1 use crate::infer::{InferCtxt, TyOrConstInferVar};
2 use rustc_data_structures::fx::FxHashMap;
3 use rustc_data_structures::obligation_forest::ProcessResult;
4 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
5 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
6 use rustc_infer::traits::ProjectionCacheKey;
7 use rustc_infer::traits::{SelectionError, TraitEngine, TraitObligation};
8 use rustc_middle::mir::interpret::ErrorHandled;
9 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
10 use rustc_middle::ty::error::{ExpectedFound, TypeError};
11 use rustc_middle::ty::subst::SubstsRef;
12 use rustc_middle::ty::{self, Binder, Const, TypeVisitable};
13 use std::marker::PhantomData;
15 use super::const_evaluatable;
16 use super::project::{self, ProjectAndUnifyResult};
17 use super::select::SelectionContext;
19 use super::CodeAmbiguity;
20 use super::CodeProjectionError;
21 use super::CodeSelectionError;
22 use super::EvaluationResult;
23 use super::PredicateObligation;
24 use super::Unimplemented;
25 use super::{FulfillmentError, FulfillmentErrorCode};
27 use crate::traits::project::PolyProjectionObligation;
28 use crate::traits::project::ProjectionCacheKeyExt as _;
29 use crate::traits::query::evaluate_obligation::InferCtxtExt;
31 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
32 /// Note that we include both the `ParamEnv` and the `Predicate`,
33 /// as the `ParamEnv` can influence whether fulfillment succeeds
35 type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
37 fn as_cache_key(&self) -> Self::CacheKey {
38 self.obligation.param_env.and(self.obligation.predicate)
42 /// The fulfillment context is used to drive trait resolution. It
43 /// consists of a list of obligations that must be (eventually)
44 /// satisfied. The job is to track which are satisfied, which yielded
45 /// errors, and which are still pending. At any point, users can call
46 /// `select_where_possible`, and the fulfillment context will try to do
47 /// selection, retaining only those obligations that remain
48 /// ambiguous. This may be helpful in pushing type inference
49 /// along. Once all type inference constraints have been generated, the
50 /// method `select_all_or_error` can be used to report any remaining
51 /// ambiguous cases as errors.
52 pub struct FulfillmentContext<'tcx> {
53 // A list of all obligations that have been registered with this
54 // fulfillment context.
55 predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
57 relationships: FxHashMap<ty::TyVid, ty::FoundRelationships>,
59 // Is it OK to register obligations into this infcx inside
62 // The "primary fulfillment" in many cases in typeck lives
63 // outside of any snapshot, so any use of it inside a snapshot
64 // will lead to trouble and therefore is checked against, but
65 // other fulfillment contexts sometimes do live inside of
66 // a snapshot (they don't *straddle* a snapshot, so there
67 // is no trouble there).
68 usable_in_snapshot: bool,
71 #[derive(Clone, Debug)]
72 pub struct PendingPredicateObligation<'tcx> {
73 pub obligation: PredicateObligation<'tcx>,
74 // This is far more often read than modified, meaning that we
75 // should mostly optimize for reading speed, while modifying is not as relevant.
77 // For whatever reason using a boxed slice is slower than using a `Vec` here.
78 pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
81 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
82 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
83 static_assert_size!(PendingPredicateObligation<'_>, 72);
85 impl<'a, 'tcx> FulfillmentContext<'tcx> {
86 /// Creates a new fulfillment context.
87 pub(super) fn new() -> FulfillmentContext<'tcx> {
89 predicates: ObligationForest::new(),
90 relationships: FxHashMap::default(),
91 usable_in_snapshot: false,
95 pub(super) fn new_in_snapshot() -> FulfillmentContext<'tcx> {
97 predicates: ObligationForest::new(),
98 relationships: FxHashMap::default(),
99 usable_in_snapshot: true,
103 /// Attempts to select obligations using `selcx`.
104 fn select(&mut self, selcx: SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
105 let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
106 let _enter = span.enter();
108 // Process pending obligations.
109 let outcome: Outcome<_, _> =
110 self.predicates.process_obligations(&mut FulfillProcessor { selcx });
112 // FIXME: if we kept the original cache key, we could mark projection
113 // obligations as complete for the projection cache here.
115 let errors: Vec<FulfillmentError<'tcx>> =
116 outcome.errors.into_iter().map(to_fulfillment_error).collect();
119 "select({} predicates remaining, {} errors) done",
120 self.predicates.len(),
128 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
129 fn register_predicate_obligation(
131 infcx: &InferCtxt<'tcx>,
132 obligation: PredicateObligation<'tcx>,
134 // this helps to reduce duplicate errors, as well as making
135 // debug output much nicer to read and so on.
