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 // FIXME: The `map` field in ProvisionalEvaluationCache should be changed to
6 // a `FxIndexMap` to avoid query instability, but right now it causes a perf regression. This would be
7 // fixed or at least lightened by the addition of the `drain_filter` method to `FxIndexMap`
8 // Relevant: https://github.com/rust-lang/rust/pull/103723 and https://github.com/bluss/indexmap/issues/242
9 #![allow(rustc::potential_query_instability)]
11 use self::EvaluationResult::*;
12 use self::SelectionCandidate::*;
14 use super::coherence::{self, Conflict};
15 use super::const_evaluatable;
17 use super::project::normalize_with_depth_to;
18 use super::project::ProjectionTyObligation;
20 use super::util::{closure_trait_ref_and_return_type, predicate_for_trait_def};
23 ErrorReporting, ImplDerivedObligation, ImplDerivedObligationCause, Normalized, Obligation,
24 ObligationCause, ObligationCauseCode, Overflow, PredicateObligation, Selection, SelectionError,
25 SelectionResult, TraitObligation, TraitQueryMode,
28 use crate::infer::{InferCtxt, InferOk, TypeFreshener};
29 use crate::traits::error_reporting::TypeErrCtxtExt;
30 use crate::traits::project::ProjectAndUnifyResult;
31 use crate::traits::project::ProjectionCacheKeyExt;
32 use crate::traits::ProjectionCacheKey;
33 use crate::traits::Unimplemented;
34 use rustc_data_structures::fx::FxHashMap;
35 use rustc_data_structures::fx::{FxHashSet, FxIndexSet};
36 use rustc_data_structures::stack::ensure_sufficient_stack;
37 use rustc_errors::Diagnostic;
39 use rustc_hir::def_id::DefId;
40 use rustc_infer::infer::LateBoundRegionConversionTime;
41 use rustc_middle::dep_graph::{DepKind, DepNodeIndex};
42 use rustc_middle::mir::interpret::ErrorHandled;
43 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
44 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
45 use rustc_middle::ty::fold::BottomUpFolder;
46 use rustc_middle::ty::print::{FmtPrinter, Print};
47 use rustc_middle::ty::relate::TypeRelation;
48 use rustc_middle::ty::SubstsRef;
49 use rustc_middle::ty::{self, EarlyBinder, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate};
50 use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable, TypeVisitable};
51 use rustc_span::symbol::sym;
53 use std::cell::{Cell, RefCell};
55 use std::fmt::{self, Display};
58 pub use rustc_middle::traits::select::*;
59 use rustc_middle::ty::print::with_no_trimmed_paths;
61 mod candidate_assembly;
64 #[derive(Clone, Debug, Eq, PartialEq, Hash)]
65 pub enum IntercrateAmbiguityCause {
66 DownstreamCrate { trait_desc: String, self_desc: Option<String> },
67 UpstreamCrateUpdate { trait_desc: String, self_desc: Option<String> },
68 ReservationImpl { message: String },
71 impl IntercrateAmbiguityCause {
72 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
73 /// See #23980 for details.
74 pub fn add_intercrate_ambiguity_hint(&self, err: &mut Diagnostic) {
75 err.note(&self.intercrate_ambiguity_hint());
78 pub fn intercrate_ambiguity_hint(&self) -> String {
80 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc } => {
81 let self_desc = if let Some(ty) = self_desc {
82 format!(" for type `{}`", ty)
86 format!("downstream crates may implement trait `{}`{}", trait_desc, self_desc)
88 IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc } => {
89 let self_desc = if let Some(ty) = self_desc {
90 format!(" for type `{}`", ty)
95 "upstream crates may add a new impl of trait `{}`{} \
100 IntercrateAmbiguityCause::ReservationImpl { message } => message.clone(),
105 pub struct SelectionContext<'cx, 'tcx> {
106 pub infcx: &'cx InferCtxt<'tcx>,
108 /// Freshener used specifically for entries on the obligation
109 /// stack. This ensures that all entries on the stack at one time
110 /// will have the same set of placeholder entries, which is
111 /// important for checking for trait bounds that recursively
112 /// require themselves.
113 freshener: TypeFreshener<'cx, 'tcx>,
115 /// If `intercrate` is set, we remember predicates which were
116 /// considered ambiguous because of impls potentially added in other crates.
117 /// This is used in coherence to give improved diagnostics.
118 /// We don't do his until we detect a coherence error because it can
119 /// lead to false overflow results (#47139) and because always
120 /// computing it may negatively impact performance.
121 intercrate_ambiguity_causes: Option<FxIndexSet<IntercrateAmbiguityCause>>,
123 /// The mode that trait queries run in, which informs our error handling
124 /// policy. In essence, canonicalized queries need their errors propagated
125 /// rather than immediately reported because we do not have accurate spans.
126 query_mode: TraitQueryMode,
129 // A stack that walks back up the stack frame.
130 struct TraitObligationStack<'prev, 'tcx> {
131 obligation: &'prev TraitObligation<'tcx>,
133 /// The trait predicate from `obligation` but "freshened" with the
134 /// selection-context's freshener. Used to check for recursion.
135 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
137 /// Starts out equal to `depth` -- if, during evaluation, we
138 /// encounter a cycle, then we will set this flag to the minimum
139 /// depth of that cycle for all participants in the cycle. These
140 /// participants will then forego caching their results. This is
141 /// not the most efficient solution, but it addresses #60010. The
142 /// problem we are trying to prevent:
144 /// - If you have `A: AutoTrait` requires `B: AutoTrait` and `C: NonAutoTrait`
145 /// - `B: AutoTrait` requires `A: AutoTrait` (coinductive cycle, ok)
146 /// - `C: NonAutoTrait` requires `A: AutoTrait` (non-coinductive cycle, not ok)
148 /// you don't want to cache that `B: AutoTrait` or `A: AutoTrait`
149 /// is `EvaluatedToOk`; this is because they were only considered
150 /// ok on the premise that if `A: AutoTrait` held, but we indeed
151 /// encountered a problem (later on) with `A: AutoTrait. So we
152 /// currently set a flag on the stack node for `B: AutoTrait` (as
153 /// well as the second instance of `A: AutoTrait`) to suppress
156 /// This is a simple, targeted fix. A more-performant fix requires
157 /// deeper changes, but would permit more caching: we could
158 /// basically defer caching until we have fully evaluated the
159 /// tree, and then cache the entire tree at once. In any case, the
160 /// performance impact here shouldn't be so horrible: every time
161 /// this is hit, we do cache at least one trait, so we only
162 /// evaluate each member of a cycle up to N times, where N is the
163 /// length of the cycle. This means the performance impact is
164 /// bounded and we shouldn't have any terrible worst-cases.
165 reached_depth: Cell<usize>,
167 previous: TraitObligationStackList<'prev, 'tcx>,
169 /// The number of parent frames plus one (thus, the topmost frame has depth 1).
172 /// The depth-first number of this node in the search graph -- a
173 /// pre-order index. Basically, a freshly incremented counter.
177 struct SelectionCandidateSet<'tcx> {
178 // A list of candidates that definitely apply to the current
179 // obligation (meaning: types unify).
180 vec: Vec<SelectionCandidate<'tcx>>,
182 // If `true`, then there were candidates that might or might
183 // not have applied, but we couldn't tell. This occurs when some
184 // of the input types are type variables, in which case there are
185 // various "builtin" rules that might or might not trigger.
189 #[derive(PartialEq, Eq, Debug, Clone)]
190 struct EvaluatedCandidate<'tcx> {
191 candidate: SelectionCandidate<'tcx>,
192 evaluation: EvaluationResult,
195 /// When does the builtin impl for `T: Trait` apply?
197 enum BuiltinImplConditions<'tcx> {
198 /// The impl is conditional on `T1, T2, ...: Trait`.
199 Where(ty::Binder<'tcx, Vec<Ty<'tcx>>>),
200 /// There is no built-in impl. There may be some other
201 /// candidate (a where-clause or user-defined impl).
203 /// It is unknown whether there is an impl.
207 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
208 pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
211 freshener: infcx.freshener_keep_static(),
212 intercrate_ambiguity_causes: None,
213 query_mode: TraitQueryMode::Standard,
217 pub fn with_query_mode(
218 infcx: &'cx InferCtxt<'tcx>,
219 query_mode: TraitQueryMode,
220 ) -> SelectionContext<'cx, 'tcx> {
221 debug!(?query_mode, "with_query_mode");
222 SelectionContext { query_mode, ..SelectionContext::new(infcx) }
225 /// Enables tracking of intercrate ambiguity causes. See
226 /// the documentation of [`Self::intercrate_ambiguity_causes`] for more.
227 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
228 assert!(self.is_intercrate());
229 assert!(self.intercrate_ambiguity_causes.is_none());
230 self.intercrate_ambiguity_causes = Some(FxIndexSet::default());
231 debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
234 /// Gets the intercrate ambiguity causes collected since tracking
235 /// was enabled and disables tracking at the same time. If
236 /// tracking is not enabled, just returns an empty vector.
237 pub fn take_intercrate_ambiguity_causes(&mut self) -> FxIndexSet<IntercrateAmbiguityCause> {
238 assert!(self.is_intercrate());
239 self.intercrate_ambiguity_causes.take().unwrap_or_default()
242 pub fn tcx(&self) -> TyCtxt<'tcx> {
246 pub fn is_intercrate(&self) -> bool {
247 self.infcx.intercrate
250 ///////////////////////////////////////////////////////////////////////////
253 // The selection phase tries to identify *how* an obligation will
254 // be resolved. For example, it will identify which impl or
255 // parameter bound is to be used. The process can be inconclusive
256 // if the self type in the obligation is not fully inferred. Selection
257 // can result in an error in one of two ways:
259 // 1. If no applicable impl or parameter bound can be found.
260 // 2. If the output type parameters in the obligation do not match
261 // those specified by the impl/bound. For example, if the obligation
262 // is `Vec<Foo>: Iterable<Bar>`, but the impl specifies
263 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
265 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
266 /// type environment by performing unification.
267 #[instrument(level = "debug", skip(self), ret)]
270 obligation: &TraitObligation<'tcx>,
271 ) -> SelectionResult<'tcx, Selection<'tcx>> {
272 let candidate = match self.select_from_obligation(obligation) {
273 Err(SelectionError::Overflow(OverflowError::Canonical)) => {
274 // In standard mode, overflow must have been caught and reported
276 assert!(self.query_mode == TraitQueryMode::Canonical);
277 return Err(SelectionError::Overflow(OverflowError::Canonical));
285 Ok(Some(candidate)) => candidate,
288 match self.confirm_candidate(obligation, candidate) {
289 Err(SelectionError::Overflow(OverflowError::Canonical)) => {
290 assert!(self.query_mode == TraitQueryMode::Canonical);
291 Err(SelectionError::Overflow(OverflowError::Canonical))
294 Ok(candidate) => Ok(Some(candidate)),
298 pub(crate) fn select_from_obligation(
300 obligation: &TraitObligation<'tcx>,
301 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
302 debug_assert!(!obligation.predicate.has_escaping_bound_vars());
304 let pec = &ProvisionalEvaluationCache::default();
305 let stack = self.push_stack(TraitObligationStackList::empty(pec), obligation);
307 self.candidate_from_obligation(&stack)
310 #[instrument(level = "debug", skip(self), ret)]
311 fn candidate_from_obligation<'o>(
313 stack: &TraitObligationStack<'o, 'tcx>,
314 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
315 // Watch out for overflow. This intentionally bypasses (and does
316 // not update) the cache.
