1 //! Support code for rustdoc and external tools.
2 //! You really don't want to be using this unless you need to.
6 use crate::infer
::region_constraints
::{Constraint, RegionConstraintData}
;
7 use crate::infer
::InferCtxt
;
8 use rustc_middle
::ty
::fold
::TypeFolder
;
9 use rustc_middle
::ty
::{Region, RegionVid}
;
11 use rustc_data_structures
::fx
::{FxHashMap, FxHashSet}
;
13 use std
::collections
::hash_map
::Entry
;
14 use std
::collections
::VecDeque
;
16 // FIXME(twk): this is obviously not nice to duplicate like that
17 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
18 pub enum RegionTarget
<'tcx
> {
23 #[derive(Default, Debug, Clone)]
24 pub struct RegionDeps
<'tcx
> {
25 larger
: FxHashSet
<RegionTarget
<'tcx
>>,
26 smaller
: FxHashSet
<RegionTarget
<'tcx
>>,
29 pub enum AutoTraitResult
<A
> {
35 impl<A
> AutoTraitResult
<A
> {
36 fn is_auto(&self) -> bool
{
38 AutoTraitResult
::PositiveImpl(_
) | AutoTraitResult
::NegativeImpl
=> true,
44 pub struct AutoTraitInfo
<'cx
> {
45 pub full_user_env
: ty
::ParamEnv
<'cx
>,
46 pub region_data
: RegionConstraintData
<'cx
>,
47 pub vid_to_region
: FxHashMap
<ty
::RegionVid
, ty
::Region
<'cx
>>,
50 pub struct AutoTraitFinder
<'tcx
> {
54 impl<'tcx
> AutoTraitFinder
<'tcx
> {
55 pub fn new(tcx
: TyCtxt
<'tcx
>) -> Self {
56 AutoTraitFinder { tcx }
59 /// Makes a best effort to determine whether and under which conditions an auto trait is
60 /// implemented for a type. For example, if you have
63 /// struct Foo<T> { data: Box<T> }
66 /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type).
67 /// The analysis attempts to account for custom impls as well as other complex cases. This
68 /// result is intended for use by rustdoc and other such consumers.
70 /// (Note that due to the coinductive nature of Send, the full and correct result is actually
71 /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
72 /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send.
73 /// But this is often not the best way to present to the user.)
75 /// Warning: The API should be considered highly unstable, and it may be refactored or removed
77 pub fn find_auto_trait_generics
<A
>(
80 orig_env
: ty
::ParamEnv
<'tcx
>,
82 auto_trait_callback
: impl Fn(&InferCtxt
<'_
, 'tcx
>, AutoTraitInfo
<'tcx
>) -> A
,
83 ) -> AutoTraitResult
<A
> {
86 let trait_ref
= ty
::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) }
;
88 let trait_pred
= ty
::Binder
::bind(trait_ref
);
90 let bail_out
= tcx
.infer_ctxt().enter(|infcx
| {
91 let mut selcx
= SelectionContext
::with_negative(&infcx
, true);
92 let result
= selcx
.select(&Obligation
::new(
93 ObligationCause
::dummy(),
95 trait_pred
.to_poly_trait_predicate(),
99 Ok(Some(Vtable
::VtableImpl(_
))) => {
101 "find_auto_trait_generics({:?}): \
102 manual impl found, bailing out",
111 // If an explicit impl exists, it always takes priority over an auto impl
113 return AutoTraitResult
::ExplicitImpl
;
116 tcx
.infer_ctxt().enter(|infcx
| {
117 let mut fresh_preds
= FxHashSet
::default();
119 // Due to the way projections are handled by SelectionContext, we need to run
120 // evaluate_predicates twice: once on the original param env, and once on the result of
121 // the first evaluate_predicates call.
123 // The problem is this: most of rustc, including SelectionContext and traits::project,
124 // are designed to work with a concrete usage of a type (e.g., Vec<u8>
125 // fn<T>() { Vec<T> }. This information will generally never change - given
126 // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
127 // If we're unable to prove that 'T' implements a particular trait, we're done -
128 // there's nothing left to do but error out.
130 // However, synthesizing an auto trait impl works differently. Here, we start out with
131 // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
132 // with - and progressively discover the conditions we need to fulfill for it to
133 // implement a certain auto trait. This ends up breaking two assumptions made by trait
134 // selection and projection:
136 // * We can always cache the result of a particular trait selection for the lifetime of
138 // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
139 // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
141 // We fix the first assumption by manually clearing out all of the InferCtxt's caches
142 // in between calls to SelectionContext.select. This allows us to keep all of the
143 // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
144 // them between calls.
