
, ty
, span
).unwrap_or(vec
![]);
152 // From the full set of obligations, just filter down to the
153 // region relationships.
154 implied_bounds
.extend(
157 .flat_map(|obligation
| {
158 assert
!(!obligation
.has_escaping_regions());
159 match obligation
.predicate
{
160 ty
::Predicate
::Trait(..) |
161 ty
::Predicate
::Rfc1592(..) |
162 ty
::Predicate
::Equate(..) |
163 ty
::Predicate
::Projection(..) |
164 ty
::Predicate
::ClosureKind(..) |
165 ty
::Predicate
::ObjectSafe(..) =>
168 ty
::Predicate
::WellFormed(subty
) => {
169 wf_types
.push(subty
);
173 ty
::Predicate
::RegionOutlives(ref data
) =>
174 match infcx
.tcx
.no_late_bound_regions(data
) {
177 Some(ty
::OutlivesPredicate(r_a
, r_b
)) =>
178 vec
![ImpliedBound
::RegionSubRegion(r_b
, r_a
)],
181 ty
::Predicate
::TypeOutlives(ref data
) =>
182 match infcx
.tcx
.no_late_bound_regions(data
) {
184 Some(ty
::OutlivesPredicate(ty_a
, r_b
)) => {
185 let components
= infcx
.outlives_components(ty_a
);
186 implied_bounds_from_components(r_b
, components
)
195 /// When we have an implied bound that `T: 'a`, we can further break
196 /// this down to determine what relationships would have to hold for
197 /// `T: 'a` to hold. We get to assume that the caller has validated
198 /// those relationships.
199 fn implied_bounds_from_components
<'tcx
>(sub_region
: ty
::Region
,
200 sup_components
: Vec
<Component
<'tcx
>>)
201 -> Vec
<ImpliedBound
<'tcx
>>
205 .flat_map(|component
| {
207 Component
::Region(r
) =>
208 vec
!(ImpliedBound
::RegionSubRegion(sub_region
, r
)),
209 Component
::Param(p
) =>
210 vec
!(ImpliedBound
::RegionSubParam(sub_region
, p
)),
211 Component
::Projection(p
) =>
212 vec
!(ImpliedBound
::RegionSubProjection(sub_region
, p
)),
213 Component
::EscapingProjection(_
) =>
214 // If the projection has escaping regions, don't
215 // try to infer any implied bounds even for its
216 // free components. This is conservative, because
217 // the caller will still have to prove that those
218 // free components outlive `sub_region`. But the
219 // idea is that the WAY that the caller proves
220 // that may change in the future and we want to
221 // give ourselves room to get smarter here.
223 Component
::UnresolvedInferenceVariable(..) =>
230 struct WfPredicates
<'a
, 'gcx
: 'a
+'tcx
, 'tcx
: 'a
> {
231 infcx
: &'a InferCtxt
<'a
, 'gcx
, 'tcx
>,
232 body_id
: ast
::NodeId
,
234 out
: Vec
<traits
::PredicateObligation
<'tcx
>>,
237 impl<'a
, 'gcx
, 'tcx
> WfPredicates
<'a
, 'gcx
, 'tcx
> {
238 fn cause(&mut self, code
: traits
::ObligationCauseCode
<'tcx
>) -> traits
::ObligationCause
<'tcx
> {
239 traits
::ObligationCause
::new(self.span
, self.body_id
, code
)
242 fn normalize(&mut self) -> Vec
<traits
::PredicateObligation
<'tcx
>> {
243 let cause
= self.cause(traits
::MiscObligation
);
244 let infcx
= &mut self.infcx
;
246 .inspect(|pred
| assert
!(!pred
.has_escaping_regions()))
248 let mut selcx
= traits
::SelectionContext
::new(infcx
);
249 let pred
= traits
::normalize(&mut selcx
, cause
.clone(), pred
);
250 once(pred
.value
).chain(pred
.obligations
)
255 /// Pushes the obligations required for `trait_ref` to be WF into
257 fn compute_trait_ref(&mut self, trait_ref
: &ty
::TraitRef
<'tcx
>) {
258 let obligations
= self.nominal_obligations(trait_ref
.def_id
, trait_ref
.substs
);
259 self.out
.extend(obligations
);
261 let cause
= self.cause(traits
::MiscObligation
);
263 trait_ref
.substs
.types
266 .filter(|ty
| !ty
.has_escaping_regions())
267 .map(|ty
| traits
::Obligation
::new(cause
.clone(),
268 ty
::Predicate
::WellFormed(ty
))));
271 /// Pushes the obligations required for `trait_ref::Item` to be WF
273 fn compute_projection(&mut self, data
: ty
::ProjectionTy
<'tcx
>) {
274 // A projection is well-formed if (a) the trait ref itself is
275 // WF and (b) the trait-ref holds. (It may also be
276 // normalizable and be WF that way.)
