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::relate::TypeRelation;
47 use rustc_middle::ty::SubstsRef;
48 use rustc_middle::ty::{self, EarlyBinder, PolyProjectionPredicate, ToPolyTraitRef, ToPredicate};
49 use rustc_middle::ty::{Ty, TyCtxt, TypeFoldable, TypeVisitable};
50 use rustc_span::symbol::sym;
52 use std::cell::{Cell, RefCell};
54 use std::fmt::{self, Display};
57 pub use rustc_middle::traits::select::*;
58 use rustc_middle::ty::print::with_no_trimmed_paths;
60 mod candidate_assembly;
63 #[derive(Clone, Debug, Eq, PartialEq, Hash)]
64 pub enum IntercrateAmbiguityCause {
65 DownstreamCrate { trait_desc: String, self_desc: Option<String> },
66 UpstreamCrateUpdate { trait_desc: String, self_desc: Option<String> },
67 ReservationImpl { message: String },
70 impl IntercrateAmbiguityCause {
71 /// Emits notes when the overlap is caused by complex intercrate ambiguities.
72 /// See #23980 for details.
73 pub fn add_intercrate_ambiguity_hint(&self, err: &mut Diagnostic) {
74 err.note(&self.intercrate_ambiguity_hint());
77 pub fn intercrate_ambiguity_hint(&self) -> String {
79 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc } => {
80 let self_desc = if let Some(ty) = self_desc {
81 format!(" for type `{}`", ty)
85 format!("downstream crates may implement trait `{}`{}", trait_desc, self_desc)
87 IntercrateAmbiguityCause::UpstreamCrateUpdate { trait_desc, self_desc } => {
88 let self_desc = if let Some(ty) = self_desc {
89 format!(" for type `{}`", ty)
94 "upstream crates may add a new impl of trait `{}`{} \
99 IntercrateAmbiguityCause::ReservationImpl { message } => message.clone(),
104 pub struct SelectionContext<'cx, 'tcx> {
105 pub infcx: &'cx InferCtxt<'tcx>,
107 /// Freshener used specifically for entries on the obligation
108 /// stack. This ensures that all entries on the stack at one time
109 /// will have the same set of placeholder entries, which is
110 /// important for checking for trait bounds that recursively
111 /// require themselves.
112 freshener: TypeFreshener<'cx, 'tcx>,
114 /// If `intercrate` is set, we remember predicates which were
115 /// considered ambiguous because of impls potentially added in other crates.
116 /// This is used in coherence to give improved diagnostics.
117 /// We don't do his until we detect a coherence error because it can
118 /// lead to false overflow results (#47139) and because always
119 /// computing it may negatively impact performance.
120 intercrate_ambiguity_causes: Option<FxIndexSet<IntercrateAmbiguityCause>>,
122 /// The mode that trait queries run in, which informs our error handling
123 /// policy. In essence, canonicalized queries need their errors propagated
124 /// rather than immediately reported because we do not have accurate spans.
125 query_mode: TraitQueryMode,
128 // A stack that walks back up the stack frame.
129 struct TraitObligationStack<'prev, 'tcx> {
130 obligation: &'prev TraitObligation<'tcx>,
132 /// The trait predicate from `obligation` but "freshened" with the
133 /// selection-context's freshener. Used to check for recursion.
134 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
136 /// Starts out equal to `depth` -- if, during evaluation, we
137 /// encounter a cycle, then we will set this flag to the minimum
138 /// depth of that cycle for all participants in the cycle. These
139 /// participants will then forego caching their results. This is
140 /// not the most efficient solution, but it addresses #60010. The
141 /// problem we are trying to prevent:
143 /// - If you have `A: AutoTrait` requires `B: AutoTrait` and `C: NonAutoTrait`
144 /// - `B: AutoTrait` requires `A: AutoTrait` (coinductive cycle, ok)
145 /// - `C: NonAutoTrait` requires `A: AutoTrait` (non-coinductive cycle, not ok)
147 /// you don't want to cache that `B: AutoTrait` or `A: AutoTrait`
148 /// is `EvaluatedToOk`; this is because they were only considered
149 /// ok on the premise that if `A: AutoTrait` held, but we indeed
150 /// encountered a problem (later on) with `A: AutoTrait. So we
151 /// currently set a flag on the stack node for `B: AutoTrait` (as
152 /// well as the second instance of `A: AutoTrait`) to suppress
155 /// This is a simple, targeted fix. A more-performant fix requires
156 /// deeper changes, but would permit more caching: we could
157 /// basically defer caching until we have fully evaluated the
158 /// tree, and then cache the entire tree at once. In any case, the
159 /// performance impact here shouldn't be so horrible: every time
160 /// this is hit, we do cache at least one trait, so we only
161 /// evaluate each member of a cycle up to N times, where N is the
162 /// length of the cycle. This means the performance impact is
163 /// bounded and we shouldn't have any terrible worst-cases.
