1 //! Candidate selection. See the [rustc dev guide] for more information on how this works.
3 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html#selection
5 use self::EvaluationResult::*;
6 use self::SelectionCandidate::*;
8 use super::coherence::{self, Conflict};
9 use super::const_evaluatable;
11 use super::project::normalize_with_depth_to;
12 use super::project::ProjectionTyObligation;
14 use super::util::{closure_trait_ref_and_return_type, predicate_for_trait_def};
16 use super::DerivedObligationCause;
17 use super::Normalized;
18 use super::Obligation;
19 use super::ObligationCauseCode;
21 use super::SelectionResult;
22 use super::TraitQueryMode;
23 use super::{ErrorReporting, Overflow, SelectionError};
24 use super::{ObligationCause, PredicateObligation, TraitObligation};
26 use crate::infer::{InferCtxt, InferOk, TypeFreshener};
27 use crate::traits::error_reporting::InferCtxtExt;
28 use crate::traits::project::ProjectionCacheKeyExt;
29 use crate::traits::ProjectionCacheKey;
30 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
31 use rustc_data_structures::stack::ensure_sufficient_stack;
32 use rustc_errors::ErrorReported;
34 use rustc_hir::def_id::DefId;
35 use rustc_infer::infer::LateBoundRegionConversionTime;
36 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
37 use rustc_middle::mir::interpret::ErrorHandled;
38 use rustc_middle::thir::abstract_const::NotConstEvaluatable;
39 use rustc_middle::ty::fast_reject::{self, SimplifyParams, StripReferences};
40 use rustc_middle::ty::print::with_no_trimmed_paths;
41 use rustc_middle::ty::relate::TypeRelation;
42 use rustc_middle::ty::subst::{GenericArgKind, Subst, SubstsRef};
43 use rustc_middle::ty::{self, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate};
44 use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable};
45 use rustc_span::symbol::sym;
47 use std::cell::{Cell, RefCell};
49 use std::fmt::{self, Display};
52 pub use rustc_middle::traits::select::*;
54 mod candidate_assembly;
57 #[derive(Clone, Debug)]
58 pub enum IntercrateAmbiguityCause {
59 DownstreamCrate { trait_desc: String, self_desc: Option<String> },
60 UpstreamCrateUpdate { trait_desc: String, self_desc: Option<String> },
61 ReservationImpl { message: String },
64 impl IntercrateAmbiguityCause {
65 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
66 /// See #23980 for details.
67 pub fn add_intercrate_ambiguity_hint(&self, err: &mut rustc_errors::DiagnosticBuilder<'_>) {
68 err.note(&self.intercrate_ambiguity_hint());
71 pub fn intercrate_ambiguity_hint(&self) -> String {
73 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc } => {
74 let self_desc = if let Some(ty) = self_desc {
75 format!(" for type `{}`", ty)
79 format!("downstream crates may implement trait `{}`{}", trait_desc, self_desc)
81 IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc } => {
82 let self_desc = if let Some(ty) = self_desc {
83 format!(" for type `{}`", ty)
88 "upstream crates may add a new impl of trait `{}`{} \
93 IntercrateAmbiguityCause::ReservationImpl { message } => message.clone(),
98 pub struct SelectionContext<'cx, 'tcx> {
99 infcx: &'cx InferCtxt<'cx, 'tcx>,
101 /// Freshener used specifically for entries on the obligation
102 /// stack. This ensures that all entries on the stack at one time
103 /// will have the same set of placeholder entries, which is
104 /// important for checking for trait bounds that recursively
105 /// require themselves.
106 freshener: TypeFreshener<'cx, 'tcx>,
108 /// If `true`, indicates that the evaluation should be conservative
109 /// and consider the possibility of types outside this crate.
110 /// This comes up primarily when resolving ambiguity. Imagine
111 /// there is some trait reference `$0: Bar` where `$0` is an
112 /// inference variable. If `intercrate` is true, then we can never
113 /// say for sure that this reference is not implemented, even if
114 /// there are *no impls at all for `Bar`*, because `$0` could be
115 /// bound to some type that in a downstream crate that implements
116 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
117 /// though, we set this to false, because we are only interested
118 /// in types that the user could actually have written --- in
119 /// other words, we consider `$0: Bar` to be unimplemented if
120 /// there is no type that the user could *actually name* that
121 /// would satisfy it. This avoids crippling inference, basically.
124 intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
126 /// Controls whether or not to filter out negative impls when selecting.
127 /// This is used in librustdoc to distinguish between the lack of an impl
128 /// and a negative impl
129 allow_negative_impls: bool,
131 /// The mode that trait queries run in, which informs our error handling
132 /// policy. In essence, canonicalized queries need their errors propagated
133 /// rather than immediately reported because we do not have accurate spans.
134 query_mode: TraitQueryMode,
137 // A stack that walks back up the stack frame.
138 struct TraitObligationStack<'prev, 'tcx> {
139 obligation: &'prev TraitObligation<'tcx>,
141 /// The trait predicate from `obligation` but "freshened" with the
142 /// selection-context's freshener. Used to check for recursion.
143 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
145 /// Starts out equal to `depth` -- if, during evaluation, we
146 /// encounter a cycle, then we will set this flag to the minimum
147 /// depth of that cycle for all participants in the cycle. These
148 /// participants will then forego caching their results. This is
149 /// not the most efficient solution, but it addresses #60010. The
150 /// problem we are trying to prevent:
152 /// - If you have `A: AutoTrait` requires `B: AutoTrait` and `C: NonAutoTrait`
153 /// - `B: AutoTrait` requires `A: AutoTrait` (coinductive cycle, ok)
154 /// - `C: NonAutoTrait` requires `A: AutoTrait` (non-coinductive cycle, not ok)
156 /// you don't want to cache that `B: AutoTrait` or `A: AutoTrait`
157 /// is `EvaluatedToOk`; this is because they were only considered
158 /// ok on the premise that if `A: AutoTrait` held, but we indeed
159 /// encountered a problem (later on) with `A: AutoTrait. So we
160 /// currently set a flag on the stack node for `B: AutoTrait` (as
161 /// well as the second instance of `A: AutoTrait`) to suppress
164 /// This is a simple, targeted fix. A more-performant fix requires
165 /// deeper changes, but would permit more caching: we could
166 /// basically defer caching until we have fully evaluated the
167 /// tree, and then cache the entire tree at once. In any case, the
168 /// performance impact here shouldn't be so horrible: every time
169 /// this is hit, we do cache at least one trait, so we only
170 /// evaluate each member of a cycle up to N times, where N is the
171 /// length of the cycle. This means the performance impact is
172 /// bounded and we shouldn't have any terrible worst-cases.
173 reached_depth: Cell<usize>,
175 previous: TraitObligationStackList<'prev, 'tcx>,
177 /// The number of parent frames plus one (thus, the topmost frame has depth 1).
180 /// The depth-first number of this node in the search graph -- a
181 /// pre-order index. Basically, a freshly incremented counter.
185 struct SelectionCandidateSet<'tcx> {
186 // A list of candidates that definitely apply to the current
187 // obligation (meaning: types unify).
188 vec: Vec<SelectionCandidate<'tcx>>,
190 // If `true`, then there were candidates that might or might
191 // not have applied, but we couldn't tell. This occurs when some
192 // of the input types are type variables, in which case there are
193 // various "builtin" rules that might or might not trigger.
197 #[derive(PartialEq, Eq, Debug, Clone)]
198 struct EvaluatedCandidate<'tcx> {
199 candidate: SelectionCandidate<'tcx>,
200 evaluation: EvaluationResult,
203 /// When does the builtin impl for `T: Trait` apply?
205 enum BuiltinImplConditions<'tcx> {
206 /// The impl is conditional on `T1, T2, ...: Trait`.
207 Where(ty::Binder<'tcx, Vec<Ty<'tcx>>>),
208 /// There is no built-in impl. There may be some other
209 /// candidate (a where-clause or user-defined impl).
211 /// It is unknown whether there is an impl.
215 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
216 pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
219 freshener: infcx.freshener_keep_static(),
221 intercrate_ambiguity_causes: None,
222 allow_negative_impls: false,
223 query_mode: TraitQueryMode::Standard,
227 pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
230 freshener: infcx.freshener_keep_static(),
232 intercrate_ambiguity_causes: None,
233 allow_negative_impls: false,
234 query_mode: TraitQueryMode::Standard,
238 pub fn with_negative(
239 infcx: &'cx InferCtxt<'cx, 'tcx>,
240 allow_negative_impls: bool,
241 ) -> SelectionContext<'cx, 'tcx> {
242 debug!(?allow_negative_impls, "with_negative");
245 freshener: infcx.freshener_keep_static(),
247 intercrate_ambiguity_causes: None,
248 allow_negative_impls,
249 query_mode: TraitQueryMode::Standard,
253 pub fn with_query_mode(
254 infcx: &'cx InferCtxt<'cx, 'tcx>,
255 query_mode: TraitQueryMode,
256 ) -> SelectionContext<'cx, 'tcx> {
257 debug!(?query_mode, "with_query_mode");
260 freshener: infcx.freshener_keep_static(),
262 intercrate_ambiguity_causes: None,
263 allow_negative_impls: false,
268 /// Enables tracking of intercrate ambiguity causes. These are
269 /// used in coherence to give improved diagnostics. We don't do
270 /// this until we detect a coherence error because it can lead to
271 /// false overflow results (#47139) and because it costs
272 /// computation time.
273 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
274 assert!(self.intercrate);
275 assert!(self.intercrate_ambiguity_causes.is_none());
276 self.intercrate_ambiguity_causes = Some(vec![]);
277 debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
280 /// Gets the intercrate ambiguity causes collected since tracking
281 /// was enabled and disables tracking at the same time. If
282 /// tracking is not enabled, just returns an empty vector.
283 pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> {
284 assert!(self.intercrate);
285 self.intercrate_ambiguity_causes.take().unwrap_or_default()
288 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'tcx> {
292 pub fn tcx(&self) -> TyCtxt<'tcx> {
296 pub fn is_intercrate(&self) -> bool {
300 ///////////////////////////////////////////////////////////////////////////
303 // The selection phase tries to identify *how* an obligation will
304 // be resolved. For example, it will identify which impl or
305 // parameter bound is to be used. The process can be inconclusive
306 // if the self type in the obligation is not fully inferred. Selection
307 // can result in an error in one of two ways:
309 // 1. If no applicable impl or parameter bound can be found.