278 self.compute_trait_ref(&data
.trait_ref
);
280 if !data
.has_escaping_regions() {
281 let predicate
= data
.trait_ref
.to_predicate();
282 let cause
= self.cause(traits
::ProjectionWf(data
));
283 self.out
.push(traits
::Obligation
::new(cause
, predicate
));
287 fn require_sized(&mut self, subty
: Ty
<'tcx
>, cause
: traits
::ObligationCauseCode
<'tcx
>,
289 if !subty
.has_escaping_regions() {
290 let cause
= self.cause(cause
);
291 match self.infcx
.tcx
.trait_ref_for_builtin_bound(ty
::BoundSized
, subty
) {
293 let predicate
= trait_ref
.to_predicate();
294 let predicate
= if rfc1592
{
295 ty
::Predicate
::Rfc1592(box predicate
)
300 traits
::Obligation
::new(cause
,
303 Err(ErrorReported
) => { }
308 /// Push new obligations into `out`. Returns true if it was able
309 /// to generate all the predicates needed to validate that `ty0`
310 /// is WF. Returns false if `ty0` is an unresolved type variable,
311 /// in which case we are not able to simplify at all.
312 fn compute(&mut self, ty0
: Ty
<'tcx
>) -> bool
{
313 let tcx
= self.infcx
.tcx
;
314 let mut subtys
= ty0
.walk();
315 while let Some(ty
) = subtys
.next() {
326 // WfScalar, WfParameter, etc
330 ty
::TyArray(subty
, _
) => {
331 self.require_sized(subty
, traits
::SliceOrArrayElem
, false);
334 ty
::TyTuple(ref tys
) => {
335 if let Some((_last
, rest
)) = tys
.split_last() {
337 self.require_sized(elem
, traits
::TupleElem
, true);
344 // simple cases that are WF if their type args are WF
347 ty
::TyProjection(data
) => {
348 subtys
.skip_current_subtree(); // subtree handled by compute_projection
349 self.compute_projection(data
);
352 ty
::TyEnum(def
, substs
) |
353 ty
::TyStruct(def
, substs
) => {
355 let obligations
= self.nominal_obligations(def
.did
, substs
);
356 self.out
.extend(obligations
);
359 ty
::TyRef(r
, mt
) => {
361 if !r
.has_escaping_regions() && !mt
.ty
.has_escaping_regions() {
362 let cause
= self.cause(traits
::ReferenceOutlivesReferent(ty
));
364 traits
::Obligation
::new(
366 ty
::Predicate
::TypeOutlives(
368 ty
::OutlivesPredicate(mt
.ty
, *r
)))));
372 ty
::TyClosure(..) => {
373 // the types in a closure are always the types of
374 // local variables (or possibly references to local
375 // variables), we'll walk those.
377 // (Though, local variables are probably not
378 // needed, as they are separately checked w/r/t
382 ty
::TyFnDef(..) | ty
::TyFnPtr(_
) => {
383 // let the loop iterate into the argument/return
384 // types appearing in the fn signature
388 // all of the requirements on type parameters
389 // should've been checked by the instantiation
390 // of whatever returned this exact `impl Trait`.
393 ty
::TyTrait(ref data
) => {
396 // Here, we defer WF checking due to higher-ranked
397 // regions. This is perhaps not ideal.
398 self.from_object_ty(ty
, data
);
400 // FIXME(#27579) RFC also considers adding trait
401 // obligations that don't refer to Self and
404 let cause
= self.cause(traits
::MiscObligation
);
406 // FIXME(#33243): remove RFC1592
407 self.out
.push(traits
::Obligation
::new(
409 ty
::Predicate
::ObjectSafe(data
.principal_def_id())
411 let component_traits
=
412 data
.bounds
.builtin_bounds
.iter().flat_map(|bound
| {
413 tcx
.lang_items
.from_builtin_kind(bound
).ok()
415 // .chain(Some(data.principal_def_id()));
417 component_traits
.map(|did
| { traits
::Obligation
::new(
419 ty
::Predicate
::Rfc1592(
420 box ty
::Predicate
::ObjectSafe(did
)
426 // Inference variables are the complicated case, since we don't
427 // know what type they are. We do two things:
429 // 1. Check if they have been resolved, and if so proceed with
431 // 2. If not, check whether this is the type that we
432 // started with (ty0). In that case, we've made no
433 // progress at all, so return false. Otherwise,
434 // we've at least simplified things (i.e., we went
435 // from `Vec<$0>: WF` to `$0: WF`, so we can
436 // register a pending obligation and keep
437 // moving. (Goal is that an "inductive hypothesis"
438 // is satisfied to ensure termination.)
