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
::TyAdt(def
, substs
) => {
341 let obligations
= self.nominal_obligations(def
.did
, substs
);
342 self.out
.extend(obligations
);
345 ty
::TyRef(r
, mt
) => {
347 if !r
.has_escaping_regions() && !mt
.ty
.has_escaping_regions() {
348 let cause
= self.cause(traits
::ReferenceOutlivesReferent(ty
));
350 traits
::Obligation
::new(
352 ty
::Predicate
::TypeOutlives(
354 ty
::OutlivesPredicate(mt
.ty
, r
)))));
358 ty
::TyClosure(..) => {
359 // the types in a closure are always the types of
360 // local variables (or possibly references to local
361 // variables), we'll walk those.
363 // (Though, local variables are probably not
364 // needed, as they are separately checked w/r/t
368 ty
::TyFnDef(..) | ty
::TyFnPtr(_
) => {
369 // let the loop iterate into the argument/return
370 // types appearing in the fn signature
374 // all of the requirements on type parameters
375 // should've been checked by the instantiation
376 // of whatever returned this exact `impl Trait`.
379 ty
::TyTrait(ref data
) => {
382 // Here, we defer WF checking due to higher-ranked
383 // regions. This is perhaps not ideal.
384 self.from_object_ty(ty
, data
);
386 // FIXME(#27579) RFC also considers adding trait
387 // obligations that don't refer to Self and
390 let cause
= self.cause(traits
::MiscObligation
);
392 let component_traits
=
393 data
.builtin_bounds
.iter().flat_map(|bound
| {
394 tcx
.lang_items
.from_builtin_kind(bound
).ok()
396 .chain(Some(data
.principal
.def_id()));
398 component_traits
.map(|did
| { traits
::Obligation
::new(
400 ty
::Predicate
::ObjectSafe(did
)
405 // Inference variables are the complicated case, since we don't
406 // know what type they are. We do two things:
408 // 1. Check if they have been resolved, and if so proceed with
410 // 2. If not, check whether this is the type that we
411 // started with (ty0). In that case, we've made no
412 // progress at all, so return false. Otherwise,
413 // we've at least simplified things (i.e., we went
414 // from `Vec<$0>: WF` to `$0: WF`, so we can
415 // register a pending obligation and keep
416 // moving. (Goal is that an "inductive hypothesis"
417 // is satisfied to ensure termination.)
419 let ty
= self.infcx
.shallow_resolve(ty
);
420 if let ty
::TyInfer(_
) = ty
.sty
{ // not yet resolved...
421 if ty
== ty0
{ // ...this is the type we started from! no progress.
425 let cause
= self.cause(traits
::MiscObligation
);
426 self.out
.push( // ...not the type we started from, so we made progress.
427 traits
::Obligation
::new(cause
, ty
::Predicate
::WellFormed(ty
)));
429 // Yes, resolved, proceed with the
430 // result. Should never return false because
431 // `ty` is not a TyInfer.
432 assert
!(self.compute(ty
));
438 // if we made it through that loop above, we made progress!
442 fn nominal_obligations(&mut self,
444 substs
: &Substs
<'tcx
>)
445 -> Vec
<traits
::PredicateObligation
<'tcx
>>
448 self.infcx
.tcx
.lookup_predicates(def_id
)
449 .instantiate(self.infcx
.tcx
, substs
);
450 let cause
= self.cause(traits
::ItemObligation(def_id
));
451 predicates
.predicates
453 .map(|pred
| traits
::Obligation
::new(cause
.clone(), pred
))
454 .filter(|pred
| !pred
.has_escaping_regions())
458 fn from_object_ty(&mut self, ty
: Ty
<'tcx
>, data
: &ty
::TraitObject
<'tcx
>) {
459 // Imagine a type like this:
462 // trait Bar<'c> : 'c { }
464 // &'b (Foo+'c+Bar<'d>)
467 // In this case, the following relationships must hold:
472 // The first conditions is due to the normal region pointer
473 // rules, which say that a reference cannot outlive its
476 // The final condition may be a bit surprising. In particular,
477 // you may expect that it would have been `'c <= 'd`, since
478 // usually lifetimes of outer things are conservative
479 // approximations for inner things. However, it works somewhat
480 // differently with trait objects: here the idea is that if the
481 // user specifies a region bound (`'c`, in this case) it is the
482 // "master bound" that *implies* that bounds from other traits are
483 // all met. (Remember that *all bounds* in a type like
484 // `Foo+Bar+Zed` must be met, not just one, hence if we write
485 // `Foo<'x>+Bar<'y>`, we know that the type outlives *both* 'x and
488 // Note: in fact we only permit builtin traits, not `Bar<'d>`, I
489 // am looking forward to the future here.
491 if !data
.has_escaping_regions() {
492 let implicit_bounds
=
493 object_region_bounds(self.infcx
.tcx
,
495 data
.builtin_bounds
);
497 let explicit_bound
= data
.region_bound
;
499 for implicit_bound
in implicit_bounds
{
500 let cause
= self.cause(traits
::ReferenceOutlivesReferent(ty
));
501 let outlives
= ty
::Binder(ty
::OutlivesPredicate(explicit_bound
, implicit_bound
));
502 self.out
.push(traits
::Obligation
::new(cause
, 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 principal
: ty
::PolyExistentialTraitRef
<'tcx
>,
517 others
: ty
::BuiltinBounds
)
518 -> Vec
<&'tcx ty
::Region
>
520 // Since we don't actually *know* the self type for an object,
521 // this "open(err)" serves as a kind of dummy standin -- basically
522 // a skolemized type.
523 let open_ty
= tcx
.mk_infer(ty
::FreshTy(0));
525 let mut predicates
= others
.to_predicates(tcx
, open_ty
);
526 predicates
.push(principal
.with_self_ty(tcx
, open_ty
).to_predicate());
528 tcx
.required_region_bounds(open_ty
, predicates
)