310 // 2. If the output type parameters in the obligation do not match
311 // those specified by the impl/bound. For example, if the obligation
312 // is `Vec<Foo>: Iterable<Bar>`, but the impl specifies
313 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
315 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
316 /// type environment by performing unification.
317 #[instrument(level = "debug", skip(self))]
320 obligation: &TraitObligation<'tcx>,
321 ) -> SelectionResult<'tcx, Selection<'tcx>> {
322 let candidate = match self.select_from_obligation(obligation) {
323 Err(SelectionError::Overflow) => {
324 // In standard mode, overflow must have been caught and reported
326 assert!(self.query_mode == TraitQueryMode::Canonical);
327 return Err(SelectionError::Overflow);
329 Err(SelectionError::Ambiguous(_)) => {
338 Ok(Some(candidate)) => candidate,
341 match self.confirm_candidate(obligation, candidate) {
342 Err(SelectionError::Overflow) => {
343 assert!(self.query_mode == TraitQueryMode::Canonical);
344 Err(SelectionError::Overflow)
348 debug!(?candidate, "confirmed");
354 crate fn select_from_obligation(
356 obligation: &TraitObligation<'tcx>,
357 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
358 debug_assert!(!obligation.predicate.has_escaping_bound_vars());
360 let pec = &ProvisionalEvaluationCache::default();
361 let stack = self.push_stack(TraitObligationStackList::empty(pec), obligation);
363 self.candidate_from_obligation(&stack)
366 ///////////////////////////////////////////////////////////////////////////
369 // Tests whether an obligation can be selected or whether an impl
370 // can be applied to particular types. It skips the "confirmation"
371 // step and hence completely ignores output type parameters.
373 // The result is "true" if the obligation *may* hold and "false" if
374 // we can be sure it does not.
376 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
377 pub fn predicate_may_hold_fatal(&mut self, obligation: &PredicateObligation<'tcx>) -> bool {
378 debug!(?obligation, "predicate_may_hold_fatal");
380 // This fatal query is a stopgap that should only be used in standard mode,
381 // where we do not expect overflow to be propagated.
382 assert!(self.query_mode == TraitQueryMode::Standard);
384 self.evaluate_root_obligation(obligation)
385 .expect("Overflow should be caught earlier in standard query mode")
389 /// Evaluates whether the obligation `obligation` can be satisfied
390 /// and returns an `EvaluationResult`. This is meant for the
392 pub fn evaluate_root_obligation(
394 obligation: &PredicateObligation<'tcx>,
395 ) -> Result<EvaluationResult, OverflowError> {
396 self.evaluation_probe(|this| {
397 this.evaluate_predicate_recursively(
398 TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
406 op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
407 ) -> Result<EvaluationResult, OverflowError> {
408 self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
409 let result = op(self)?;
411 match self.infcx.leak_check(true, snapshot) {
413 Err(_) => return Ok(EvaluatedToErr),
416 match self.infcx.region_constraints_added_in_snapshot(snapshot) {
418 Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)),
423 /// Evaluates the predicates in `predicates` recursively. Note that
424 /// this applies projections in the predicates, and therefore
425 /// is run within an inference probe.
426 #[instrument(skip(self, stack), level = "debug")]
427 fn evaluate_predicates_recursively<'o, I>(
429 stack: TraitObligationStackList<'o, 'tcx>,
431 ) -> Result<EvaluationResult, OverflowError>
433 I: IntoIterator<Item = PredicateObligation<'tcx>> + std::fmt::Debug,
435 let mut result = EvaluatedToOk;
436 for obligation in predicates {
437 let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
438 if let EvaluatedToErr = eval {
439 // fast-path - EvaluatedToErr is the top of the lattice,
440 // so we don't need to look on the other predicates.
441 return Ok(EvaluatedToErr);
443 result = cmp::max(result, eval);
451 skip(self, previous_stack),
452 fields(previous_stack = ?previous_stack.head())
454 fn evaluate_predicate_recursively<'o>(
456 previous_stack: TraitObligationStackList<'o, 'tcx>,
457 obligation: PredicateObligation<'tcx>,
458 ) -> Result<EvaluationResult, OverflowError> {
459 // `previous_stack` stores a `TraitObligation`, while `obligation` is
460 // a `PredicateObligation`. These are distinct types, so we can't
461 // use any `Option` combinator method that would force them to be
463 match previous_stack.head() {
464 Some(h) => self.check_recursion_limit(&obligation, h.obligation)?,
465 None => self.check_recursion_limit(&obligation, &obligation)?,
468 let result = ensure_sufficient_stack(|| {
469 let bound_predicate = obligation.predicate.kind();
470 match bound_predicate.skip_binder() {
471 ty::PredicateKind::Trait(t) => {
472 let t = bound_predicate.rebind(t);
473 debug_assert!(!t.has_escaping_bound_vars());
474 let obligation = obligation.with(t);
475 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
478 ty::PredicateKind::Subtype(p) => {
479 let p = bound_predicate.rebind(p);
480 // Does this code ever run?
481 match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
482 Some(Ok(InferOk { mut obligations, .. })) => {
483 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
484 self.evaluate_predicates_recursively(
486 obligations.into_iter(),
489 Some(Err(_)) => Ok(EvaluatedToErr),
490 None => Ok(EvaluatedToAmbig),
494 ty::PredicateKind::Coerce(p) => {
495 let p = bound_predicate.rebind(p);
496 // Does this code ever run?
497 match self.infcx.coerce_predicate(&obligation.cause, obligation.param_env, p) {
498 Some(Ok(InferOk { mut obligations, .. })) => {
499 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
500 self.evaluate_predicates_recursively(
502 obligations.into_iter(),
505 Some(Err(_)) => Ok(EvaluatedToErr),
506 None => Ok(EvaluatedToAmbig),
510 ty::PredicateKind::WellFormed(arg) => match wf::obligations(
512 obligation.param_env,
513 obligation.cause.body_id,
514 obligation.recursion_depth + 1,
516 obligation.cause.span,
518 Some(mut obligations) => {
519 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
520 self.evaluate_predicates_recursively(previous_stack, obligations)
522 None => Ok(EvaluatedToAmbig),
525 ty::PredicateKind::TypeOutlives(pred) => {
526 // A global type with no late-bound regions can only
527 // contain the "'static" lifetime (any other lifetime
528 // would either be late-bound or local), so it is guaranteed
529 // to outlive any other lifetime
530 if pred.0.is_global() && !pred.0.has_late_bound_regions() {
533 Ok(EvaluatedToOkModuloRegions)
537 ty::PredicateKind::RegionOutlives(..) => {
538 // We do not consider region relationships when evaluating trait matches.
539 Ok(EvaluatedToOkModuloRegions)
542 ty::PredicateKind::ObjectSafe(trait_def_id) => {
543 if self.tcx().is_object_safe(trait_def_id) {
550 ty::PredicateKind::Projection(data) => {
551 let data = bound_predicate.rebind(data);
552 let project_obligation = obligation.with(data);
553 match project::poly_project_and_unify_type(self, &project_obligation) {
554 Ok(Ok(Some(mut subobligations))) => {
556 // If we've previously marked this projection as 'complete', thne
557 // use the final cached result (either `EvaluatedToOk` or
558 // `EvaluatedToOkModuloRegions`), and skip re-evaluating the
561 ProjectionCacheKey::from_poly_projection_predicate(self, data)
563 if let Some(cached_res) = self
570 break 'compute_res Ok(cached_res);
575 subobligations.iter_mut(),
576 obligation.recursion_depth,
578 let res = self.evaluate_predicates_recursively(
582 if let Ok(res) = res {
583 if res == EvaluatedToOk || res == EvaluatedToOkModuloRegions {
585 ProjectionCacheKey::from_poly_projection_predicate(
589 // If the result is something that we can cache, then mark this
590 // entry as 'complete'. This will allow us to skip evaluating the
591 // suboligations at all the next time we evaluate the projection
604 Ok(Ok(None)) => Ok(EvaluatedToAmbig),
605 Ok(Err(project::InProgress)) => Ok(EvaluatedToRecur),
606 Err(_) => Ok(EvaluatedToErr),
610 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
611 match self.infcx.closure_kind(closure_substs) {
612 Some(closure_kind) => {
613 if closure_kind.extends(kind) {
619 None => Ok(EvaluatedToAmbig),
623 ty::PredicateKind::ConstEvaluatable(uv) => {
624 match const_evaluatable::is_const_evaluatable(
627 obligation.param_env,
628 obligation.cause.span,
630 Ok(()) => Ok(EvaluatedToOk),
631 Err(NotConstEvaluatable::MentionsInfer) => Ok(EvaluatedToAmbig),
632 Err(NotConstEvaluatable::MentionsParam) => Ok(EvaluatedToErr),
633 Err(_) => Ok(EvaluatedToErr),
637 ty::PredicateKind::ConstEquate(c1, c2) => {
638 debug!(?c1, ?c2, "evaluate_predicate_recursively: equating consts");
640 if self.tcx().features().generic_const_exprs {
641 // FIXME: we probably should only try to unify abstract constants
642 // if the constants depend on generic parameters.
644 // Let's just see where this breaks :shrug:
645 if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
648 if self.infcx.try_unify_abstract_consts(a.shrink(), b.shrink()) {
649 return Ok(EvaluatedToOk);
654 let evaluate = |c: &'tcx ty::Const<'tcx>| {
655 if let ty::ConstKind::Unevaluated(unevaluated) = c.val {
658 obligation.param_env,
660 Some(obligation.cause.span),
662 .map(|val| ty::Const::from_value(self.tcx(), val, c.ty))
668 match (evaluate(c1), evaluate(c2)) {
669 (Ok(c1), Ok(c2)) => {
672 .at(&obligation.cause, obligation.param_env)
675 Ok(_) => Ok(EvaluatedToOk),
676 Err(_) => Ok(EvaluatedToErr),
679 (Err(ErrorHandled::Reported(ErrorReported)), _)
680 | (_, Err(ErrorHandled::Reported(ErrorReported))) => Ok(EvaluatedToErr),
681 (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
683 obligation.cause.span(self.tcx()),
684 "ConstEquate: const_eval_resolve returned an unexpected error"
687 (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
688 if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
691 // Two different constants using generic parameters ~> error.