440 let ty
= self.infcx
.shallow_resolve(ty
);
441 if let ty
::TyInfer(_
) = ty
.sty
{ // not yet resolved...
442 if ty
== ty0
{ // ...this is the type we started from! no progress.
446 let cause
= self.cause(traits
::MiscObligation
);
447 self.out
.push( // ...not the type we started from, so we made progress.
448 traits
::Obligation
::new(cause
, ty
::Predicate
::WellFormed(ty
)));
450 // Yes, resolved, proceed with the
451 // result. Should never return false because
452 // `ty` is not a TyInfer.
453 assert
!(self.compute(ty
));
459 // if we made it through that loop above, we made progress!
463 fn nominal_obligations(&mut self,
465 substs
: &Substs
<'tcx
>)
466 -> Vec
<traits
::PredicateObligation
<'tcx
>>
469 self.infcx
.tcx
.lookup_predicates(def_id
)
470 .instantiate(self.infcx
.tcx
, substs
);
471 let cause
= self.cause(traits
::ItemObligation(def_id
));
472 predicates
.predicates
474 .map(|pred
| traits
::Obligation
::new(cause
.clone(), pred
))
475 .filter(|pred
| !pred
.has_escaping_regions())
479 fn from_object_ty(&mut self, ty
: Ty
<'tcx
>, data
: &ty
::TraitTy
<'tcx
>) {
480 // Imagine a type like this:
483 // trait Bar<'c> : 'c { }
485 // &'b (Foo+'c+Bar<'d>)
488 // In this case, the following relationships must hold:
493 // The first conditions is due to the normal region pointer
494 // rules, which say that a reference cannot outlive its
497 // The final condition may be a bit surprising. In particular,
498 // you may expect that it would have been `'c <= 'd`, since
499 // usually lifetimes of outer things are conservative
500 // approximations for inner things. However, it works somewhat
501 // differently with trait objects: here the idea is that if the
502 // user specifies a region bound (`'c`, in this case) it is the
503 // "master bound" that *implies* that bounds from other traits are
504 // all met. (Remember that *all bounds* in a type like
505 // `Foo+Bar+Zed` must be met, not just one, hence if we write
506 // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
509 // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
510 // am looking forward to the future here.
512 if !data
.has_escaping_regions() {
513 let implicit_bounds
=
514 object_region_bounds(self.infcx
.tcx
,
516 data
.bounds
.builtin_bounds
);
518 let explicit_bound
= data
.bounds
.region_bound
;
520 for implicit_bound
in implicit_bounds
{
521 let cause
= self.cause(traits
::ReferenceOutlivesReferent(ty
));
522 let outlives
= ty
::Binder(ty
::OutlivesPredicate(explicit_bound
, implicit_bound
));
523 self.out
.push(traits
::Obligation
::new(cause
, outlives
.to_predicate()));
529 /// Given an object type like `SomeTrait+Send`, computes the lifetime
530 /// bounds that must hold on the elided self type. These are derived
531 /// from the declarations of `SomeTrait`, `Send`, and friends -- if
532 /// they declare `trait SomeTrait : 'static`, for example, then
533 /// `'static` would appear in the list. The hard work is done by
534 /// `ty::required_region_bounds`, see that for more information.
535 pub fn object_region_bounds
<'a
, 'gcx
, 'tcx
>(
536 tcx
: TyCtxt
<'a
, 'gcx
, 'tcx
>,
537 principal
: &ty
::PolyTraitRef
<'tcx
>,
538 others
: ty
::BuiltinBounds
)
541 // Since we don't actually *know* the self type for an object,
542 // this "open(err)" serves as a kind of dummy standin -- basically
543 // a skolemized type.
544 let open_ty
= tcx
.mk_infer(ty
::FreshTy(0));
546 // Note that we preserve the overall binding levels here.
547 assert
!(!open_ty
.has_escaping_regions());
548 let substs
= tcx
.mk_substs(principal
.0.substs
.with_self_ty(open_ty
));
549 let trait_refs
= vec
!(ty
::Binder(ty
::TraitRef
::new(principal
.0.def_id
, substs
)));
551 let mut predicates
= others
.to_predicates(tcx
, open_ty
);
552 predicates
.extend(trait_refs
.iter().map(|t
| t
.to_predicate()));
554 tcx
.required_region_bounds(open_ty
, predicates
)