317 self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
319 // Check the cache. Note that we freshen the trait-ref
320 // separately rather than using `stack.fresh_trait_ref` --
321 // this is because we want the unbound variables to be
322 // replaced with fresh types starting from index 0.
323 let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate);
324 debug!(?cache_fresh_trait_pred);
325 debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
328 self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred)
334 // If no match, compute result and insert into cache.
336 // FIXME(nikomatsakis) -- this cache is not taking into
337 // account cycles that may have occurred in forming the
338 // candidate. I don't know of any specific problems that
339 // result but it seems awfully suspicious.
340 let (candidate, dep_node) =
341 self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
343 debug!("CACHE MISS");
344 self.insert_candidate_cache(
345 stack.obligation.param_env,
346 cache_fresh_trait_pred,
353 fn candidate_from_obligation_no_cache<'o>(
355 stack: &TraitObligationStack<'o, 'tcx>,
356 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
357 if let Err(conflict) = self.is_knowable(stack) {
358 debug!("coherence stage: not knowable");
359 if self.intercrate_ambiguity_causes.is_some() {
360 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
361 // Heuristics: show the diagnostics when there are no candidates in crate.
362 if let Ok(candidate_set) = self.assemble_candidates(stack) {
363 let mut no_candidates_apply = true;
365 for c in candidate_set.vec.iter() {
366 if self.evaluate_candidate(stack, &c)?.may_apply() {
367 no_candidates_apply = false;
372 if !candidate_set.ambiguous && no_candidates_apply {
373 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
374 if !trait_ref.references_error() {
375 let self_ty = trait_ref.self_ty();
376 let (trait_desc, self_desc) = with_no_trimmed_paths!({
377 let trait_desc = trait_ref.print_only_trait_path().to_string();
378 let self_desc = if self_ty.has_concrete_skeleton() {
379 Some(self_ty.to_string())
383 (trait_desc, self_desc)
385 let cause = if let Conflict::Upstream = conflict {
386 IntercrateAmbiguityCause::UpstreamCrateUpdate {
391 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
393 debug!(?cause, "evaluate_stack: pushing cause");
394 self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause);
402 let candidate_set = self.assemble_candidates(stack)?;
404 if candidate_set.ambiguous {
405 debug!("candidate set contains ambig");
409 let candidates = candidate_set.vec;
411 debug!(?stack, ?candidates, "assembled {} candidates", candidates.len());
413 // At this point, we know that each of the entries in the
414 // candidate set is *individually* applicable. Now we have to
415 // figure out if they contain mutual incompatibilities. This
416 // frequently arises if we have an unconstrained input type --
417 // for example, we are looking for `$0: Eq` where `$0` is some
418 // unconstrained type variable. In that case, we'll get a
419 // candidate which assumes $0 == int, one that assumes `$0 ==
420 // usize`, etc. This spells an ambiguity.
422 let mut candidates = self.filter_impls(candidates, stack.obligation);
424 // If there is more than one candidate, first winnow them down
425 // by considering extra conditions (nested obligations and so
426 // forth). We don't winnow if there is exactly one
427 // candidate. This is a relatively minor distinction but it
428 // can lead to better inference and error-reporting. An
429 // example would be if there was an impl:
431 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
433 // and we were to see some code `foo.push_clone()` where `boo`
434 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
435 // we were to winnow, we'd wind up with zero candidates.
436 // Instead, we select the right impl now but report "`Bar` does
437 // not implement `Clone`".
438 if candidates.len() == 1 {
439 return self.filter_reservation_impls(candidates.pop().unwrap(), stack.obligation);
442 // Winnow, but record the exact outcome of evaluation, which
443 // is needed for specialization. Propagate overflow if it occurs.
444 let mut candidates = candidates
446 .map(|c| match self.evaluate_candidate(stack, &c) {
447 Ok(eval) if eval.may_apply() => {
448 Ok(Some(EvaluatedCandidate { candidate: c, evaluation: eval }))
451 Err(OverflowError::Canonical) => Err(Overflow(OverflowError::Canonical)),
452 Err(OverflowError::ErrorReporting) => Err(ErrorReporting),
453 Err(OverflowError::Error(e)) => Err(Overflow(OverflowError::Error(e))),
455 .flat_map(Result::transpose)
456 .collect::<Result<Vec<_>, _>>()?;
458 debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len());
460 let needs_infer = stack.obligation.predicate.has_non_region_infer();
462 // If there are STILL multiple candidates, we can further
463 // reduce the list by dropping duplicates -- including
464 // resolving specializations.
465 if candidates.len() > 1 {
467 while i < candidates.len() {
468 let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| {
469 self.candidate_should_be_dropped_in_favor_of(
476 debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
477 candidates.swap_remove(i);
479 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
482 // If there are *STILL* multiple candidates, give up
483 // and report ambiguity.
485 debug!("multiple matches, ambig");
492 // If there are *NO* candidates, then there are no impls --
493 // that we know of, anyway. Note that in the case where there
494 // are unbound type variables within the obligation, it might
495 // be the case that you could still satisfy the obligation
496 // from another crate by instantiating the type variables with
497 // a type from another crate that does have an impl. This case
498 // is checked for in `evaluate_stack` (and hence users
499 // who might care about this case, like coherence, should use
501 if candidates.is_empty() {
502 // If there's an error type, 'downgrade' our result from
503 // `Err(Unimplemented)` to `Ok(None)`. This helps us avoid
504 // emitting additional spurious errors, since we're guaranteed
505 // to have emitted at least one.
506 if stack.obligation.predicate.references_error() {
507 debug!(?stack.obligation.predicate, "found error type in predicate, treating as ambiguous");
510 return Err(Unimplemented);
513 // Just one candidate left.
514 self.filter_reservation_impls(candidates.pop().unwrap().candidate, stack.obligation)
517 ///////////////////////////////////////////////////////////////////////////
520 // Tests whether an obligation can be selected or whether an impl
521 // can be applied to particular types. It skips the "confirmation"
522 // step and hence completely ignores output type parameters.
524 // The result is "true" if the obligation *may* hold and "false" if
525 // we can be sure it does not.
527 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
528 pub fn predicate_may_hold_fatal(&mut self, obligation: &PredicateObligation<'tcx>) -> bool {
529 debug!(?obligation, "predicate_may_hold_fatal");
531 // This fatal query is a stopgap that should only be used in standard mode,
532 // where we do not expect overflow to be propagated.
533 assert!(self.query_mode == TraitQueryMode::Standard);
535 self.evaluate_root_obligation(obligation)
536 .expect("Overflow should be caught earlier in standard query mode")
540 /// Evaluates whether the obligation `obligation` can be satisfied
541 /// and returns an `EvaluationResult`. This is meant for the
543 pub fn evaluate_root_obligation(
545 obligation: &PredicateObligation<'tcx>,
546 ) -> Result<EvaluationResult, OverflowError> {
547 self.evaluation_probe(|this| {
548 this.evaluate_predicate_recursively(
549 TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
557 op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
558 ) -> Result<EvaluationResult, OverflowError> {
559 self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
560 let result = op(self)?;
562 match self.infcx.leak_check(true, snapshot) {
564 Err(_) => return Ok(EvaluatedToErr),
567 if self.infcx.opaque_types_added_in_snapshot(snapshot) {
568 return Ok(result.max(EvaluatedToOkModuloOpaqueTypes));
571 match self.infcx.region_constraints_added_in_snapshot(snapshot) {
573 Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)),
578 /// Evaluates the predicates in `predicates` recursively. Note that
579 /// this applies projections in the predicates, and therefore
580 /// is run within an inference probe.
581 #[instrument(skip(self, stack), level = "debug")]
582 fn evaluate_predicates_recursively<'o, I>(
584 stack: TraitObligationStackList<'o, 'tcx>,
586 ) -> Result<EvaluationResult, OverflowError>
588 I: IntoIterator<Item = PredicateObligation<'tcx>> + std::fmt::Debug,
590 let mut result = EvaluatedToOk;
591 for obligation in predicates {
592 let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
593 if let EvaluatedToErr = eval {
594 // fast-path - EvaluatedToErr is the top of the lattice,
595 // so we don't need to look on the other predicates.
596 return Ok(EvaluatedToErr);
598 result = cmp::max(result, eval);
606 skip(self, previous_stack),
607 fields(previous_stack = ?previous_stack.head())
610 fn evaluate_predicate_recursively<'o>(
612 previous_stack: TraitObligationStackList<'o, 'tcx>,
613 obligation: PredicateObligation<'tcx>,
614 ) -> Result<EvaluationResult, OverflowError> {
615 // `previous_stack` stores a `TraitObligation`, while `obligation` is
616 // a `PredicateObligation`. These are distinct types, so we can't
617 // use any `Option` combinator method that would force them to be
619 match previous_stack.head() {
620 Some(h) => self.check_recursion_limit(&obligation, h.obligation)?,
621 None => self.check_recursion_limit(&obligation, &obligation)?,
624 ensure_sufficient_stack(|| {
625 let bound_predicate = obligation.predicate.kind();
626 match bound_predicate.skip_binder() {
627 ty::PredicateKind::Clause(ty::Clause::Trait(t)) => {
628 let t = bound_predicate.rebind(t);
629 debug_assert!(!t.has_escaping_bound_vars());
630 let obligation = obligation.with(self.tcx(), t);
631 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
634 ty::PredicateKind::Subtype(p) => {
635 let p = bound_predicate.rebind(p);
636 // Does this code ever run?
637 match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
638 Ok(Ok(InferOk { mut obligations, .. })) => {
639 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
640 self.evaluate_predicates_recursively(
642 obligations.into_iter(),
645 Ok(Err(_)) => Ok(EvaluatedToErr),
646 Err(..) => Ok(EvaluatedToAmbig),
650 ty::PredicateKind::Coerce(p) => {
651 let p = bound_predicate.rebind(p);
652 // Does this code ever run?
653 match self.infcx.coerce_predicate(&obligation.cause, obligation.param_env, p) {
654 Ok(Ok(InferOk { mut obligations, .. })) => {
655 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
656 self.evaluate_predicates_recursively(
658 obligations.into_iter(),
661 Ok(Err(_)) => Ok(EvaluatedToErr),
662 Err(..) => Ok(EvaluatedToAmbig),
666 ty::PredicateKind::WellFormed(arg) => {
667 // So, there is a bit going on here. First, `WellFormed` predicates
668 // are coinductive, like trait predicates with auto traits.
669 // This means that we need to detect if we have recursively
670 // evaluated `WellFormed(X)`. Otherwise, we would run into
671 // a "natural" overflow error.
673 // Now, the next question is whether we need to do anything
674 // special with caching. Considering the following tree:
679 // In this case, the innermost `WF(Foo<T>)` should return
680 // `EvaluatedToOk`, since it's coinductive. Then if
681 // `Bar<T>: Send` is resolved to `EvaluatedToOk`, it can be
682 // inserted into a cache (because without thinking about `WF`
683 // goals, it isn't in a cycle). If `Foo<T>: Trait` later doesn't
684 // hold, then `Bar<T>: Send` shouldn't hold. Therefore, we
685 // *do* need to keep track of coinductive cycles.