146 // We fix the second assumption by reprocessing the result of our first call to
147 // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
148 // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
149 // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
150 // SelectionContext to return it back to us.
152 let (new_env
, user_env
) = match self.evaluate_predicates(
162 None
=> return AutoTraitResult
::NegativeImpl
,
165 let (full_env
, full_user_env
) = self
166 .evaluate_predicates(
176 panic
!("Failed to fully process: {:?} {:?} {:?}", ty
, trait_did
, orig_env
)
180 "find_auto_trait_generics({:?}): fulfilling \
184 infcx
.clear_caches();
186 // At this point, we already have all of the bounds we need. FulfillmentContext is used
187 // to store all of the necessary region/lifetime bounds in the InferContext, as well as
188 // an additional sanity check.
189 let mut fulfill
= FulfillmentContext
::new();
190 fulfill
.register_bound(&infcx
, full_env
, ty
, trait_did
, ObligationCause
::dummy());
191 fulfill
.select_all_or_error(&infcx
).unwrap_or_else(|e
| {
192 panic
!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did
, ty
, e
)
195 let body_id_map
: FxHashMap
<_
, _
> = infcx
198 .region_obligations()
200 .map(|&(id
, _
)| (id
, vec
![]))
203 infcx
.process_registered_region_obligations(&body_id_map
, None
, full_env
);
205 let region_data
= infcx
208 .unwrap_region_constraints()
209 .region_constraint_data()
212 let vid_to_region
= self.map_vid_to_region(®ion_data
);
214 let info
= AutoTraitInfo { full_user_env, region_data, vid_to_region }
;
216 AutoTraitResult
::PositiveImpl(auto_trait_callback(&infcx
, info
))
221 impl AutoTraitFinder
<'tcx
> {
222 /// The core logic responsible for computing the bounds for our synthesized impl.
224 /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
225 /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
226 /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
227 /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
228 /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
230 /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
231 /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
232 /// user code. According, it considers all possible ways that a `Predicate` could be met, which
233 /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
234 /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
235 /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
236 /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
237 /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
240 /// impl<T> Send for Foo<T> where T: IntoIterator
242 /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
243 /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
245 /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
246 /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
247 /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
248 /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
251 /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
252 /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
253 /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
254 /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
255 /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
256 /// else `SelectionContext` will choke on the missing predicates. However, this should never
257 /// show up in the final synthesized generics: we don't want our generated docs page to contain
258 /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
259 /// separate `user_env`, which only holds the predicates that will actually be displayed to the
261 fn evaluate_predicates(
263 infcx
: &InferCtxt
<'_
, 'tcx
>,
266 param_env
: ty
::ParamEnv
<'tcx
>,
267 user_env
: ty
::ParamEnv
<'tcx
>,
268 fresh_preds
: &mut FxHashSet
<ty
::Predicate
<'tcx
>>,
269 only_projections
: bool
,
270 ) -> Option
<(ty
::ParamEnv
<'tcx
>, ty
::ParamEnv
<'tcx
>)> {
273 let mut select
= SelectionContext
::with_negative(&infcx
, true);
275 let mut already_visited
= FxHashSet
::default();
276 let mut predicates
= VecDeque
::new();
277 predicates
.push_back(ty
::Binder
::bind(ty
::TraitPredicate
{
278 trait_ref
: ty
::TraitRef
{
280 substs
: infcx
.tcx
.mk_substs_trait(ty
, &[]),
284 let computed_preds
= param_env
.caller_bounds
.iter();
285 let mut user_computed_preds
: FxHashSet
<_
> = user_env
.caller_bounds
.iter().collect();
287 let mut new_env
= param_env
;
288 let dummy_cause
= ObligationCause
::dummy();
290 while let Some(pred
) = predicates
.pop_front() {
291 infcx
.clear_caches();
293 if !already_visited
.insert(pred
) {
297 // Call `infcx.resolve_vars_if_possible` to see if we can
298 // get rid of any inference variables.
299 let obligation
= infcx
.resolve_vars_if_possible(&Obligation
::new(
304 let result
= select
.select(&obligation
);
307 &Ok(Some(ref vtable
)) => {
308 // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
309 // we immediately bail out, since it's impossible for us to continue.