164 reached_depth: Cell<usize>,
166 previous: TraitObligationStackList<'prev, 'tcx>,
168 /// The number of parent frames plus one (thus, the topmost frame has depth 1).
171 /// The depth-first number of this node in the search graph -- a
172 /// pre-order index. Basically, a freshly incremented counter.
176 struct SelectionCandidateSet<'tcx> {
177 // A list of candidates that definitely apply to the current
178 // obligation (meaning: types unify).
179 vec: Vec<SelectionCandidate<'tcx>>,
181 // If `true`, then there were candidates that might or might
182 // not have applied, but we couldn't tell. This occurs when some
183 // of the input types are type variables, in which case there are
184 // various "builtin" rules that might or might not trigger.
188 #[derive(PartialEq, Eq, Debug, Clone)]
189 struct EvaluatedCandidate<'tcx> {
190 candidate: SelectionCandidate<'tcx>,
191 evaluation: EvaluationResult,
194 /// When does the builtin impl for `T: Trait` apply?
196 enum BuiltinImplConditions<'tcx> {
197 /// The impl is conditional on `T1, T2, ...: Trait`.
198 Where(ty::Binder<'tcx, Vec<Ty<'tcx>>>),
199 /// There is no built-in impl. There may be some other
200 /// candidate (a where-clause or user-defined impl).
202 /// It is unknown whether there is an impl.
206 impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
207 pub fn new(infcx: &'cx InferCtxt<'tcx>) -> SelectionContext<'cx, 'tcx> {
210 freshener: infcx.freshener_keep_static(),
211 intercrate_ambiguity_causes: None,
212 query_mode: TraitQueryMode::Standard,
216 pub fn with_query_mode(
217 infcx: &'cx InferCtxt<'tcx>,
218 query_mode: TraitQueryMode,
219 ) -> SelectionContext<'cx, 'tcx> {
220 debug!(?query_mode, "with_query_mode");
221 SelectionContext { query_mode, ..SelectionContext::new(infcx) }
224 /// Enables tracking of intercrate ambiguity causes. See
225 /// the documentation of [`Self::intercrate_ambiguity_causes`] for more.
226 pub fn enable_tracking_intercrate_ambiguity_causes(&mut self) {
227 assert!(self.is_intercrate());
228 assert!(self.intercrate_ambiguity_causes.is_none());
229 self.intercrate_ambiguity_causes = Some(FxIndexSet::default());
230 debug!("selcx: enable_tracking_intercrate_ambiguity_causes");
233 /// Gets the intercrate ambiguity causes collected since tracking
234 /// was enabled and disables tracking at the same time. If
235 /// tracking is not enabled, just returns an empty vector.
236 pub fn take_intercrate_ambiguity_causes(&mut self) -> FxIndexSet<IntercrateAmbiguityCause> {
237 assert!(self.is_intercrate());
238 self.intercrate_ambiguity_causes.take().unwrap_or_default()
241 pub fn tcx(&self) -> TyCtxt<'tcx> {
245 pub fn is_intercrate(&self) -> bool {
246 self.infcx.intercrate
249 ///////////////////////////////////////////////////////////////////////////
252 // The selection phase tries to identify *how* an obligation will
253 // be resolved. For example, it will identify which impl or
254 // parameter bound is to be used. The process can be inconclusive
255 // if the self type in the obligation is not fully inferred. Selection
256 // can result in an error in one of two ways:
258 // 1. If no applicable impl or parameter bound can be found.
259 // 2. If the output type parameters in the obligation do not match
260 // those specified by the impl/bound. For example, if the obligation
261 // is `Vec<Foo>: Iterable<Bar>`, but the impl specifies
262 // `impl<T> Iterable<T> for Vec<T>`, than an error would result.
264 /// Attempts to satisfy the obligation. If successful, this will affect the surrounding
265 /// type environment by performing unification.