697 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
698 bug!("TypeWellFormedFromEnv is only used for chalk")
703 debug!("finished: {:?} from {:?}", result, obligation);
708 #[instrument(skip(self, previous_stack), level = "debug")]
709 fn evaluate_trait_predicate_recursively<'o>(
711 previous_stack: TraitObligationStackList<'o, 'tcx>,
712 mut obligation: TraitObligation<'tcx>,
713 ) -> Result<EvaluationResult, OverflowError> {
715 && obligation.is_global()
716 && obligation.param_env.caller_bounds().iter().all(|bound| bound.needs_subst())
718 // If a param env has no global bounds, global obligations do not
719 // depend on its particular value in order to work, so we can clear
720 // out the param env and get better caching.
722 obligation.param_env = obligation.param_env.without_caller_bounds();
725 let stack = self.push_stack(previous_stack, &obligation);
726 let mut fresh_trait_pred = stack.fresh_trait_pred;
727 let mut param_env = obligation.param_env;
729 fresh_trait_pred = fresh_trait_pred.map_bound(|mut pred| {
730 pred.remap_constness(self.tcx(), &mut param_env);
734 debug!(?fresh_trait_pred);
736 if let Some(result) = self.check_evaluation_cache(param_env, fresh_trait_pred) {
737 debug!(?result, "CACHE HIT");
741 if let Some(result) = stack.cache().get_provisional(fresh_trait_pred) {
742 debug!(?result, "PROVISIONAL CACHE HIT");
743 stack.update_reached_depth(result.reached_depth);
744 return Ok(result.result);
747 // Check if this is a match for something already on the
748 // stack. If so, we don't want to insert the result into the
749 // main cache (it is cycle dependent) nor the provisional
750 // cache (which is meant for things that have completed but
751 // for a "backedge" -- this result *is* the backedge).
752 if let Some(cycle_result) = self.check_evaluation_cycle(&stack) {
753 return Ok(cycle_result);
756 let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
757 let result = result?;
759 if !result.must_apply_modulo_regions() {
760 stack.cache().on_failure(stack.dfn);
763 let reached_depth = stack.reached_depth.get();
764 if reached_depth >= stack.depth {
765 debug!(?result, "CACHE MISS");
766 self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
768 stack.cache().on_completion(stack.dfn, |fresh_trait_pred, provisional_result| {
769 self.insert_evaluation_cache(
773 provisional_result.max(result),
777 debug!(?result, "PROVISIONAL");
779 "caching provisionally because {:?} \
780 is a cycle participant (at depth {}, reached depth {})",
781 fresh_trait_pred, stack.depth, reached_depth,
784 stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_pred, result);
790 /// If there is any previous entry on the stack that precisely
791 /// matches this obligation, then we can assume that the
792 /// obligation is satisfied for now (still all other conditions
793 /// must be met of course). One obvious case this comes up is
794 /// marker traits like `Send`. Think of a linked list:
796 /// struct List<T> { data: T, next: Option<Box<List<T>>> }
798 /// `Box<List<T>>` will be `Send` if `T` is `Send` and
799 /// `Option<Box<List<T>>>` is `Send`, and in turn
800 /// `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
803 /// Note that we do this comparison using the `fresh_trait_ref`
804 /// fields. Because these have all been freshened using
805 /// `self.freshener`, we can be sure that (a) this will not
806 /// affect the inferencer state and (b) that if we see two
807 /// fresh regions with the same index, they refer to the same
808 /// unbound type variable.
809 fn check_evaluation_cycle(
811 stack: &TraitObligationStack<'_, 'tcx>,
812 ) -> Option<EvaluationResult> {
813 if let Some(cycle_depth) = stack
815 .skip(1) // Skip top-most frame.
817 stack.obligation.param_env == prev.obligation.param_env
818 && stack.fresh_trait_pred == prev.fresh_trait_pred
820 .map(|stack| stack.depth)
822 debug!("evaluate_stack --> recursive at depth {}", cycle_depth);
824 // If we have a stack like `A B C D E A`, where the top of
825 // the stack is the final `A`, then this will iterate over
826 // `A, E, D, C, B` -- i.e., all the participants apart
827 // from the cycle head. We mark them as participating in a
828 // cycle. This suppresses caching for those nodes. See
829 // `in_cycle` field for more details.
830 stack.update_reached_depth(cycle_depth);
832 // Subtle: when checking for a coinductive cycle, we do
833 // not compare using the "freshened trait refs" (which
834 // have erased regions) but rather the fully explicit
835 // trait refs. This is important because it's only a cycle
836 // if the regions match exactly.
837 let cycle = stack.iter().skip(1).take_while(|s| s.depth >= cycle_depth);
838 let tcx = self.tcx();
839 let cycle = cycle.map(|stack| stack.obligation.predicate.to_predicate(tcx));
840 if self.coinductive_match(cycle) {
841 debug!("evaluate_stack --> recursive, coinductive");
844 debug!("evaluate_stack --> recursive, inductive");
845 Some(EvaluatedToRecur)
852 fn evaluate_stack<'o>(
854 stack: &TraitObligationStack<'o, 'tcx>,
855 ) -> Result<EvaluationResult, OverflowError> {
856 // In intercrate mode, whenever any of the generics are unbound,
857 // there can always be an impl. Even if there are no impls in
858 // this crate, perhaps the type would be unified with
859 // something from another crate that does provide an impl.
861 // In intra mode, we must still be conservative. The reason is
862 // that we want to avoid cycles. Imagine an impl like:
864 // impl<T:Eq> Eq for Vec<T>
866 // and a trait reference like `$0 : Eq` where `$0` is an
867 // unbound variable. When we evaluate this trait-reference, we
868 // will unify `$0` with `Vec<$1>` (for some fresh variable
869 // `$1`), on the condition that `$1 : Eq`. We will then wind
870 // up with many candidates (since that are other `Eq` impls
871 // that apply) and try to winnow things down. This results in
872 // a recursive evaluation that `$1 : Eq` -- as you can
873 // imagine, this is just where we started. To avoid that, we
874 // check for unbound variables and return an ambiguous (hence possible)
875 // match if we've seen this trait before.
877 // This suffices to allow chains like `FnMut` implemented in
878 // terms of `Fn` etc, but we could probably make this more
880 let unbound_input_types =
881 stack.fresh_trait_pred.skip_binder().trait_ref.substs.types().any(|ty| ty.is_fresh());
883 if stack.obligation.polarity() != ty::ImplPolarity::Negative {
884 // This check was an imperfect workaround for a bug in the old
885 // intercrate mode; it should be removed when that goes away.
886 if unbound_input_types && self.intercrate {
887 debug!("evaluate_stack --> unbound argument, intercrate --> ambiguous",);
888 // Heuristics: show the diagnostics when there are no candidates in crate.
889 if self.intercrate_ambiguity_causes.is_some() {
890 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
891 if let Ok(candidate_set) = self.assemble_candidates(stack) {
892 if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
893 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
894 let self_ty = trait_ref.self_ty();
895 let cause = with_no_trimmed_paths(|| {
896 IntercrateAmbiguityCause::DownstreamCrate {
897 trait_desc: trait_ref.print_only_trait_path().to_string(),
898 self_desc: if self_ty.has_concrete_skeleton() {
899 Some(self_ty.to_string())
906 debug!(?cause, "evaluate_stack: pushing cause");
907 self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
911 return Ok(EvaluatedToAmbig);
915 if unbound_input_types
916 && stack.iter().skip(1).any(|prev| {
917 stack.obligation.param_env == prev.obligation.param_env
918 && self.match_fresh_trait_refs(
919 stack.fresh_trait_pred,
920 prev.fresh_trait_pred,
921 prev.obligation.param_env,
925 debug!("evaluate_stack --> unbound argument, recursive --> giving up",);
926 return Ok(EvaluatedToUnknown);
929 match self.candidate_from_obligation(stack) {
930 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
931 Err(SelectionError::Ambiguous(_)) => Ok(EvaluatedToAmbig),
932 Ok(None) => Ok(EvaluatedToAmbig),
933 Err(Overflow) => Err(OverflowError::Canonical),
934 Err(ErrorReporting) => Err(OverflowError::ErrorReporting),
935 Err(..) => Ok(EvaluatedToErr),
939 /// For defaulted traits, we use a co-inductive strategy to solve, so
940 /// that recursion is ok. This routine returns `true` if the top of the
941 /// stack (`cycle[0]`):
943 /// - is a defaulted trait,
944 /// - it also appears in the backtrace at some position `X`,
945 /// - all the predicates at positions `X..` between `X` and the top are
946 /// also defaulted traits.
947 pub fn coinductive_match<I>(&mut self, mut cycle: I) -> bool
949 I: Iterator<Item = ty::Predicate<'tcx>>,
951 cycle.all(|predicate| self.coinductive_predicate(predicate))
954 fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
955 let result = match predicate.kind().skip_binder() {
956 ty::PredicateKind::Trait(ref data) => self.tcx().trait_is_auto(data.def_id()),
959 debug!(?predicate, ?result, "coinductive_predicate");
963 /// Further evaluates `candidate` to decide whether all type parameters match and whether nested
964 /// obligations are met. Returns whether `candidate` remains viable after this further
969 fields(depth = stack.obligation.recursion_depth)
971 fn evaluate_candidate<'o>(
973 stack: &TraitObligationStack<'o, 'tcx>,
974 candidate: &SelectionCandidate<'tcx>,
975 ) -> Result<EvaluationResult, OverflowError> {
976 let mut result = self.evaluation_probe(|this| {
977 let candidate = (*candidate).clone();
978 match this.confirm_candidate(stack.obligation, candidate) {
981 this.evaluate_predicates_recursively(
983 selection.nested_obligations().into_iter(),
986 Err(..) => Ok(EvaluatedToErr),
990 // If we erased any lifetimes, then we want to use
991 // `EvaluatedToOkModuloRegions` instead of `EvaluatedToOk`
992 // as your final result. The result will be cached using
993 // the freshened trait predicate as a key, so we need
994 // our result to be correct by *any* choice of original lifetimes,
995 // not just the lifetime choice for this particular (non-erased)
998 if stack.fresh_trait_pred.has_erased_regions() {
999 result = result.max(EvaluatedToOkModuloRegions);
1006 fn check_evaluation_cache(
1008 param_env: ty::ParamEnv<'tcx>,
1009 trait_pred: ty::PolyTraitPredicate<'tcx>,
1010 ) -> Option<EvaluationResult> {
1011 // Neither the global nor local cache is aware of intercrate
1012 // mode, so don't do any caching. In particular, we might
1013 // re-use the same `InferCtxt` with both an intercrate
1014 // and non-intercrate `SelectionContext`
1015 if self.intercrate {
1019 let tcx = self.tcx();
1020 if self.can_use_global_caches(param_env) {
1021 if let Some(res) = tcx.evaluation_cache.get(¶m_env.and(trait_pred), tcx) {
1025 self.infcx.evaluation_cache.get(¶m_env.and(trait_pred), tcx)
1028 fn insert_evaluation_cache(
1030 param_env: ty::ParamEnv<'tcx>,
1031 trait_pred: ty::PolyTraitPredicate<'tcx>,
1032 dep_node: DepNodeIndex,
1033 result: EvaluationResult,
1035 // Avoid caching results that depend on more than just the trait-ref
1036 // - the stack can create recursion.