687 let cache = previous_stack.cache;
688 let dfn = cache.next_dfn();
690 for stack_arg in previous_stack.cache.wf_args.borrow().iter().rev() {
691 if stack_arg.0 != arg {
694 debug!("WellFormed({:?}) on stack", arg);
695 if let Some(stack) = previous_stack.head {
696 // Okay, let's imagine we have two different stacks:
697 // `T: NonAutoTrait -> WF(T) -> T: NonAutoTrait`
698 // `WF(T) -> T: NonAutoTrait -> WF(T)`
699 // Because of this, we need to check that all
700 // predicates between the WF goals are coinductive.
701 // Otherwise, we can say that `T: NonAutoTrait` is
703 // Let's imagine we have a predicate stack like
704 // `Foo: Bar -> WF(T) -> T: NonAutoTrait -> T: Auto
706 // and the current predicate is `WF(T)`. `wf_args`
707 // would contain `(T, 1)`. We want to check all
708 // trait predicates greater than `1`. The previous
709 // stack would be `T: Auto`.
710 let cycle = stack.iter().take_while(|s| s.depth > stack_arg.1);
711 let tcx = self.tcx();
713 cycle.map(|stack| stack.obligation.predicate.to_predicate(tcx));
714 if self.coinductive_match(cycle) {
715 stack.update_reached_depth(stack_arg.1);
716 return Ok(EvaluatedToOk);
718 return Ok(EvaluatedToRecur);
721 return Ok(EvaluatedToOk);
724 match wf::obligations(
726 obligation.param_env,
727 obligation.cause.body_id,
728 obligation.recursion_depth + 1,
730 obligation.cause.span,
732 Some(mut obligations) => {
733 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
735 cache.wf_args.borrow_mut().push((arg, previous_stack.depth()));
737 self.evaluate_predicates_recursively(previous_stack, obligations);
738 cache.wf_args.borrow_mut().pop();
740 let result = result?;
742 if !result.must_apply_modulo_regions() {
743 cache.on_failure(dfn);
746 cache.on_completion(dfn);
750 None => Ok(EvaluatedToAmbig),
754 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(pred)) => {
755 // A global type with no late-bound regions can only
756 // contain the "'static" lifetime (any other lifetime
757 // would either be late-bound or local), so it is guaranteed
758 // to outlive any other lifetime
759 if pred.0.is_global() && !pred.0.has_late_bound_regions() {
762 Ok(EvaluatedToOkModuloRegions)
766 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..)) => {
767 // We do not consider region relationships when evaluating trait matches.
768 Ok(EvaluatedToOkModuloRegions)
771 ty::PredicateKind::ObjectSafe(trait_def_id) => {
772 if self.tcx().is_object_safe(trait_def_id) {
779 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
780 let data = bound_predicate.rebind(data);
781 let project_obligation = obligation.with(self.tcx(), data);
782 match project::poly_project_and_unify_type(self, &project_obligation) {
783 ProjectAndUnifyResult::Holds(mut subobligations) => {
785 // If we've previously marked this projection as 'complete', then
786 // use the final cached result (either `EvaluatedToOk` or
787 // `EvaluatedToOkModuloRegions`), and skip re-evaluating the
790 ProjectionCacheKey::from_poly_projection_predicate(self, data)
792 if let Some(cached_res) = self
799 break 'compute_res Ok(cached_res);
804 subobligations.iter_mut(),
805 obligation.recursion_depth,
807 let res = self.evaluate_predicates_recursively(
811 if let Ok(eval_rslt) = res
812 && (eval_rslt == EvaluatedToOk || eval_rslt == EvaluatedToOkModuloRegions)
814 ProjectionCacheKey::from_poly_projection_predicate(
818 // If the result is something that we can cache, then mark this
819 // entry as 'complete'. This will allow us to skip evaluating the
820 // subobligations at all the next time we evaluate the projection
826 .complete(key, eval_rslt);
831 ProjectAndUnifyResult::FailedNormalization => Ok(EvaluatedToAmbig),
832 ProjectAndUnifyResult::Recursive => Ok(EvaluatedToRecur),
833 ProjectAndUnifyResult::MismatchedProjectionTypes(_) => Ok(EvaluatedToErr),
837 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
838 match self.infcx.closure_kind(closure_substs) {
839 Some(closure_kind) => {
840 if closure_kind.extends(kind) {
846 None => Ok(EvaluatedToAmbig),
850 ty::PredicateKind::ConstEvaluatable(uv) => {
851 match const_evaluatable::is_const_evaluatable(
854 obligation.param_env,
855 obligation.cause.span,
857 Ok(()) => Ok(EvaluatedToOk),
858 Err(NotConstEvaluatable::MentionsInfer) => Ok(EvaluatedToAmbig),
859 Err(NotConstEvaluatable::MentionsParam) => Ok(EvaluatedToErr),
860 Err(_) => Ok(EvaluatedToErr),
864 ty::PredicateKind::ConstEquate(c1, c2) => {
865 let tcx = self.tcx();
867 tcx.features().generic_const_exprs,
868 "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
872 let c1 = tcx.expand_abstract_consts(c1);
873 let c2 = tcx.expand_abstract_consts(c2);
875 "evalaute_predicate_recursively: equating consts:\nc1= {:?}\nc2= {:?}",
879 use rustc_hir::def::DefKind;
880 use ty::ConstKind::Unevaluated;
881 match (c1.kind(), c2.kind()) {
882 (Unevaluated(a), Unevaluated(b))
883 if a.def.did == b.def.did
884 && tcx.def_kind(a.def.did) == DefKind::AssocConst =>
886 if let Ok(new_obligations) = self
888 .at(&obligation.cause, obligation.param_env)
890 .eq(a.substs, b.substs)
892 let mut obligations = new_obligations.obligations;
894 obligations.iter_mut(),
895 obligation.recursion_depth,
897 return self.evaluate_predicates_recursively(
899 obligations.into_iter(),
903 (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
905 if let Ok(new_obligations) = self
907 .at(&obligation.cause, obligation.param_env)
910 let mut obligations = new_obligations.obligations;
912 obligations.iter_mut(),
913 obligation.recursion_depth,
915 return self.evaluate_predicates_recursively(
917 obligations.into_iter(),
924 let evaluate = |c: ty::Const<'tcx>| {
925 if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
926 match self.infcx.try_const_eval_resolve(
927 obligation.param_env,
930 Some(obligation.cause.span),
940 match (evaluate(c1), evaluate(c2)) {
941 (Ok(c1), Ok(c2)) => {
942 match self.infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
944 Ok(inf_ok) => self.evaluate_predicates_recursively(
946 inf_ok.into_obligations(),
948 Err(_) => Ok(EvaluatedToErr),
951 (Err(ErrorHandled::Reported(_)), _)
952 | (_, Err(ErrorHandled::Reported(_))) => Ok(EvaluatedToErr),
953 (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
954 if c1.has_non_region_infer() || c2.has_non_region_infer() {
957 // Two different constants using generic parameters ~> error.
963 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
964 bug!("TypeWellFormedFromEnv is only used for chalk")
966 ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
971 #[instrument(skip(self, previous_stack), level = "debug", ret)]
972 fn evaluate_trait_predicate_recursively<'o>(
974 previous_stack: TraitObligationStackList<'o, 'tcx>,
975 mut obligation: TraitObligation<'tcx>,
976 ) -> Result<EvaluationResult, OverflowError> {
977 if !self.is_intercrate()
978 && obligation.is_global()
979 && obligation.param_env.caller_bounds().iter().all(|bound| bound.needs_subst())
981 // If a param env has no global bounds, global obligations do not
982 // depend on its particular value in order to work, so we can clear
983 // out the param env and get better caching.
985 obligation.param_env = obligation.param_env.without_caller_bounds();
988 let stack = self.push_stack(previous_stack, &obligation);
989 let mut fresh_trait_pred = stack.fresh_trait_pred;
990 let mut param_env = obligation.param_env;
992 fresh_trait_pred = fresh_trait_pred.map_bound(|mut pred| {
993 pred.remap_constness(&mut param_env);
997 debug!(?fresh_trait_pred);
999 // If a trait predicate is in the (local or global) evaluation cache,
1000 // then we know it holds without cycles.
1001 if let Some(result) = self.check_evaluation_cache(param_env, fresh_trait_pred) {
1002 debug!("CACHE HIT");
1006 if let Some(result) = stack.cache().get_provisional(fresh_trait_pred) {
1007 debug!("PROVISIONAL CACHE HIT");
1008 stack.update_reached_depth(result.reached_depth);
1009 return Ok(result.result);
1012 // Check if this is a match for something already on the
1013 // stack. If so, we don't want to insert the result into the
1014 // main cache (it is cycle dependent) nor the provisional
1015 // cache (which is meant for things that have completed but
1016 // for a "backedge" -- this result *is* the backedge).
1017 if let Some(cycle_result) = self.check_evaluation_cycle(&stack) {
1018 return Ok(cycle_result);
1021 let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
1022 let result = result?;
1024 if !result.must_apply_modulo_regions() {
1025 stack.cache().on_failure(stack.dfn);
1028 let reached_depth = stack.reached_depth.get();
1029 if reached_depth >= stack.depth {
1030 debug!("CACHE MISS");
1031 self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
1032 stack.cache().on_completion(stack.dfn);
1034 debug!("PROVISIONAL");
1036 "caching provisionally because {:?} \
1037 is a cycle participant (at depth {}, reached depth {})",
1038 fresh_trait_pred, stack.depth, reached_depth,
1041 stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_pred, result);
1047 /// If there is any previous entry on the stack that precisely
1048 /// matches this obligation, then we can assume that the
1049 /// obligation is satisfied for now (still all other conditions
1050 /// must be met of course). One obvious case this comes up is
1051 /// marker traits like `Send`. Think of a linked list:
1053 /// struct List<T> { data: T, next: Option<Box<List<T>>> }
1055 /// `Box<List<T>>` will be `Send` if `T` is `Send` and
1056 /// `Option<Box<List<T>>>` is `Send`, and in turn
1057 /// `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
1060 /// Note that we do this comparison using the `fresh_trait_ref`
1061 /// fields. Because these have all been freshened using
1062 /// `self.freshener`, we can be sure that (a) this will not
1063 /// affect the inferencer state and (b) that if we see two
1064 /// fresh regions with the same index, they refer to the same
1065 /// unbound type variable.
1066 fn check_evaluation_cycle(
1068 stack: &TraitObligationStack<'_, 'tcx>,
1069 ) -> Option<EvaluationResult> {
1070 if let Some(cycle_depth) = stack
1072 .skip(1) // Skip top-most frame.
1074 stack.obligation.param_env == prev.obligation.param_env
1075 && stack.fresh_trait_pred == prev.fresh_trait_pred
1077 .map(|stack| stack.depth)
1079 debug!("evaluate_stack --> recursive at depth {}", cycle_depth);
1081 // If we have a stack like `A B C D E A`, where the top of
1082 // the stack is the final `A`, then this will iterate over
1083 // `A, E, D, C, B` -- i.e., all the participants apart
1084 // from the cycle head. We mark them as participating in a
1085 // cycle. This suppresses caching for those nodes. See
1086 // `in_cycle` field for more details.
1087 stack.update_reached_depth(cycle_depth);
1089 // Subtle: when checking for a coinductive cycle, we do
1090 // not compare using the "freshened trait refs" (which
1091 // have erased regions) but rather the fully explicit
1092 // trait refs. This is important because it's only a cycle
1093 // if the regions match exactly.
