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::Obligation;
18 use super::ObligationCauseCode;
20 use super::SelectionResult;
21 use super::TraitQueryMode;
22 use super::{Normalized, ProjectionCacheKey};
23 use super::{ObligationCause, PredicateObligation, TraitObligation};
24 use super::{Overflow, SelectionError, Unimplemented};
26 use crate::infer::{InferCtxt, InferOk, TypeFreshener};
27 use crate::traits::error_reporting::InferCtxtExt;
28 use crate::traits::project::ProjectionCacheKeyExt;
29 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
30 use rustc_data_structures::stack::ensure_sufficient_stack;
31 use rustc_errors::ErrorReported;
33 use rustc_hir::def_id::DefId;
34 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
35 use rustc_middle::mir::interpret::ErrorHandled;
36 use rustc_middle::ty::fast_reject;
37 use rustc_middle::ty::print::with_no_trimmed_paths;
38 use rustc_middle::ty::relate::TypeRelation;
39 use rustc_middle::ty::subst::{GenericArgKind, Subst, SubstsRef};
40 use rustc_middle::ty::{self, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate};
41 use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable, WithConstness};
42 use rustc_span::symbol::sym;
44 use std::cell::{Cell, RefCell};
46 use std::fmt::{self, Display};
50 pub use rustc_middle::traits::select::*;
52 mod candidate_assembly;
55 #[derive(Clone, Debug)]
56 pub enum IntercrateAmbiguityCause {
57 DownstreamCrate { trait_desc: String, self_desc: Option<String> },
58 UpstreamCrateUpdate { trait_desc: String, self_desc: Option<String> },
59 ReservationImpl { message: String },
62 impl IntercrateAmbiguityCause {
63 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
64 /// See #23980 for details.
65 pub fn add_intercrate_ambiguity_hint(&self, err: &mut rustc_errors::DiagnosticBuilder<'_>) {
66 err.note(&self.intercrate_ambiguity_hint());
69 pub fn intercrate_ambiguity_hint(&self) -> String {
71 &IntercrateAmbiguityCause::DownstreamCrate { ref trait_desc, ref self_desc } => {
72 let self_desc = if let &Some(ref ty) = self_desc {
73 format!(" for type `{}`", ty)
77 format!("downstream crates may implement trait `{}`{}", trait_desc, self_desc)
79 &IntercrateAmbiguityCause::UpstreamCrateUpdate { ref trait_desc, ref self_desc } => {
80 let self_desc = if let &Some(ref ty) = self_desc {
81 format!(" for type `{}`", ty)
86 "upstream crates may add a new impl of trait `{}`{} \
91 &IntercrateAmbiguityCause::ReservationImpl { ref message } => message.clone(),
96 pub struct SelectionContext<'cx, 'tcx> {
97 infcx: &'cx InferCtxt<'cx, 'tcx>,
99 /// Freshener used specifically for entries on the obligation
100 /// stack. This ensures that all entries on the stack at one time
101 /// will have the same set of placeholder entries, which is
102 /// important for checking for trait bounds that recursively
103 /// require themselves.
104 freshener: TypeFreshener<'cx, 'tcx>,
106 /// If `true`, indicates that the evaluation should be conservative
107 /// and consider the possibility of types outside this crate.
108 /// This comes up primarily when resolving ambiguity. Imagine
109 /// there is some trait reference `$0: Bar` where `$0` is an
110 /// inference variable. If `intercrate` is true, then we can never
111 /// say for sure that this reference is not implemented, even if
112 /// there are *no impls at all for `Bar`*, because `$0` could be
113 /// bound to some type that in a downstream crate that implements
114 /// `Bar`. This is the suitable mode for coherence. Elsewhere,
115 /// though, we set this to false, because we are only interested
116 /// in types that the user could actually have written --- in
117 /// other words, we consider `$0: Bar` to be unimplemented if
118 /// there is no type that the user could *actually name* that
119 /// would satisfy it. This avoids crippling inference, basically.
122 intercrate_ambiguity_causes: Option<Vec<IntercrateAmbiguityCause>>,
124 /// Controls whether or not to filter out negative impls when selecting.
125 /// This is used in librustdoc to distinguish between the lack of an impl
126 /// and a negative impl
127 allow_negative_impls: bool,
129 /// The mode that trait queries run in, which informs our error handling
130 /// policy. In essence, canonicalized queries need their errors propagated
131 /// rather than immediately reported because we do not have accurate spans.
132 query_mode: TraitQueryMode,
135 // A stack that walks back up the stack frame.
136 struct TraitObligationStack<'prev, 'tcx> {
137 obligation: &'prev TraitObligation<'tcx>,
139 /// The trait ref from `obligation` but "freshened" with the
140 /// selection-context's freshener. Used to check for recursion.
141 fresh_trait_ref: ty::PolyTraitRef<'tcx>,
143 /// Starts out equal to `depth` -- if, during evaluation, we
144 /// encounter a cycle, then we will set this flag to the minimum
145 /// depth of that cycle for all participants in the cycle. These
146 /// participants will then forego caching their results. This is
147 /// not the most efficient solution, but it addresses #60010. The
148 /// problem we are trying to prevent:
150 /// - If you have `A: AutoTrait` requires `B: AutoTrait` and `C: NonAutoTrait`
151 /// - `B: AutoTrait` requires `A: AutoTrait` (coinductive cycle, ok)
152 /// - `C: NonAutoTrait` requires `A: AutoTrait` (non-coinductive cycle, not ok)
154 /// you don't want to cache that `B: AutoTrait` or `A: AutoTrait`
155 /// is `EvaluatedToOk`; this is because they were only considered
156 /// ok on the premise that if `A: AutoTrait` held, but we indeed
157 /// encountered a problem (later on) with `A: AutoTrait. So we
158 /// currently set a flag on the stack node for `B: AutoTrait` (as
159 /// well as the second instance of `A: AutoTrait`) to suppress
162 /// This is a simple, targeted fix. A more-performant fix requires
163 /// deeper changes, but would permit more caching: we could
164 /// basically defer caching until we have fully evaluated the
165 /// tree, and then cache the entire tree at once. In any case, the
166 /// performance impact here shouldn't be so horrible: every time
167 /// this is hit, we do cache at least one trait, so we only
168 /// evaluate each member of a cycle up to N times, where N is the
169 /// length of the cycle. This means the performance impact is
170 /// bounded and we shouldn't have any terrible worst-cases.
171 reached_depth: Cell<usize>,
173 previous: TraitObligationStackList<'prev, 'tcx>,
175 /// The number of parent frames plus one (thus, the topmost frame has depth 1).
178 /// The depth-first number of this node in the search graph -- a
179 /// pre-order index. Basically, a freshly incremented counter.
183 struct SelectionCandidateSet<'tcx> {
184 // A list of candidates that definitely apply to the current
185 // obligation (meaning: types unify).
186 vec: Vec<SelectionCandidate<'tcx>>,
188 // If `true`, then there were candidates that might or might
189 // not have applied, but we couldn't tell. This occurs when some
190 // of the input types are type variables, in which case there are
191 // various "builtin" rules that might or might not trigger.
195 #[derive(PartialEq, Eq, Debug, Clone)]
196 struct EvaluatedCandidate<'tcx> {
197 candidate: SelectionCandidate<'tcx>,
198 evaluation: EvaluationResult,
201 /// When does the builtin impl for `T: Trait` apply?
202 enum BuiltinImplConditions<'tcx> {
203 /// The impl is conditional on `T1, T2, ...: Trait`.
204 Where(ty::Binder<Vec<Ty<'tcx>>>),
205 /// There is no built-in impl. There may be some other
206 /// candidate (a where-clause or user-defined impl).
208 /// It is unknown whether there is an impl.
212 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
213 pub fn new(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
216 freshener: infcx.freshener(),
218 intercrate_ambiguity_causes: None,
219 allow_negative_impls: false,
220 query_mode: TraitQueryMode::Standard,
224 pub fn intercrate(infcx: &'cx InferCtxt<'cx, 'tcx>) -> SelectionContext<'cx, 'tcx> {
227 freshener: infcx.freshener(),
229 intercrate_ambiguity_causes: None,
230 allow_negative_impls: false,
231 query_mode: TraitQueryMode::Standard,
235 pub fn with_negative(
236 infcx: &'cx InferCtxt<'cx, 'tcx>,
237 allow_negative_impls: bool,
238 ) -> SelectionContext<'cx, 'tcx> {
239 debug!("with_negative({:?})", allow_negative_impls);
242 freshener: infcx.freshener(),
244 intercrate_ambiguity_causes: None,
245 allow_negative_impls,
246 query_mode: TraitQueryMode::Standard,
250 pub fn with_query_mode(
251 infcx: &'cx InferCtxt<'cx, 'tcx>,
252 query_mode: TraitQueryMode,
253 ) -> SelectionContext<'cx, 'tcx> {
254 debug!("with_query_mode({:?})", query_mode);
257 freshener: infcx.freshener(),
259 intercrate_ambiguity_causes: None,
260 allow_negative_impls: false,
265 /// Enables tracking of intercrate ambiguity causes. These are
266 /// used in coherence to give improved diagnostics. We don't do
267 /// this until we detect a coherence error because it can lead to
268 /// false overflow results (#47139) and because it costs
269 /// computation time.
270 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
271 assert!(self.intercrate);
272 assert!(self.intercrate_ambiguity_causes.is_none());
273 self.intercrate_ambiguity_causes = Some(vec![]);
274 debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
277 /// Gets the intercrate ambiguity causes collected since tracking
278 /// was enabled and disables tracking at the same time. If
279 /// tracking is not enabled, just returns an empty vector.