1037 if result.is_stack_dependent() {
1041 // Neither the global nor local cache is aware of intercrate
1042 // mode, so don't do any caching. In particular, we might
1043 // re-use the same `InferCtxt` with both an intercrate
1044 // and non-intercrate `SelectionContext`
1045 if self.intercrate {
1049 if self.can_use_global_caches(param_env) {
1050 if !trait_pred.needs_infer() {
1051 debug!(?trait_pred, ?result, "insert_evaluation_cache global");
1052 // This may overwrite the cache with the same value
1053 // FIXME: Due to #50507 this overwrites the different values
1054 // This should be changed to use HashMapExt::insert_same
1055 // when that is fixed
1056 self.tcx().evaluation_cache.insert(param_env.and(trait_pred), dep_node, result);
1061 debug!(?trait_pred, ?result, "insert_evaluation_cache");
1062 self.infcx.evaluation_cache.insert(param_env.and(trait_pred), dep_node, result);
1065 /// For various reasons, it's possible for a subobligation
1066 /// to have a *lower* recursion_depth than the obligation used to create it.
1067 /// Projection sub-obligations may be returned from the projection cache,
1068 /// which results in obligations with an 'old' `recursion_depth`.
1069 /// Additionally, methods like `InferCtxt.subtype_predicate` produce
1070 /// subobligations without taking in a 'parent' depth, causing the
1071 /// generated subobligations to have a `recursion_depth` of `0`.
1073 /// To ensure that obligation_depth never decreases, we force all subobligations
1074 /// to have at least the depth of the original obligation.
1075 fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>(
1080 it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
1083 fn check_recursion_depth<T: Display + TypeFoldable<'tcx>>(
1086 error_obligation: &Obligation<'tcx, T>,
1087 ) -> Result<(), OverflowError> {
1088 if !self.infcx.tcx.recursion_limit().value_within_limit(depth) {
1089 match self.query_mode {
1090 TraitQueryMode::Standard => {
1091 if self.infcx.is_tainted_by_errors() {
1092 return Err(OverflowError::ErrorReporting);
1094 self.infcx.report_overflow_error(error_obligation, true);
1096 TraitQueryMode::Canonical => {
1097 return Err(OverflowError::Canonical);
1104 /// Checks that the recursion limit has not been exceeded.
1106 /// The weird return type of this function allows it to be used with the `try` (`?`)
1107 /// operator within certain functions.
1109 fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V: Display + TypeFoldable<'tcx>>(
1111 obligation: &Obligation<'tcx, T>,
1112 error_obligation: &Obligation<'tcx, V>,
1113 ) -> Result<(), OverflowError> {
1114 self.check_recursion_depth(obligation.recursion_depth, error_obligation)
1117 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1119 OP: FnOnce(&mut Self) -> R,
1121 let (result, dep_node) =
1122 self.tcx().dep_graph.with_anon_task(self.tcx(), DepKind::TraitSelect, || op(self));
1123 self.tcx().dep_graph.read_index(dep_node);
1127 /// filter_impls filters constant trait obligations and candidates that have a positive impl
1128 /// for a negative goal and a negative impl for a positive goal
1129 #[instrument(level = "debug", skip(self))]
1132 candidates: Vec<SelectionCandidate<'tcx>>,
1133 obligation: &TraitObligation<'tcx>,
1134 ) -> Vec<SelectionCandidate<'tcx>> {
1135 let tcx = self.tcx();
1136 let mut result = Vec::with_capacity(candidates.len());
1138 for candidate in candidates {
1139 // Respect const trait obligations
1140 if obligation.is_const() {
1143 ImplCandidate(def_id)
1144 if tcx.impl_constness(def_id) == hir::Constness::Const => {}
1146 ParamCandidate(trait_pred)
1147 if trait_pred.skip_binder().constness
1148 == ty::BoundConstness::ConstIfConst => {}
1150 AutoImplCandidate(..) => {}
1151 // generator, this will raise error in other places
1152 // or ignore error with const_async_blocks feature
1153 GeneratorCandidate => {}
1154 // FnDef where the function is const
1155 FnPointerCandidate { is_const: true } => {}
1156 ConstDropCandidate => {}
1158 // reject all other types of candidates
1164 if let ImplCandidate(def_id) = candidate {
1165 if ty::ImplPolarity::Reservation == tcx.impl_polarity(def_id)
1166 || obligation.polarity() == tcx.impl_polarity(def_id)
1167 || self.allow_negative_impls
1169 result.push(candidate);
1172 result.push(candidate);
1179 /// filter_reservation_impls filter reservation impl for any goal as ambiguous
1180 #[instrument(level = "debug", skip(self))]
1181 fn filter_reservation_impls(
1183 candidate: SelectionCandidate<'tcx>,
1184 obligation: &TraitObligation<'tcx>,
1185 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1186 let tcx = self.tcx();
1187 // Treat reservation impls as ambiguity.
1188 if let ImplCandidate(def_id) = candidate {
1189 if let ty::ImplPolarity::Reservation = tcx.impl_polarity(def_id) {
1190 if let Some(intercrate_ambiguity_clauses) = &mut self.intercrate_ambiguity_causes {
1191 let attrs = tcx.get_attrs(def_id);
1192 let attr = tcx.sess.find_by_name(&attrs, sym::rustc_reservation_impl);
1193 let value = attr.and_then(|a| a.value_str());
1194 if let Some(value) = value {
1196 "filter_reservation_impls: \
1197 reservation impl ambiguity on {:?}",
1200 intercrate_ambiguity_clauses.push(
1201 IntercrateAmbiguityCause::ReservationImpl {
1202 message: value.to_string(),
1213 fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> {
1214 debug!("is_knowable(intercrate={:?})", self.intercrate);
1216 if !self.intercrate || stack.obligation.polarity() == ty::ImplPolarity::Negative {
1220 let obligation = &stack.obligation;
1221 let predicate = self.infcx().resolve_vars_if_possible(obligation.predicate);
1223 // Okay to skip binder because of the nature of the
1224 // trait-ref-is-knowable check, which does not care about
1226 let trait_ref = predicate.skip_binder().trait_ref;
1228 coherence::trait_ref_is_knowable(self.tcx(), trait_ref)
1231 /// Returns `true` if the global caches can be used.
1232 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1233 // If there are any inference variables in the `ParamEnv`, then we
1234 // always use a cache local to this particular scope. Otherwise, we
1235 // switch to a global cache.
1236 if param_env.needs_infer() {
1240 // Avoid using the master cache during coherence and just rely
1241 // on the local cache. This effectively disables caching
1242 // during coherence. It is really just a simplification to
1243 // avoid us having to fear that coherence results "pollute"
1244 // the master cache. Since coherence executes pretty quickly,
1245 // it's not worth going to more trouble to increase the
1246 // hit-rate, I don't think.
1247 if self.intercrate {
1251 // Otherwise, we can use the global cache.
1255 fn check_candidate_cache(
1257 mut param_env: ty::ParamEnv<'tcx>,
1258 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1259 ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
1260 // Neither the global nor local cache is aware of intercrate
1261 // mode, so don't do any caching. In particular, we might
1262 // re-use the same `InferCtxt` with both an intercrate
1263 // and non-intercrate `SelectionContext`
1264 if self.intercrate {
1267 let tcx = self.tcx();
1268 let mut pred = cache_fresh_trait_pred.skip_binder();
1269 pred.remap_constness(tcx, &mut param_env);
1271 if self.can_use_global_caches(param_env) {
1272 if let Some(res) = tcx.selection_cache.get(¶m_env.and(pred), tcx) {
1276 self.infcx.selection_cache.get(¶m_env.and(pred), tcx)
1279 /// Determines whether can we safely cache the result
1280 /// of selecting an obligation. This is almost always `true`,
1281 /// except when dealing with certain `ParamCandidate`s.
1283 /// Ordinarily, a `ParamCandidate` will contain no inference variables,
1284 /// since it was usually produced directly from a `DefId`. However,
1285 /// certain cases (currently only librustdoc's blanket impl finder),
1286 /// a `ParamEnv` may be explicitly constructed with inference types.
1287 /// When this is the case, we do *not* want to cache the resulting selection
1288 /// candidate. This is due to the fact that it might not always be possible
1289 /// to equate the obligation's trait ref and the candidate's trait ref,
1290 /// if more constraints end up getting added to an inference variable.
1292 /// Because of this, we always want to re-run the full selection
1293 /// process for our obligation the next time we see it, since
1294 /// we might end up picking a different `SelectionCandidate` (or none at all).
1295 fn can_cache_candidate(
1297 result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1299 // Neither the global nor local cache is aware of intercrate
1300 // mode, so don't do any caching. In particular, we might
1301 // re-use the same `InferCtxt` with both an intercrate
1302 // and non-intercrate `SelectionContext`
1303 if self.intercrate {
1307 Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => !trait_ref.needs_infer(),
1312 fn insert_candidate_cache(
1314 mut param_env: ty::ParamEnv<'tcx>,
1315 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1316 dep_node: DepNodeIndex,
1317 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1319 let tcx = self.tcx();
1320 let mut pred = cache_fresh_trait_pred.skip_binder();
1322 pred.remap_constness(tcx, &mut param_env);
1324 if !self.can_cache_candidate(&candidate) {
1325 debug!(?pred, ?candidate, "insert_candidate_cache - candidate is not cacheable");
1329 if self.can_use_global_caches(param_env) {
1330 if let Err(Overflow) = candidate {
1331 // Don't cache overflow globally; we only produce this in certain modes.