136 let obligation = infcx.resolve_vars_if_possible(obligation);
138 debug!(?obligation, "register_predicate_obligation");
140 assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
142 super::relationships::update(self, infcx, &obligation);
145 .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
148 fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
150 let errors = self.select_where_possible(infcx);
151 if !errors.is_empty() {
156 self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
159 fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
160 let selcx = SelectionContext::new(infcx);
164 fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
165 self.predicates.map_pending_obligations(|o| o.obligation.clone())
168 fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
169 &mut self.relationships
173 struct FulfillProcessor<'a, 'tcx> {
174 selcx: SelectionContext<'a, 'tcx>,
177 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
179 .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
183 impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
184 type Obligation = PendingPredicateObligation<'tcx>;
185 type Error = FulfillmentErrorCode<'tcx>;
186 type OUT = Outcome<Self::Obligation, Self::Error>;
188 /// Identifies whether a predicate obligation needs processing.
190 /// This is always inlined, despite its size, because it has a single
191 /// callsite and it is called *very* frequently.
193 fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
194 // If we were stalled on some unresolved variables, first check whether
195 // any of them have been resolved; if not, don't bother doing more work
197 match pending_obligation.stalled_on.len() {
198 // Match arms are in order of frequency, which matters because this
199 // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
201 let infer_var = pending_obligation.stalled_on[0];
202 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
205 // In this case we haven't changed, but wish to make a change.
209 // This `for` loop was once a call to `all()`, but this lower-level
210 // form was a perf win. See #64545 for details.
212 for &infer_var in &pending_obligation.stalled_on {
213 if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
223 /// Processes a predicate obligation and returns either:
224 /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
225 /// - `Unchanged` if we don't have enough info to be sure
226 /// - `Error(e)` if the predicate does not hold
228 /// This is called much less often than `needs_process_obligation`, so we
231 #[instrument(level = "debug", skip(self, pending_obligation))]
232 fn process_obligation(
234 pending_obligation: &mut PendingPredicateObligation<'tcx>,
235 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
236 pending_obligation.stalled_on.truncate(0);
238 let obligation = &mut pending_obligation.obligation;
240 debug!(?obligation, "pre-resolve");
242 if obligation.predicate.has_non_region_infer() {
243 obligation.predicate =
244 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
247 let obligation = &pending_obligation.obligation;
249 let infcx = self.selcx.infcx();
251 if obligation.predicate.has_projections() {
252 let mut obligations = Vec::new();
253 let predicate = crate::traits::project::try_normalize_with_depth_to(
255 obligation.param_env,
256 obligation.cause.clone(),
257 obligation.recursion_depth + 1,
258 obligation.predicate,
261 if predicate != obligation.predicate {
262 obligations.push(obligation.with(infcx.tcx, predicate));
263 return ProcessResult::Changed(mk_pending(obligations));
266 let binder = obligation.predicate.kind();
267 match binder.no_bound_vars() {
268 None => match binder.skip_binder() {
269 // Evaluation will discard candidates using the leak check.
270 // This means we need to pass it the bound version of our
272 ty::PredicateKind::Clause(ty::Clause::Trait(trait_ref)) => {
273 let trait_obligation = obligation.with(infcx.tcx, binder.rebind(trait_ref));
275 self.process_trait_obligation(
278 &mut pending_obligation.stalled_on,
281 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
282 let project_obligation = obligation.with(infcx.tcx, binder.rebind(data));
284 self.process_projection_obligation(
287 &mut pending_obligation.stalled_on,
290 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(_))
291 | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(_))
292 | ty::PredicateKind::WellFormed(_)
293 | ty::PredicateKind::ObjectSafe(_)
294 | ty::PredicateKind::ClosureKind(..)
295 | ty::PredicateKind::Subtype(_)
296 | ty::PredicateKind::Coerce(_)
297 | ty::PredicateKind::ConstEvaluatable(..)