266 #[instrument(level = "debug", skip(self), ret)]
269 obligation: &TraitObligation<'tcx>,
270 ) -> SelectionResult<'tcx, Selection<'tcx>> {
271 let candidate = match self.select_from_obligation(obligation) {
272 Err(SelectionError::Overflow(OverflowError::Canonical)) => {
273 // In standard mode, overflow must have been caught and reported
275 assert!(self.query_mode == TraitQueryMode::Canonical);
276 return Err(SelectionError::Overflow(OverflowError::Canonical));
284 Ok(Some(candidate)) => candidate,
287 match self.confirm_candidate(obligation, candidate) {
288 Err(SelectionError::Overflow(OverflowError::Canonical)) => {
289 assert!(self.query_mode == TraitQueryMode::Canonical);
290 Err(SelectionError::Overflow(OverflowError::Canonical))
293 Ok(candidate) => Ok(Some(candidate)),
297 pub(crate) fn select_from_obligation(
299 obligation: &TraitObligation<'tcx>,
300 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
301 debug_assert!(!obligation.predicate.has_escaping_bound_vars());
303 let pec = &ProvisionalEvaluationCache::default();
304 let stack = self.push_stack(TraitObligationStackList::empty(pec), obligation);
306 self.candidate_from_obligation(&stack)
309 #[instrument(level = "debug", skip(self), ret)]
310 fn candidate_from_obligation<'o>(
312 stack: &TraitObligationStack<'o, 'tcx>,
313 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
314 // Watch out for overflow. This intentionally bypasses (and does
315 // not update) the cache.
316 self.check_recursion_limit(&stack.obligation, &stack.obligation)?;
318 // Check the cache. Note that we freshen the trait-ref
319 // separately rather than using `stack.fresh_trait_ref` --
320 // this is because we want the unbound variables to be
321 // replaced with fresh types starting from index 0.
322 let cache_fresh_trait_pred = self.infcx.freshen(stack.obligation.predicate);
323 debug!(?cache_fresh_trait_pred);
324 debug_assert!(!stack.obligation.predicate.has_escaping_bound_vars());
327 self.check_candidate_cache(stack.obligation.param_env, cache_fresh_trait_pred)
333 // If no match, compute result and insert into cache.
335 // FIXME(nikomatsakis) -- this cache is not taking into
336 // account cycles that may have occurred in forming the
337 // candidate. I don't know of any specific problems that
338 // result but it seems awfully suspicious.
339 let (candidate, dep_node) =
340 self.in_task(|this| this.candidate_from_obligation_no_cache(stack));
342 debug!("CACHE MISS");
343 self.insert_candidate_cache(
344 stack.obligation.param_env,
345 cache_fresh_trait_pred,
352 fn candidate_from_obligation_no_cache<'o>(
354 stack: &TraitObligationStack<'o, 'tcx>,
355 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
356 if let Err(conflict) = self.is_knowable(stack) {
357 debug!("coherence stage: not knowable");
358 if self.intercrate_ambiguity_causes.is_some() {
359 debug!("evaluate_stack: intercrate_ambiguity_causes is some");
360 // Heuristics: show the diagnostics when there are no candidates in crate.
361 if let Ok(candidate_set) = self.assemble_candidates(stack) {
362 let mut no_candidates_apply = true;
364 for c in candidate_set.vec.iter() {
365 if self.evaluate_candidate(stack, &c)?.may_apply() {
366 no_candidates_apply = false;
371 if !candidate_set.ambiguous && no_candidates_apply {
372 let trait_ref = stack.obligation.predicate.skip_binder().trait_ref;
373 if !trait_ref.references_error() {
374 let self_ty = trait_ref.self_ty();
375 let (trait_desc, self_desc) = with_no_trimmed_paths!({
376 let trait_desc = trait_ref.print_only_trait_path().to_string();
377 let self_desc = if self_ty.has_concrete_skeleton() {
378 Some(self_ty.to_string())
382 (trait_desc, self_desc)
384 let cause = if let Conflict::Upstream = conflict {
385 IntercrateAmbiguityCause::UpstreamCrateUpdate {
390 IntercrateAmbiguityCause::DownstreamCrate { trait_desc, self_desc }
392 debug!(?cause, "evaluate_stack: pushing cause");
393 self.intercrate_ambiguity_causes.as_mut().unwrap().insert(cause);
401 let candidate_set = self.assemble_candidates(stack)?;
403 if candidate_set.ambiguous {
404 debug!("candidate set contains ambig");
408 let candidates = candidate_set.vec;
410 debug!(?stack, ?candidates, "assembled {} candidates", candidates.len());
412 // At this point, we know that each of the entries in the
413 // candidate set is *individually* applicable. Now we have to
414 // figure out if they contain mutual incompatibilities. This
415 // frequently arises if we have an unconstrained input type --
416 // for example, we are looking for `$0: Eq` where `$0` is some
417 // unconstrained type variable. In that case, we'll get a
418 // candidate which assumes $0 == int, one that assumes `$0 ==
419 // usize`, etc. This spells an ambiguity.