280 pub fn take_intercrate_ambiguity_causes(&mut self) -> Vec<IntercrateAmbiguityCause> {
281 assert!(self.intercrate);
282 self.intercrate_ambiguity_causes.take().unwrap_or(vec![])
285 pub fn infcx(&self) -> &'cx InferCtxt<'cx, 'tcx> {
289 pub fn tcx(&self) -> TyCtxt<'tcx> {
293 pub fn closure_typer(&self) -> &'cx InferCtxt<'cx, 'tcx> {
297 ///////////////////////////////////////////////////////////////////////////
300 // The selection phase tries to identify *how* an obligation will
301 // be resolved. For example, it will identify which impl or
302 // parameter bound is to be used. The process can be inconclusive
303 // if the self type in the obligation is not fully inferred. Selection
304 // can result in an error in one of two ways:
306 // 1. If no applicable impl or parameter bound can be found.
307 // 2. If the output type parameters in the obligation do not match
308 // those specified by the impl/bound. For example, if the obligation
309 // is `Vec<Foo>: Iterable<Bar>`, but the impl specifies
310 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
312 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
313 /// type environment by performing unification.
316 obligation: &TraitObligation<'tcx>,
317 ) -> SelectionResult<'tcx, Selection<'tcx>> {
318 debug!("select({:?})", obligation);
319 debug_assert!(!obligation.predicate.has_escaping_bound_vars());
321 let pec = &ProvisionalEvaluationCache::default();
322 let stack = self.push_stack(TraitObligationStackList::empty(pec), obligation);
324 let candidate = match self.candidate_from_obligation(&stack) {
325 Err(SelectionError::Overflow) => {
326 // In standard mode, overflow must have been caught and reported
328 assert!(self.query_mode == TraitQueryMode::Canonical);
329 return Err(SelectionError::Overflow);
337 Ok(Some(candidate)) => candidate,
340 match self.confirm_candidate(obligation, candidate) {
341 Err(SelectionError::Overflow) => {
342 assert!(self.query_mode == TraitQueryMode::Canonical);
343 Err(SelectionError::Overflow)
347 debug!("select: candidate = {:?}", candidate);
353 ///////////////////////////////////////////////////////////////////////////
356 // Tests whether an obligation can be selected or whether an impl
357 // can be applied to particular types. It skips the "confirmation"
358 // step and hence completely ignores output type parameters.
360 // The result is "true" if the obligation *may* hold and "false" if
361 // we can be sure it does not.
363 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
364 pub fn predicate_may_hold_fatal(&mut self, obligation: &PredicateObligation<'tcx>) -> bool {
365 debug!("predicate_may_hold_fatal({:?})", obligation);
367 // This fatal query is a stopgap that should only be used in standard mode,
368 // where we do not expect overflow to be propagated.
369 assert!(self.query_mode == TraitQueryMode::Standard);
371 self.evaluate_root_obligation(obligation)
372 .expect("Overflow should be caught earlier in standard query mode")
376 /// Evaluates whether the obligation `obligation` can be satisfied
377 /// and returns an `EvaluationResult`. This is meant for the
379 pub fn evaluate_root_obligation(
381 obligation: &PredicateObligation<'tcx>,
382 ) -> Result<EvaluationResult, OverflowError> {
383 self.evaluation_probe(|this| {
384 this.evaluate_predicate_recursively(
385 TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
393 op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
394 ) -> Result<EvaluationResult, OverflowError> {
395 self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
396 let result = op(self)?;
398 match self.infcx.leak_check(true, snapshot) {
400 Err(_) => return Ok(EvaluatedToErr),
403 match self.infcx.region_constraints_added_in_snapshot(snapshot) {
405 Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)),
410 /// Evaluates the predicates in `predicates` recursively. Note that
411 /// this applies projections in the predicates, and therefore
412 /// is run within an inference probe.
413 fn evaluate_predicates_recursively<'o, I>(
415 stack: TraitObligationStackList<'o, 'tcx>,
417 ) -> Result<EvaluationResult, OverflowError>
419 I: IntoIterator<Item = PredicateObligation<'tcx>> + std::fmt::Debug,
421 let mut result = EvaluatedToOk;
422 debug!("evaluate_predicates_recursively({:?})", predicates);
423 for obligation in predicates {
424 let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
425 debug!("evaluate_predicate_recursively({:?}) = {:?}", obligation, eval);
426 if let EvaluatedToErr = eval {
427 // fast-path - EvaluatedToErr is the top of the lattice,
428 // so we don't need to look on the other predicates.
429 return Ok(EvaluatedToErr);
431 result = cmp::max(result, eval);
437 fn evaluate_predicate_recursively<'o>(
439 previous_stack: TraitObligationStackList<'o, 'tcx>,
440 obligation: PredicateObligation<'tcx>,
441 ) -> Result<EvaluationResult, OverflowError> {
443 "evaluate_predicate_recursively(obligation={:?}, previous_stack={:?})",
445 previous_stack.head()
448 // `previous_stack` stores a `TraitObligation`, while `obligation` is
449 // a `PredicateObligation`. These are distinct types, so we can't
450 // use any `Option` combinator method that would force them to be
452 match previous_stack.head() {
453 Some(h) => self.check_recursion_limit(&obligation, h.obligation)?,
454 None => self.check_recursion_limit(&obligation, &obligation)?,
457 ensure_sufficient_stack(|| {
458 match obligation.predicate.skip_binders() {
459 ty::PredicateAtom::Trait(t, _) => {
460 let t = ty::Binder::bind(t);
461 debug_assert!(!t.has_escaping_bound_vars());
462 let obligation = obligation.with(t);
463 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
466 ty::PredicateAtom::Subtype(p) => {
467 let p = ty::Binder::bind(p);
468 // Does this code ever run?
469 match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
470 Some(Ok(InferOk { mut obligations, .. })) => {
471 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
472 self.evaluate_predicates_recursively(
474 obligations.into_iter(),
477 Some(Err(_)) => Ok(EvaluatedToErr),
478 None => Ok(EvaluatedToAmbig),
482 ty::PredicateAtom::WellFormed(arg) => match wf::obligations(
484 obligation.param_env,
485 obligation.cause.body_id,
486 obligation.recursion_depth + 1,
488 obligation.cause.span,
490 Some(mut obligations) => {
491 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
492 self.evaluate_predicates_recursively(previous_stack, obligations)
494 None => Ok(EvaluatedToAmbig),
497 ty::PredicateAtom::TypeOutlives(..) | ty::PredicateAtom::RegionOutlives(..) => {
498 // We do not consider region relationships when evaluating trait matches.
499 Ok(EvaluatedToOkModuloRegions)
502 ty::PredicateAtom::ObjectSafe(trait_def_id) => {
503 if self.tcx().is_object_safe(trait_def_id) {
510 ty::PredicateAtom::Projection(data) => {
511 let data = ty::Binder::bind(data);
512 let project_obligation = obligation.with(data);
513 match project::poly_project_and_unify_type(self, &project_obligation) {
514 Ok(Ok(Some(mut subobligations))) => {
515 self.add_depth(subobligations.iter_mut(), obligation.recursion_depth);
517 .evaluate_predicates_recursively(previous_stack, subobligations);
519 ProjectionCacheKey::from_poly_projection_predicate(self, data)
521 self.infcx.inner.borrow_mut().projection_cache().complete(key);
525 Ok(Ok(None)) => Ok(EvaluatedToAmbig),
526 // EvaluatedToRecur might also be acceptable here, but use
527 // Unknown for now because it means that we won't dismiss a
528 // selection candidate solely because it has a projection
529 // cycle. This is closest to the previous behavior of
530 // immediately erroring.
531 Ok(Err(project::InProgress)) => Ok(EvaluatedToUnknown),
532 Err(_) => Ok(EvaluatedToErr),
536 ty::PredicateAtom::ClosureKind(_, closure_substs, kind) => {
537 match self.infcx.closure_kind(closure_substs) {
538 Some(closure_kind) => {
539 if closure_kind.extends(kind) {
545 None => Ok(EvaluatedToAmbig),
549 ty::PredicateAtom::ConstEvaluatable(def_id, substs) => {
550 match const_evaluatable::is_const_evaluatable(
554 obligation.param_env,
555 obligation.cause.span,
557 Ok(()) => Ok(EvaluatedToOk),
558 Err(ErrorHandled::TooGeneric) => Ok(EvaluatedToAmbig),
559 Err(_) => Ok(EvaluatedToErr),
563 ty::PredicateAtom::ConstEquate(c1, c2) => {
565 "evaluate_predicate_recursively: equating consts c1={:?} c2={:?}",
569 let evaluate = |c: &'tcx ty::Const<'tcx>| {
570 if let ty::ConstKind::Unevaluated(def, substs, promoted) = c.val {
573 obligation.param_env,
577 Some(obligation.cause.span),
579 .map(|val| ty::Const::from_value(self.tcx(), val, c.ty))
585 match (evaluate(c1), evaluate(c2)) {
586 (Ok(c1), Ok(c2)) => {
589 .at(&obligation.cause, obligation.param_env)
592 Ok(_) => Ok(EvaluatedToOk),
593 Err(_) => Ok(EvaluatedToErr),
596 (Err(ErrorHandled::Reported(ErrorReported)), _)
597 | (_, Err(ErrorHandled::Reported(ErrorReported))) => Ok(EvaluatedToErr),
598 (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
600 obligation.cause.span(self.tcx()),
601 "ConstEquate: const_eval_resolve returned an unexpected error"
604 (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
609 ty::PredicateAtom::TypeWellFormedFromEnv(..) => {
610 bug!("TypeWellFormedFromEnv is only used for chalk")
616 fn evaluate_trait_predicate_recursively<'o>(
618 previous_stack: TraitObligationStackList<'o, 'tcx>,
619 mut obligation: TraitObligation<'tcx>,
620 ) -> Result<EvaluationResult, OverflowError> {
621 debug!("evaluate_trait_predicate_recursively({:?})", obligation);
624 && obligation.is_global()
625 && obligation.param_env.caller_bounds().iter().all(|bound| bound.needs_subst())
627 // If a param env has no global bounds, global obligations do not
628 // depend on its particular value in order to work, so we can clear
629 // out the param env and get better caching.
