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, TraitEngineExt as _, 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, Ty, 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::Unimplemented;
24 use super::{FulfillmentError, FulfillmentErrorCode};
25 use super::{ObligationCause, PredicateObligation};
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 /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
130 /// creating a fresh type variable `$0` as well as a projection
131 /// predicate `<SomeType as SomeTrait>::X == $0`. When the
132 /// inference engine runs, it will attempt to find an impl of
133 /// `SomeTrait` or a where-clause that lets us unify `$0` with
134 /// something concrete. If this fails, we'll unify `$0` with
135 /// `projection_ty` again.
136 #[instrument(level = "debug", skip(self, infcx, param_env, cause))]
137 fn normalize_projection_type(
139 infcx: &InferCtxt<'tcx>,
140 param_env: ty::ParamEnv<'tcx>,
141 projection_ty: ty::ProjectionTy<'tcx>,
142 cause: ObligationCause<'tcx>,
144 debug_assert!(!projection_ty.has_escaping_bound_vars());
146 // FIXME(#20304) -- cache
148 let mut selcx = SelectionContext::new(infcx);
149 let mut obligations = vec![];
150 let normalized_ty = project::normalize_projection_type(
158 self.register_predicate_obligations(infcx, obligations);
160 debug!(?normalized_ty);
162 normalized_ty.ty().unwrap()
165 fn register_predicate_obligation(
167 infcx: &InferCtxt<'tcx>,
168 obligation: PredicateObligation<'tcx>,
170 // this helps to reduce duplicate errors, as well as making
171 // debug output much nicer to read and so on.
172 let obligation = infcx.resolve_vars_if_possible(obligation);
174 debug!(?obligation, "register_predicate_obligation");
176 assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
178 super::relationships::update(self, infcx, &obligation);
181 .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
184 fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
186 let errors = self.select_where_possible(infcx);
187 if !errors.is_empty() {
192 self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
195 fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
196 let selcx = SelectionContext::new(infcx);
200 fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
201 self.predicates.map_pending_obligations(|o| o.obligation.clone())
204 fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
205 &mut self.relationships
209 struct FulfillProcessor<'a, 'tcx> {
210 selcx: SelectionContext<'a, 'tcx>,
213 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
215 .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
219 impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
220 type Obligation = PendingPredicateObligation<'tcx>;
221 type Error = FulfillmentErrorCode<'tcx>;
222 type OUT = Outcome<Self::Obligation, Self::Error>;
224 /// Identifies whether a predicate obligation needs processing.
226 /// This is always inlined, despite its size, because it has a single
227 /// callsite and it is called *very* frequently.
229 fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
230 // If we were stalled on some unresolved variables, first check whether
231 // any of them have been resolved; if not, don't bother doing more work
233 match pending_obligation.stalled_on.len() {
234 // Match arms are in order of frequency, which matters because this
235 // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
237 let infer_var = pending_obligation.stalled_on[0];
238 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
241 // In this case we haven't changed, but wish to make a change.
245 // This `for` loop was once a call to `all()`, but this lower-level
246 // form was a perf win. See #64545 for details.
248 for &infer_var in &pending_obligation.stalled_on {
249 if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
259 /// Processes a predicate obligation and returns either:
260 /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
261 /// - `Unchanged` if we don't have enough info to be sure
262 /// - `Error(e)` if the predicate does not hold
264 /// This is called much less often than `needs_process_obligation`, so we
267 #[instrument(level = "debug", skip(self, pending_obligation))]
268 fn process_obligation(
270 pending_obligation: &mut PendingPredicateObligation<'tcx>,
271 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
272 pending_obligation.stalled_on.truncate(0);
274 let obligation = &mut pending_obligation.obligation;
276 debug!(?obligation, "pre-resolve");
278 if obligation.predicate.has_non_region_infer() {
279 obligation.predicate =
280 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
283 let obligation = &pending_obligation.obligation;
285 let infcx = self.selcx.infcx();
287 if obligation.predicate.has_projections() {
288 let mut obligations = Vec::new();
289 let predicate = crate::traits::project::try_normalize_with_depth_to(
291 obligation.param_env,
292 obligation.cause.clone(),
293 obligation.recursion_depth + 1,
294 obligation.predicate,
297 if predicate != obligation.predicate {
298 obligations.push(obligation.with(infcx.tcx, predicate));
299 return ProcessResult::Changed(mk_pending(obligations));
302 let binder = obligation.predicate.kind();
303 match binder.no_bound_vars() {
304 None => match binder.skip_binder() {
305 // Evaluation will discard candidates using the leak check.