421 let mut candidates = self.filter_impls(candidates, stack.obligation);
423 // If there is more than one candidate, first winnow them down
424 // by considering extra conditions (nested obligations and so
425 // forth). We don't winnow if there is exactly one
426 // candidate. This is a relatively minor distinction but it
427 // can lead to better inference and error-reporting. An
428 // example would be if there was an impl:
430 // impl<T:Clone> Vec<T> { fn push_clone(...) { ... } }
432 // and we were to see some code `foo.push_clone()` where `boo`
433 // is a `Vec<Bar>` and `Bar` does not implement `Clone`. If
434 // we were to winnow, we'd wind up with zero candidates.
435 // Instead, we select the right impl now but report "`Bar` does
436 // not implement `Clone`".
437 if candidates.len() == 1 {
438 return self.filter_reservation_impls(candidates.pop().unwrap(), stack.obligation);
441 // Winnow, but record the exact outcome of evaluation, which
442 // is needed for specialization. Propagate overflow if it occurs.
443 let mut candidates = candidates
445 .map(|c| match self.evaluate_candidate(stack, &c) {
446 Ok(eval) if eval.may_apply() => {
447 Ok(Some(EvaluatedCandidate { candidate: c, evaluation: eval }))
450 Err(OverflowError::Canonical) => Err(Overflow(OverflowError::Canonical)),
451 Err(OverflowError::ErrorReporting) => Err(ErrorReporting),
452 Err(OverflowError::Error(e)) => Err(Overflow(OverflowError::Error(e))),
454 .flat_map(Result::transpose)
455 .collect::<Result<Vec<_>, _>>()?;
457 debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len());
459 let needs_infer = stack.obligation.predicate.has_non_region_infer();
461 // If there are STILL multiple candidates, we can further
462 // reduce the list by dropping duplicates -- including
463 // resolving specializations.
464 if candidates.len() > 1 {
466 while i < candidates.len() {
467 let is_dup = (0..candidates.len()).filter(|&j| i != j).any(|j| {
468 self.candidate_should_be_dropped_in_favor_of(
475 debug!(candidate = ?candidates[i], "Dropping candidate #{}/{}", i, candidates.len());
476 candidates.swap_remove(i);
478 debug!(candidate = ?candidates[i], "Retaining candidate #{}/{}", i, candidates.len());
481 // If there are *STILL* multiple candidates, give up
482 // and report ambiguity.
484 debug!("multiple matches, ambig");
491 // If there are *NO* candidates, then there are no impls --
492 // that we know of, anyway. Note that in the case where there
493 // are unbound type variables within the obligation, it might
494 // be the case that you could still satisfy the obligation
495 // from another crate by instantiating the type variables with
496 // a type from another crate that does have an impl. This case
497 // is checked for in `evaluate_stack` (and hence users
498 // who might care about this case, like coherence, should use
500 if candidates.is_empty() {
501 // If there's an error type, 'downgrade' our result from
502 // `Err(Unimplemented)` to `Ok(None)`. This helps us avoid
503 // emitting additional spurious errors, since we're guaranteed
504 // to have emitted at least one.
505 if stack.obligation.predicate.references_error() {
506 debug!(?stack.obligation.predicate, "found error type in predicate, treating as ambiguous");
509 return Err(Unimplemented);
512 // Just one candidate left.
513 self.filter_reservation_impls(candidates.pop().unwrap().candidate, stack.obligation)
516 ///////////////////////////////////////////////////////////////////////////
519 // Tests whether an obligation can be selected or whether an impl
520 // can be applied to particular types. It skips the "confirmation"
521 // step and hence completely ignores output type parameters.
523 // The result is "true" if the obligation *may* hold and "false" if
524 // we can be sure it does not.
526 /// Evaluates whether the obligation `obligation` can be satisfied (by any means).
527 pub fn predicate_may_hold_fatal(&mut self, obligation: &PredicateObligation<'tcx>) -> bool {
528 debug!(?obligation, "predicate_may_hold_fatal");
530 // This fatal query is a stopgap that should only be used in standard mode,
531 // where we do not expect overflow to be propagated.