630 debug!("evaluate_trait_predicate_recursively({:?}) - in global", obligation);
631 obligation.param_env = obligation.param_env.without_caller_bounds();
634 let stack = self.push_stack(previous_stack, &obligation);
635 let fresh_trait_ref = stack.fresh_trait_ref;
636 if let Some(result) = self.check_evaluation_cache(obligation.param_env, fresh_trait_ref) {
637 debug!("CACHE HIT: EVAL({:?})={:?}", fresh_trait_ref, result);
641 if let Some(result) = stack.cache().get_provisional(fresh_trait_ref) {
642 debug!("PROVISIONAL CACHE HIT: EVAL({:?})={:?}", fresh_trait_ref, result);
643 stack.update_reached_depth(stack.cache().current_reached_depth());
647 // Check if this is a match for something already on the
648 // stack. If so, we don't want to insert the result into the
649 // main cache (it is cycle dependent) nor the provisional
650 // cache (which is meant for things that have completed but
651 // for a "backedge" -- this result *is* the backedge).
652 if let Some(cycle_result) = self.check_evaluation_cycle(&stack) {
653 return Ok(cycle_result);
656 let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
657 let result = result?;
659 if !result.must_apply_modulo_regions() {
660 stack.cache().on_failure(stack.dfn);
663 let reached_depth = stack.reached_depth.get();
664 if reached_depth >= stack.depth {
665 debug!("CACHE MISS: EVAL({:?})={:?}", fresh_trait_ref, result);
666 self.insert_evaluation_cache(obligation.param_env, fresh_trait_ref, dep_node, result);
668 stack.cache().on_completion(stack.depth, |fresh_trait_ref, provisional_result| {
669 self.insert_evaluation_cache(
670 obligation.param_env,
673 provisional_result.max(result),
677 debug!("PROVISIONAL: {:?}={:?}", fresh_trait_ref, result);
679 "evaluate_trait_predicate_recursively: caching provisionally because {:?} \
680 is a cycle participant (at depth {}, reached depth {})",
681 fresh_trait_ref, stack.depth, reached_depth,
684 stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_ref, result);
690 /// If there is any previous entry on the stack that precisely
691 /// matches this obligation, then we can assume that the
692 /// obligation is satisfied for now (still all other conditions
693 /// must be met of course). One obvious case this comes up is
694 /// marker traits like `Send`. Think of a linked list:
696 /// struct List<T> { data: T, next: Option<Box<List<T>>> }
698 /// `Box<List<T>>` will be `Send` if `T` is `Send` and
699 /// `Option<Box<List<T>>>` is `Send`, and in turn
700 /// `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
703 /// Note that we do this comparison using the `fresh_trait_ref`
704 /// fields. Because these have all been freshened using
705 /// `self.freshener`, we can be sure that (a) this will not
706 /// affect the inferencer state and (b) that if we see two
707 /// fresh regions with the same index, they refer to the same
708 /// unbound type variable.
709 fn check_evaluation_cycle(
711 stack: &TraitObligationStack<'_, 'tcx>,
712 ) -> Option<EvaluationResult> {
713 if let Some(cycle_depth) = stack
715 .skip(1) // Skip top-most frame.
717 stack.obligation.param_env == prev.obligation.param_env
718 && stack.fresh_trait_ref == prev.fresh_trait_ref
720 .map(|stack| stack.depth)
723 "evaluate_stack({:?}) --> recursive at depth {}",
724 stack.fresh_trait_ref, cycle_depth,
727 // If we have a stack like `A B C D E A`, where the top of
728 // the stack is the final `A`, then this will iterate over
729 // `A, E, D, C, B` -- i.e., all the participants apart
730 // from the cycle head. We mark them as participating in a
731 // cycle. This suppresses caching for those nodes. See
732 // `in_cycle` field for more details.
733 stack.update_reached_depth(cycle_depth);
735 // Subtle: when checking for a coinductive cycle, we do
736 // not compare using the "freshened trait refs" (which
737 // have erased regions) but rather the fully explicit
738 // trait refs. This is important because it's only a cycle
739 // if the regions match exactly.
740 let cycle = stack.iter().skip(1).take_while(|s| s.depth >= cycle_depth);
741 let tcx = self.tcx();
743 cycle.map(|stack| stack.obligation.predicate.without_const().to_predicate(tcx));
744 if self.coinductive_match(cycle) {
745 debug!("evaluate_stack({:?}) --> recursive, coinductive", stack.fresh_trait_ref);
748 debug!("evaluate_stack({:?}) --> recursive, inductive", stack.fresh_trait_ref);
749 Some(EvaluatedToRecur)
756 fn evaluate_stack<'o>(
758 stack: &TraitObligationStack<'o, 'tcx>,
759 ) -> Result<EvaluationResult, OverflowError> {
760 // In intercrate mode, whenever any of the generics are unbound,
761 // there can always be an impl. Even if there are no impls in
762 // this crate, perhaps the type would be unified with
763 // something from another crate that does provide an impl.
765 // In intra mode, we must still be conservative. The reason is
766 // that we want to avoid cycles. Imagine an impl like:
768 // impl<T:Eq> Eq for Vec<T>
770 // and a trait reference like `$0 : Eq` where `$0` is an
771 // unbound variable. When we evaluate this trait-reference, we
772 // will unify `$0` with `Vec<$1>` (for some fresh variable
773 // `$1`), on the condition that `$1 : Eq`. We will then wind
774 // up with many candidates (since that are other `Eq` impls
775 // that apply) and try to winnow things down. This results in
776 // a recursive evaluation that `$1 : Eq` -- as you can
777 // imagine, this is just where we started. To avoid that, we
778 // check for unbound variables and return an ambiguous (hence possible)
779 // match if we've seen this trait before.
781 // This suffices to allow chains like `FnMut` implemented in
782 // terms of `Fn` etc, but we could probably make this more
784 let unbound_input_types =
785 stack.fresh_trait_ref.skip_binder().substs.types().any(|ty| ty.is_fresh());
786 // This check was an imperfect workaround for a bug in the old
787 // intercrate mode; it should be removed when that goes away.
788 if unbound_input_types && self.intercrate {
790 "evaluate_stack({:?}) --> unbound argument, intercrate --> ambiguous",
791 stack.fresh_trait_ref
793 // Heuristics: show the diagnostics when there are no candidates in crate.
794 if self.intercrate_ambiguity_causes.is_some() {
795 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
796 if let Ok(candidate_set) = self.assemble_candidates(stack) {
797 if !candidate_set.ambiguous && candidate_set.vec.is_empty() {
798 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
799 let self_ty = trait_ref.self_ty();
801 with_no_trimmed_paths(|| IntercrateAmbiguityCause::DownstreamCrate {
802 trait_desc: trait_ref.print_only_trait_path().to_string(),
803 self_desc: if self_ty.has_concrete_skeleton() {
804 Some(self_ty.to_string())
810 debug!("evaluate_stack: pushing cause = {:?}", cause);
811 self.intercrate_ambiguity_causes.as_mut().unwrap().push(cause);
815 return Ok(EvaluatedToAmbig);
817 if unbound_input_types
818 && stack.iter().skip(1).any(|prev| {
819 stack.obligation.param_env == prev.obligation.param_env
820 && self.match_fresh_trait_refs(
821 stack.fresh_trait_ref,
822 prev.fresh_trait_ref,
823 prev.obligation.param_env,
828 "evaluate_stack({:?}) --> unbound argument, recursive --> giving up",
829 stack.fresh_trait_ref
831 return Ok(EvaluatedToUnknown);
834 match self.candidate_from_obligation(stack) {
835 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
836 Ok(None) => Ok(EvaluatedToAmbig),
837 Err(Overflow) => Err(OverflowError),
838 Err(..) => Ok(EvaluatedToErr),
842 /// For defaulted traits, we use a co-inductive strategy to solve, so
843 /// that recursion is ok. This routine returns `true` if the top of the
844 /// stack (`cycle[0]`):
846 /// - is a defaulted trait,
847 /// - it also appears in the backtrace at some position `X`,
848 /// - all the predicates at positions `X..` between `X` and the top are
849 /// also defaulted traits.