306 // This means we need to pass it the bound version of our
308 ty::PredicateKind::Trait(trait_ref) => {
309 let trait_obligation = obligation.with(infcx.tcx, binder.rebind(trait_ref));
311 self.process_trait_obligation(
314 &mut pending_obligation.stalled_on,
317 ty::PredicateKind::Projection(data) => {
318 let project_obligation = obligation.with(infcx.tcx, binder.rebind(data));
320 self.process_projection_obligation(
323 &mut pending_obligation.stalled_on,
326 ty::PredicateKind::RegionOutlives(_)
327 | ty::PredicateKind::TypeOutlives(_)
328 | ty::PredicateKind::WellFormed(_)
329 | ty::PredicateKind::ObjectSafe(_)
330 | ty::PredicateKind::ClosureKind(..)
331 | ty::PredicateKind::Subtype(_)
332 | ty::PredicateKind::Coerce(_)
333 | ty::PredicateKind::ConstEvaluatable(..)
334 | ty::PredicateKind::ConstEquate(..) => {
336 ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
337 ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
339 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
340 bug!("TypeWellFormedFromEnv is only used for Chalk")
343 Some(pred) => match pred {
344 ty::PredicateKind::Trait(data) => {
345 let trait_obligation = obligation.with(infcx.tcx, Binder::dummy(data));
347 self.process_trait_obligation(
350 &mut pending_obligation.stalled_on,
354 ty::PredicateKind::RegionOutlives(data) => {
355 if infcx.considering_regions {
356 infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
359 ProcessResult::Changed(vec![])
362 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
363 if infcx.considering_regions {
364 infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
366 ProcessResult::Changed(vec![])
369 ty::PredicateKind::Projection(ref data) => {
370 let project_obligation = obligation.with(infcx.tcx, Binder::dummy(*data));
372 self.process_projection_obligation(
375 &mut pending_obligation.stalled_on,
379 ty::PredicateKind::ObjectSafe(trait_def_id) => {
380 if !self.selcx.tcx().is_object_safe(trait_def_id) {
381 ProcessResult::Error(CodeSelectionError(Unimplemented))
383 ProcessResult::Changed(vec![])
387 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
388 match self.selcx.infcx().closure_kind(closure_substs) {
389 Some(closure_kind) => {
390 if closure_kind.extends(kind) {
391 ProcessResult::Changed(vec![])
393 ProcessResult::Error(CodeSelectionError(Unimplemented))
396 None => ProcessResult::Unchanged,
400 ty::PredicateKind::WellFormed(arg) => {
401 match wf::obligations(
403 obligation.param_env,
404 obligation.cause.body_id,
405 obligation.recursion_depth + 1,
407 obligation.cause.span,
410 pending_obligation.stalled_on =
411 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
412 ProcessResult::Unchanged
414 Some(os) => ProcessResult::Changed(mk_pending(os)),
418 ty::PredicateKind::Subtype(subtype) => {
419 match self.selcx.infcx().subtype_predicate(
421 obligation.param_env,
422 Binder::dummy(subtype),
425 // None means that both are unresolved.
426 pending_obligation.stalled_on =
427 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
428 ProcessResult::Unchanged
430 Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
433 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
434 ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
442 ty::PredicateKind::Coerce(coerce) => {
443 match self.selcx.infcx().coerce_predicate(
445 obligation.param_env,
446 Binder::dummy(coerce),
449 // None means that both are unresolved.
450 pending_obligation.stalled_on =
451 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
452 ProcessResult::Unchanged
454 Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
456 let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
457 ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
465 ty::PredicateKind::ConstEvaluatable(uv) => {
466 match const_evaluatable::is_const_evaluatable(
469 obligation.param_env,
470 obligation.cause.span,
472 Ok(()) => ProcessResult::Changed(vec![]),
473 Err(NotConstEvaluatable::MentionsInfer) => {
474 pending_obligation.stalled_on.clear();
475 pending_obligation.stalled_on.extend(
476 uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
478 ProcessResult::Unchanged
481 e @ NotConstEvaluatable::MentionsParam
482 | e @ NotConstEvaluatable::Error(_),
483 ) => ProcessResult::Error(CodeSelectionError(
484 SelectionError::NotConstEvaluatable(e),
489 ty::PredicateKind::ConstEquate(c1, c2) => {
491 self.selcx.tcx().features().generic_const_exprs,
492 "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
494 debug!(?c1, ?c2, "equating consts");
495 // FIXME: we probably should only try to unify abstract constants
496 // if the constants depend on generic parameters.