532 assert!(self.query_mode == TraitQueryMode::Standard);
534 self.evaluate_root_obligation(obligation)
535 .expect("Overflow should be caught earlier in standard query mode")
539 /// Evaluates whether the obligation `obligation` can be satisfied
540 /// and returns an `EvaluationResult`. This is meant for the
542 pub fn evaluate_root_obligation(
544 obligation: &PredicateObligation<'tcx>,
545 ) -> Result<EvaluationResult, OverflowError> {
546 self.evaluation_probe(|this| {
547 this.evaluate_predicate_recursively(
548 TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
556 op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
557 ) -> Result<EvaluationResult, OverflowError> {
558 self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
559 let result = op(self)?;
561 match self.infcx.leak_check(true, snapshot) {
563 Err(_) => return Ok(EvaluatedToErr),
566 if self.infcx.opaque_types_added_in_snapshot(snapshot) {
567 return Ok(result.max(EvaluatedToOkModuloOpaqueTypes));
570 match self.infcx.region_constraints_added_in_snapshot(snapshot) {
572 Some(_) => Ok(result.max(EvaluatedToOkModuloRegions)),
577 /// Evaluates the predicates in `predicates` recursively. Note that
578 /// this applies projections in the predicates, and therefore
579 /// is run within an inference probe.
580 #[instrument(skip(self, stack), level = "debug")]
581 fn evaluate_predicates_recursively<'o, I>(
583 stack: TraitObligationStackList<'o, 'tcx>,
585 ) -> Result<EvaluationResult, OverflowError>
587 I: IntoIterator<Item = PredicateObligation<'tcx>> + std::fmt::Debug,
589 let mut result = EvaluatedToOk;
590 for obligation in predicates {
591 let eval = self.evaluate_predicate_recursively(stack, obligation.clone())?;
592 if let EvaluatedToErr = eval {
593 // fast-path - EvaluatedToErr is the top of the lattice,
594 // so we don't need to look on the other predicates.
595 return Ok(EvaluatedToErr);
597 result = cmp::max(result, eval);
605 skip(self, previous_stack),
606 fields(previous_stack = ?previous_stack.head())
609 fn evaluate_predicate_recursively<'o>(
611 previous_stack: TraitObligationStackList<'o, 'tcx>,
612 obligation: PredicateObligation<'tcx>,
613 ) -> Result<EvaluationResult, OverflowError> {
614 // `previous_stack` stores a `TraitObligation`, while `obligation` is
615 // a `PredicateObligation`. These are distinct types, so we can't
616 // use any `Option` combinator method that would force them to be
618 match previous_stack.head() {
619 Some(h) => self.check_recursion_limit(&obligation, h.obligation)?,
620 None => self.check_recursion_limit(&obligation, &obligation)?,
623 ensure_sufficient_stack(|| {
624 let bound_predicate = obligation.predicate.kind();
625 match bound_predicate.skip_binder() {
626 ty::PredicateKind::Clause(ty::Clause::Trait(t)) => {
627 let t = bound_predicate.rebind(t);
628 debug_assert!(!t.has_escaping_bound_vars());
629 let obligation = obligation.with(self.tcx(), t);
630 self.evaluate_trait_predicate_recursively(previous_stack, obligation)
633 ty::PredicateKind::Subtype(p) => {
634 let p = bound_predicate.rebind(p);
635 // Does this code ever run?
636 match self.infcx.subtype_predicate(&obligation.cause, obligation.param_env, p) {
637 Ok(Ok(InferOk { mut obligations, .. })) => {
638 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
639 self.evaluate_predicates_recursively(
641 obligations.into_iter(),
644 Ok(Err(_)) => Ok(EvaluatedToErr),
645 Err(..) => Ok(EvaluatedToAmbig),
649 ty::PredicateKind::Coerce(p) => {
650 let p = bound_predicate.rebind(p);
651 // Does this code ever run?
652 match self.infcx.coerce_predicate(&obligation.cause, obligation.param_env, p) {
653 Ok(Ok(InferOk { mut obligations, .. })) => {
654 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
655 self.evaluate_predicates_recursively(
657 obligations.into_iter(),
660 Ok(Err(_)) => Ok(EvaluatedToErr),
661 Err(..) => Ok(EvaluatedToAmbig),
665 ty::PredicateKind::WellFormed(arg) => {
666 // So, there is a bit going on here. First, `WellFormed` predicates
667 // are coinductive, like trait predicates with auto traits.