850 pub fn coinductive_match<I>(&mut self, cycle: I) -> bool
852 I: Iterator<Item = ty::Predicate<'tcx>>,
854 let mut cycle = cycle;
855 cycle.all(|predicate| self.coinductive_predicate(predicate))
858 fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
859 let result = match predicate.skip_binders() {
860 ty::PredicateAtom::Trait(ref data, _) => self.tcx().trait_is_auto(data.def_id()),
863 debug!("coinductive_predicate({:?}) = {:?}", predicate, result);
867 /// Further evaluates `candidate` to decide whether all type parameters match and whether nested
868 /// obligations are met. Returns whether `candidate` remains viable after this further
870 fn evaluate_candidate<'o>(
872 stack: &TraitObligationStack<'o, 'tcx>,
873 candidate: &SelectionCandidate<'tcx>,
874 ) -> Result<EvaluationResult, OverflowError> {
876 "evaluate_candidate: depth={} candidate={:?}",
877 stack.obligation.recursion_depth, candidate
879 let result = self.evaluation_probe(|this| {
880 let candidate = (*candidate).clone();
881 match this.confirm_candidate(stack.obligation, candidate) {
883 debug!("evaluate_candidate: selection = {:?}", selection);
884 this.evaluate_predicates_recursively(
886 selection.nested_obligations().into_iter(),
889 Err(..) => Ok(EvaluatedToErr),
893 "evaluate_candidate: depth={} result={:?}",
894 stack.obligation.recursion_depth, result
899 fn check_evaluation_cache(
901 param_env: ty::ParamEnv<'tcx>,
902 trait_ref: ty::PolyTraitRef<'tcx>,
903 ) -> Option<EvaluationResult> {
904 let tcx = self.tcx();
905 if self.can_use_global_caches(param_env) {
906 if let Some(res) = tcx.evaluation_cache.get(¶m_env.and(trait_ref), tcx) {
910 self.infcx.evaluation_cache.get(¶m_env.and(trait_ref), tcx)
913 fn insert_evaluation_cache(
915 param_env: ty::ParamEnv<'tcx>,
916 trait_ref: ty::PolyTraitRef<'tcx>,
917 dep_node: DepNodeIndex,
918 result: EvaluationResult,
920 // Avoid caching results that depend on more than just the trait-ref
921 // - the stack can create recursion.
922 if result.is_stack_dependent() {
926 if self.can_use_global_caches(param_env) {
927 if !trait_ref.needs_infer() {
929 "insert_evaluation_cache(trait_ref={:?}, candidate={:?}) global",
932 // This may overwrite the cache with the same value
933 // FIXME: Due to #50507 this overwrites the different values
934 // This should be changed to use HashMapExt::insert_same
935 // when that is fixed
936 self.tcx().evaluation_cache.insert(param_env.and(trait_ref), dep_node, result);
941 debug!("insert_evaluation_cache(trait_ref={:?}, candidate={:?})", trait_ref, result,);
942 self.infcx.evaluation_cache.insert(param_env.and(trait_ref), dep_node, result);
945 /// For various reasons, it's possible for a subobligation
946 /// to have a *lower* recursion_depth than the obligation used to create it.
947 /// Projection sub-obligations may be returned from the projection cache,
948 /// which results in obligations with an 'old' `recursion_depth`.
949 /// Additionally, methods like `InferCtxt.subtype_predicate` produce
950 /// subobligations without taking in a 'parent' depth, causing the
951 /// generated subobligations to have a `recursion_depth` of `0`.
953 /// To ensure that obligation_depth never decreasees, we force all subobligations
954 /// to have at least the depth of the original obligation.
955 fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>(
960 it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
963 /// Checks that the recursion limit has not been exceeded.
965 /// The weird return type of this function allows it to be used with the `try` (`?`)
966 /// operator within certain functions.
967 fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V: Display + TypeFoldable<'tcx>>(
969 obligation: &Obligation<'tcx, T>,
970 error_obligation: &Obligation<'tcx, V>,
971 ) -> Result<(), OverflowError> {
972 if !self.infcx.tcx.sess.recursion_limit().value_within_limit(obligation.recursion_depth) {
973 match self.query_mode {
974 TraitQueryMode::Standard => {
975 self.infcx().report_overflow_error(error_obligation, true);
977 TraitQueryMode::Canonical => {
978 return Err(OverflowError);
985 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
987 OP: FnOnce(&mut Self) -> R,
989 let (result, dep_node) =
990 self.tcx().dep_graph.with_anon_task(DepKind::TraitSelect, || op(self));
991 self.tcx().dep_graph.read_index(dep_node);
995 // Treat negative impls as unimplemented, and reservation impls as ambiguity.
996 fn filter_negative_and_reservation_impls(
998 candidate: SelectionCandidate<'tcx>,
999 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1000 if let ImplCandidate(def_id) = candidate {
1001 let tcx = self.tcx();
1002 match tcx.impl_polarity(def_id) {
1003 ty::ImplPolarity::Negative if !self.allow_negative_impls => {
1004 return Err(Unimplemented);
1006 ty::ImplPolarity::Reservation => {
1007 if let Some(intercrate_ambiguity_clauses) =
1008 &mut self.intercrate_ambiguity_causes
1010 let attrs = tcx.get_attrs(def_id);
1011 let attr = tcx.sess.find_by_name(&attrs, sym::rustc_reservation_impl);
1012 let value = attr.and_then(|a| a.value_str());
1013 if let Some(value) = value {
1015 "filter_negative_and_reservation_impls: \
1016 reservation impl ambiguity on {:?}",
1019 intercrate_ambiguity_clauses.push(
1020 IntercrateAmbiguityCause::ReservationImpl {
1021 message: value.to_string(),
1034 fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Option<Conflict> {
1035 debug!("is_knowable(intercrate={:?})", self.intercrate);
1037 if !self.intercrate {
1041 let obligation = &stack.obligation;
1042 let predicate = self.infcx().resolve_vars_if_possible(&obligation.predicate);
1044 // Okay to skip binder because of the nature of the
1045 // trait-ref-is-knowable check, which does not care about
1047 let trait_ref = predicate.skip_binder().trait_ref;
1049 coherence::trait_ref_is_knowable(self.tcx(), trait_ref)
1052 /// Returns `true` if the global caches can be used.
1053 /// Do note that if the type itself is not in the
1054 /// global tcx, the local caches will be used.
1055 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1056 // If there are any inference variables in the `ParamEnv`, then we
1057 // always use a cache local to this particular scope. Otherwise, we
1058 // switch to a global cache.
1059 if param_env.needs_infer() {
1063 // Avoid using the master cache during coherence and just rely
1064 // on the local cache. This effectively disables caching
1065 // during coherence. It is really just a simplification to
1066 // avoid us having to fear that coherence results "pollute"
1067 // the master cache. Since coherence executes pretty quickly,
1068 // it's not worth going to more trouble to increase the
1069 // hit-rate, I don't think.
1070 if self.intercrate {
1074 // Otherwise, we can use the global cache.
1078 fn check_candidate_cache(
1080 param_env: ty::ParamEnv<'tcx>,
1081 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1082 ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
1083 let tcx = self.tcx();
1084 let trait_ref = &cache_fresh_trait_pred.skip_binder().trait_ref;
1085 if self.can_use_global_caches(param_env) {
1086 if let Some(res) = tcx.selection_cache.get(¶m_env.and(*trait_ref), tcx) {
1090 self.infcx.selection_cache.get(¶m_env.and(*trait_ref), tcx)
1093 /// Determines whether can we safely cache the result
1094 /// of selecting an obligation. This is almost always `true`,
1095 /// except when dealing with certain `ParamCandidate`s.
1097 /// Ordinarily, a `ParamCandidate` will contain no inference variables,
1098 /// since it was usually produced directly from a `DefId`. However,
1099 /// certain cases (currently only librustdoc's blanket impl finder),
1100 /// a `ParamEnv` may be explicitly constructed with inference types.
1101 /// When this is the case, we do *not* want to cache the resulting selection
1102 /// candidate. This is due to the fact that it might not always be possible
1103 /// to equate the obligation's trait ref and the candidate's trait ref,
1104 /// if more constraints end up getting added to an inference variable.
1106 /// Because of this, we always want to re-run the full selection
1107 /// process for our obligation the next time we see it, since
1108 /// we might end up picking a different `SelectionCandidate` (or none at all).
1109 fn can_cache_candidate(
1111 result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1114 Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => !trait_ref.needs_infer(),
1119 fn insert_candidate_cache(
1121 param_env: ty::ParamEnv<'tcx>,
1122 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1123 dep_node: DepNodeIndex,
1124 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1126 let tcx = self.tcx();
1127 let trait_ref = cache_fresh_trait_pred.skip_binder().trait_ref;
1129 if !self.can_cache_candidate(&candidate) {
1131 "insert_candidate_cache(trait_ref={:?}, candidate={:?} -\
1132 candidate is not cacheable",
1133 trait_ref, candidate
1138 if self.can_use_global_caches(param_env) {
1139 if let Err(Overflow) = candidate {
1140 // Don't cache overflow globally; we only produce this in certain modes.
1141 } else if !trait_ref.needs_infer() {
1142 if !candidate.needs_infer() {
1144 "insert_candidate_cache(trait_ref={:?}, candidate={:?}) global",
1145 trait_ref, candidate,
1147 // This may overwrite the cache with the same value.
1148 tcx.selection_cache.insert(param_env.and(trait_ref), dep_node, candidate);
1155 "insert_candidate_cache(trait_ref={:?}, candidate={:?}) local",
1156 trait_ref, candidate,
1158 self.infcx.selection_cache.insert(param_env.and(trait_ref), dep_node, candidate);
1161 /// Matches a predicate against the bounds of its self type.
1163 /// Given an obligation like `<T as Foo>::Bar: Baz` where the self type is
1164 /// a projection, look at the bounds of `T::Bar`, see if we can find a
1165 /// `Baz` bound. We return indexes into the list returned by
1166 /// `tcx.item_bounds` for any applicable bounds.