1332 } else if !pred.needs_infer() {
1333 if !candidate.needs_infer() {
1334 debug!(?pred, ?candidate, "insert_candidate_cache global");
1335 // This may overwrite the cache with the same value.
1336 tcx.selection_cache.insert(param_env.and(pred), dep_node, candidate);
1342 debug!(?pred, ?candidate, "insert_candidate_cache local");
1343 self.infcx.selection_cache.insert(param_env.and(pred), dep_node, candidate);
1346 /// Matches a predicate against the bounds of its self type.
1348 /// Given an obligation like `<T as Foo>::Bar: Baz` where the self type is
1349 /// a projection, look at the bounds of `T::Bar`, see if we can find a
1350 /// `Baz` bound. We return indexes into the list returned by
1351 /// `tcx.item_bounds` for any applicable bounds.
1352 fn match_projection_obligation_against_definition_bounds(
1354 obligation: &TraitObligation<'tcx>,
1355 ) -> smallvec::SmallVec<[usize; 2]> {
1356 let poly_trait_predicate = self.infcx().resolve_vars_if_possible(obligation.predicate);
1357 let placeholder_trait_predicate =
1358 self.infcx().replace_bound_vars_with_placeholders(poly_trait_predicate);
1360 ?placeholder_trait_predicate,
1361 "match_projection_obligation_against_definition_bounds"
1364 let tcx = self.infcx.tcx;
1365 let (def_id, substs) = match *placeholder_trait_predicate.trait_ref.self_ty().kind() {
1366 ty::Projection(ref data) => (data.item_def_id, data.substs),
1367 ty::Opaque(def_id, substs) => (def_id, substs),
1370 obligation.cause.span,
1371 "match_projection_obligation_against_definition_bounds() called \
1372 but self-ty is not a projection: {:?}",
1373 placeholder_trait_predicate.trait_ref.self_ty()
1377 let bounds = tcx.item_bounds(def_id).subst(tcx, substs);
1379 // The bounds returned by `item_bounds` may contain duplicates after
1380 // normalization, so try to deduplicate when possible to avoid
1381 // unnecessary ambiguity.
1382 let mut distinct_normalized_bounds = FxHashSet::default();
1384 let matching_bounds = bounds
1387 .filter_map(|(idx, bound)| {
1388 let bound_predicate = bound.kind();
1389 if let ty::PredicateKind::Trait(pred) = bound_predicate.skip_binder() {
1390 let bound = bound_predicate.rebind(pred.trait_ref);
1391 if self.infcx.probe(|_| {
1392 match self.match_normalize_trait_ref(
1395 placeholder_trait_predicate.trait_ref,
1398 Ok(Some(normalized_trait))
1399 if distinct_normalized_bounds.insert(normalized_trait) =>
1413 debug!(?matching_bounds, "match_projection_obligation_against_definition_bounds");
1417 /// Equates the trait in `obligation` with trait bound. If the two traits
1418 /// can be equated and the normalized trait bound doesn't contain inference
1419 /// variables or placeholders, the normalized bound is returned.
1420 fn match_normalize_trait_ref(
1422 obligation: &TraitObligation<'tcx>,
1423 trait_bound: ty::PolyTraitRef<'tcx>,
1424 placeholder_trait_ref: ty::TraitRef<'tcx>,
1425 ) -> Result<Option<ty::PolyTraitRef<'tcx>>, ()> {
1426 debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
1427 if placeholder_trait_ref.def_id != trait_bound.def_id() {
1428 // Avoid unnecessary normalization
1432 let Normalized { value: trait_bound, obligations: _ } = ensure_sufficient_stack(|| {
1433 project::normalize_with_depth(
1435 obligation.param_env,
1436 obligation.cause.clone(),
1437 obligation.recursion_depth + 1,
1442 .at(&obligation.cause, obligation.param_env)
1443 .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
1444 .map(|InferOk { obligations: _, value: () }| {
1445 // This method is called within a probe, so we can't have
1446 // inference variables and placeholders escape.
1447 if !trait_bound.needs_infer() && !trait_bound.has_placeholders() {
1456 fn evaluate_where_clause<'o>(
1458 stack: &TraitObligationStack<'o, 'tcx>,
1459 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
1460 ) -> Result<EvaluationResult, OverflowError> {
1461 self.evaluation_probe(|this| {
1462 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1463 Ok(obligations) => this.evaluate_predicates_recursively(stack.list(), obligations),
1464 Err(()) => Ok(EvaluatedToErr),
1469 pub(super) fn match_projection_projections(
1471 obligation: &ProjectionTyObligation<'tcx>,
1472 env_predicate: PolyProjectionPredicate<'tcx>,
1473 potentially_unnormalized_candidates: bool,
1475 let mut nested_obligations = Vec::new();
1476 let (infer_predicate, _) = self.infcx.replace_bound_vars_with_fresh_vars(
1477 obligation.cause.span,
1478 LateBoundRegionConversionTime::HigherRankedType,
1481 let infer_projection = if potentially_unnormalized_candidates {
1482 ensure_sufficient_stack(|| {
1483 project::normalize_with_depth_to(
1485 obligation.param_env,
1486 obligation.cause.clone(),
1487 obligation.recursion_depth + 1,
1488 infer_predicate.projection_ty,
1489 &mut nested_obligations,
1493 infer_predicate.projection_ty
1497 .at(&obligation.cause, obligation.param_env)
1498 .sup(obligation.predicate, infer_projection)
1499 .map_or(false, |InferOk { obligations, value: () }| {
1500 self.evaluate_predicates_recursively(
1501 TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
1502 nested_obligations.into_iter().chain(obligations),
1504 .map_or(false, |res| res.may_apply())
1508 ///////////////////////////////////////////////////////////////////////////
1511 // Winnowing is the process of attempting to resolve ambiguity by
1512 // probing further. During the winnowing process, we unify all
1513 // type variables and then we also attempt to evaluate recursive
1514 // bounds to see if they are satisfied.
1516 /// Returns `true` if `victim` should be dropped in favor of
1517 /// `other`. Generally speaking we will drop duplicate
1518 /// candidates and prefer where-clause candidates.
1520 /// See the comment for "SelectionCandidate" for more details.
1521 fn candidate_should_be_dropped_in_favor_of(
1523 sized_predicate: bool,
1524 victim: &EvaluatedCandidate<'tcx>,
1525 other: &EvaluatedCandidate<'tcx>,
1528 if victim.candidate == other.candidate {
1532 // Check if a bound would previously have been removed when normalizing
1533 // the param_env so that it can be given the lowest priority. See
1534 // #50825 for the motivation for this.
1535 let is_global = |cand: &ty::PolyTraitPredicate<'tcx>| {
1536 cand.is_global() && !cand.has_late_bound_regions()
1539 // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
1540 // and `DiscriminantKindCandidate` to anything else.
1542 // This is a fix for #53123 and prevents winnowing from accidentally extending the
1543 // lifetime of a variable.
1544 match (&other.candidate, &victim.candidate) {
1545 (_, AutoImplCandidate(..)) | (AutoImplCandidate(..), _) => {
1547 "default implementations shouldn't be recorded \
1548 when there are other valid candidates"
1554 BuiltinCandidate { has_nested: false }
1555 | DiscriminantKindCandidate
1557 | ConstDropCandidate,
1562 BuiltinCandidate { has_nested: false }
1563 | DiscriminantKindCandidate
1565 | ConstDropCandidate,
1568 (ParamCandidate(other), ParamCandidate(victim)) => {
1569 let same_except_bound_vars = other.skip_binder().trait_ref
1570 == victim.skip_binder().trait_ref
1571 && other.skip_binder().constness == victim.skip_binder().constness
1572 && other.skip_binder().polarity == victim.skip_binder().polarity
1573 && !other.skip_binder().trait_ref.has_escaping_bound_vars();
1574 if same_except_bound_vars {
1575 // See issue #84398. In short, we can generate multiple ParamCandidates which are
1576 // the same except for unused bound vars. Just pick the one with the fewest bound vars
1577 // or the current one if tied (they should both evaluate to the same answer). This is
1578 // probably best characterized as a "hack", since we might prefer to just do our
1579 // best to *not* create essentially duplicate candidates in the first place.
1580 other.bound_vars().len() <= victim.bound_vars().len()
1581 } else if other.skip_binder().trait_ref == victim.skip_binder().trait_ref
1582 && victim.skip_binder().constness == ty::BoundConstness::NotConst
1583 && other.skip_binder().polarity == victim.skip_binder().polarity
1585 // Drop otherwise equivalent non-const candidates in favor of const candidates.
1592 // Drop otherwise equivalent non-const fn pointer candidates
1593 (FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => true,
1595 // If obligation is a sized predicate or the where-clause bound is
1596 // global, prefer the projection or object candidate. See issue
1597 // #50825 and #89352.
1598 (ObjectCandidate(_) | ProjectionCandidate(_), ParamCandidate(ref cand)) => {
1599 sized_predicate || is_global(cand)
1601 (ParamCandidate(ref cand), ObjectCandidate(_) | ProjectionCandidate(_)) => {
1602 !(sized_predicate || is_global(cand))
1605 // Global bounds from the where clause should be ignored
1606 // here (see issue #50825). Otherwise, we have a where
1607 // clause so don't go around looking for impls.
1608 // Arbitrarily give param candidates priority
1609 // over projection and object candidates.
1611 ParamCandidate(ref cand),
1614 | GeneratorCandidate
1615 | FnPointerCandidate { .. }
1616 | BuiltinObjectCandidate
1617 | BuiltinUnsizeCandidate
1618 | TraitUpcastingUnsizeCandidate(_)
1619 | BuiltinCandidate { .. }
1620 | TraitAliasCandidate(..),
1621 ) => !is_global(cand),
1625 | GeneratorCandidate
1626 | FnPointerCandidate { .. }
1627 | BuiltinObjectCandidate
1628 | BuiltinUnsizeCandidate
1629 | TraitUpcastingUnsizeCandidate(_)
1630 | BuiltinCandidate { has_nested: true }
1631 | TraitAliasCandidate(..),
1632 ParamCandidate(ref cand),
1634 // Prefer these to a global where-clause bound
1635 // (see issue #50825).