298 | ty::PredicateKind::ConstEquate(..) => {
300 ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
301 ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
303 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
304 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
305 bug!("TypeWellFormedFromEnv is only used for Chalk")
308 Some(pred) => match pred {
309 ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
310 let trait_obligation = obligation.with(infcx.tcx, Binder::dummy(data));
312 self.process_trait_obligation(
315 &mut pending_obligation.stalled_on,
319 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(data)) => {
320 if infcx.considering_regions {
321 infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
324 ProcessResult::Changed(vec![])
327 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
331 if infcx.considering_regions {
332 infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
334 ProcessResult::Changed(vec![])
337 ty::PredicateKind::Clause(ty::Clause::Projection(ref data)) => {
338 let project_obligation = obligation.with(infcx.tcx, Binder::dummy(*data));
340 self.process_projection_obligation(
343 &mut pending_obligation.stalled_on,
347 ty::PredicateKind::ObjectSafe(trait_def_id) => {
348 if !self.selcx.tcx().is_object_safe(trait_def_id) {
349 ProcessResult::Error(CodeSelectionError(Unimplemented))
351 ProcessResult::Changed(vec![])
355 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
356 match self.selcx.infcx().closure_kind(closure_substs) {
357 Some(closure_kind) => {
358 if closure_kind.extends(kind) {
359 ProcessResult::Changed(vec![])
361 ProcessResult::Error(CodeSelectionError(Unimplemented))
364 None => ProcessResult::Unchanged,
368 ty::PredicateKind::WellFormed(arg) => {
369 match wf::obligations(
371 obligation.param_env,
372 obligation.cause.body_id,
373 obligation.recursion_depth + 1,
375 obligation.cause.span,
378 pending_obligation.stalled_on =
379 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
380 ProcessResult::Unchanged
382 Some(os) => ProcessResult::Changed(mk_pending(os)),
386 ty::PredicateKind::Subtype(subtype) => {
387 match self.selcx.infcx().subtype_predicate(
389 obligation.param_env,
390 Binder::dummy(subtype),
393 // None means that both are unresolved.
394 pending_obligation.stalled_on =
395 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
396 ProcessResult::Unchanged
398 Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
401 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
402 ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
410 ty::PredicateKind::Coerce(coerce) => {
411 match self.selcx.infcx().coerce_predicate(
413 obligation.param_env,
414 Binder::dummy(coerce),
417 // None means that both are unresolved.
418 pending_obligation.stalled_on =
419 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
420 ProcessResult::Unchanged
422 Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
424 let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
425 ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
433 ty::PredicateKind::ConstEvaluatable(uv) => {
434 match const_evaluatable::is_const_evaluatable(
437 obligation.param_env,
438 obligation.cause.span,
440 Ok(()) => ProcessResult::Changed(vec![]),
441 Err(NotConstEvaluatable::MentionsInfer) => {
442 pending_obligation.stalled_on.clear();
443 pending_obligation.stalled_on.extend(
444 uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
446 ProcessResult::Unchanged
449 e @ NotConstEvaluatable::MentionsParam
450 | e @ NotConstEvaluatable::Error(_),
451 ) => ProcessResult::Error(CodeSelectionError(
452 SelectionError::NotConstEvaluatable(e),
457 ty::PredicateKind::ConstEquate(c1, c2) => {
458 let tcx = self.selcx.tcx();
460 tcx.features().generic_const_exprs,
461 "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
463 // FIXME: we probably should only try to unify abstract constants
464 // if the constants depend on generic parameters.
466 // Let's just see where this breaks :shrug:
468 let c1 = tcx.expand_abstract_consts(c1);
469 let c2 = tcx.expand_abstract_consts(c2);
470 debug!("equating consts:\nc1= {:?}\nc2= {:?}", c1, c2);
472 use rustc_hir::def::DefKind;
473 use ty::ConstKind::Unevaluated;
474 match (c1.kind(), c2.kind()) {
475 (Unevaluated(a), Unevaluated(b))
476 if a.def.did == b.def.did
477 && tcx.def_kind(a.def.did) == DefKind::AssocConst =>
479 if let Ok(new_obligations) = infcx
480 .at(&obligation.cause, obligation.param_env)
482 .eq(a.substs, b.substs)
484 return ProcessResult::Changed(mk_pending(
485 new_obligations.into_obligations(),
489 (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
491 if let Ok(new_obligations) =
492 infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
494 return ProcessResult::Changed(mk_pending(
495 new_obligations.into_obligations(),
502 let stalled_on = &mut pending_obligation.stalled_on;
504 let mut evaluate = |c: Const<'tcx>| {
505 if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
506 match self.selcx.infcx().try_const_eval_resolve(
507 obligation.param_env,
510 Some(obligation.cause.span),
514 ErrorHandled::TooGeneric => {
516 unevaluated.substs.iter().filter_map(
517 TyOrConstInferVar::maybe_from_generic_arg,
520 Err(ErrorHandled::TooGeneric)
530 match (evaluate(c1), evaluate(c2)) {
531 (Ok(c1), Ok(c2)) => {
535 .at(&obligation.cause, obligation.param_env)
539 ProcessResult::Changed(mk_pending(inf_ok.into_obligations()))
541 Err(err) => ProcessResult::Error(
542 FulfillmentErrorCode::CodeConstEquateError(
543 ExpectedFound::new(true, c1, c2),
549 (Err(ErrorHandled::Reported(reported)), _)
550 | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
551 CodeSelectionError(SelectionError::NotConstEvaluatable(
552 NotConstEvaluatable::Error(reported),
555 (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
556 if c1.has_non_region_infer() || c2.has_non_region_infer() {
557 ProcessResult::Unchanged
559 // Two different constants using generic parameters ~> error.