1167 fn match_projection_obligation_against_definition_bounds(
1169 obligation: &TraitObligation<'tcx>,
1170 ) -> smallvec::SmallVec<[usize; 2]> {
1171 let poly_trait_predicate = self.infcx().resolve_vars_if_possible(&obligation.predicate);
1172 let placeholder_trait_predicate =
1173 self.infcx().replace_bound_vars_with_placeholders(&poly_trait_predicate);
1175 "match_projection_obligation_against_definition_bounds: \
1176 placeholder_trait_predicate={:?}",
1177 placeholder_trait_predicate,
1180 let tcx = self.infcx.tcx;
1181 let (def_id, substs) = match *placeholder_trait_predicate.trait_ref.self_ty().kind() {
1182 ty::Projection(ref data) => (data.item_def_id, data.substs),
1183 ty::Opaque(def_id, substs) => (def_id, substs),
1186 obligation.cause.span,
1187 "match_projection_obligation_against_definition_bounds() called \
1188 but self-ty is not a projection: {:?}",
1189 placeholder_trait_predicate.trait_ref.self_ty()
1193 let bounds = tcx.item_bounds(def_id).subst(tcx, substs);
1195 // The bounds returned by `item_bounds` may contain duplicates after
1196 // normalization, so try to deduplicate when possible to avoid
1197 // unnecessary ambiguity.
1198 let mut distinct_normalized_bounds = FxHashSet::default();
1200 let matching_bounds = bounds
1203 .filter_map(|(idx, bound)| {
1204 if let ty::PredicateAtom::Trait(pred, _) = bound.skip_binders() {
1205 let bound = ty::Binder::bind(pred.trait_ref);
1206 if self.infcx.probe(|_| {
1207 match self.match_projection(
1210 placeholder_trait_predicate.trait_ref,
1213 Ok(Some(normalized_trait))
1214 if distinct_normalized_bounds.insert(normalized_trait) =>
1229 "match_projection_obligation_against_definition_bounds: \
1230 matching_bounds={:?}",
1236 /// Equates the trait in `obligation` with trait bound. If the two traits
1237 /// can be equated and the normalized trait bound doesn't contain inference
1238 /// variables or placeholders, the normalized bound is returned.
1239 fn match_projection(
1241 obligation: &TraitObligation<'tcx>,
1242 trait_bound: ty::PolyTraitRef<'tcx>,
1243 placeholder_trait_ref: ty::TraitRef<'tcx>,
1244 ) -> Result<Option<ty::PolyTraitRef<'tcx>>, ()> {
1245 debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
1246 if placeholder_trait_ref.def_id != trait_bound.def_id() {
1247 // Avoid unnecessary normalization
1251 let Normalized { value: trait_bound, obligations: _ } = ensure_sufficient_stack(|| {
1252 project::normalize_with_depth(
1254 obligation.param_env,
1255 obligation.cause.clone(),
1256 obligation.recursion_depth + 1,
1261 .at(&obligation.cause, obligation.param_env)
1262 .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
1263 .map(|InferOk { obligations: _, value: () }| {
1264 // This method is called within a probe, so we can't have
1265 // inference variables and placeholders escape.
1266 if !trait_bound.needs_infer() && !trait_bound.has_placeholders() {
1275 fn evaluate_where_clause<'o>(
1277 stack: &TraitObligationStack<'o, 'tcx>,
1278 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
1279 ) -> Result<EvaluationResult, OverflowError> {
1280 self.evaluation_probe(|this| {
1281 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1282 Ok(obligations) => this.evaluate_predicates_recursively(stack.list(), obligations),
1283 Err(()) => Ok(EvaluatedToErr),
1288 pub(super) fn match_projection_projections(
1290 obligation: &ProjectionTyObligation<'tcx>,
1291 obligation_trait_ref: &ty::TraitRef<'tcx>,
1292 data: &PolyProjectionPredicate<'tcx>,
1293 potentially_unnormalized_candidates: bool,
1295 let mut nested_obligations = Vec::new();
1296 let projection_ty = if potentially_unnormalized_candidates {
1297 ensure_sufficient_stack(|| {
1298 project::normalize_with_depth_to(
1300 obligation.param_env,
1301 obligation.cause.clone(),
1302 obligation.recursion_depth + 1,
1303 &data.map_bound_ref(|data| data.projection_ty),
1304 &mut nested_obligations,
1308 data.map_bound_ref(|data| data.projection_ty)
1311 // FIXME(generic_associated_types): Compare the whole projections
1312 let data_poly_trait_ref = projection_ty.map_bound(|proj| proj.trait_ref(self.tcx()));
1313 let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1315 .at(&obligation.cause, obligation.param_env)
1316 .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1317 .map_or(false, |InferOk { obligations, value: () }| {
1318 self.evaluate_predicates_recursively(
1319 TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
1320 nested_obligations.into_iter().chain(obligations),
1322 .map_or(false, |res| res.may_apply())
1326 ///////////////////////////////////////////////////////////////////////////
1329 // Winnowing is the process of attempting to resolve ambiguity by
1330 // probing further. During the winnowing process, we unify all
1331 // type variables and then we also attempt to evaluate recursive
1332 // bounds to see if they are satisfied.
1334 /// Returns `true` if `victim` should be dropped in favor of
1335 /// `other`. Generally speaking we will drop duplicate
1336 /// candidates and prefer where-clause candidates.
1338 /// See the comment for "SelectionCandidate" for more details.
1339 fn candidate_should_be_dropped_in_favor_of(
1341 victim: &EvaluatedCandidate<'tcx>,
1342 other: &EvaluatedCandidate<'tcx>,
1345 if victim.candidate == other.candidate {
1349 // Check if a bound would previously have been removed when normalizing
1350 // the param_env so that it can be given the lowest priority. See
1351 // #50825 for the motivation for this.
1353 |cand: &ty::PolyTraitRef<'_>| cand.is_global() && !cand.has_late_bound_regions();
1355 // (*) Prefer `BuiltinCandidate { has_nested: false }` and `DiscriminantKindCandidate`
1356 // to anything else.
1358 // This is a fix for #53123 and prevents winnowing from accidentally extending the
1359 // lifetime of a variable.
1360 match (&other.candidate, &victim.candidate) {
1361 (_, AutoImplCandidate(..)) | (AutoImplCandidate(..), _) => {
1363 "default implementations shouldn't be recorded \
1364 when there are other valid candidates"
1369 (BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate, _) => true,
1370 (_, BuiltinCandidate { has_nested: false } | DiscriminantKindCandidate) => false,
1372 (ParamCandidate(..), ParamCandidate(..)) => false,
1374 // Global bounds from the where clause should be ignored
1375 // here (see issue #50825). Otherwise, we have a where
1376 // clause so don't go around looking for impls.
1377 // Arbitrarily give param candidates priority
1378 // over projection and object candidates.
1380 ParamCandidate(ref cand),
1383 | GeneratorCandidate
1384 | FnPointerCandidate
1385 | BuiltinObjectCandidate
1386 | BuiltinUnsizeCandidate
1387 | BuiltinCandidate { .. }
1388 | TraitAliasCandidate(..)
1390 | ProjectionCandidate(_),
1391 ) => !is_global(cand),
1392 (ObjectCandidate | ProjectionCandidate(_), ParamCandidate(ref cand)) => {
1393 // Prefer these to a global where-clause bound
1394 // (see issue #50825).
1400 | GeneratorCandidate
1401 | FnPointerCandidate
1402 | BuiltinObjectCandidate
1403 | BuiltinUnsizeCandidate
1404 | BuiltinCandidate { has_nested: true }
1405 | TraitAliasCandidate(..),
1406 ParamCandidate(ref cand),
1408 // Prefer these to a global where-clause bound
1409 // (see issue #50825).
1410 is_global(cand) && other.evaluation.must_apply_modulo_regions()
1413 (ProjectionCandidate(i), ProjectionCandidate(j)) => {
1414 // Arbitrarily pick the first candidate for backwards
1415 // compatibility reasons. Don't let this affect inference.
1416 i > j && !needs_infer
1418 (ObjectCandidate, ObjectCandidate) => bug!("Duplicate object candidate"),
1419 (ObjectCandidate, ProjectionCandidate(_))
1420 | (ProjectionCandidate(_), ObjectCandidate) => {
1421 bug!("Have both object and projection candidate")
1424 // Arbitrarily give projection and object candidates priority.
1426 ObjectCandidate | ProjectionCandidate(_),
1429 | GeneratorCandidate
1430 | FnPointerCandidate
1431 | BuiltinObjectCandidate
1432 | BuiltinUnsizeCandidate
1433 | BuiltinCandidate { .. }
1434 | TraitAliasCandidate(..),
1440 | GeneratorCandidate
1441 | FnPointerCandidate
1442 | BuiltinObjectCandidate
1443 | BuiltinUnsizeCandidate
1444 | BuiltinCandidate { .. }
1445 | TraitAliasCandidate(..),
1446 ObjectCandidate | ProjectionCandidate(_),
1449 (&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
1450 // See if we can toss out `victim` based on specialization.
1451 // This requires us to know *for sure* that the `other` impl applies
1452 // i.e., `EvaluatedToOk`.
1453 if other.evaluation.must_apply_modulo_regions() {
1454 let tcx = self.tcx();
1455 if tcx.specializes((other_def, victim_def)) {
1458 return match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
1459 Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
1460 // Subtle: If the predicate we are evaluating has inference
1461 // variables, do *not* allow discarding candidates due to
1462 // marker trait impls.
1464 // Without this restriction, we could end up accidentally
1465 // constrainting inference variables based on an arbitrarily
1466 // chosen trait impl.
1468 // Imagine we have the following code:
1471 // #[marker] trait MyTrait {}
1472 // impl MyTrait for u8 {}
1473 // impl MyTrait for bool {}
1476 // And we are evaluating the predicate `<_#0t as MyTrait>`.
1478 // During selection, we will end up with one candidate for each
1479 // impl of `MyTrait`. If we were to discard one impl in favor
1480 // of the other, we would be left with one candidate, causing
1481 // us to "successfully" select the predicate, unifying
1482 // _#0t with (for example) `u8`.