668 // This means that we need to detect if we have recursively
669 // evaluated `WellFormed(X)`. Otherwise, we would run into
670 // a "natural" overflow error.
672 // Now, the next question is whether we need to do anything
673 // special with caching. Considering the following tree:
678 // In this case, the innermost `WF(Foo<T>)` should return
679 // `EvaluatedToOk`, since it's coinductive. Then if
680 // `Bar<T>: Send` is resolved to `EvaluatedToOk`, it can be
681 // inserted into a cache (because without thinking about `WF`
682 // goals, it isn't in a cycle). If `Foo<T>: Trait` later doesn't
683 // hold, then `Bar<T>: Send` shouldn't hold. Therefore, we
684 // *do* need to keep track of coinductive cycles.
686 let cache = previous_stack.cache;
687 let dfn = cache.next_dfn();
689 for stack_arg in previous_stack.cache.wf_args.borrow().iter().rev() {
690 if stack_arg.0 != arg {
693 debug!("WellFormed({:?}) on stack", arg);
694 if let Some(stack) = previous_stack.head {
695 // Okay, let's imagine we have two different stacks:
696 // `T: NonAutoTrait -> WF(T) -> T: NonAutoTrait`
697 // `WF(T) -> T: NonAutoTrait -> WF(T)`
698 // Because of this, we need to check that all
699 // predicates between the WF goals are coinductive.
700 // Otherwise, we can say that `T: NonAutoTrait` is
702 // Let's imagine we have a predicate stack like
703 // `Foo: Bar -> WF(T) -> T: NonAutoTrait -> T: Auto
705 // and the current predicate is `WF(T)`. `wf_args`
706 // would contain `(T, 1)`. We want to check all
707 // trait predicates greater than `1`. The previous
708 // stack would be `T: Auto`.
709 let cycle = stack.iter().take_while(|s| s.depth > stack_arg.1);
710 let tcx = self.tcx();
712 cycle.map(|stack| stack.obligation.predicate.to_predicate(tcx));
713 if self.coinductive_match(cycle) {
714 stack.update_reached_depth(stack_arg.1);
715 return Ok(EvaluatedToOk);
717 return Ok(EvaluatedToRecur);
720 return Ok(EvaluatedToOk);
723 match wf::obligations(
725 obligation.param_env,
726 obligation.cause.body_id,
727 obligation.recursion_depth + 1,
729 obligation.cause.span,
731 Some(mut obligations) => {
732 self.add_depth(obligations.iter_mut(), obligation.recursion_depth);
734 cache.wf_args.borrow_mut().push((arg, previous_stack.depth()));
736 self.evaluate_predicates_recursively(previous_stack, obligations);
737 cache.wf_args.borrow_mut().pop();
739 let result = result?;
741 if !result.must_apply_modulo_regions() {
742 cache.on_failure(dfn);
745 cache.on_completion(dfn);
749 None => Ok(EvaluatedToAmbig),
753 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(pred)) => {
754 // A global type with no late-bound regions can only
755 // contain the "'static" lifetime (any other lifetime
756 // would either be late-bound or local), so it is guaranteed
757 // to outlive any other lifetime
758 if pred.0.is_global() && !pred.0.has_late_bound_regions() {
761 Ok(EvaluatedToOkModuloRegions)
765 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..)) => {
766 // We do not consider region relationships when evaluating trait matches.