1094 let cycle = stack.iter().skip(1).take_while(|s| s.depth >= cycle_depth);
1095 let tcx = self.tcx();
1096 let cycle = cycle.map(|stack| stack.obligation.predicate.to_predicate(tcx));
1097 if self.coinductive_match(cycle) {
1098 debug!("evaluate_stack --> recursive, coinductive");
1101 debug!("evaluate_stack --> recursive, inductive");
1102 Some(EvaluatedToRecur)
1109 fn evaluate_stack<'o>(
1111 stack: &TraitObligationStack<'o, 'tcx>,
1112 ) -> Result<EvaluationResult, OverflowError> {
1113 // In intercrate mode, whenever any of the generics are unbound,
1114 // there can always be an impl. Even if there are no impls in
1115 // this crate, perhaps the type would be unified with
1116 // something from another crate that does provide an impl.
1118 // In intra mode, we must still be conservative. The reason is
1119 // that we want to avoid cycles. Imagine an impl like:
1121 // impl<T:Eq> Eq for Vec<T>
1123 // and a trait reference like `$0 : Eq` where `$0` is an
1124 // unbound variable. When we evaluate this trait-reference, we
1125 // will unify `$0` with `Vec<$1>` (for some fresh variable
1126 // `$1`), on the condition that `$1 : Eq`. We will then wind
1127 // up with many candidates (since that are other `Eq` impls
1128 // that apply) and try to winnow things down. This results in
1129 // a recursive evaluation that `$1 : Eq` -- as you can
1130 // imagine, this is just where we started. To avoid that, we
1131 // check for unbound variables and return an ambiguous (hence possible)
1132 // match if we've seen this trait before.
1134 // This suffices to allow chains like `FnMut` implemented in
1135 // terms of `Fn` etc, but we could probably make this more
1137 let unbound_input_types =
1138 stack.fresh_trait_pred.skip_binder().trait_ref.substs.types().any(|ty| ty.is_fresh());
1140 if unbound_input_types
1141 && stack.iter().skip(1).any(|prev| {
1142 stack.obligation.param_env == prev.obligation.param_env
1143 && self.match_fresh_trait_refs(
1144 stack.fresh_trait_pred,
1145 prev.fresh_trait_pred,
1146 prev.obligation.param_env,
1150 debug!("evaluate_stack --> unbound argument, recursive --> giving up",);
1151 return Ok(EvaluatedToUnknown);
1154 match self.candidate_from_obligation(stack) {
1155 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
1156 Ok(None) => Ok(EvaluatedToAmbig),
1157 Err(Overflow(OverflowError::Canonical)) => Err(OverflowError::Canonical),
1158 Err(ErrorReporting) => Err(OverflowError::ErrorReporting),
1159 Err(..) => Ok(EvaluatedToErr),
1163 /// For defaulted traits, we use a co-inductive strategy to solve, so
1164 /// that recursion is ok. This routine returns `true` if the top of the
1165 /// stack (`cycle[0]`):
1167 /// - is a defaulted trait,
1168 /// - it also appears in the backtrace at some position `X`,
1169 /// - all the predicates at positions `X..` between `X` and the top are
1170 /// also defaulted traits.
1171 pub(crate) fn coinductive_match<I>(&mut self, mut cycle: I) -> bool
1173 I: Iterator<Item = ty::Predicate<'tcx>>,
1175 cycle.all(|predicate| self.coinductive_predicate(predicate))
1178 fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
1179 let result = match predicate.kind().skip_binder() {
1180 ty::PredicateKind::Clause(ty::Clause::Trait(ref data)) => {
1181 self.tcx().trait_is_coinductive(data.def_id())
1183 ty::PredicateKind::WellFormed(_) => true,
1186 debug!(?predicate, ?result, "coinductive_predicate");
1190 /// Further evaluates `candidate` to decide whether all type parameters match and whether nested
1191 /// obligations are met. Returns whether `candidate` remains viable after this further
1196 fields(depth = stack.obligation.recursion_depth),
1199 fn evaluate_candidate<'o>(
1201 stack: &TraitObligationStack<'o, 'tcx>,
1202 candidate: &SelectionCandidate<'tcx>,
1203 ) -> Result<EvaluationResult, OverflowError> {
1204 let mut result = self.evaluation_probe(|this| {
1205 let candidate = (*candidate).clone();
1206 match this.confirm_candidate(stack.obligation, candidate) {
1209 this.evaluate_predicates_recursively(
1211 selection.nested_obligations().into_iter(),
1214 Err(..) => Ok(EvaluatedToErr),
1218 // If we erased any lifetimes, then we want to use
1219 // `EvaluatedToOkModuloRegions` instead of `EvaluatedToOk`
1220 // as your final result. The result will be cached using
1221 // the freshened trait predicate as a key, so we need
1222 // our result to be correct by *any* choice of original lifetimes,
1223 // not just the lifetime choice for this particular (non-erased)
1226 if stack.fresh_trait_pred.has_erased_regions() {
1227 result = result.max(EvaluatedToOkModuloRegions);
1233 fn check_evaluation_cache(
1235 param_env: ty::ParamEnv<'tcx>,
1236 trait_pred: ty::PolyTraitPredicate<'tcx>,
1237 ) -> Option<EvaluationResult> {
1238 // Neither the global nor local cache is aware of intercrate
1239 // mode, so don't do any caching. In particular, we might
1240 // re-use the same `InferCtxt` with both an intercrate
1241 // and non-intercrate `SelectionContext`
1242 if self.is_intercrate() {
1246 let tcx = self.tcx();
1247 if self.can_use_global_caches(param_env) {
1248 if let Some(res) = tcx.evaluation_cache.get(&(param_env, trait_pred), tcx) {
1252 self.infcx.evaluation_cache.get(&(param_env, trait_pred), tcx)
1255 fn insert_evaluation_cache(
1257 param_env: ty::ParamEnv<'tcx>,
1258 trait_pred: ty::PolyTraitPredicate<'tcx>,
1259 dep_node: DepNodeIndex,
1260 result: EvaluationResult,
1262 // Avoid caching results that depend on more than just the trait-ref
1263 // - the stack can create recursion.
1264 if result.is_stack_dependent() {
1268 // Neither the global nor local cache is aware of intercrate
1269 // mode, so don't do any caching. In particular, we might
1270 // re-use the same `InferCtxt` with both an intercrate
1271 // and non-intercrate `SelectionContext`
1272 if self.is_intercrate() {
1276 if self.can_use_global_caches(param_env) {
1277 if !trait_pred.needs_infer() {
1278 debug!(?trait_pred, ?result, "insert_evaluation_cache global");
1279 // This may overwrite the cache with the same value
1280 // FIXME: Due to #50507 this overwrites the different values
1281 // This should be changed to use HashMapExt::insert_same
1282 // when that is fixed
1283 self.tcx().evaluation_cache.insert((param_env, trait_pred), dep_node, result);
1288 debug!(?trait_pred, ?result, "insert_evaluation_cache");
1289 self.infcx.evaluation_cache.insert((param_env, trait_pred), dep_node, result);
1292 /// For various reasons, it's possible for a subobligation
1293 /// to have a *lower* recursion_depth than the obligation used to create it.
1294 /// Projection sub-obligations may be returned from the projection cache,
1295 /// which results in obligations with an 'old' `recursion_depth`.
1296 /// Additionally, methods like `InferCtxt.subtype_predicate` produce
1297 /// subobligations without taking in a 'parent' depth, causing the
1298 /// generated subobligations to have a `recursion_depth` of `0`.
1300 /// To ensure that obligation_depth never decreases, we force all subobligations
1301 /// to have at least the depth of the original obligation.
1302 fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>(
1307 it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
1310 fn check_recursion_depth<T>(
1313 error_obligation: &Obligation<'tcx, T>,
1314 ) -> Result<(), OverflowError>
1317 + TypeFoldable<'tcx>
1318 + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
1319 <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug,
1321 if !self.infcx.tcx.recursion_limit().value_within_limit(depth) {
1322 match self.query_mode {
1323 TraitQueryMode::Standard => {
1324 if let Some(e) = self.infcx.tainted_by_errors() {
1325 return Err(OverflowError::Error(e));
1327 self.infcx.err_ctxt().report_overflow_error(error_obligation, true);
1329 TraitQueryMode::Canonical => {
1330 return Err(OverflowError::Canonical);
1337 /// Checks that the recursion limit has not been exceeded.
1339 /// The weird return type of this function allows it to be used with the `try` (`?`)
1340 /// operator within certain functions.
1342 fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V>(
1344 obligation: &Obligation<'tcx, T>,
1345 error_obligation: &Obligation<'tcx, V>,
1346 ) -> Result<(), OverflowError>
1349 + TypeFoldable<'tcx>
1350 + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
1351 <V as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug,
1353 self.check_recursion_depth(obligation.recursion_depth, error_obligation)
1356 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1358 OP: FnOnce(&mut Self) -> R,
1360 let (result, dep_node) =
1361 self.tcx().dep_graph.with_anon_task(self.tcx(), DepKind::TraitSelect, || op(self));
1362 self.tcx().dep_graph.read_index(dep_node);
1366 /// filter_impls filters constant trait obligations and candidates that have a positive impl
1367 /// for a negative goal and a negative impl for a positive goal
1368 #[instrument(level = "debug", skip(self, candidates))]
1371 candidates: Vec<SelectionCandidate<'tcx>>,
1372 obligation: &TraitObligation<'tcx>,
1373 ) -> Vec<SelectionCandidate<'tcx>> {
1374 trace!("{candidates:#?}");
1375 let tcx = self.tcx();
1376 let mut result = Vec::with_capacity(candidates.len());
1378 for candidate in candidates {
1379 // Respect const trait obligations
1380 if obligation.is_const() {
1383 ImplCandidate(def_id) if tcx.constness(def_id) == hir::Constness::Const => {}
1385 ParamCandidate(trait_pred) if trait_pred.is_const_if_const() => {}
1387 ProjectionCandidate(_, ty::BoundConstness::ConstIfConst) => {}
1389 AutoImplCandidate => {}
1390 // generator / future, this will raise error in other places
1391 // or ignore error with const_async_blocks feature
1392 GeneratorCandidate => {}
1393 FutureCandidate => {}
1394 // FnDef where the function is const
1395 FnPointerCandidate { is_const: true } => {}
1396 ConstDestructCandidate(_) => {}
1398 // reject all other types of candidates
1404 if let ImplCandidate(def_id) = candidate {
1405 if ty::ImplPolarity::Reservation == tcx.impl_polarity(def_id)
1406 || obligation.polarity() == tcx.impl_polarity(def_id)
1408 result.push(candidate);
1411 result.push(candidate);
1415 trace!("{result:#?}");
1419 /// filter_reservation_impls filter reservation impl for any goal as ambiguous
1420 #[instrument(level = "debug", skip(self))]
1421 fn filter_reservation_impls(
1423 candidate: SelectionCandidate<'tcx>,
1424 obligation: &TraitObligation<'tcx>,
1425 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1426 let tcx = self.tcx();
1427 // Treat reservation impls as ambiguity.