1484 // However, we have no reason to believe that this unification
1485 // is correct - we've essentially just picked an arbitrary
1486 // *possibility* for _#0t, and required that this be the *only*
1489 // Eventually, we will either:
1490 // 1) Unify all inference variables in the predicate through
1491 // some other means (e.g. type-checking of a function). We will
1492 // then be in a position to drop marker trait candidates
1493 // without constraining inference variables (since there are
1494 // none left to constrin)
1495 // 2) Be left with some unconstrained inference variables. We
1496 // will then correctly report an inference error, since the
1497 // existence of multiple marker trait impls tells us nothing
1498 // about which one should actually apply.
1509 // Everything else is ambiguous
1513 | GeneratorCandidate
1514 | FnPointerCandidate
1515 | BuiltinObjectCandidate
1516 | BuiltinUnsizeCandidate
1517 | BuiltinCandidate { has_nested: true }
1518 | TraitAliasCandidate(..),
1521 | GeneratorCandidate
1522 | FnPointerCandidate
1523 | BuiltinObjectCandidate
1524 | BuiltinUnsizeCandidate
1525 | BuiltinCandidate { has_nested: true }
1526 | TraitAliasCandidate(..),
1531 fn sized_conditions(
1533 obligation: &TraitObligation<'tcx>,
1534 ) -> BuiltinImplConditions<'tcx> {
1535 use self::BuiltinImplConditions::{Ambiguous, None, Where};
1537 // NOTE: binder moved to (*)
1538 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
1540 match self_ty.kind() {
1541 ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
1552 | ty::GeneratorWitness(..)
1557 // safe for everything
1558 Where(ty::Binder::dummy(Vec::new()))
1561 ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
1564 Where(ty::Binder::bind(tys.last().into_iter().map(|k| k.expect_ty()).collect()))
1567 ty::Adt(def, substs) => {
1568 let sized_crit = def.sized_constraint(self.tcx());
1569 // (*) binder moved here
1570 Where(ty::Binder::bind(
1571 sized_crit.iter().map(|ty| ty.subst(self.tcx(), substs)).collect(),
1575 ty::Projection(_) | ty::Param(_) | ty::Opaque(..) => None,
1576 ty::Infer(ty::TyVar(_)) => Ambiguous,
1580 | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
1581 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
1586 fn copy_clone_conditions(
1588 obligation: &TraitObligation<'tcx>,
1589 ) -> BuiltinImplConditions<'tcx> {
1590 // NOTE: binder moved to (*)
1591 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
1593 use self::BuiltinImplConditions::{Ambiguous, None, Where};
1595 match self_ty.kind() {
1596 ty::Infer(ty::IntVar(_))
1597 | ty::Infer(ty::FloatVar(_))
1600 | ty::Error(_) => Where(ty::Binder::dummy(Vec::new())),
1609 | ty::Ref(_, _, hir::Mutability::Not) => {
1610 // Implementations provided in libcore
1618 | ty::GeneratorWitness(..)
1620 | ty::Ref(_, _, hir::Mutability::Mut) => None,
1622 ty::Array(element_ty, _) => {
1623 // (*) binder moved here
1624 Where(ty::Binder::bind(vec![element_ty]))
1628 // (*) binder moved here
1629 Where(ty::Binder::bind(tys.iter().map(|k| k.expect_ty()).collect()))
1632 ty::Closure(_, substs) => {
1633 // (*) binder moved here
1634 Where(ty::Binder::bind(substs.as_closure().upvar_tys().collect()))
1637 ty::Adt(..) | ty::Projection(..) | ty::Param(..) | ty::Opaque(..) => {
1638 // Fallback to whatever user-defined impls exist in this case.
1642 ty::Infer(ty::TyVar(_)) => {
1643 // Unbound type variable. Might or might not have
1644 // applicable impls and so forth, depending on what
1645 // those type variables wind up being bound to.
1651 | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
1652 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
1657 /// For default impls, we need to break apart a type into its
1658 /// "constituent types" -- meaning, the types that it contains.
1660 /// Here are some (simple) examples:
1663 /// (i32, u32) -> [i32, u32]
1664 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
1665 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
1666 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
1668 fn constituent_types_for_ty(&self, t: Ty<'tcx>) -> Vec<Ty<'tcx>> {
1678 | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
1680 | ty::Char => Vec::new(),
1686 | ty::Projection(..)
1688 | ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
1689 bug!("asked to assemble constituent types of unexpected type: {:?}", t);
1692 ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => {
1696 ty::Array(element_ty, _) | ty::Slice(element_ty) => vec![element_ty],
1698 ty::Tuple(ref tys) => {
1699 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
1700 tys.iter().map(|k| k.expect_ty()).collect()
1703 ty::Closure(_, ref substs) => substs.as_closure().upvar_tys().collect(),
1705 ty::Generator(_, ref substs, _) => {
1706 let witness = substs.as_generator().witness();
1707 substs.as_generator().upvar_tys().chain(iter::once(witness)).collect()
1710 ty::GeneratorWitness(types) => {
1711 // This is sound because no regions in the witness can refer to
1712 // the binder outside the witness. So we'll effectivly reuse
1713 // the implicit binder around the witness.
1714 types.skip_binder().to_vec()
1717 // For `PhantomData<T>`, we pass `T`.
1718 ty::Adt(def, substs) if def.is_phantom_data() => substs.types().collect(),
1720 ty::Adt(def, substs) => def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect(),
1722 ty::Opaque(def_id, substs) => {
1723 // We can resolve the `impl Trait` to its concrete type,
1724 // which enforces a DAG between the functions requiring
1725 // the auto trait bounds in question.
1726 vec![self.tcx().type_of(def_id).subst(self.tcx(), substs)]
1731 fn collect_predicates_for_types(
1733 param_env: ty::ParamEnv<'tcx>,
1734 cause: ObligationCause<'tcx>,
1735 recursion_depth: usize,
1736 trait_def_id: DefId,
1737 types: ty::Binder<Vec<Ty<'tcx>>>,
1738 ) -> Vec<PredicateObligation<'tcx>> {
1739 // Because the types were potentially derived from
1740 // higher-ranked obligations they may reference late-bound
1741 // regions. For example, `for<'a> Foo<&'a i32> : Copy` would
1742 // yield a type like `for<'a> &'a i32`. In general, we
1743 // maintain the invariant that we never manipulate bound
1744 // regions, so we have to process these bound regions somehow.
1746 // The strategy is to:
1748 // 1. Instantiate those regions to placeholder regions (e.g.,
1749 // `for<'a> &'a i32` becomes `&0 i32`.
1750 // 2. Produce something like `&'0 i32 : Copy`
1751 // 3. Re-bind the regions back to `for<'a> &'a i32 : Copy`
1754 .skip_binder() // binder moved -\
1757 let ty: ty::Binder<Ty<'tcx>> = ty::Binder::bind(ty); // <----/
1759 self.infcx.commit_unconditionally(|_| {
1760 let placeholder_ty = self.infcx.replace_bound_vars_with_placeholders(&ty);
1761 let Normalized { value: normalized_ty, mut obligations } =
1762 ensure_sufficient_stack(|| {
1763 project::normalize_with_depth(
1771 let placeholder_obligation = predicate_for_trait_def(
1780 obligations.push(placeholder_obligation);
1787 ///////////////////////////////////////////////////////////////////////////
1790 // Matching is a common path used for both evaluation and
1791 // confirmation. It basically unifies types that appear in impls
1792 // and traits. This does affect the surrounding environment;
1793 // therefore, when used during evaluation, match routines must be
1794 // run inside of a `probe()` so that their side-effects are
1800 obligation: &TraitObligation<'tcx>,
1801 ) -> Normalized<'tcx, SubstsRef<'tcx>> {
1802 match self.match_impl(impl_def_id, obligation) {
1803 Ok(substs) => substs,
1806 "Impl {:?} was matchable against {:?} but now is not",
1817 obligation: &TraitObligation<'tcx>,
1818 ) -> Result<Normalized<'tcx, SubstsRef<'tcx>>, ()> {
1819 let impl_trait_ref = self.tcx().impl_trait_ref(impl_def_id).unwrap();
1821 // Before we create the substitutions and everything, first
1822 // consider a "quick reject". This avoids creating more types
1823 // and so forth that we need to.
1824 if self.fast_reject_trait_refs(obligation, &impl_trait_ref) {
1828 let placeholder_obligation =
1829 self.infcx().replace_bound_vars_with_placeholders(&obligation.predicate);
1830 let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
1832 let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
1834 let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs);
1836 let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
1837 ensure_sufficient_stack(|| {
1838 project::normalize_with_depth(
1840 obligation.param_env,
1841 obligation.cause.clone(),
1842 obligation.recursion_depth + 1,
1848 "match_impl(impl_def_id={:?}, obligation={:?}, \
1849 impl_trait_ref={:?}, placeholder_obligation_trait_ref={:?})",
1850 impl_def_id, obligation, impl_trait_ref, placeholder_obligation_trait_ref
1853 let InferOk { obligations, .. } = self
1855 .at(&obligation.cause, obligation.param_env)
1856 .eq(placeholder_obligation_trait_ref, impl_trait_ref)
1857 .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{}`", e))?;
1858 nested_obligations.extend(obligations);
1861 && self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation
1863 debug!("match_impl: reservation impls only apply in intercrate mode");
1867 debug!("match_impl: success impl_substs={:?}", impl_substs);
1868 Ok(Normalized { value: impl_substs, obligations: nested_obligations })
1871 fn fast_reject_trait_refs(
1873 obligation: &TraitObligation<'_>,
1874 impl_trait_ref: &ty::TraitRef<'_>,
1876 // We can avoid creating type variables and doing the full
1877 // substitution if we find that any of the input types, when
1878 // simplified, do not match.