767 Ok(EvaluatedToOkModuloRegions)
770 ty::PredicateKind::ObjectSafe(trait_def_id) => {
771 if self.tcx().is_object_safe(trait_def_id) {
778 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
779 let data = bound_predicate.rebind(data);
780 let project_obligation = obligation.with(self.tcx(), data);
781 match project::poly_project_and_unify_type(self, &project_obligation) {
782 ProjectAndUnifyResult::Holds(mut subobligations) => {
784 // If we've previously marked this projection as 'complete', then
785 // use the final cached result (either `EvaluatedToOk` or
786 // `EvaluatedToOkModuloRegions`), and skip re-evaluating the
789 ProjectionCacheKey::from_poly_projection_predicate(self, data)
791 if let Some(cached_res) = self
798 break 'compute_res Ok(cached_res);
803 subobligations.iter_mut(),
804 obligation.recursion_depth,
806 let res = self.evaluate_predicates_recursively(
810 if let Ok(eval_rslt) = res
811 && (eval_rslt == EvaluatedToOk || eval_rslt == EvaluatedToOkModuloRegions)
813 ProjectionCacheKey::from_poly_projection_predicate(
817 // If the result is something that we can cache, then mark this
818 // entry as 'complete'. This will allow us to skip evaluating the
819 // subobligations at all the next time we evaluate the projection
825 .complete(key, eval_rslt);
830 ProjectAndUnifyResult::FailedNormalization => Ok(EvaluatedToAmbig),
831 ProjectAndUnifyResult::Recursive => Ok(EvaluatedToRecur),
832 ProjectAndUnifyResult::MismatchedProjectionTypes(_) => Ok(EvaluatedToErr),
836 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
837 match self.infcx.closure_kind(closure_substs) {
838 Some(closure_kind) => {
839 if closure_kind.extends(kind) {
845 None => Ok(EvaluatedToAmbig),
849 ty::PredicateKind::ConstEvaluatable(uv) => {
850 match const_evaluatable::is_const_evaluatable(
853 obligation.param_env,
854 obligation.cause.span,
856 Ok(()) => Ok(EvaluatedToOk),
857 Err(NotConstEvaluatable::MentionsInfer) => Ok(EvaluatedToAmbig),
858 Err(NotConstEvaluatable::MentionsParam) => Ok(EvaluatedToErr),
859 Err(_) => Ok(EvaluatedToErr),
863 ty::PredicateKind::ConstEquate(c1, c2) => {
864 let tcx = self.tcx();
866 tcx.features().generic_const_exprs,
867 "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
871 let c1 = tcx.expand_abstract_consts(c1);
872 let c2 = tcx.expand_abstract_consts(c2);
874 "evalaute_predicate_recursively: equating consts:\nc1= {:?}\nc2= {:?}",
878 use rustc_hir::def::DefKind;
879 use ty::ConstKind::Unevaluated;
880 match (c1.kind(), c2.kind()) {
881 (Unevaluated(a), Unevaluated(b))
882 if a.def.did == b.def.did
883 && tcx.def_kind(a.def.did) == DefKind::AssocConst =>
885 if let Ok(new_obligations) = self
887 .at(&obligation.cause, obligation.param_env)
889 .eq(a.substs, b.substs)
891 let mut obligations = new_obligations.obligations;
893 obligations.iter_mut(),
894 obligation.recursion_depth,
896 return self.evaluate_predicates_recursively(
898 obligations.into_iter(),
902 (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
904 if let Ok(new_obligations) = self
906 .at(&obligation.cause, obligation.param_env)
909 let mut obligations = new_obligations.obligations;
911 obligations.iter_mut(),
912 obligation.recursion_depth,
914 return self.evaluate_predicates_recursively(
916 obligations.into_iter(),
923 let evaluate = |c: ty::Const<'tcx>| {
924 if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
925 match self.infcx.try_const_eval_resolve(
926 obligation.param_env,
929 Some(obligation.cause.span),
939 match (evaluate(c1), evaluate(c2)) {
940 (Ok(c1), Ok(c2)) => {
941 match self.infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
943 Ok(inf_ok) => self.evaluate_predicates_recursively(
945 inf_ok.into_obligations(),
947 Err(_) => Ok(EvaluatedToErr),
950 (Err(ErrorHandled::Reported(_)), _)
951 | (_, Err(ErrorHandled::Reported(_))) => Ok(EvaluatedToErr),
952 (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
953 if c1.has_non_region_infer() || c2.has_non_region_infer() {
956 // Two different constants using generic parameters ~> error.
962 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
963 bug!("TypeWellFormedFromEnv is only used for chalk")
965 ty::PredicateKind::Ambiguous => Ok(EvaluatedToAmbig),
970 #[instrument(skip(self, previous_stack), level = "debug", ret)]
971 fn evaluate_trait_predicate_recursively<'o>(
973 previous_stack: TraitObligationStackList<'o, 'tcx>,
974 mut obligation: TraitObligation<'tcx>,
975 ) -> Result<EvaluationResult, OverflowError> {
976 if !self.is_intercrate()
977 && obligation.is_global()
978 && obligation.param_env.caller_bounds().iter().all(|bound| bound.needs_subst())
980 // If a param env has no global bounds, global obligations do not
981 // depend on its particular value in order to work, so we can clear
982 // out the param env and get better caching.
984 obligation.param_env = obligation.param_env.without_caller_bounds();
987 let stack = self.push_stack(previous_stack, &obligation);
988 let mut fresh_trait_pred = stack.fresh_trait_pred;
989 let mut param_env = obligation.param_env;
991 fresh_trait_pred = fresh_trait_pred.map_bound(|mut pred| {
992 pred.remap_constness(&mut param_env);
996 debug!(?fresh_trait_pred);
998 // If a trait predicate is in the (local or global) evaluation cache,
999 // then we know it holds without cycles.