560 let expected_found = ExpectedFound::new(true, c1, c2);
561 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
563 TypeError::ConstMismatch(expected_found),
569 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
570 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
571 bug!("TypeWellFormedFromEnv is only used for Chalk")
578 fn process_backedge<'c, I>(
581 _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
582 ) -> Result<(), FulfillmentErrorCode<'tcx>>
584 I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
586 if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
587 debug!("process_child_obligations: coinductive match");
590 let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
591 Err(FulfillmentErrorCode::CodeCycle(cycle))
596 impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
597 #[instrument(level = "debug", skip(self, obligation, stalled_on))]
598 fn process_trait_obligation(
600 obligation: &PredicateObligation<'tcx>,
601 trait_obligation: TraitObligation<'tcx>,
602 stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
603 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
604 let infcx = self.selcx.infcx();
605 if obligation.predicate.is_global() {
606 // no type variables present, can use evaluation for better caching.
607 // FIXME: consider caching errors too.
608 if infcx.predicate_must_hold_considering_regions(obligation) {
610 "selecting trait at depth {} evaluated to holds",
611 obligation.recursion_depth
613 return ProcessResult::Changed(vec![]);
617 match self.selcx.select(&trait_obligation) {
618 Ok(Some(impl_source)) => {
619 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
620 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
623 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
625 // This is a bit subtle: for the most part, the
626 // only reason we can fail to make progress on
627 // trait selection is because we don't have enough
628 // information about the types in the trait.
630 stalled_on.extend(substs_infer_vars(
632 trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
636 "process_predicate: pending obligation {:?} now stalled on {:?}",
637 infcx.resolve_vars_if_possible(obligation.clone()),
641 ProcessResult::Unchanged
643 Err(selection_err) => {
644 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
646 ProcessResult::Error(CodeSelectionError(selection_err))
651 fn process_projection_obligation(
653 obligation: &PredicateObligation<'tcx>,
654 project_obligation: PolyProjectionObligation<'tcx>,
655 stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
656 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
657 let tcx = self.selcx.tcx();
659 if obligation.predicate.is_global() {
660 // no type variables present, can use evaluation for better caching.
661 // FIXME: consider caching errors too.
662 if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
663 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
665 project_obligation.predicate,
667 // If `predicate_must_hold_considering_regions` succeeds, then we've
668 // evaluated all sub-obligations. We can therefore mark the 'root'
669 // obligation as complete, and skip evaluating sub-obligations.
675 .complete(key, EvaluationResult::EvaluatedToOk);
677 return ProcessResult::Changed(vec![]);
679 debug!("Does NOT hold: {:?}", obligation);
683 match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
684 ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
685 ProjectAndUnifyResult::FailedNormalization => {
687 stalled_on.extend(substs_infer_vars(
689 project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
691 ProcessResult::Unchanged
693 // Let the caller handle the recursion
694 ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
695 project_obligation.with(tcx, project_obligation.predicate),
697 ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
698 ProcessResult::Error(CodeProjectionError(e))
704 /// Returns the set of inference variables contained in `substs`.
705 fn substs_infer_vars<'a, 'tcx>(
706 selcx: &SelectionContext<'a, 'tcx>,
707 substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
708 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
711 .resolve_vars_if_possible(substs)
712 .skip_binder() // ok because this check doesn't care about regions
714 .filter(|arg| arg.has_non_region_infer())
716 let mut walker = arg.walk();
717 while let Some(c) = walker.next() {
718 if !c.has_non_region_infer() {
719 walker.visited.remove(&c);
720 walker.skip_current_subtree();
723 walker.visited.into_iter()
725 .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
728 fn to_fulfillment_error<'tcx>(
729 error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
730 ) -> FulfillmentError<'tcx> {
731 let mut iter = error.backtrace.into_iter();
732 let obligation = iter.next().unwrap().obligation;
733 // The root obligation is the last item in the backtrace - if there's only
734 // one item, then it's the same as the main obligation
735 let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
736 FulfillmentError::new(obligation, error.error, root_obligation)