311 if let Vtable
::VtableImpl(VtableImplData { impl_def_id, .. }
) = vtable
{
312 // Blame 'tidy' for the weird bracket placement.
313 if infcx
.tcx
.impl_polarity(*impl_def_id
) == ty
::ImplPolarity
::Negative
{
315 "evaluate_nested_obligations: found explicit negative impl\
323 let obligations
= vtable
.clone().nested_obligations().into_iter();
325 if !self.evaluate_nested_obligations(
328 &mut user_computed_preds
,
338 &Err(SelectionError
::Unimplemented
) => {
339 if self.is_param_no_infer(pred
.skip_binder().trait_ref
.substs
) {
340 already_visited
.remove(&pred
);
342 &mut user_computed_preds
,
343 ty
::PredicateKind
::Trait(pred
, hir
::Constness
::NotConst
)
344 .to_predicate(self.tcx
),
346 predicates
.push_back(pred
);
349 "evaluate_nested_obligations: `Unimplemented` found, bailing: \
353 pred
.skip_binder().trait_ref
.substs
358 _
=> panic
!("Unexpected error for '{:?}': {:?}", ty
, result
),
361 let normalized_preds
= elaborate_predicates(
363 computed_preds
.clone().chain(user_computed_preds
.iter().cloned()),
365 .map(|o
| o
.predicate
);
367 ty
::ParamEnv
::new(tcx
.mk_predicates(normalized_preds
), param_env
.reveal
, None
);
370 let final_user_env
= ty
::ParamEnv
::new(
371 tcx
.mk_predicates(user_computed_preds
.into_iter()),
376 "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
378 ty
, trait_did
, new_env
, final_user_env
381 Some((new_env
, final_user_env
))
384 /// This method is designed to work around the following issue:
385 /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
386 /// progressively building a `ParamEnv` based on the results we get.
387 /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
388 /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
390 /// This can lead to a corner case when dealing with region parameters.
391 /// During our selection loop in `evaluate_predicates`, we might end up with
392 /// two trait predicates that differ only in their region parameters:
393 /// one containing a HRTB lifetime parameter, and one containing a 'normal'
394 /// lifetime parameter. For example:
397 /// T as MyTrait<'static>
399 /// If we put both of these predicates in our computed `ParamEnv`, we'll
400 /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
402 /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
403 /// Our end goal is to generate a user-visible description of the conditions
404 /// under which a type implements an auto trait. A trait predicate involving
405 /// a HRTB means that the type needs to work with any choice of lifetime,
406 /// not just one specific lifetime (e.g., `'static`).
407 fn add_user_pred
<'c
>(
409 user_computed_preds
: &mut FxHashSet
<ty
::Predicate
<'c
>>,
410 new_pred
: ty
::Predicate
<'c
>,
412 let mut should_add_new
= true;
413 user_computed_preds
.retain(|&old_pred
| {
415 ty
::PredicateKind
::Trait(new_trait
, _
),
416 ty
::PredicateKind
::Trait(old_trait
, _
),
417 ) = (new_pred
.kind(), old_pred
.kind())
419 if new_trait
.def_id() == old_trait
.def_id() {
420 let new_substs
= new_trait
.skip_binder().trait_ref
.substs
;
421 let old_substs
= old_trait
.skip_binder().trait_ref
.substs
;
423 if !new_substs
.types().eq(old_substs
.types()) {
424 // We can't compare lifetimes if the types are different,
425 // so skip checking `old_pred`.
429 for (new_region
, old_region
) in new_substs
.regions().zip(old_substs
.regions()) {
430 match (new_region
, old_region
) {
431 // If both predicates have an `ReLateBound` (a HRTB) in the
432 // same spot, we do nothing.
434 ty
::RegionKind
::ReLateBound(_
, _
),
435 ty
::RegionKind
::ReLateBound(_
, _
),
438 (ty
::RegionKind
::ReLateBound(_
, _
), _
)
439 | (_
, ty
::RegionKind
::ReVar(_
)) => {
440 // One of these is true:
441 // The new predicate has a HRTB in a spot where the old
442 // predicate does not (if they both had a HRTB, the previous
443 // match arm would have executed). A HRBT is a 'stricter'
444 // bound than anything else, so we want to keep the newer
445 // predicate (with the HRBT) in place of the old predicate.