1636 is_global(cand) && other.evaluation.must_apply_modulo_regions()
1639 (ProjectionCandidate(i), ProjectionCandidate(j))
1640 | (ObjectCandidate(i), ObjectCandidate(j)) => {
1641 // Arbitrarily pick the lower numbered candidate for backwards
1642 // compatibility reasons. Don't let this affect inference.
1643 i < j && !needs_infer
1645 (ObjectCandidate(_), ProjectionCandidate(_))
1646 | (ProjectionCandidate(_), ObjectCandidate(_)) => {
1647 bug!("Have both object and projection candidate")
1650 // Arbitrarily give projection and object candidates priority.
1652 ObjectCandidate(_) | ProjectionCandidate(_),
1655 | GeneratorCandidate
1656 | FnPointerCandidate { .. }
1657 | BuiltinObjectCandidate
1658 | BuiltinUnsizeCandidate
1659 | TraitUpcastingUnsizeCandidate(_)
1660 | BuiltinCandidate { .. }
1661 | TraitAliasCandidate(..),
1667 | GeneratorCandidate
1668 | FnPointerCandidate { .. }
1669 | BuiltinObjectCandidate
1670 | BuiltinUnsizeCandidate
1671 | TraitUpcastingUnsizeCandidate(_)
1672 | BuiltinCandidate { .. }
1673 | TraitAliasCandidate(..),
1674 ObjectCandidate(_) | ProjectionCandidate(_),
1677 (&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
1678 // See if we can toss out `victim` based on specialization.
1679 // This requires us to know *for sure* that the `other` impl applies
1680 // i.e., `EvaluatedToOk`.
1682 // FIXME(@lcnr): Using `modulo_regions` here seems kind of scary
1683 // to me but is required for `std` to compile, so I didn't change it
1685 let tcx = self.tcx();
1686 if other.evaluation.must_apply_modulo_regions() {
1687 if tcx.specializes((other_def, victim_def)) {
1692 if other.evaluation.must_apply_considering_regions() {
1693 match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
1694 Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
1695 // Subtle: If the predicate we are evaluating has inference
1696 // variables, do *not* allow discarding candidates due to
1697 // marker trait impls.
1699 // Without this restriction, we could end up accidentally
1700 // constrainting inference variables based on an arbitrarily
1701 // chosen trait impl.
1703 // Imagine we have the following code:
1706 // #[marker] trait MyTrait {}
1707 // impl MyTrait for u8 {}
1708 // impl MyTrait for bool {}
1711 // And we are evaluating the predicate `<_#0t as MyTrait>`.
1713 // During selection, we will end up with one candidate for each
1714 // impl of `MyTrait`. If we were to discard one impl in favor
1715 // of the other, we would be left with one candidate, causing
1716 // us to "successfully" select the predicate, unifying
1717 // _#0t with (for example) `u8`.
1719 // However, we have no reason to believe that this unification
1720 // is correct - we've essentially just picked an arbitrary
1721 // *possibility* for _#0t, and required that this be the *only*
1724 // Eventually, we will either:
1725 // 1) Unify all inference variables in the predicate through
1726 // some other means (e.g. type-checking of a function). We will
1727 // then be in a position to drop marker trait candidates
1728 // without constraining inference variables (since there are
1729 // none left to constrin)
1730 // 2) Be left with some unconstrained inference variables. We
1731 // will then correctly report an inference error, since the
1732 // existence of multiple marker trait impls tells us nothing
1733 // about which one should actually apply.
1744 // Everything else is ambiguous
1748 | GeneratorCandidate
1749 | FnPointerCandidate { .. }
1750 | BuiltinObjectCandidate
1751 | BuiltinUnsizeCandidate
1752 | TraitUpcastingUnsizeCandidate(_)
1753 | BuiltinCandidate { has_nested: true }
1754 | TraitAliasCandidate(..),
1757 | GeneratorCandidate
1758 | FnPointerCandidate { .. }
1759 | BuiltinObjectCandidate
1760 | BuiltinUnsizeCandidate
1761 | TraitUpcastingUnsizeCandidate(_)
1762 | BuiltinCandidate { has_nested: true }
1763 | TraitAliasCandidate(..),
1768 fn sized_conditions(
1770 obligation: &TraitObligation<'tcx>,
1771 ) -> BuiltinImplConditions<'tcx> {
1772 use self::BuiltinImplConditions::{Ambiguous, None, Where};
1774 // NOTE: binder moved to (*)
1775 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
1777 match self_ty.kind() {
1778 ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
1789 | ty::GeneratorWitness(..)
1794 // safe for everything
1795 Where(ty::Binder::dummy(Vec::new()))
1798 ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
1800 ty::Tuple(tys) => Where(
1803 .rebind(tys.last().into_iter().map(|k| k.expect_ty()).collect()),
1806 ty::Adt(def, substs) => {
1807 let sized_crit = def.sized_constraint(self.tcx());
1808 // (*) binder moved here
1810 obligation.predicate.rebind({
1811 sized_crit.iter().map(|ty| ty.subst(self.tcx(), substs)).collect()
1816 ty::Projection(_) | ty::Param(_) | ty::Opaque(..) => None,
1817 ty::Infer(ty::TyVar(_)) => Ambiguous,
1821 | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
1822 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
1827 fn copy_clone_conditions(
1829 obligation: &TraitObligation<'tcx>,
1830 ) -> BuiltinImplConditions<'tcx> {
1831 // NOTE: binder moved to (*)
1832 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
1834 use self::BuiltinImplConditions::{Ambiguous, None, Where};
1836 match *self_ty.kind() {
1837 ty::Infer(ty::IntVar(_))
1838 | ty::Infer(ty::FloatVar(_))
1841 | ty::Error(_) => Where(ty::Binder::dummy(Vec::new())),
1850 | ty::Ref(_, _, hir::Mutability::Not)
1851 | ty::Array(..) => {
1852 // Implementations provided in libcore
1860 | ty::GeneratorWitness(..)
1862 | ty::Ref(_, _, hir::Mutability::Mut) => None,
1865 // (*) binder moved here
1866 Where(obligation.predicate.rebind(tys.iter().map(|k| k.expect_ty()).collect()))
1869 ty::Closure(_, substs) => {
1870 // (*) binder moved here
1871 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
1872 if let ty::Infer(ty::TyVar(_)) = ty.kind() {
1873 // Not yet resolved.
1876 Where(obligation.predicate.rebind(substs.as_closure().upvar_tys().collect()))
1880 ty::Adt(..) | ty::Projection(..) | ty::Param(..) | ty::Opaque(..) => {
1881 // Fallback to whatever user-defined impls exist in this case.
1885 ty::Infer(ty::TyVar(_)) => {
1886 // Unbound type variable. Might or might not have
1887 // applicable impls and so forth, depending on what
1888 // those type variables wind up being bound to.
1894 | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
1895 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
1900 /// For default impls, we need to break apart a type into its
1901 /// "constituent types" -- meaning, the types that it contains.
1903 /// Here are some (simple) examples:
1906 /// (i32, u32) -> [i32, u32]
1907 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
1908 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
1909 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
1911 fn constituent_types_for_ty(
1913 t: ty::Binder<'tcx, Ty<'tcx>>,
1914 ) -> ty::Binder<'tcx, Vec<Ty<'tcx>>> {
1915 match *t.skip_binder().kind() {
1924 | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
1926 | ty::Char => ty::Binder::dummy(Vec::new()),
1932 | ty::Projection(..)
1934 | ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
1935 bug!("asked to assemble constituent types of unexpected type: {:?}", t);
1938 ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => {
1939 t.rebind(vec![element_ty])
1942 ty::Array(element_ty, _) | ty::Slice(element_ty) => t.rebind(vec![element_ty]),
1944 ty::Tuple(ref tys) => {
1945 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
1946 t.rebind(tys.iter().map(|k| k.expect_ty()).collect())
1949 ty::Closure(_, ref substs) => {
1950 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
1954 ty::Generator(_, ref substs, _) => {
1955 let ty = self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
1956 let witness = substs.as_generator().witness();
1957 t.rebind([ty].into_iter().chain(iter::once(witness)).collect())
1960 ty::GeneratorWitness(types) => {
1961 debug_assert!(!types.has_escaping_bound_vars());
1962 types.map_bound(|types| types.to_vec())
1965 // For `PhantomData<T>`, we pass `T`.
1966 ty::Adt(def, substs) if def.is_phantom_data() => t.rebind(substs.types().collect()),
1968 ty::Adt(def, substs) => {
1969 t.rebind(def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect())
1972 ty::Opaque(def_id, substs) => {
1973 // We can resolve the `impl Trait` to its concrete type,
1974 // which enforces a DAG between the functions requiring
1975 // the auto trait bounds in question.
1976 t.rebind(vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)])
1981 fn collect_predicates_for_types(
1983 param_env: ty::ParamEnv<'tcx>,
1984 cause: ObligationCause<'tcx>,
1985 recursion_depth: usize,
1986 trait_def_id: DefId,
1987 types: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
1988 ) -> Vec<PredicateObligation<'tcx>> {
1989 // Because the types were potentially derived from
1990 // higher-ranked obligations they may reference late-bound
1991 // regions. For example, `for<'a> Foo<&'a i32> : Copy` would
1992 // yield a type like `for<'a> &'a i32`. In general, we
1993 // maintain the invariant that we never manipulate bound
1994 // regions, so we have to process these bound regions somehow.
1996 // The strategy is to:
1998 // 1. Instantiate those regions to placeholder regions (e.g.,
1999 // `for<'a> &'a i32` becomes `&0 i32`.