1428 if let ImplCandidate(def_id) = candidate {
1429 if let ty::ImplPolarity::Reservation = tcx.impl_polarity(def_id) {
1430 if let Some(intercrate_ambiguity_clauses) = &mut self.intercrate_ambiguity_causes {
1432 .get_attr(def_id, sym::rustc_reservation_impl)
1433 .and_then(|a| a.value_str());
1434 if let Some(value) = value {
1436 "filter_reservation_impls: \
1437 reservation impl ambiguity on {:?}",
1440 intercrate_ambiguity_clauses.insert(
1441 IntercrateAmbiguityCause::ReservationImpl {
1442 message: value.to_string(),
1453 fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result<(), Conflict> {
1454 debug!("is_knowable(intercrate={:?})", self.is_intercrate());
1456 if !self.is_intercrate() || stack.obligation.polarity() == ty::ImplPolarity::Negative {
1460 let obligation = &stack.obligation;
1461 let predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
1463 // Okay to skip binder because of the nature of the
1464 // trait-ref-is-knowable check, which does not care about
1466 let trait_ref = predicate.skip_binder().trait_ref;
1468 coherence::trait_ref_is_knowable(self.tcx(), trait_ref)
1471 /// Returns `true` if the global caches can be used.
1472 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1473 // If there are any inference variables in the `ParamEnv`, then we
1474 // always use a cache local to this particular scope. Otherwise, we
1475 // switch to a global cache.
1476 if param_env.needs_infer() {
1480 // Avoid using the master cache during coherence and just rely
1481 // on the local cache. This effectively disables caching
1482 // during coherence. It is really just a simplification to
1483 // avoid us having to fear that coherence results "pollute"
1484 // the master cache. Since coherence executes pretty quickly,
1485 // it's not worth going to more trouble to increase the
1486 // hit-rate, I don't think.
1487 if self.is_intercrate() {
1491 // Otherwise, we can use the global cache.
1495 fn check_candidate_cache(
1497 mut param_env: ty::ParamEnv<'tcx>,
1498 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1499 ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
1500 // Neither the global nor local cache is aware of intercrate
1501 // mode, so don't do any caching. In particular, we might
1502 // re-use the same `InferCtxt` with both an intercrate
1503 // and non-intercrate `SelectionContext`
1504 if self.is_intercrate() {
1507 let tcx = self.tcx();
1508 let mut pred = cache_fresh_trait_pred.skip_binder();
1509 pred.remap_constness(&mut param_env);
1511 if self.can_use_global_caches(param_env) {
1512 if let Some(res) = tcx.selection_cache.get(&(param_env, pred), tcx) {
1516 self.infcx.selection_cache.get(&(param_env, pred), tcx)
1519 /// Determines whether can we safely cache the result
1520 /// of selecting an obligation. This is almost always `true`,
1521 /// except when dealing with certain `ParamCandidate`s.
1523 /// Ordinarily, a `ParamCandidate` will contain no inference variables,
1524 /// since it was usually produced directly from a `DefId`. However,
1525 /// certain cases (currently only librustdoc's blanket impl finder),
1526 /// a `ParamEnv` may be explicitly constructed with inference types.
1527 /// When this is the case, we do *not* want to cache the resulting selection
1528 /// candidate. This is due to the fact that it might not always be possible
1529 /// to equate the obligation's trait ref and the candidate's trait ref,
1530 /// if more constraints end up getting added to an inference variable.
1532 /// Because of this, we always want to re-run the full selection
1533 /// process for our obligation the next time we see it, since
1534 /// we might end up picking a different `SelectionCandidate` (or none at all).
1535 fn can_cache_candidate(
1537 result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1539 // Neither the global nor local cache is aware of intercrate
1540 // mode, so don't do any caching. In particular, we might
1541 // re-use the same `InferCtxt` with both an intercrate
1542 // and non-intercrate `SelectionContext`
1543 if self.is_intercrate() {
1547 Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => !trait_ref.needs_infer(),
1552 #[instrument(skip(self, param_env, cache_fresh_trait_pred, dep_node), level = "debug")]
1553 fn insert_candidate_cache(
1555 mut param_env: ty::ParamEnv<'tcx>,
1556 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1557 dep_node: DepNodeIndex,
1558 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1560 let tcx = self.tcx();
1561 let mut pred = cache_fresh_trait_pred.skip_binder();
1563 pred.remap_constness(&mut param_env);
1565 if !self.can_cache_candidate(&candidate) {
1566 debug!(?pred, ?candidate, "insert_candidate_cache - candidate is not cacheable");
1570 if self.can_use_global_caches(param_env) {
1571 if let Err(Overflow(OverflowError::Canonical)) = candidate {
1572 // Don't cache overflow globally; we only produce this in certain modes.
1573 } else if !pred.needs_infer() {
1574 if !candidate.needs_infer() {
1575 debug!(?pred, ?candidate, "insert_candidate_cache global");
1576 // This may overwrite the cache with the same value.
1577 tcx.selection_cache.insert((param_env, pred), dep_node, candidate);
1583 debug!(?pred, ?candidate, "insert_candidate_cache local");
1584 self.infcx.selection_cache.insert((param_env, pred), dep_node, candidate);
1587 /// Matches a predicate against the bounds of its self type.
1589 /// Given an obligation like `<T as Foo>::Bar: Baz` where the self type is
1590 /// a projection, look at the bounds of `T::Bar`, see if we can find a
1591 /// `Baz` bound. We return indexes into the list returned by
1592 /// `tcx.item_bounds` for any applicable bounds.
1593 #[instrument(level = "debug", skip(self), ret)]
1594 fn match_projection_obligation_against_definition_bounds(
1596 obligation: &TraitObligation<'tcx>,
1597 ) -> smallvec::SmallVec<[(usize, ty::BoundConstness); 2]> {
1598 let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
1599 let placeholder_trait_predicate =
1600 self.infcx.replace_bound_vars_with_placeholders(poly_trait_predicate);
1601 debug!(?placeholder_trait_predicate);
1603 let tcx = self.infcx.tcx;
1604 let (def_id, substs) = match *placeholder_trait_predicate.trait_ref.self_ty().kind() {
1605 ty::Projection(ref data) => (data.item_def_id, data.substs),
1606 ty::Opaque(def_id, substs) => (def_id, substs),
1609 obligation.cause.span,
1610 "match_projection_obligation_against_definition_bounds() called \
1611 but self-ty is not a projection: {:?}",
1612 placeholder_trait_predicate.trait_ref.self_ty()
1616 let bounds = tcx.bound_item_bounds(def_id).subst(tcx, substs);
1618 // The bounds returned by `item_bounds` may contain duplicates after
1619 // normalization, so try to deduplicate when possible to avoid
1620 // unnecessary ambiguity.
1621 let mut distinct_normalized_bounds = FxHashSet::default();
1626 .filter_map(|(idx, bound)| {
1627 let bound_predicate = bound.kind();
1628 if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) =
1629 bound_predicate.skip_binder()
1631 let bound = bound_predicate.rebind(pred.trait_ref);
1632 if self.infcx.probe(|_| {
1633 match self.match_normalize_trait_ref(
1636 placeholder_trait_predicate.trait_ref,
1639 Ok(Some(normalized_trait))
1640 if distinct_normalized_bounds.insert(normalized_trait) =>
1647 return Some((idx, pred.constness));
1655 /// Equates the trait in `obligation` with trait bound. If the two traits
1656 /// can be equated and the normalized trait bound doesn't contain inference
1657 /// variables or placeholders, the normalized bound is returned.
1658 fn match_normalize_trait_ref(
1660 obligation: &TraitObligation<'tcx>,
1661 trait_bound: ty::PolyTraitRef<'tcx>,
1662 placeholder_trait_ref: ty::TraitRef<'tcx>,
1663 ) -> Result<Option<ty::PolyTraitRef<'tcx>>, ()> {
1664 debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
1665 if placeholder_trait_ref.def_id != trait_bound.def_id() {
1666 // Avoid unnecessary normalization
1670 let Normalized { value: trait_bound, obligations: _ } = ensure_sufficient_stack(|| {
1671 project::normalize_with_depth(
1673 obligation.param_env,
1674 obligation.cause.clone(),
1675 obligation.recursion_depth + 1,
1680 .at(&obligation.cause, obligation.param_env)
1681 .define_opaque_types(false)
1682 .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
1683 .map(|InferOk { obligations: _, value: () }| {
1684 // This method is called within a probe, so we can't have
1685 // inference variables and placeholders escape.
1686 if !trait_bound.needs_infer() && !trait_bound.has_placeholders() {
1695 fn where_clause_may_apply<'o>(
1697 stack: &TraitObligationStack<'o, 'tcx>,
1698 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
1699 ) -> Result<EvaluationResult, OverflowError> {
1700 self.evaluation_probe(|this| {
1701 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1702 Ok(obligations) => this.evaluate_predicates_recursively(stack.list(), obligations),
1703 Err(()) => Ok(EvaluatedToErr),
1708 /// Return `Yes` if the obligation's predicate type applies to the env_predicate, and
1709 /// `No` if it does not. Return `Ambiguous` in the case that the projection type is a GAT,
1710 /// and applying this env_predicate constrains any of the obligation's GAT substitutions.
1712 /// This behavior is a somewhat of a hack to prevent over-constraining inference variables
1713 /// in cases like #91762.
1714 pub(super) fn match_projection_projections(
1716 obligation: &ProjectionTyObligation<'tcx>,
1717 env_predicate: PolyProjectionPredicate<'tcx>,
1718 potentially_unnormalized_candidates: bool,
1719 ) -> ProjectionMatchesProjection {
1720 let mut nested_obligations = Vec::new();
1721 let infer_predicate = self.infcx.replace_bound_vars_with_fresh_vars(
1722 obligation.cause.span,
1723 LateBoundRegionConversionTime::HigherRankedType,
1726 let infer_projection = if potentially_unnormalized_candidates {
1727 ensure_sufficient_stack(|| {
1728 project::normalize_with_depth_to(
1730 obligation.param_env,
1731 obligation.cause.clone(),
1732 obligation.recursion_depth + 1,
1733 infer_predicate.projection_ty,
1734 &mut nested_obligations,
1738 infer_predicate.projection_ty
1743 .at(&obligation.cause, obligation.param_env)
1744 .define_opaque_types(false)
1745 .sup(obligation.predicate, infer_projection)
1746 .map_or(false, |InferOk { obligations, value: () }| {
1747 self.evaluate_predicates_recursively(
1748 TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
1749 nested_obligations.into_iter().chain(obligations),
1751 .map_or(false, |res| res.may_apply())
1755 let generics = self.tcx().generics_of(obligation.predicate.item_def_id);
1756 // FIXME(generic-associated-types): Addresses aggressive inference in #92917.
1757 // If this type is a GAT, and of the GAT substs resolve to something new,
1758 // that means that we must have newly inferred something about the GAT.
1759 // We should give up in that case.
1760 if !generics.params.is_empty()
1761 && obligation.predicate.substs[generics.parent_count..]
1763 .any(|&p| p.has_non_region_infer() && self.infcx.shallow_resolve(p) != p)
1765 ProjectionMatchesProjection::Ambiguous
1767 ProjectionMatchesProjection::Yes
1770 ProjectionMatchesProjection::No
1774 ///////////////////////////////////////////////////////////////////////////
1777 // Winnowing is the process of attempting to resolve ambiguity by
1778 // probing further. During the winnowing process, we unify all
1779 // type variables and then we also attempt to evaluate recursive
1780 // bounds to see if they are satisfied.
1782 /// Returns `true` if `victim` should be dropped in favor of
1783 /// `other`. Generally speaking we will drop duplicate
1784 /// candidates and prefer where-clause candidates.
1786 /// See the comment for "SelectionCandidate" for more details.