449 // The old predicate has a region variable where the new
450 // predicate has some other kind of region. An region
451 // variable isn't something we can actually display to a user,
452 // so we choose their new predicate (which doesn't have a region
455 // In both cases, we want to remove the old predicate,
456 // from `user_computed_preds`, and replace it with the new
457 // one. Having both the old and the new
458 // predicate in a `ParamEnv` would confuse `SelectionContext`.
460 // We're currently in the predicate passed to 'retain',
461 // so we return `false` to remove the old predicate from
462 // `user_computed_preds`.
465 (_
, ty
::RegionKind
::ReLateBound(_
, _
))
466 | (ty
::RegionKind
::ReVar(_
), _
) => {
467 // This is the opposite situation as the previous arm.
468 // One of these is true:
470 // The old predicate has a HRTB lifetime in a place where the
471 // new predicate does not.
475 // The new predicate has a region variable where the old
476 // predicate has some other type of region.
478 // We want to leave the old
479 // predicate in `user_computed_preds`, and skip adding
480 // new_pred to `user_computed_params`.
481 should_add_new
= false
492 user_computed_preds
.insert(new_pred
);
496 /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
497 /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
498 fn map_vid_to_region
<'cx
>(
500 regions
: &RegionConstraintData
<'cx
>,
501 ) -> FxHashMap
<ty
::RegionVid
, ty
::Region
<'cx
>> {
502 let mut vid_map
: FxHashMap
<RegionTarget
<'cx
>, RegionDeps
<'cx
>> = FxHashMap
::default();
503 let mut finished_map
= FxHashMap
::default();
505 for constraint
in regions
.constraints
.keys() {
507 &Constraint
::VarSubVar(r1
, r2
) => {
509 let deps1
= vid_map
.entry(RegionTarget
::RegionVid(r1
)).or_default();
510 deps1
.larger
.insert(RegionTarget
::RegionVid(r2
));
513 let deps2
= vid_map
.entry(RegionTarget
::RegionVid(r2
)).or_default();
514 deps2
.smaller
.insert(RegionTarget
::RegionVid(r1
));
516 &Constraint
::RegSubVar(region
, vid
) => {
518 let deps1
= vid_map
.entry(RegionTarget
::Region(region
)).or_default();
519 deps1
.larger
.insert(RegionTarget
::RegionVid(vid
));
522 let deps2
= vid_map
.entry(RegionTarget
::RegionVid(vid
)).or_default();
523 deps2
.smaller
.insert(RegionTarget
::Region(region
));
525 &Constraint
::VarSubReg(vid
, region
) => {
526 finished_map
.insert(vid
, region
);
528 &Constraint
::RegSubReg(r1
, r2
) => {
530 let deps1
= vid_map
.entry(RegionTarget
::Region(r1
)).or_default();
531 deps1
.larger
.insert(RegionTarget
::Region(r2
));
534 let deps2
= vid_map
.entry(RegionTarget
::Region(r2
)).or_default();
535 deps2
.smaller
.insert(RegionTarget
::Region(r1
));
540 while !vid_map
.is_empty() {
541 let target
= *vid_map
.keys().next().expect("Keys somehow empty");
542 let deps
= vid_map
.remove(&target
).expect("Entry somehow missing");
544 for smaller
in deps
.smaller
.iter() {
545 for larger
in deps
.larger
.iter() {
546 match (smaller
, larger
) {
547 (&RegionTarget
::Region(_
), &RegionTarget
::Region(_
)) => {
548 if let Entry
::Occupied(v
) = vid_map
.entry(*smaller
) {
549 let smaller_deps
= v
.into_mut();
550 smaller_deps
.larger
.insert(*larger
);
551 smaller_deps
.larger
.remove(&target
);
554 if let Entry
::Occupied(v
) = vid_map
.entry(*larger
) {
555 let larger_deps
= v
.into_mut();
556 larger_deps
.smaller
.insert(*smaller
);
557 larger_deps
.smaller
.remove(&target
);
560 (&RegionTarget
::RegionVid(v1
), &RegionTarget
::Region(r1
)) => {
561 finished_map
.insert(v1
, r1
);
563 (&RegionTarget
::Region(_
), &RegionTarget
::RegionVid(_
)) => {
564 // Do nothing; we don't care about regions that are smaller than vids.