1880 obligation.predicate.skip_binder().trait_ref.substs.iter().zip(impl_trait_ref.substs).any(
1881 |(obligation_arg, impl_arg)| {
1882 match (obligation_arg.unpack(), impl_arg.unpack()) {
1883 (GenericArgKind::Type(obligation_ty), GenericArgKind::Type(impl_ty)) => {
1884 let simplified_obligation_ty =
1885 fast_reject::simplify_type(self.tcx(), obligation_ty, true);
1886 let simplified_impl_ty =
1887 fast_reject::simplify_type(self.tcx(), impl_ty, false);
1889 simplified_obligation_ty.is_some()
1890 && simplified_impl_ty.is_some()
1891 && simplified_obligation_ty != simplified_impl_ty
1893 (GenericArgKind::Lifetime(_), GenericArgKind::Lifetime(_)) => {
1894 // Lifetimes can never cause a rejection.
1897 (GenericArgKind::Const(_), GenericArgKind::Const(_)) => {
1898 // Conservatively ignore consts (i.e. assume they might
1899 // unify later) until we have `fast_reject` support for
1900 // them (if we'll ever need it, even).
1903 _ => unreachable!(),
1909 /// Normalize `where_clause_trait_ref` and try to match it against
1910 /// `obligation`. If successful, return any predicates that
1911 /// result from the normalization. Normalization is necessary
1912 /// because where-clauses are stored in the parameter environment
1914 fn match_where_clause_trait_ref(
1916 obligation: &TraitObligation<'tcx>,
1917 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
1918 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
1919 self.match_poly_trait_ref(obligation, where_clause_trait_ref)
1922 /// Returns `Ok` if `poly_trait_ref` being true implies that the
1923 /// obligation is satisfied.
1924 fn match_poly_trait_ref(
1926 obligation: &TraitObligation<'tcx>,
1927 poly_trait_ref: ty::PolyTraitRef<'tcx>,
1928 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
1930 "match_poly_trait_ref: obligation={:?} poly_trait_ref={:?}",
1931 obligation, poly_trait_ref
1935 .at(&obligation.cause, obligation.param_env)
1936 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
1937 .map(|InferOk { obligations, .. }| obligations)
1941 ///////////////////////////////////////////////////////////////////////////
1944 fn match_fresh_trait_refs(
1946 previous: ty::PolyTraitRef<'tcx>,
1947 current: ty::PolyTraitRef<'tcx>,
1948 param_env: ty::ParamEnv<'tcx>,
1950 let mut matcher = ty::_match::Match::new(self.tcx(), param_env);
1951 matcher.relate(previous, current).is_ok()
1956 previous_stack: TraitObligationStackList<'o, 'tcx>,
1957 obligation: &'o TraitObligation<'tcx>,
1958 ) -> TraitObligationStack<'o, 'tcx> {
1959 let fresh_trait_ref =
1960 obligation.predicate.to_poly_trait_ref().fold_with(&mut self.freshener);
1962 let dfn = previous_stack.cache.next_dfn();
1963 let depth = previous_stack.depth() + 1;
1964 TraitObligationStack {
1967 reached_depth: Cell::new(depth),
1968 previous: previous_stack,
1974 fn closure_trait_ref_unnormalized(
1976 obligation: &TraitObligation<'tcx>,
1977 substs: SubstsRef<'tcx>,
1978 ) -> ty::PolyTraitRef<'tcx> {
1979 debug!("closure_trait_ref_unnormalized(obligation={:?}, substs={:?})", obligation, substs);
1980 let closure_sig = substs.as_closure().sig();
1982 debug!("closure_trait_ref_unnormalized: closure_sig = {:?}", closure_sig);
1984 // (1) Feels icky to skip the binder here, but OTOH we know
1985 // that the self-type is an unboxed closure type and hence is
1986 // in fact unparameterized (or at least does not reference any
1987 // regions bound in the obligation). Still probably some
1988 // refactoring could make this nicer.
1989 closure_trait_ref_and_return_type(
1991 obligation.predicate.def_id(),
1992 obligation.predicate.skip_binder().self_ty(), // (1)
1994 util::TupleArgumentsFlag::No,
1996 .map_bound(|(trait_ref, _)| trait_ref)
1999 fn generator_trait_ref_unnormalized(
2001 obligation: &TraitObligation<'tcx>,
2002 substs: SubstsRef<'tcx>,
2003 ) -> ty::PolyTraitRef<'tcx> {
2004 let gen_sig = substs.as_generator().poly_sig();
2006 // (1) Feels icky to skip the binder here, but OTOH we know
2007 // that the self-type is an generator type and hence is
2008 // in fact unparameterized (or at least does not reference any
2009 // regions bound in the obligation). Still probably some
2010 // refactoring could make this nicer.
2012 super::util::generator_trait_ref_and_outputs(
2014 obligation.predicate.def_id(),
2015 obligation.predicate.skip_binder().self_ty(), // (1)
2018 .map_bound(|(trait_ref, ..)| trait_ref)
2021 /// Returns the obligations that are implied by instantiating an
2022 /// impl or trait. The obligations are substituted and fully
2023 /// normalized. This is used when confirming an impl or default
2025 fn impl_or_trait_obligations(
2027 cause: ObligationCause<'tcx>,
2028 recursion_depth: usize,
2029 param_env: ty::ParamEnv<'tcx>,
2030 def_id: DefId, // of impl or trait
2031 substs: SubstsRef<'tcx>, // for impl or trait
2032 ) -> Vec<PredicateObligation<'tcx>> {
2033 debug!("impl_or_trait_obligations(def_id={:?})", def_id);
2034 let tcx = self.tcx();
2036 // To allow for one-pass evaluation of the nested obligation,
2037 // each predicate must be preceded by the obligations required
2039 // for example, if we have:
2040 // impl<U: Iterator<Item: Copy>, V: Iterator<Item = U>> Foo for V
2041 // the impl will have the following predicates:
2042 // <V as Iterator>::Item = U,
2043 // U: Iterator, U: Sized,
2044 // V: Iterator, V: Sized,
2045 // <U as Iterator>::Item: Copy
2046 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2047 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2048 // `$1: Copy`, so we must ensure the obligations are emitted in
2050 let predicates = tcx.predicates_of(def_id);
2051 assert_eq!(predicates.parent, None);
2052 let mut obligations = Vec::with_capacity(predicates.predicates.len());
2053 for (predicate, _) in predicates.predicates {
2054 let predicate = normalize_with_depth_to(
2059 &predicate.subst(tcx, substs),
2062 obligations.push(Obligation {
2063 cause: cause.clone(),
2070 // We are performing deduplication here to avoid exponential blowups
2071 // (#38528) from happening, but the real cause of the duplication is
2072 // unknown. What we know is that the deduplication avoids exponential
2073 // amount of predicates being propagated when processing deeply nested
2076 // This code is hot enough that it's worth avoiding the allocation
2077 // required for the FxHashSet when possible. Special-casing lengths 0,
2078 // 1 and 2 covers roughly 75-80% of the cases.
2079 if obligations.len() <= 1 {
2080 // No possibility of duplicates.
2081 } else if obligations.len() == 2 {
2082 // Only two elements. Drop the second if they are equal.
2083 if obligations[0] == obligations[1] {
2084 obligations.truncate(1);
2087 // Three or more elements. Use a general deduplication process.
2088 let mut seen = FxHashSet::default();
2089 obligations.retain(|i| seen.insert(i.clone()));
2096 trait TraitObligationExt<'tcx> {
2099 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
2100 ) -> ObligationCause<'tcx>;
2103 impl<'tcx> TraitObligationExt<'tcx> for TraitObligation<'tcx> {
2106 variant: fn(DerivedObligationCause<'tcx>) -> ObligationCauseCode<'tcx>,
2107 ) -> ObligationCause<'tcx> {
2109 * Creates a cause for obligations that are derived from
2110 * `obligation` by a recursive search (e.g., for a builtin
2111 * bound, or eventually a `auto trait Foo`). If `obligation`
2112 * is itself a derived obligation, this is just a clone, but
2113 * otherwise we create a "derived obligation" cause so as to
2114 * keep track of the original root obligation for error
2118 let obligation = self;
2120 // NOTE(flaper87): As of now, it keeps track of the whole error
2121 // chain. Ideally, we should have a way to configure this either
2122 // by using -Z verbose or just a CLI argument.
2123 let derived_cause = DerivedObligationCause {
2124 parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
2125 parent_code: Rc::new(obligation.cause.code.clone()),
2127 let derived_code = variant(derived_cause);
2128 ObligationCause::new(obligation.cause.span, obligation.cause.body_id, derived_code)
2132 impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
2133 fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2134 TraitObligationStackList::with(self)
2137 fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx> {
2141 fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2145 /// Indicates that attempting to evaluate this stack entry
2146 /// required accessing something from the stack at depth `reached_depth`.