1787 fn candidate_should_be_dropped_in_favor_of(
1789 victim: &EvaluatedCandidate<'tcx>,
1790 other: &EvaluatedCandidate<'tcx>,
1793 if victim.candidate == other.candidate {
1797 // Check if a bound would previously have been removed when normalizing
1798 // the param_env so that it can be given the lowest priority. See
1799 // #50825 for the motivation for this.
1800 let is_global = |cand: &ty::PolyTraitPredicate<'tcx>| {
1801 cand.is_global() && !cand.has_late_bound_regions()
1804 // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
1805 // `DiscriminantKindCandidate`, `ConstDestructCandidate`
1806 // to anything else.
1808 // This is a fix for #53123 and prevents winnowing from accidentally extending the
1809 // lifetime of a variable.
1810 match (&other.candidate, &victim.candidate) {
1811 (_, AutoImplCandidate) | (AutoImplCandidate, _) => {
1813 "default implementations shouldn't be recorded \
1814 when there are other valid candidates"
1818 // FIXME(@jswrenn): this should probably be more sophisticated
1819 (TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => false,
1822 (BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_), _) => true,
1823 (_, BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_)) => false,
1825 (ParamCandidate(other), ParamCandidate(victim)) => {
1826 let same_except_bound_vars = other.skip_binder().trait_ref
1827 == victim.skip_binder().trait_ref
1828 && other.skip_binder().constness == victim.skip_binder().constness
1829 && other.skip_binder().polarity == victim.skip_binder().polarity
1830 && !other.skip_binder().trait_ref.has_escaping_bound_vars();
1831 if same_except_bound_vars {
1832 // See issue #84398. In short, we can generate multiple ParamCandidates which are
1833 // the same except for unused bound vars. Just pick the one with the fewest bound vars
1834 // or the current one if tied (they should both evaluate to the same answer). This is
1835 // probably best characterized as a "hack", since we might prefer to just do our
1836 // best to *not* create essentially duplicate candidates in the first place.
1837 other.bound_vars().len() <= victim.bound_vars().len()
1838 } else if other.skip_binder().trait_ref == victim.skip_binder().trait_ref
1839 && victim.skip_binder().constness == ty::BoundConstness::NotConst
1840 && other.skip_binder().polarity == victim.skip_binder().polarity
1842 // Drop otherwise equivalent non-const candidates in favor of const candidates.
1849 // Drop otherwise equivalent non-const fn pointer candidates
1850 (FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => true,
1852 // Global bounds from the where clause should be ignored
1853 // here (see issue #50825). Otherwise, we have a where
1854 // clause so don't go around looking for impls.
1855 // Arbitrarily give param candidates priority
1856 // over projection and object candidates.
1858 ParamCandidate(ref cand),
1861 | GeneratorCandidate
1863 | FnPointerCandidate { .. }
1864 | BuiltinObjectCandidate
1865 | BuiltinUnsizeCandidate
1866 | TraitUpcastingUnsizeCandidate(_)
1867 | BuiltinCandidate { .. }
1868 | TraitAliasCandidate
1869 | ObjectCandidate(_)
1870 | ProjectionCandidate(..),
1871 ) => !is_global(cand),
1872 (ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref cand)) => {
1873 // Prefer these to a global where-clause bound
1874 // (see issue #50825).
1880 | GeneratorCandidate
1882 | FnPointerCandidate { .. }
1883 | BuiltinObjectCandidate
1884 | BuiltinUnsizeCandidate
1885 | TraitUpcastingUnsizeCandidate(_)
1886 | BuiltinCandidate { has_nested: true }
1887 | TraitAliasCandidate,
1888 ParamCandidate(ref cand),
1890 // Prefer these to a global where-clause bound
1891 // (see issue #50825).
1892 is_global(cand) && other.evaluation.must_apply_modulo_regions()
1895 (ProjectionCandidate(i, _), ProjectionCandidate(j, _))
1896 | (ObjectCandidate(i), ObjectCandidate(j)) => {
1897 // Arbitrarily pick the lower numbered candidate for backwards
1898 // compatibility reasons. Don't let this affect inference.
1899 i < j && !needs_infer
1901 (ObjectCandidate(_), ProjectionCandidate(..))
1902 | (ProjectionCandidate(..), ObjectCandidate(_)) => {
1903 bug!("Have both object and projection candidate")
1906 // Arbitrarily give projection and object candidates priority.
1908 ObjectCandidate(_) | ProjectionCandidate(..),
1911 | GeneratorCandidate
1913 | FnPointerCandidate { .. }
1914 | BuiltinObjectCandidate
1915 | BuiltinUnsizeCandidate
1916 | TraitUpcastingUnsizeCandidate(_)
1917 | BuiltinCandidate { .. }
1918 | TraitAliasCandidate,
1924 | GeneratorCandidate
1926 | FnPointerCandidate { .. }
1927 | BuiltinObjectCandidate
1928 | BuiltinUnsizeCandidate
1929 | TraitUpcastingUnsizeCandidate(_)
1930 | BuiltinCandidate { .. }
1931 | TraitAliasCandidate,
1932 ObjectCandidate(_) | ProjectionCandidate(..),
1935 (&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
1936 // See if we can toss out `victim` based on specialization.
1937 // While this requires us to know *for sure* that the `other` impl applies
1938 // we still use modulo regions here.
1940 // This is fine as specialization currently assumes that specializing
1941 // impls have to be always applicable, meaning that the only allowed
1942 // region constraints may be constraints also present on the default impl.
1943 let tcx = self.tcx();
1944 if other.evaluation.must_apply_modulo_regions() {
1945 if tcx.specializes((other_def, victim_def)) {
1950 if other.evaluation.must_apply_considering_regions() {
1951 match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
1952 Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
1953 // Subtle: If the predicate we are evaluating has inference
1954 // variables, do *not* allow discarding candidates due to
1955 // marker trait impls.
1957 // Without this restriction, we could end up accidentally
1958 // constraining inference variables based on an arbitrarily
1959 // chosen trait impl.
1961 // Imagine we have the following code:
1964 // #[marker] trait MyTrait {}
1965 // impl MyTrait for u8 {}
1966 // impl MyTrait for bool {}
1969 // And we are evaluating the predicate `<_#0t as MyTrait>`.
1971 // During selection, we will end up with one candidate for each
1972 // impl of `MyTrait`. If we were to discard one impl in favor
1973 // of the other, we would be left with one candidate, causing
1974 // us to "successfully" select the predicate, unifying
1975 // _#0t with (for example) `u8`.
1977 // However, we have no reason to believe that this unification
1978 // is correct - we've essentially just picked an arbitrary
1979 // *possibility* for _#0t, and required that this be the *only*
1982 // Eventually, we will either:
1983 // 1) Unify all inference variables in the predicate through
1984 // some other means (e.g. type-checking of a function). We will
1985 // then be in a position to drop marker trait candidates
1986 // without constraining inference variables (since there are
1987 // none left to constrain)
1988 // 2) Be left with some unconstrained inference variables. We
1989 // will then correctly report an inference error, since the
1990 // existence of multiple marker trait impls tells us nothing
1991 // about which one should actually apply.
2002 // Everything else is ambiguous
2006 | GeneratorCandidate
2008 | FnPointerCandidate { .. }
2009 | BuiltinObjectCandidate
2010 | BuiltinUnsizeCandidate
2011 | TraitUpcastingUnsizeCandidate(_)
2012 | BuiltinCandidate { has_nested: true }
2013 | TraitAliasCandidate,
2016 | GeneratorCandidate
2018 | FnPointerCandidate { .. }
2019 | BuiltinObjectCandidate
2020 | BuiltinUnsizeCandidate
2021 | TraitUpcastingUnsizeCandidate(_)
2022 | BuiltinCandidate { has_nested: true }
2023 | TraitAliasCandidate,
2028 fn sized_conditions(
2030 obligation: &TraitObligation<'tcx>,
2031 ) -> BuiltinImplConditions<'tcx> {
2032 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2034 // NOTE: binder moved to (*)
2035 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2037 match self_ty.kind() {
2038 ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
2049 | ty::GeneratorWitness(..)
2053 | ty::Dynamic(_, _, ty::DynStar)
2055 // safe for everything
2056 Where(ty::Binder::dummy(Vec::new()))
2059 ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
2061 ty::Tuple(tys) => Where(
2062 obligation.predicate.rebind(tys.last().map_or_else(Vec::new, |&last| vec![last])),
2065 ty::Adt(def, substs) => {
2066 let sized_crit = def.sized_constraint(self.tcx());
2067 // (*) binder moved here
2068 Where(obligation.predicate.rebind({
2072 .map(|ty| sized_crit.rebind(*ty).subst(self.tcx(), substs))
2077 ty::Projection(_) | ty::Param(_) | ty::Opaque(..) => None,
2078 ty::Infer(ty::TyVar(_)) => Ambiguous,
2082 | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2083 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
2088 fn copy_clone_conditions(
2090 obligation: &TraitObligation<'tcx>,
2091 ) -> BuiltinImplConditions<'tcx> {
2092 // NOTE: binder moved to (*)
2093 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2095 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2097 match *self_ty.kind() {
2098 ty::Infer(ty::IntVar(_))
2099 | ty::Infer(ty::FloatVar(_))
2102 | ty::Error(_) => Where(ty::Binder::dummy(Vec::new())),
2111 | ty::Ref(_, _, hir::Mutability::Not)
2112 | ty::Array(..) => {
2113 // Implementations provided in libcore
2120 | ty::Generator(_, _, hir::Movability::Static)
2122 | ty::Ref(_, _, hir::Mutability::Mut) => None,
2125 // (*) binder moved here
2126 Where(obligation.predicate.rebind(tys.iter().collect()))
2129 ty::Generator(_, substs, hir::Movability::Movable) => {
2130 if self.tcx().features().generator_clone {
2131 let resolved_upvars =
2132 self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
2133 let resolved_witness =
2134 self.infcx.shallow_resolve(substs.as_generator().witness());
2135 if resolved_upvars.is_ty_var() || resolved_witness.is_ty_var() {
2136 // Not yet resolved.
2142 .chain(iter::once(substs.as_generator().witness()))
2143 .collect::<Vec<_>>();
2144 Where(obligation.predicate.rebind(all))
2151 ty::GeneratorWitness(binder) => {
2152 let witness_tys = binder.skip_binder();
2153 for witness_ty in witness_tys.iter() {
2154 let resolved = self.infcx.shallow_resolve(witness_ty);
2155 if resolved.is_ty_var() {
2159 // (*) binder moved here
2160 let all_vars = self.tcx().mk_bound_variable_kinds(
2161 obligation.predicate.bound_vars().iter().chain(binder.bound_vars().iter()),
2163 Where(ty::Binder::bind_with_vars(witness_tys.to_vec(), all_vars))
2166 ty::Closure(_, substs) => {
2167 // (*) binder moved here
2168 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
2169 if let ty::Infer(ty::TyVar(_)) = ty.kind() {
2170 // Not yet resolved.
2173 Where(obligation.predicate.rebind(substs.as_closure().upvar_tys().collect()))
2177 ty::Adt(..) | ty::Projection(..) | ty::Param(..) | ty::Opaque(..) => {
2178 // Fallback to whatever user-defined impls exist in this case.
2182 ty::Infer(ty::TyVar(_)) => {
2183 // Unbound type variable. Might or might not have
2184 // applicable impls and so forth, depending on what
2185 // those type variables wind up being bound to.