566 (&RegionTarget
::RegionVid(_
), &RegionTarget
::RegionVid(_
)) => {
567 if let Entry
::Occupied(v
) = vid_map
.entry(*smaller
) {
568 let smaller_deps
= v
.into_mut();
569 smaller_deps
.larger
.insert(*larger
);
570 smaller_deps
.larger
.remove(&target
);
573 if let Entry
::Occupied(v
) = vid_map
.entry(*larger
) {
574 let larger_deps
= v
.into_mut();
575 larger_deps
.smaller
.insert(*smaller
);
576 larger_deps
.smaller
.remove(&target
);
586 fn is_param_no_infer(&self, substs
: SubstsRef
<'_
>) -> bool
{
587 self.is_of_param(substs
.type_at(0)) && !substs
.types().any(|t
| t
.has_infer_types())
590 pub fn is_of_param(&self, ty
: Ty
<'_
>) -> bool
{
592 ty
::Param(_
) => true,
593 ty
::Projection(p
) => self.is_of_param(p
.self_ty()),
598 fn is_self_referential_projection(&self, p
: ty
::PolyProjectionPredicate
<'_
>) -> bool
{
599 match p
.ty().skip_binder().kind
{
600 ty
::Projection(proj
) if proj
== p
.skip_binder().projection_ty
=> true,
605 fn evaluate_nested_obligations(
608 nested
: impl Iterator
<Item
= Obligation
<'tcx
, ty
::Predicate
<'tcx
>>>,
609 computed_preds
: &mut FxHashSet
<ty
::Predicate
<'tcx
>>,
610 fresh_preds
: &mut FxHashSet
<ty
::Predicate
<'tcx
>>,
611 predicates
: &mut VecDeque
<ty
::PolyTraitPredicate
<'tcx
>>,
612 select
: &mut SelectionContext
<'_
, 'tcx
>,
613 only_projections
: bool
,
615 let dummy_cause
= ObligationCause
::dummy();
617 for (obligation
, mut predicate
) in nested
.map(|o
| (o
.clone(), o
.predicate
)) {
618 let is_new_pred
= fresh_preds
.insert(self.clean_pred(select
.infcx(), predicate
));
620 // Resolve any inference variables that we can, to help selection succeed
621 predicate
= select
.infcx().resolve_vars_if_possible(&predicate
);
623 // We only add a predicate as a user-displayable bound if
624 // it involves a generic parameter, and doesn't contain
625 // any inference variables.
627 // Displaying a bound involving a concrete type (instead of a generic
628 // parameter) would be pointless, since it's always true
630 // Displaying an inference variable is impossible, since they're
631 // an internal compiler detail without a defined visual representation
633 // We check this by calling is_of_param on the relevant types
634 // from the various possible predicates
635 match predicate
.kind() {
636 &ty
::PredicateKind
::Trait(p
, _
) => {
637 if self.is_param_no_infer(p
.skip_binder().trait_ref
.substs
)
641 self.add_user_pred(computed_preds
, predicate
);
643 predicates
.push_back(p
);
645 &ty
::PredicateKind
::Projection(p
) => {
647 "evaluate_nested_obligations: examining projection predicate {:?}",
651 // As described above, we only want to display
652 // bounds which include a generic parameter but don't include
653 // an inference variable.
654 // Additionally, we check if we've seen this predicate before,
655 // to avoid rendering duplicate bounds to the user.
656 if self.is_param_no_infer(p
.skip_binder().projection_ty
.substs
)
657 && !p
.ty().skip_binder().has_infer_types()
661 "evaluate_nested_obligations: adding projection predicate\
662 to computed_preds: {:?}",
666 // Under unusual circumstances, we can end up with a self-refeential
667 // projection predicate. For example:
668 // <T as MyType>::Value == <T as MyType>::Value
669 // Not only is displaying this to the user pointless,
670 // having it in the ParamEnv will cause an issue if we try to call
671 // poly_project_and_unify_type on the predicate, since this kind of
672 // predicate will normally never end up in a ParamEnv.
674 // For these reasons, we ignore these weird predicates,
675 // ensuring that we're able to properly synthesize an auto trait impl
676 if self.is_self_referential_projection(p
) {
678 "evaluate_nested_obligations: encountered a projection
679 predicate equating a type with itself! Skipping"
682 self.add_user_pred(computed_preds
, predicate
);
686 // There are three possible cases when we project a predicate:
688 // 1. We encounter an error. This means that it's impossible for
689 // our current type to implement the auto trait - there's bound
690 // that we could add to our ParamEnv that would 'fix' this kind
691 // of error, as it's not caused by an unimplemented type.