2000 // 2. Produce something like `&'0 i32 : Copy`
2001 // 3. Re-bind the regions back to `for<'a> &'a i32 : Copy`
2005 .skip_binder() // binder moved -\
2008 let ty: ty::Binder<'tcx, Ty<'tcx>> = types.rebind(ty); // <----/
2010 self.infcx.commit_unconditionally(|_| {
2011 let placeholder_ty = self.infcx.replace_bound_vars_with_placeholders(ty);
2012 let Normalized { value: normalized_ty, mut obligations } =
2013 ensure_sufficient_stack(|| {
2014 project::normalize_with_depth(
2022 let placeholder_obligation = predicate_for_trait_def(
2031 obligations.push(placeholder_obligation);
2038 ///////////////////////////////////////////////////////////////////////////
2041 // Matching is a common path used for both evaluation and
2042 // confirmation. It basically unifies types that appear in impls
2043 // and traits. This does affect the surrounding environment;
2044 // therefore, when used during evaluation, match routines must be
2045 // run inside of a `probe()` so that their side-effects are
2051 obligation: &TraitObligation<'tcx>,
2052 ) -> Normalized<'tcx, SubstsRef<'tcx>> {
2053 match self.match_impl(impl_def_id, obligation) {
2054 Ok(substs) => substs,
2057 "Impl {:?} was matchable against {:?} but now is not",
2065 #[tracing::instrument(level = "debug", skip(self))]
2069 obligation: &TraitObligation<'tcx>,
2070 ) -> Result<Normalized<'tcx, SubstsRef<'tcx>>, ()> {
2071 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
2073 // Before we create the substitutions and everything, first
2074 // consider a "quick reject". This avoids creating more types
2075 // and so forth that we need to.
2076 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
2080 let placeholder_obligation =
2081 self.infcx().replace_bound_vars_with_placeholders(obligation.predicate);
2082 let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
2084 let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
2086 let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs);
2088 debug!(?impl_trait_ref);
2090 let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
2091 ensure_sufficient_stack(|| {
2092 project::normalize_with_depth(
2094 obligation.param_env,
2095 obligation.cause.clone(),
2096 obligation.recursion_depth + 1,
2101 debug!(?impl_trait_ref, ?placeholder_obligation_trait_ref);
2103 let cause = ObligationCause::new(
2104 obligation.cause.span,
2105 obligation.cause.body_id,
2106 ObligationCauseCode::MatchImpl(obligation.cause.clone(), impl_def_id),
2109 let InferOk { obligations, .. } = self
2111 .at(&cause, obligation.param_env)
2112 .eq(placeholder_obligation_trait_ref, impl_trait_ref)
2113 .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
2114 nested_obligations.extend(obligations);
2117 && self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation
2119 debug!("match_impl: reservation impls only apply in intercrate mode");
2123 debug!(?impl_substs, ?nested_obligations, "match_impl: success");
2124 Ok(Normalized { value: impl_substs, obligations: nested_obligations })
2127 fn fast_reject_trait_refs(
2129 obligation: &TraitObligation<'_>,
2130 impl_trait_ref: &ty::TraitRef<'_>,
2132 // We can avoid creating type variables and doing the full
2133 // substitution if we find that any of the input types, when
2134 // simplified, do not match.
2136 iter::zip(obligation.predicate.skip_binder().trait_ref.substs, impl_trait_ref.substs).any(
2137 |(obligation_arg, impl_arg)| {
2138 match (obligation_arg.unpack(), impl_arg.unpack()) {
2139 (GenericArgKind::Type(obligation_ty), GenericArgKind::Type(impl_ty)) => {
2140 // Note, we simplify parameters for the obligation but not the
2141 // impl so that we do not reject a blanket impl but do reject
2142 // more concrete impls if we're searching for `T: Trait`.
2143 let simplified_obligation_ty = fast_reject::simplify_type(
2146 SimplifyParams::Yes,
2147 StripReferences::No,
2149 let simplified_impl_ty = fast_reject::simplify_type(
2153 StripReferences::No,
2156 simplified_obligation_ty.is_some()
2157 && simplified_impl_ty.is_some()
2158 && simplified_obligation_ty != simplified_impl_ty
2160 (GenericArgKind::Lifetime(_), GenericArgKind::Lifetime(_)) => {
2161 // Lifetimes can never cause a rejection.
2164 (GenericArgKind::Const(_), GenericArgKind::Const(_)) => {
2165 // Conservatively ignore consts (i.e. assume they might
2166 // unify later) until we have `fast_reject` support for
2167 // them (if we'll ever need it, even).
2170 _ => unreachable!(),
2176 /// Normalize `where_clause_trait_ref` and try to match it against
2177 /// `obligation`. If successful, return any predicates that
2178 /// result from the normalization.
2179 fn match_where_clause_trait_ref(
2181 obligation: &TraitObligation<'tcx>,
2182 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
2183 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
2184 self.match_poly_trait_ref(obligation, where_clause_trait_ref)
2187 /// Returns `Ok` if `poly_trait_ref` being true implies that the
2188 /// obligation is satisfied.
2189 #[instrument(skip(self), level = "debug")]
2190 fn match_poly_trait_ref(
2192 obligation: &TraitObligation<'tcx>,
2193 poly_trait_ref: ty::PolyTraitRef<'tcx>,
2194 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
2196 .at(&obligation.cause, obligation.param_env)
2197 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
2198 .map(|InferOk { obligations, .. }| obligations)
2202 ///////////////////////////////////////////////////////////////////////////
2205 fn match_fresh_trait_refs(
2207 previous: ty::PolyTraitPredicate<'tcx>,
2208 current: ty::PolyTraitPredicate<'tcx>,
2209 param_env: ty::ParamEnv<'tcx>,
2211 let mut matcher = ty::_match::Match::new(self.tcx(), param_env);
2212 matcher.relate(previous, current).is_ok()
2217 previous_stack: TraitObligationStackList<'o, 'tcx>,
2218 obligation: &'o TraitObligation<'tcx>,
2219 ) -> TraitObligationStack<'o, 'tcx> {
2220 let fresh_trait_pred = obligation.predicate.fold_with(&mut self.freshener);
2222 let dfn = previous_stack.cache.next_dfn();
2223 let depth = previous_stack.depth() + 1;
2224 TraitObligationStack {
2227 reached_depth: Cell::new(depth),
2228 previous: previous_stack,
2234 #[instrument(skip(self), level = "debug")]
2235 fn closure_trait_ref_unnormalized(
2237 obligation: &TraitObligation<'tcx>,
2238 substs: SubstsRef<'tcx>,
2239 ) -> ty::PolyTraitRef<'tcx> {
2240 let closure_sig = substs.as_closure().sig();
2242 debug!(?closure_sig);
2244 // (1) Feels icky to skip the binder here, but OTOH we know
2245 // that the self-type is an unboxed closure type and hence is
2246 // in fact unparameterized (or at least does not reference any
2247 // regions bound in the obligation). Still probably some
2248 // refactoring could make this nicer.
2249 closure_trait_ref_and_return_type(
2251 obligation.predicate.def_id(),
2252 obligation.predicate.skip_binder().self_ty(), // (1)
2254 util::TupleArgumentsFlag::No,
2256 .map_bound(|(trait_ref, _)| trait_ref)
2259 fn generator_trait_ref_unnormalized(
2261 obligation: &TraitObligation<'tcx>,
2262 substs: SubstsRef<'tcx>,
2263 ) -> ty::PolyTraitRef<'tcx> {
2264 let gen_sig = substs.as_generator().poly_sig();
2266 // (1) Feels icky to skip the binder here, but OTOH we know
2267 // that the self-type is an generator type and hence is
2268 // in fact unparameterized (or at least does not reference any
2269 // regions bound in the obligation). Still probably some
2270 // refactoring could make this nicer.
2272 super::util::generator_trait_ref_and_outputs(
2274 obligation.predicate.def_id(),
2275 obligation.predicate.skip_binder().self_ty(), // (1)
2278 .map_bound(|(trait_ref, ..)| trait_ref)
2281 /// Returns the obligations that are implied by instantiating an
2282 /// impl or trait. The obligations are substituted and fully
2283 /// normalized. This is used when confirming an impl or default
2285 #[tracing::instrument(level = "debug", skip(self, cause, param_env))]
2286 fn impl_or_trait_obligations(
2288 cause: ObligationCause<'tcx>,
2289 recursion_depth: usize,
2290 param_env: ty::ParamEnv<'tcx>,
2291 def_id: DefId, // of impl or trait
2292 substs: SubstsRef<'tcx>, // for impl or trait
2293 ) -> Vec<PredicateObligation<'tcx>> {
2294 let tcx = self.tcx();
2296 // To allow for one-pass evaluation of the nested obligation,
2297 // each predicate must be preceded by the obligations required
2299 // for example, if we have:
2300 // impl<U: Iterator<Item: Copy>, V: Iterator<Item = U>> Foo for V
2301 // the impl will have the following predicates:
2302 // <V as Iterator>::Item = U,
2303 // U: Iterator, U: Sized,
2304 // V: Iterator, V: Sized,
2305 // <U as Iterator>::Item: Copy
2306 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2307 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2308 // `$1: Copy`, so we must ensure the obligations are emitted in
2310 let predicates = tcx.predicates_of(def_id);
2311 debug!(?predicates);
2312 assert_eq!(predicates.parent, None);
2313 let mut obligations = Vec::with_capacity(predicates.predicates.len());
2314 for (predicate, _) in predicates.predicates {
2316 let predicate = normalize_with_depth_to(
2321 predicate.subst(tcx, substs),
2324 obligations.push(Obligation {
2325 cause: cause.clone(),
2332 // We are performing deduplication here to avoid exponential blowups
2333 // (#38528) from happening, but the real cause of the duplication is
2334 // unknown. What we know is that the deduplication avoids exponential
2335 // amount of predicates being propagated when processing deeply nested
2338 // This code is hot enough that it's worth avoiding the allocation
2339 // required for the FxHashSet when possible. Special-casing lengths 0,
2340 // 1 and 2 covers roughly 75-80% of the cases.
2341 if obligations.len() <= 1 {
2342 // No possibility of duplicates.
2343 } else if obligations.len() == 2 {
2344 // Only two elements. Drop the second if they are equal.
2345 if obligations[0] == obligations[1] {
2346 obligations.truncate(1);
2349 // Three or more elements. Use a general deduplication process.
2350 let mut seen = FxHashSet::default();
2351 obligations.retain(|i| seen.insert(i.clone()));
2358 trait TraitObligationExt<'tcx> {
2361 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
2362 ) -> ObligationCause<'tcx>;
2365 impl<'tcx> TraitObligationExt<'tcx> for TraitObligation<'tcx> {
2368 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
2369 ) -> ObligationCause<'tcx> {
2371 * Creates a cause for obligations that are derived from
2372 * `obligation` by a recursive search (e.g., for a builtin
2373 * bound, or eventually a `auto trait Foo`). If `obligation`
2374 * is itself a derived obligation, this is just a clone, but
2375 * otherwise we create a "derived obligation" cause so as to
2376 * keep track of the original root obligation for error
2380 let obligation = self;
2382 // NOTE(flaper87): As of now, it keeps track of the whole error
2383 // chain. Ideally, we should have a way to configure this either
2384 // by using -Z verbose or just a CLI argument.