2191 | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2192 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
2197 /// For default impls, we need to break apart a type into its
2198 /// "constituent types" -- meaning, the types that it contains.
2200 /// Here are some (simple) examples:
2202 /// ```ignore (illustrative)
2203 /// (i32, u32) -> [i32, u32]
2204 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2205 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2206 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2208 #[instrument(level = "debug", skip(self), ret)]
2209 fn constituent_types_for_ty(
2211 t: ty::Binder<'tcx, Ty<'tcx>>,
2212 ) -> ty::Binder<'tcx, Vec<Ty<'tcx>>> {
2213 match *t.skip_binder().kind() {
2222 | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
2224 | ty::Char => ty::Binder::dummy(Vec::new()),
2230 | ty::Projection(..)
2232 | ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2233 bug!("asked to assemble constituent types of unexpected type: {:?}", t);
2236 ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => {
2237 t.rebind(vec![element_ty])
2240 ty::Array(element_ty, _) | ty::Slice(element_ty) => t.rebind(vec![element_ty]),
2242 ty::Tuple(ref tys) => {
2243 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2244 t.rebind(tys.iter().collect())
2247 ty::Closure(_, ref substs) => {
2248 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
2252 ty::Generator(_, ref substs, _) => {
2253 let ty = self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
2254 let witness = substs.as_generator().witness();
2255 t.rebind([ty].into_iter().chain(iter::once(witness)).collect())
2258 ty::GeneratorWitness(types) => {
2259 debug_assert!(!types.has_escaping_bound_vars());
2260 types.map_bound(|types| types.to_vec())
2263 // For `PhantomData<T>`, we pass `T`.
2264 ty::Adt(def, substs) if def.is_phantom_data() => t.rebind(substs.types().collect()),
2266 ty::Adt(def, substs) => {
2267 t.rebind(def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect())
2270 ty::Opaque(def_id, substs) => {
2271 // We can resolve the `impl Trait` to its concrete type,
2272 // which enforces a DAG between the functions requiring
2273 // the auto trait bounds in question.
2274 t.rebind(vec![self.tcx().bound_type_of(def_id).subst(self.tcx(), substs)])
2279 fn collect_predicates_for_types(
2281 param_env: ty::ParamEnv<'tcx>,
2282 cause: ObligationCause<'tcx>,
2283 recursion_depth: usize,
2284 trait_def_id: DefId,
2285 types: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
2286 ) -> Vec<PredicateObligation<'tcx>> {
2287 // Because the types were potentially derived from
2288 // higher-ranked obligations they may reference late-bound
2289 // regions. For example, `for<'a> Foo<&'a i32> : Copy` would
2290 // yield a type like `for<'a> &'a i32`. In general, we
2291 // maintain the invariant that we never manipulate bound
2292 // regions, so we have to process these bound regions somehow.
2294 // The strategy is to:
2296 // 1. Instantiate those regions to placeholder regions (e.g.,
2297 // `for<'a> &'a i32` becomes `&0 i32`.
2298 // 2. Produce something like `&'0 i32 : Copy`
2299 // 3. Re-bind the regions back to `for<'a> &'a i32 : Copy`
2303 .skip_binder() // binder moved -\
2306 let ty: ty::Binder<'tcx, Ty<'tcx>> = types.rebind(*ty); // <----/
2308 let placeholder_ty = self.infcx.replace_bound_vars_with_placeholders(ty);
2309 let Normalized { value: normalized_ty, mut obligations } =
2310 ensure_sufficient_stack(|| {
2311 project::normalize_with_depth(
2319 let placeholder_obligation = predicate_for_trait_def(
2327 obligations.push(placeholder_obligation);
2333 ///////////////////////////////////////////////////////////////////////////
2336 // Matching is a common path used for both evaluation and
2337 // confirmation. It basically unifies types that appear in impls
2338 // and traits. This does affect the surrounding environment;
2339 // therefore, when used during evaluation, match routines must be
2340 // run inside of a `probe()` so that their side-effects are
2346 obligation: &TraitObligation<'tcx>,
2347 ) -> Normalized<'tcx, SubstsRef<'tcx>> {
2348 let impl_trait_ref = self.tcx().bound_impl_trait_ref(impl_def_id).unwrap();
2349 match self.match_impl(impl_def_id, impl_trait_ref, obligation) {
2350 Ok(substs) => substs,
2352 // FIXME: A rematch may fail when a candidate cache hit occurs
2353 // on thefreshened form of the trait predicate, but the match
2354 // fails for some reason that is not captured in the freshened
2355 // cache key. For example, equating an impl trait ref against
2356 // the placeholder trait ref may fail due the Generalizer relation
2357 // raising a CyclicalTy error due to a sub_root_var relation
2358 // for a variable being generalized...
2359 self.infcx.tcx.sess.delay_span_bug(
2360 obligation.cause.span,
2362 "Impl {:?} was matchable against {:?} but now is not",
2363 impl_def_id, obligation
2366 let value = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
2367 let err = self.tcx().ty_error();
2368 let value = value.fold_with(&mut BottomUpFolder {
2374 Normalized { value, obligations: vec![] }
2379 #[instrument(level = "debug", skip(self), ret)]
2383 impl_trait_ref: EarlyBinder<ty::TraitRef<'tcx>>,
2384 obligation: &TraitObligation<'tcx>,
2385 ) -> Result<Normalized<'tcx, SubstsRef<'tcx>>, ()> {
2386 let placeholder_obligation =
2387 self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
2388 let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
2390 let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
2392 let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs);
2394 debug!(?impl_trait_ref);
2396 let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
2397 ensure_sufficient_stack(|| {
2398 project::normalize_with_depth(
2400 obligation.param_env,
2401 obligation.cause.clone(),
2402 obligation.recursion_depth + 1,
2407 debug!(?impl_trait_ref, ?placeholder_obligation_trait_ref);
2409 let cause = ObligationCause::new(
2410 obligation.cause.span,
2411 obligation.cause.body_id,
2412 ObligationCauseCode::MatchImpl(obligation.cause.clone(), impl_def_id),
2415 let InferOk { obligations, .. } = self
2417 .at(&cause, obligation.param_env)
2418 .define_opaque_types(false)
2419 .eq(placeholder_obligation_trait_ref, impl_trait_ref)
2420 .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{e}`"))?;
2421 nested_obligations.extend(obligations);
2423 if !self.is_intercrate()
2424 && self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation
2426 debug!("reservation impls only apply in intercrate mode");
2430 Ok(Normalized { value: impl_substs, obligations: nested_obligations })
2433 fn fast_reject_trait_refs(
2435 obligation: &TraitObligation<'tcx>,
2436 impl_trait_ref: &ty::TraitRef<'tcx>,
2438 // We can avoid creating type variables and doing the full
2439 // substitution if we find that any of the input types, when
2440 // simplified, do not match.
2441 let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
2442 iter::zip(obligation.predicate.skip_binder().trait_ref.substs, impl_trait_ref.substs)
2443 .any(|(obl, imp)| !drcx.generic_args_may_unify(obl, imp))
2446 /// Normalize `where_clause_trait_ref` and try to match it against
2447 /// `obligation`. If successful, return any predicates that
2448 /// result from the normalization.
2449 fn match_where_clause_trait_ref(
2451 obligation: &TraitObligation<'tcx>,
2452 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
2453 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
2454 self.match_poly_trait_ref(obligation, where_clause_trait_ref)
2457 /// Returns `Ok` if `poly_trait_ref` being true implies that the
2458 /// obligation is satisfied.
2459 #[instrument(skip(self), level = "debug")]
2460 fn match_poly_trait_ref(
2462 obligation: &TraitObligation<'tcx>,
2463 poly_trait_ref: ty::PolyTraitRef<'tcx>,
2464 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
2466 .at(&obligation.cause, obligation.param_env)
2467 // We don't want predicates for opaque types to just match all other types,
2468 // if there is an obligation on the opaque type, then that obligation must be met
2469 // opaquely. Otherwise we'd match any obligation to the opaque type and then error
2471 .define_opaque_types(false)
2472 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
2473 .map(|InferOk { obligations, .. }| obligations)
2477 ///////////////////////////////////////////////////////////////////////////
2480 fn match_fresh_trait_refs(
2482 previous: ty::PolyTraitPredicate<'tcx>,
2483 current: ty::PolyTraitPredicate<'tcx>,
2484 param_env: ty::ParamEnv<'tcx>,
2486 let mut matcher = ty::_match::Match::new(self.tcx(), param_env);
2487 matcher.relate(previous, current).is_ok()
2492 previous_stack: TraitObligationStackList<'o, 'tcx>,
2493 obligation: &'o TraitObligation<'tcx>,
2494 ) -> TraitObligationStack<'o, 'tcx> {
2495 let fresh_trait_pred = obligation.predicate.fold_with(&mut self.freshener);
2497 let dfn = previous_stack.cache.next_dfn();
2498 let depth = previous_stack.depth() + 1;
2499 TraitObligationStack {
2502 reached_depth: Cell::new(depth),
2503 previous: previous_stack,
2509 #[instrument(skip(self), level = "debug")]
2510 fn closure_trait_ref_unnormalized(
2512 obligation: &TraitObligation<'tcx>,
2513 substs: SubstsRef<'tcx>,
2514 ) -> ty::PolyTraitRef<'tcx> {
2515 let closure_sig = substs.as_closure().sig();
2517 debug!(?closure_sig);
2519 // NOTE: The self-type is an unboxed closure type and hence is
2520 // in fact unparameterized (or at least does not reference any
2521 // regions bound in the obligation).
2522 let self_ty = obligation
2526 .expect("unboxed closure type should not capture bound vars from the predicate");
2528 closure_trait_ref_and_return_type(
2530 obligation.predicate.def_id(),
2533 util::TupleArgumentsFlag::No,
2535 .map_bound(|(trait_ref, _)| trait_ref)
2538 /// Returns the obligations that are implied by instantiating an
2539 /// impl or trait. The obligations are substituted and fully
2540 /// normalized. This is used when confirming an impl or default
2542 #[instrument(level = "debug", skip(self, cause, param_env))]
2543 fn impl_or_trait_obligations(
2545 cause: &ObligationCause<'tcx>,
2546 recursion_depth: usize,
2547 param_env: ty::ParamEnv<'tcx>,
2548 def_id: DefId, // of impl or trait
2549 substs: SubstsRef<'tcx>, // for impl or trait
2550 parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
2551 ) -> Vec<PredicateObligation<'tcx>> {
2552 let tcx = self.tcx();
2554 // To allow for one-pass evaluation of the nested obligation,
2555 // each predicate must be preceded by the obligations required
2557 // for example, if we have:
2558 // impl<U: Iterator<Item: Copy>, V: Iterator<Item = U>> Foo for V
2559 // the impl will have the following predicates:
2560 // <V as Iterator>::Item = U,
2561 // U: Iterator, U: Sized,
2562 // V: Iterator, V: Sized,
2563 // <U as Iterator>::Item: Copy
2564 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2565 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2566 // `$1: Copy`, so we must ensure the obligations are emitted in
2568 let predicates = tcx.bound_predicates_of(def_id);
2569 debug!(?predicates);
2570 assert_eq!(predicates.0.parent, None);
2571 let mut obligations = Vec::with_capacity(predicates.0.predicates.len());
2572 for (predicate, span) in predicates.0.predicates {
2574 let cause = cause.clone().derived_cause(parent_trait_pred, |derived| {
2575 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
2577 impl_def_id: def_id,
2581 let predicate = normalize_with_depth_to(
2586 predicates.rebind(*predicate).subst(tcx, substs),
2589 obligations.push(Obligation { cause, recursion_depth, param_env, predicate });
2596 impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
2597 fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2598 TraitObligationStackList::with(self)
2601 fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx> {
2605 fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2609 /// Indicates that attempting to evaluate this stack entry
2610 /// required accessing something from the stack at depth `reached_depth`.