693 // 2. We successfully project the predicate (Ok(Some(_))), generating
694 // some subobligations. We then process these subobligations
695 // like any other generated sub-obligations.
697 // 3. We receive an 'ambiguous' result (Ok(None))
698 // If we were actually trying to compile a crate,
699 // we would need to re-process this obligation later.
700 // However, all we care about is finding out what bounds
701 // are needed for our type to implement a particular auto trait.
702 // We've already added this obligation to our computed ParamEnv
703 // above (if it was necessary). Therefore, we don't need
704 // to do any further processing of the obligation.
706 // Note that we *must* try to project *all* projection predicates
707 // we encounter, even ones without inference variable.
708 // This ensures that we detect any projection errors,
709 // which indicate that our type can *never* implement the given
710 // auto trait. In that case, we will generate an explicit negative
711 // impl (e.g. 'impl !Send for MyType'). However, we don't
712 // try to process any of the generated subobligations -
713 // they contain no new information, since we already know
714 // that our type implements the projected-through trait,
715 // and can lead to weird region issues.
717 // Normally, we'll generate a negative impl as a result of encountering
718 // a type with an explicit negative impl of an auto trait
719 // (for example, raw pointers have !Send and !Sync impls)
720 // However, through some **interesting** manipulations of the type
721 // system, it's actually possible to write a type that never
722 // implements an auto trait due to a projection error, not a normal
723 // negative impl error. To properly handle this case, we need
724 // to ensure that we catch any potential projection errors,
725 // and turn them into an explicit negative impl for our type.
726 debug
!("Projecting and unifying projection predicate {:?}", predicate
);
728 match poly_project_and_unify_type(select
, &obligation
.with(p
)) {
731 "evaluate_nested_obligations: Unable to unify predicate \
732 '{:?}' '{:?}', bailing out",
738 // We only care about sub-obligations
739 // when we started out trying to unify
740 // some inference variables. See the comment above
741 // for more infomration
742 if p
.ty().skip_binder().has_infer_types() {
743 if !self.evaluate_nested_obligations(
757 // It's ok not to make progress when hvave no inference variables -
758 // in that case, we were only performing unifcation to check if an
759 // error occurred (which would indicate that it's impossible for our
760 // type to implement the auto trait).
761 // However, we should always make progress (either by generating
762 // subobligations or getting an error) when we started off with
763 // inference variables
764 if p
.ty().skip_binder().has_infer_types() {
765 panic
!("Unexpected result when selecting {:?} {:?}", ty
, obligation
)
770 &ty
::PredicateKind
::RegionOutlives(binder
) => {
771 if select
.infcx().region_outlives_predicate(&dummy_cause
, binder
).is_err() {
775 &ty
::PredicateKind
::TypeOutlives(binder
) => {
777 binder
.no_bound_vars(),
778 binder
.map_bound_ref(|pred
| pred
.0).no_bound_vars(),
780 (None
, Some(t_a
)) => {
781 select
.infcx().register_region_obligation_with_cause(
783 select
.infcx().tcx
.lifetimes
.re_static
,
787 (Some(ty
::OutlivesPredicate(t_a
, r_b
)), _
) => {
788 select
.infcx().register_region_obligation_with_cause(
797 _
=> panic
!("Unexpected predicate {:?} {:?}", ty
, predicate
),
805 infcx
: &InferCtxt
<'_
, 'tcx
>,
806 p
: ty
::Predicate
<'tcx
>,
807 ) -> ty
::Predicate
<'tcx
> {
812 // Replaces all ReVars in a type with ty::Region's, using the provided map
813 pub struct RegionReplacer
<'a
, 'tcx
> {
814 vid_to_region
: &'a FxHashMap
<ty
::RegionVid
, ty
::Region
<'tcx
>>,
818 impl<'a
, 'tcx
> TypeFolder
<'tcx
> for RegionReplacer
<'a
, 'tcx
> {
819 fn tcx
<'b
>(&'b
self) -> TyCtxt
<'tcx
> {
823 fn fold_region(&mut self, r
: ty
::Region
<'tcx
>) -> ty
::Region
<'tcx
> {
825 &ty
::ReVar(vid
) => self.vid_to_region
.get(&vid
).cloned(),
828 .unwrap_or_else(|| r
.super_fold_with(self))