498 // Let's just see where this breaks :shrug:
499 if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
500 (c1.kind(), c2.kind())
502 if infcx.try_unify_abstract_consts(a, b, obligation.param_env) {
503 return ProcessResult::Changed(vec![]);
507 let stalled_on = &mut pending_obligation.stalled_on;
509 let mut evaluate = |c: Const<'tcx>| {
510 if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
511 match self.selcx.infcx().try_const_eval_resolve(
512 obligation.param_env,
515 Some(obligation.cause.span),
519 ErrorHandled::TooGeneric => {
521 unevaluated.substs.iter().filter_map(
522 TyOrConstInferVar::maybe_from_generic_arg,
525 Err(ErrorHandled::TooGeneric)
535 match (evaluate(c1), evaluate(c2)) {
536 (Ok(c1), Ok(c2)) => {
540 .at(&obligation.cause, obligation.param_env)
543 Ok(_) => ProcessResult::Changed(vec![]),
544 Err(err) => ProcessResult::Error(
545 FulfillmentErrorCode::CodeConstEquateError(
546 ExpectedFound::new(true, c1, c2),
552 (Err(ErrorHandled::Reported(reported)), _)
553 | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
554 CodeSelectionError(SelectionError::NotConstEvaluatable(
555 NotConstEvaluatable::Error(reported),
558 (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
559 if c1.has_non_region_infer() || c2.has_non_region_infer() {
560 ProcessResult::Unchanged
562 // Two different constants using generic parameters ~> error.
563 let expected_found = ExpectedFound::new(true, c1, c2);
564 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
566 TypeError::ConstMismatch(expected_found),
572 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
573 bug!("TypeWellFormedFromEnv is only used for Chalk")
580 fn process_backedge<'c, I>(
583 _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
584 ) -> Result<(), FulfillmentErrorCode<'tcx>>
586 I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
588 if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
589 debug!("process_child_obligations: coinductive match");
592 let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
593 Err(FulfillmentErrorCode::CodeCycle(cycle))
598 impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
599 #[instrument(level = "debug", skip(self, obligation, stalled_on))]
600 fn process_trait_obligation(
602 obligation: &PredicateObligation<'tcx>,
603 trait_obligation: TraitObligation<'tcx>,
604 stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
605 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
606 let infcx = self.selcx.infcx();
607 if obligation.predicate.is_global() {
608 // no type variables present, can use evaluation for better caching.
609 // FIXME: consider caching errors too.
610 if infcx.predicate_must_hold_considering_regions(obligation) {
612 "selecting trait at depth {} evaluated to holds",
613 obligation.recursion_depth
615 return ProcessResult::Changed(vec![]);
619 match self.selcx.select(&trait_obligation) {
620 Ok(Some(impl_source)) => {
621 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
622 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
625 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
627 // This is a bit subtle: for the most part, the
628 // only reason we can fail to make progress on
629 // trait selection is because we don't have enough
630 // information about the types in the trait.
632 stalled_on.extend(substs_infer_vars(
634 trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
638 "process_predicate: pending obligation {:?} now stalled on {:?}",
639 infcx.resolve_vars_if_possible(obligation.clone()),
643 ProcessResult::Unchanged
645 Err(selection_err) => {
646 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
648 ProcessResult::Error(CodeSelectionError(selection_err))
653 fn process_projection_obligation(
655 obligation: &PredicateObligation<'tcx>,
656 project_obligation: PolyProjectionObligation<'tcx>,
657 stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
658 ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
659 let tcx = self.selcx.tcx();
661 if obligation.predicate.is_global() {
662 // no type variables present, can use evaluation for better caching.
663 // FIXME: consider caching errors too.
664 if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
665 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
667 project_obligation.predicate,
669 // If `predicate_must_hold_considering_regions` succeeds, then we've
670 // evaluated all sub-obligations. We can therefore mark the 'root'
671 // obligation as complete, and skip evaluating sub-obligations.
677 .complete(key, EvaluationResult::EvaluatedToOk);
679 return ProcessResult::Changed(vec![]);
681 debug!("Does NOT hold: {:?}", obligation);
685 match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
686 ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
687 ProjectAndUnifyResult::FailedNormalization => {
689 stalled_on.extend(substs_infer_vars(
691 project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
693 ProcessResult::Unchanged
695 // Let the caller handle the recursion
696 ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
697 project_obligation.with(tcx, project_obligation.predicate),
699 ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
700 ProcessResult::Error(CodeProjectionError(e))
706 /// Returns the set of inference variables contained in `substs`.
707 fn substs_infer_vars<'a, 'tcx>(
708 selcx: &SelectionContext<'a, 'tcx>,
709 substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
710 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
713 .resolve_vars_if_possible(substs)
714 .skip_binder() // ok because this check doesn't care about regions
716 .filter(|arg| arg.has_non_region_infer())
718 let mut walker = arg.walk();
719 while let Some(c) = walker.next() {
720 if !c.has_non_region_infer() {
721 walker.visited.remove(&c);
722 walker.skip_current_subtree();
725 walker.visited.into_iter()
727 .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
730 fn to_fulfillment_error<'tcx>(
731 error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
732 ) -> FulfillmentError<'tcx> {
733 let mut iter = error.backtrace.into_iter();
734 let obligation = iter.next().unwrap().obligation;
735 // The root obligation is the last item in the backtrace - if there's only
736 // one item, then it's the same as the main obligation
737 let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
738 FulfillmentError::new(obligation, error.error, root_obligation)