2611 fn update_reached_depth(&self, reached_depth: usize) {
2613 self.depth >= reached_depth,
2614 "invoked `update_reached_depth` with something under this stack: \
2615 self.depth={} reached_depth={}",
2619 debug!(reached_depth, "update_reached_depth");
2621 while reached_depth < p.depth {
2622 debug!(?p.fresh_trait_pred, "update_reached_depth: marking as cycle participant");
2623 p.reached_depth.set(p.reached_depth.get().min(reached_depth));
2624 p = p.previous.head.unwrap();
2629 /// The "provisional evaluation cache" is used to store intermediate cache results
2630 /// when solving auto traits. Auto traits are unusual in that they can support
2631 /// cycles. So, for example, a "proof tree" like this would be ok:
2633 /// - `Foo<T>: Send` :-
2634 /// - `Bar<T>: Send` :-
2635 /// - `Foo<T>: Send` -- cycle, but ok
2636 /// - `Baz<T>: Send`
2638 /// Here, to prove `Foo<T>: Send`, we have to prove `Bar<T>: Send` and
2639 /// `Baz<T>: Send`. Proving `Bar<T>: Send` in turn required `Foo<T>: Send`.
2640 /// For non-auto traits, this cycle would be an error, but for auto traits (because
2641 /// they are coinductive) it is considered ok.
2643 /// However, there is a complication: at the point where we have
2644 /// "proven" `Bar<T>: Send`, we have in fact only proven it
2645 /// *provisionally*. In particular, we proved that `Bar<T>: Send`
2646 /// *under the assumption* that `Foo<T>: Send`. But what if we later
2647 /// find out this assumption is wrong? Specifically, we could
2648 /// encounter some kind of error proving `Baz<T>: Send`. In that case,
2649 /// `Bar<T>: Send` didn't turn out to be true.
2651 /// In Issue #60010, we found a bug in rustc where it would cache
2652 /// these intermediate results. This was fixed in #60444 by disabling
2653 /// *all* caching for things involved in a cycle -- in our example,
2654 /// that would mean we don't cache that `Bar<T>: Send`. But this led
2655 /// to large slowdowns.
2657 /// Specifically, imagine this scenario, where proving `Baz<T>: Send`
2658 /// first requires proving `Bar<T>: Send` (which is true:
2660 /// - `Foo<T>: Send` :-
2661 /// - `Bar<T>: Send` :-
2662 /// - `Foo<T>: Send` -- cycle, but ok
2663 /// - `Baz<T>: Send`
2664 /// - `Bar<T>: Send` -- would be nice for this to be a cache hit!
2665 /// - `*const T: Send` -- but what if we later encounter an error?
2667 /// The *provisional evaluation cache* resolves this issue. It stores
2668 /// cache results that we've proven but which were involved in a cycle
2669 /// in some way. We track the minimal stack depth (i.e., the
2670 /// farthest from the top of the stack) that we are dependent on.
2671 /// The idea is that the cache results within are all valid -- so long as
2672 /// none of the nodes in between the current node and the node at that minimum
2673 /// depth result in an error (in which case the cached results are just thrown away).
2675 /// During evaluation, we consult this provisional cache and rely on
2676 /// it. Accessing a cached value is considered equivalent to accessing
2677 /// a result at `reached_depth`, so it marks the *current* solution as
2678 /// provisional as well. If an error is encountered, we toss out any
2679 /// provisional results added from the subtree that encountered the
2680 /// error. When we pop the node at `reached_depth` from the stack, we
2681 /// can commit all the things that remain in the provisional cache.
2682 struct ProvisionalEvaluationCache<'tcx> {
2683 /// next "depth first number" to issue -- just a counter
2686 /// Map from cache key to the provisionally evaluated thing.
2687 /// The cache entries contain the result but also the DFN in which they
2688 /// were added. The DFN is used to clear out values on failure.
2690 /// Imagine we have a stack like:
2692 /// - `A B C` and we add a cache for the result of C (DFN 2)
2693 /// - Then we have a stack `A B D` where `D` has DFN 3
2694 /// - We try to solve D by evaluating E: `A B D E` (DFN 4)
2695 /// - `E` generates various cache entries which have cyclic dependencies on `B`
2696 /// - `A B D E F` and so forth
2697 /// - the DFN of `F` for example would be 5
2698 /// - then we determine that `E` is in error -- we will then clear
2699 /// all cache values whose DFN is >= 4 -- in this case, that
2700 /// means the cached value for `F`.
2701 map: RefCell<FxHashMap<ty::PolyTraitPredicate<'tcx>, ProvisionalEvaluation>>,
2703 /// The stack of args that we assume to be true because a `WF(arg)` predicate
2704 /// is on the stack above (and because of wellformedness is coinductive).
2705 /// In an "ideal" world, this would share a stack with trait predicates in
2706 /// `TraitObligationStack`. However, trait predicates are *much* hotter than
2707 /// `WellFormed` predicates, and it's very likely that the additional matches
2708 /// will have a perf effect. The value here is the well-formed `GenericArg`
2709 /// and the depth of the trait predicate *above* that well-formed predicate.
2710 wf_args: RefCell<Vec<(ty::GenericArg<'tcx>, usize)>>,
2713 /// A cache value for the provisional cache: contains the depth-first
2714 /// number (DFN) and result.
2715 #[derive(Copy, Clone, Debug)]
2716 struct ProvisionalEvaluation {
2718 reached_depth: usize,
2719 result: EvaluationResult,
2722 impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> {
2723 fn default() -> Self {
2724 Self { dfn: Cell::new(0), map: Default::default(), wf_args: Default::default() }
2728 impl<'tcx> ProvisionalEvaluationCache<'tcx> {
2729 /// Get the next DFN in sequence (basically a counter).
2730 fn next_dfn(&self) -> usize {
2731 let result = self.dfn.get();
2732 self.dfn.set(result + 1);
2736 /// Check the provisional cache for any result for
2737 /// `fresh_trait_ref`. If there is a hit, then you must consider
2738 /// it an access to the stack slots at depth
2739 /// `reached_depth` (from the returned value).
2742 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
2743 ) -> Option<ProvisionalEvaluation> {
2746 "get_provisional = {:#?}",
2747 self.map.borrow().get(&fresh_trait_pred),
2749 Some(*self.map.borrow().get(&fresh_trait_pred)?)
2752 /// Insert a provisional result into the cache. The result came
2753 /// from the node with the given DFN. It accessed a minimum depth
2754 /// of `reached_depth` to compute. It evaluated `fresh_trait_pred`
2755 /// and resulted in `result`.
2756 fn insert_provisional(
2759 reached_depth: usize,
2760 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
2761 result: EvaluationResult,
2763 debug!(?from_dfn, ?fresh_trait_pred, ?result, "insert_provisional");
2765 let mut map = self.map.borrow_mut();
2767 // Subtle: when we complete working on the DFN `from_dfn`, anything
2768 // that remains in the provisional cache must be dependent on some older
2769 // stack entry than `from_dfn`. We have to update their depth with our transitive
2770 // depth in that case or else it would be referring to some popped note.
2773 // A (reached depth 0)
2775 // B // depth 1 -- reached depth = 0
2776 // C // depth 2 -- reached depth = 1 (should be 0)
2779 // D (reached depth 1)
2780 // C (cache -- reached depth = 2)
2781 for (_k, v) in &mut *map {
2782 if v.from_dfn >= from_dfn {
2783 v.reached_depth = reached_depth.min(v.reached_depth);
2787 map.insert(fresh_trait_pred, ProvisionalEvaluation { from_dfn, reached_depth, result });
2790 /// Invoked when the node with dfn `dfn` does not get a successful
2791 /// result. This will clear out any provisional cache entries
2792 /// that were added since `dfn` was created. This is because the
2793 /// provisional entries are things which must assume that the
2794 /// things on the stack at the time of their creation succeeded --
2795 /// since the failing node is presently at the top of the stack,
2796 /// these provisional entries must either depend on it or some
2798 fn on_failure(&self, dfn: usize) {
2799 debug!(?dfn, "on_failure");
2800 self.map.borrow_mut().retain(|key, eval| {
2801 if !eval.from_dfn >= dfn {
2802 debug!("on_failure: removing {:?}", key);
2810 /// Invoked when the node at depth `depth` completed without
2811 /// depending on anything higher in the stack (if that completion
2812 /// was a failure, then `on_failure` should have been invoked
2815 /// Note that we may still have provisional cache items remaining
2816 /// in the cache when this is done. For example, if there is a
2819 /// * A depends on...
2820 /// * B depends on A
2821 /// * C depends on...
2822 /// * D depends on C
2825 /// Then as we complete the C node we will have a provisional cache
2826 /// with results for A, B, C, and D. This method would clear out
2827 /// the C and D results, but leave A and B provisional.
2829 /// This is determined based on the DFN: we remove any provisional
2830 /// results created since `dfn` started (e.g., in our example, dfn
2831 /// would be 2, representing the C node, and hence we would
2832 /// remove the result for D, which has DFN 3, but not the results for
2833 /// A and B, which have DFNs 0 and 1 respectively).
2835 /// Note that we *do not* attempt to cache these cycle participants
2836 /// in the evaluation cache. Doing so would require carefully computing
2837 /// the correct `DepNode` to store in the cache entry:
2838 /// cycle participants may implicitly depend on query results
2839 /// related to other participants in the cycle, due to our logic
2840 /// which examines the evaluation stack.
2842 /// We used to try to perform this caching,
2843 /// but it lead to multiple incremental compilation ICEs
2844 /// (see #92987 and #96319), and was very hard to understand.
2845 /// Fortunately, removing the caching didn't seem to
2846 /// have a performance impact in practice.
2847 fn on_completion(&self, dfn: usize) {
2848 debug!(?dfn, "on_completion");
2850 for (fresh_trait_pred, eval) in
2851 self.map.borrow_mut().drain_filter(|_k, eval| eval.from_dfn >= dfn)
2853 debug!(?fresh_trait_pred, ?eval, "on_completion");
2858 #[derive(Copy, Clone)]
2859 struct TraitObligationStackList<'o, 'tcx> {
2860 cache: &'o ProvisionalEvaluationCache<'tcx>,
2861 head: Option<&'o TraitObligationStack<'o, 'tcx>>,
2864 impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> {
2865 fn empty(cache: &'o ProvisionalEvaluationCache<'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2866 TraitObligationStackList { cache, head: None }
2869 fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2870 TraitObligationStackList { cache: r.cache(), head: Some(r) }
2873 fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2877 fn depth(&self) -> usize {
2878 if let Some(head) = self.head { head.depth } else { 0 }
2882 impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> {
2883 type Item = &'o TraitObligationStack<'o, 'tcx>;
2885 fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2892 impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
2893 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2894 write!(f, "TraitObligationStack({:?})", self.obligation)
2898 pub enum ProjectionMatchesProjection {