2147 fn update_reached_depth(&self, reached_depth: usize) {
2149 self.depth > reached_depth,
2150 "invoked `update_reached_depth` with something under this stack: \
2151 self.depth={} reached_depth={}",
2155 debug!("update_reached_depth(reached_depth={})", reached_depth);
2157 while reached_depth < p.depth {
2158 debug!("update_reached_depth: marking {:?} as cycle participant", p.fresh_trait_ref);
2159 p.reached_depth.set(p.reached_depth.get().min(reached_depth));
2160 p = p.previous.head.unwrap();
2165 /// The "provisional evaluation cache" is used to store intermediate cache results
2166 /// when solving auto traits. Auto traits are unusual in that they can support
2167 /// cycles. So, for example, a "proof tree" like this would be ok:
2169 /// - `Foo<T>: Send` :-
2170 /// - `Bar<T>: Send` :-
2171 /// - `Foo<T>: Send` -- cycle, but ok
2172 /// - `Baz<T>: Send`
2174 /// Here, to prove `Foo<T>: Send`, we have to prove `Bar<T>: Send` and
2175 /// `Baz<T>: Send`. Proving `Bar<T>: Send` in turn required `Foo<T>: Send`.
2176 /// For non-auto traits, this cycle would be an error, but for auto traits (because
2177 /// they are coinductive) it is considered ok.
2179 /// However, there is a complication: at the point where we have
2180 /// "proven" `Bar<T>: Send`, we have in fact only proven it
2181 /// *provisionally*. In particular, we proved that `Bar<T>: Send`
2182 /// *under the assumption* that `Foo<T>: Send`. But what if we later
2183 /// find out this assumption is wrong? Specifically, we could
2184 /// encounter some kind of error proving `Baz<T>: Send`. In that case,
2185 /// `Bar<T>: Send` didn't turn out to be true.
2187 /// In Issue #60010, we found a bug in rustc where it would cache
2188 /// these intermediate results. This was fixed in #60444 by disabling
2189 /// *all* caching for things involved in a cycle -- in our example,
2190 /// that would mean we don't cache that `Bar<T>: Send`. But this led
2191 /// to large slowdowns.
2193 /// Specifically, imagine this scenario, where proving `Baz<T>: Send`
2194 /// first requires proving `Bar<T>: Send` (which is true:
2196 /// - `Foo<T>: Send` :-
2197 /// - `Bar<T>: Send` :-
2198 /// - `Foo<T>: Send` -- cycle, but ok
2199 /// - `Baz<T>: Send`
2200 /// - `Bar<T>: Send` -- would be nice for this to be a cache hit!
2201 /// - `*const T: Send` -- but what if we later encounter an error?
2203 /// The *provisional evaluation cache* resolves this issue. It stores
2204 /// cache results that we've proven but which were involved in a cycle
2205 /// in some way. We track the minimal stack depth (i.e., the
2206 /// farthest from the top of the stack) that we are dependent on.
2207 /// The idea is that the cache results within are all valid -- so long as
2208 /// none of the nodes in between the current node and the node at that minimum
2209 /// depth result in an error (in which case the cached results are just thrown away).
2211 /// During evaluation, we consult this provisional cache and rely on
2212 /// it. Accessing a cached value is considered equivalent to accessing
2213 /// a result at `reached_depth`, so it marks the *current* solution as
2214 /// provisional as well. If an error is encountered, we toss out any
2215 /// provisional results added from the subtree that encountered the
2216 /// error. When we pop the node at `reached_depth` from the stack, we
2217 /// can commit all the things that remain in the provisional cache.
2218 struct ProvisionalEvaluationCache<'tcx> {
2219 /// next "depth first number" to issue -- just a counter
2222 /// Stores the "coldest" depth (bottom of stack) reached by any of
2223 /// the evaluation entries. The idea here is that all things in the provisional
2224 /// cache are always dependent on *something* that is colder in the stack:
2225 /// therefore, if we add a new entry that is dependent on something *colder still*,
2226 /// we have to modify the depth for all entries at once.
2230 /// Imagine we have a stack `A B C D E` (with `E` being the top of
2231 /// the stack). We cache something with depth 2, which means that
2232 /// it was dependent on C. Then we pop E but go on and process a
2233 /// new node F: A B C D F. Now F adds something to the cache with
2234 /// depth 1, meaning it is dependent on B. Our original cache
2235 /// entry is also dependent on B, because there is a path from E
2236 /// to C and then from C to F and from F to B.
2237 reached_depth: Cell<usize>,
2239 /// Map from cache key to the provisionally evaluated thing.
2240 /// The cache entries contain the result but also the DFN in which they
2241 /// were added. The DFN is used to clear out values on failure.
2243 /// Imagine we have a stack like:
2245 /// - `A B C` and we add a cache for the result of C (DFN 2)
2246 /// - Then we have a stack `A B D` where `D` has DFN 3
2247 /// - We try to solve D by evaluating E: `A B D E` (DFN 4)
2248 /// - `E` generates various cache entries which have cyclic dependices on `B`
2249 /// - `A B D E F` and so forth
2250 /// - the DFN of `F` for example would be 5
2251 /// - then we determine that `E` is in error -- we will then clear
2252 /// all cache values whose DFN is >= 4 -- in this case, that
2253 /// means the cached value for `F`.
2254 map: RefCell<FxHashMap<ty::PolyTraitRef<'tcx>, ProvisionalEvaluation>>,
2257 /// A cache value for the provisional cache: contains the depth-first
2258 /// number (DFN) and result.
2259 #[derive(Copy, Clone, Debug)]
2260 struct ProvisionalEvaluation {
2262 result: EvaluationResult,
2265 impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> {
2266 fn default() -> Self {
2267 Self { dfn: Cell::new(0), reached_depth: Cell::new(usize::MAX), map: Default::default() }
2271 impl<'tcx> ProvisionalEvaluationCache<'tcx> {
2272 /// Get the next DFN in sequence (basically a counter).
2273 fn next_dfn(&self) -> usize {
2274 let result = self.dfn.get();
2275 self.dfn.set(result + 1);
2279 /// Check the provisional cache for any result for
2280 /// `fresh_trait_ref`. If there is a hit, then you must consider
2281 /// it an access to the stack slots at depth
2282 /// `self.current_reached_depth()` and above.
2283 fn get_provisional(&self, fresh_trait_ref: ty::PolyTraitRef<'tcx>) -> Option<EvaluationResult> {
2285 "get_provisional(fresh_trait_ref={:?}) = {:#?} with reached-depth {}",
2287 self.map.borrow().get(&fresh_trait_ref),
2288 self.reached_depth.get(),
2290 Some(self.map.borrow().get(&fresh_trait_ref)?.result)
2293 /// Current value of the `reached_depth` counter -- all the
2294 /// provisional cache entries are dependent on the item at this
2296 fn current_reached_depth(&self) -> usize {
2297 self.reached_depth.get()
2300 /// Insert a provisional result into the cache. The result came
2301 /// from the node with the given DFN. It accessed a minimum depth
2302 /// of `reached_depth` to compute. It evaluated `fresh_trait_ref`
2303 /// and resulted in `result`.
2304 fn insert_provisional(
2307 reached_depth: usize,
2308 fresh_trait_ref: ty::PolyTraitRef<'tcx>,
2309 result: EvaluationResult,
2312 "insert_provisional(from_dfn={}, reached_depth={}, fresh_trait_ref={:?}, result={:?})",
2313 from_dfn, reached_depth, fresh_trait_ref, result,
2315 let r_d = self.reached_depth.get();
2316 self.reached_depth.set(r_d.min(reached_depth));
2318 debug!("insert_provisional: reached_depth={:?}", self.reached_depth.get());
2320 self.map.borrow_mut().insert(fresh_trait_ref, ProvisionalEvaluation { from_dfn, result });
2323 /// Invoked when the node with dfn `dfn` does not get a successful
2324 /// result. This will clear out any provisional cache entries
2325 /// that were added since `dfn` was created. This is because the
2326 /// provisional entries are things which must assume that the
2327 /// things on the stack at the time of their creation succeeded --
2328 /// since the failing node is presently at the top of the stack,
2329 /// these provisional entries must either depend on it or some
2331 fn on_failure(&self, dfn: usize) {
2332 debug!("on_failure(dfn={:?})", dfn,);
2333 self.map.borrow_mut().retain(|key, eval| {
2334 if !eval.from_dfn >= dfn {
2335 debug!("on_failure: removing {:?}", key);
2343 /// Invoked when the node at depth `depth` completed without
2344 /// depending on anything higher in the stack (if that completion
2345 /// was a failure, then `on_failure` should have been invoked
2346 /// already). The callback `op` will be invoked for each
2347 /// provisional entry that we can now confirm.
2351 mut op: impl FnMut(ty::PolyTraitRef<'tcx>, EvaluationResult),
2353 debug!("on_completion(depth={}, reached_depth={})", depth, self.reached_depth.get(),);
2355 if self.reached_depth.get() < depth {
2356 debug!("on_completion: did not yet reach depth to complete");
2360 for (fresh_trait_ref, eval) in self.map.borrow_mut().drain() {
2361 debug!("on_completion: fresh_trait_ref={:?} eval={:?}", fresh_trait_ref, eval,);
2363 op(fresh_trait_ref, eval.result);
2366 self.reached_depth.set(usize::MAX);
2370 #[derive(Copy, Clone)]
2371 struct TraitObligationStackList<'o, 'tcx> {
2372 cache: &'o ProvisionalEvaluationCache<'tcx>,
2373 head: Option<&'o TraitObligationStack<'o, 'tcx>>,
2376 impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> {
2377 fn empty(cache: &'o ProvisionalEvaluationCache<'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2378 TraitObligationStackList { cache, head: None }
2381 fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2382 TraitObligationStackList { cache: r.cache(), head: Some(r) }
2385 fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2389 fn depth(&self) -> usize {
2390 if let Some(head) = self.head { head.depth } else { 0 }
2394 impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> {
2395 type Item = &'o TraitObligationStack<'o, 'tcx>;
2397 fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2408 impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
2409 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2410 write!(f, "TraitObligationStack({:?})", self.obligation)