2385 let derived_cause = DerivedObligationCause {
2386 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
2387 parent_code: obligation.cause.clone_code(),
2389 let derived_code = variant(derived_cause);
2390 ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
2394 impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
2395 fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2396 TraitObligationStackList::with(self)
2399 fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx> {
2403 fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2407 /// Indicates that attempting to evaluate this stack entry
2408 /// required accessing something from the stack at depth `reached_depth`.
2409 fn update_reached_depth(&self, reached_depth: usize) {
2411 self.depth >= reached_depth,
2412 "invoked `update_reached_depth` with something under this stack: \
2413 self.depth={} reached_depth={}",
2417 debug!(reached_depth, "update_reached_depth");
2419 while reached_depth < p.depth {
2420 debug!(?p.fresh_trait_pred, "update_reached_depth: marking as cycle participant");
2421 p.reached_depth.set(p.reached_depth.get().min(reached_depth));
2422 p = p.previous.head.unwrap();
2427 /// The "provisional evaluation cache" is used to store intermediate cache results
2428 /// when solving auto traits. Auto traits are unusual in that they can support
2429 /// cycles. So, for example, a "proof tree" like this would be ok:
2431 /// - `Foo<T>: Send` :-
2432 /// - `Bar<T>: Send` :-
2433 /// - `Foo<T>: Send` -- cycle, but ok
2434 /// - `Baz<T>: Send`
2436 /// Here, to prove `Foo<T>: Send`, we have to prove `Bar<T>: Send` and
2437 /// `Baz<T>: Send`. Proving `Bar<T>: Send` in turn required `Foo<T>: Send`.
2438 /// For non-auto traits, this cycle would be an error, but for auto traits (because
2439 /// they are coinductive) it is considered ok.
2441 /// However, there is a complication: at the point where we have
2442 /// "proven" `Bar<T>: Send`, we have in fact only proven it
2443 /// *provisionally*. In particular, we proved that `Bar<T>: Send`
2444 /// *under the assumption* that `Foo<T>: Send`. But what if we later
2445 /// find out this assumption is wrong? Specifically, we could
2446 /// encounter some kind of error proving `Baz<T>: Send`. In that case,
2447 /// `Bar<T>: Send` didn't turn out to be true.
2449 /// In Issue #60010, we found a bug in rustc where it would cache
2450 /// these intermediate results. This was fixed in #60444 by disabling
2451 /// *all* caching for things involved in a cycle -- in our example,
2452 /// that would mean we don't cache that `Bar<T>: Send`. But this led
2453 /// to large slowdowns.
2455 /// Specifically, imagine this scenario, where proving `Baz<T>: Send`
2456 /// first requires proving `Bar<T>: Send` (which is true:
2458 /// - `Foo<T>: Send` :-
2459 /// - `Bar<T>: Send` :-
2460 /// - `Foo<T>: Send` -- cycle, but ok
2461 /// - `Baz<T>: Send`
2462 /// - `Bar<T>: Send` -- would be nice for this to be a cache hit!
2463 /// - `*const T: Send` -- but what if we later encounter an error?
2465 /// The *provisional evaluation cache* resolves this issue. It stores
2466 /// cache results that we've proven but which were involved in a cycle
2467 /// in some way. We track the minimal stack depth (i.e., the
2468 /// farthest from the top of the stack) that we are dependent on.
2469 /// The idea is that the cache results within are all valid -- so long as
2470 /// none of the nodes in between the current node and the node at that minimum
2471 /// depth result in an error (in which case the cached results are just thrown away).
2473 /// During evaluation, we consult this provisional cache and rely on
2474 /// it. Accessing a cached value is considered equivalent to accessing
2475 /// a result at `reached_depth`, so it marks the *current* solution as
2476 /// provisional as well. If an error is encountered, we toss out any
2477 /// provisional results added from the subtree that encountered the
2478 /// error. When we pop the node at `reached_depth` from the stack, we
2479 /// can commit all the things that remain in the provisional cache.
2480 struct ProvisionalEvaluationCache<'tcx> {
2481 /// next "depth first number" to issue -- just a counter
2484 /// Map from cache key to the provisionally evaluated thing.
2485 /// The cache entries contain the result but also the DFN in which they
2486 /// were added. The DFN is used to clear out values on failure.
2488 /// Imagine we have a stack like:
2490 /// - `A B C` and we add a cache for the result of C (DFN 2)
2491 /// - Then we have a stack `A B D` where `D` has DFN 3
2492 /// - We try to solve D by evaluating E: `A B D E` (DFN 4)
2493 /// - `E` generates various cache entries which have cyclic dependices on `B`
2494 /// - `A B D E F` and so forth
2495 /// - the DFN of `F` for example would be 5
2496 /// - then we determine that `E` is in error -- we will then clear
2497 /// all cache values whose DFN is >= 4 -- in this case, that
2498 /// means the cached value for `F`.
2499 map: RefCell<FxHashMap<ty::PolyTraitPredicate<'tcx>, ProvisionalEvaluation>>,
2502 /// A cache value for the provisional cache: contains the depth-first
2503 /// number (DFN) and result.
2504 #[derive(Copy, Clone, Debug)]
2505 struct ProvisionalEvaluation {
2507 reached_depth: usize,
2508 result: EvaluationResult,
2511 impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> {
2512 fn default() -> Self {
2513 Self { dfn: Cell::new(0), map: Default::default() }
2517 impl<'tcx> ProvisionalEvaluationCache<'tcx> {
2518 /// Get the next DFN in sequence (basically a counter).
2519 fn next_dfn(&self) -> usize {
2520 let result = self.dfn.get();
2521 self.dfn.set(result + 1);
2525 /// Check the provisional cache for any result for
2526 /// `fresh_trait_ref`. If there is a hit, then you must consider
2527 /// it an access to the stack slots at depth
2528 /// `reached_depth` (from the returned value).
2531 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
2532 ) -> Option<ProvisionalEvaluation> {
2535 "get_provisional = {:#?}",
2536 self.map.borrow().get(&fresh_trait_pred),
2538 Some(*self.map.borrow().get(&fresh_trait_pred)?)
2541 /// Insert a provisional result into the cache. The result came
2542 /// from the node with the given DFN. It accessed a minimum depth
2543 /// of `reached_depth` to compute. It evaluated `fresh_trait_pred`
2544 /// and resulted in `result`.
2545 fn insert_provisional(
2548 reached_depth: usize,
2549 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
2550 result: EvaluationResult,
2552 debug!(?from_dfn, ?fresh_trait_pred, ?result, "insert_provisional");
2554 let mut map = self.map.borrow_mut();
2556 // Subtle: when we complete working on the DFN `from_dfn`, anything
2557 // that remains in the provisional cache must be dependent on some older
2558 // stack entry than `from_dfn`. We have to update their depth with our transitive
2559 // depth in that case or else it would be referring to some popped note.
2562 // A (reached depth 0)
2564 // B // depth 1 -- reached depth = 0
2565 // C // depth 2 -- reached depth = 1 (should be 0)
2568 // D (reached depth 1)
2569 // C (cache -- reached depth = 2)
2570 for (_k, v) in &mut *map {
2571 if v.from_dfn >= from_dfn {
2572 v.reached_depth = reached_depth.min(v.reached_depth);
2576 map.insert(fresh_trait_pred, ProvisionalEvaluation { from_dfn, reached_depth, result });
2579 /// Invoked when the node with dfn `dfn` does not get a successful
2580 /// result. This will clear out any provisional cache entries
2581 /// that were added since `dfn` was created. This is because the
2582 /// provisional entries are things which must assume that the
2583 /// things on the stack at the time of their creation succeeded --
2584 /// since the failing node is presently at the top of the stack,
2585 /// these provisional entries must either depend on it or some
2587 fn on_failure(&self, dfn: usize) {
2588 debug!(?dfn, "on_failure");
2589 self.map.borrow_mut().retain(|key, eval| {
2590 if !eval.from_dfn >= dfn {
2591 debug!("on_failure: removing {:?}", key);
2599 /// Invoked when the node at depth `depth` completed without
2600 /// depending on anything higher in the stack (if that completion
2601 /// was a failure, then `on_failure` should have been invoked
2602 /// already). The callback `op` will be invoked for each
2603 /// provisional entry that we can now confirm.
2605 /// Note that we may still have provisional cache items remaining
2606 /// in the cache when this is done. For example, if there is a
2609 /// * A depends on...
2610 /// * B depends on A
2611 /// * C depends on...
2612 /// * D depends on C
2615 /// Then as we complete the C node we will have a provisional cache
2616 /// with results for A, B, C, and D. This method would clear out
2617 /// the C and D results, but leave A and B provisional.
2619 /// This is determined based on the DFN: we remove any provisional
2620 /// results created since `dfn` started (e.g., in our example, dfn
2621 /// would be 2, representing the C node, and hence we would
2622 /// remove the result for D, which has DFN 3, but not the results for
2623 /// A and B, which have DFNs 0 and 1 respectively).
2627 mut op: impl FnMut(ty::PolyTraitPredicate<'tcx>, EvaluationResult),
2629 debug!(?dfn, "on_completion");
2631 for (fresh_trait_pred, eval) in
2632 self.map.borrow_mut().drain_filter(|_k, eval| eval.from_dfn >= dfn)
2634 debug!(?fresh_trait_pred, ?eval, "on_completion");
2636 op(fresh_trait_pred, eval.result);
2641 #[derive(Copy, Clone)]
2642 struct TraitObligationStackList<'o, 'tcx> {
2643 cache: &'o ProvisionalEvaluationCache<'tcx>,
2644 head: Option<&'o TraitObligationStack<'o, 'tcx>>,
2647 impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> {
2648 fn empty(cache: &'o ProvisionalEvaluationCache<'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2649 TraitObligationStackList { cache, head: None }
2652 fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2653 TraitObligationStackList { cache: r.cache(), head: Some(r) }
2656 fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2660 fn depth(&self) -> usize {
2661 if let Some(head) = self.head { head.depth } else { 0 }
2665 impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> {
2666 type Item = &'o TraitObligationStack<'o, 'tcx>;
2668 fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2675 impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
2676 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2677 write!(f, "TraitObligationStack({:?})", self.obligation)