1000 if let Some(result) = self.check_evaluation_cache(param_env, fresh_trait_pred) {
1001 debug!("CACHE HIT");
1005 if let Some(result) = stack.cache().get_provisional(fresh_trait_pred) {
1006 debug!("PROVISIONAL CACHE HIT");
1007 stack.update_reached_depth(result.reached_depth);
1008 return Ok(result.result);
1011 // Check if this is a match for something already on the
1012 // stack. If so, we don't want to insert the result into the
1013 // main cache (it is cycle dependent) nor the provisional
1014 // cache (which is meant for things that have completed but
1015 // for a "backedge" -- this result *is* the backedge).
1016 if let Some(cycle_result) = self.check_evaluation_cycle(&stack) {
1017 return Ok(cycle_result);
1020 let (result, dep_node) = self.in_task(|this| this.evaluate_stack(&stack));
1021 let result = result?;
1023 if !result.must_apply_modulo_regions() {
1024 stack.cache().on_failure(stack.dfn);
1027 let reached_depth = stack.reached_depth.get();
1028 if reached_depth >= stack.depth {
1029 debug!("CACHE MISS");
1030 self.insert_evaluation_cache(param_env, fresh_trait_pred, dep_node, result);
1031 stack.cache().on_completion(stack.dfn);
1033 debug!("PROVISIONAL");
1035 "caching provisionally because {:?} \
1036 is a cycle participant (at depth {}, reached depth {})",
1037 fresh_trait_pred, stack.depth, reached_depth,
1040 stack.cache().insert_provisional(stack.dfn, reached_depth, fresh_trait_pred, result);
1046 /// If there is any previous entry on the stack that precisely
1047 /// matches this obligation, then we can assume that the
1048 /// obligation is satisfied for now (still all other conditions
1049 /// must be met of course). One obvious case this comes up is
1050 /// marker traits like `Send`. Think of a linked list:
1052 /// struct List<T> { data: T, next: Option<Box<List<T>>> }
1054 /// `Box<List<T>>` will be `Send` if `T` is `Send` and
1055 /// `Option<Box<List<T>>>` is `Send`, and in turn
1056 /// `Option<Box<List<T>>>` is `Send` if `Box<List<T>>` is
1059 /// Note that we do this comparison using the `fresh_trait_ref`
1060 /// fields. Because these have all been freshened using
1061 /// `self.freshener`, we can be sure that (a) this will not
1062 /// affect the inferencer state and (b) that if we see two
1063 /// fresh regions with the same index, they refer to the same
1064 /// unbound type variable.
1065 fn check_evaluation_cycle(
1067 stack: &TraitObligationStack<'_, 'tcx>,
1068 ) -> Option<EvaluationResult> {
1069 if let Some(cycle_depth) = stack
1071 .skip(1) // Skip top-most frame.
1073 stack.obligation.param_env == prev.obligation.param_env
1074 && stack.fresh_trait_pred == prev.fresh_trait_pred
1076 .map(|stack| stack.depth)
1078 debug!("evaluate_stack --> recursive at depth {}", cycle_depth);
1080 // If we have a stack like `A B C D E A`, where the top of
1081 // the stack is the final `A`, then this will iterate over
1082 // `A, E, D, C, B` -- i.e., all the participants apart
1083 // from the cycle head. We mark them as participating in a
1084 // cycle. This suppresses caching for those nodes. See
1085 // `in_cycle` field for more details.
1086 stack.update_reached_depth(cycle_depth);
1088 // Subtle: when checking for a coinductive cycle, we do
1089 // not compare using the "freshened trait refs" (which
1090 // have erased regions) but rather the fully explicit
1091 // trait refs. This is important because it's only a cycle
1092 // if the regions match exactly.
1093 let cycle = stack.iter().skip(1).take_while(|s| s.depth >= cycle_depth);
1094 let tcx = self.tcx();
1095 let cycle = cycle.map(|stack| stack.obligation.predicate.to_predicate(tcx));
1096 if self.coinductive_match(cycle) {
1097 debug!("evaluate_stack --> recursive, coinductive");
1100 debug!("evaluate_stack --> recursive, inductive");
1101 Some(EvaluatedToRecur)
1108 fn evaluate_stack<'o>(
1110 stack: &TraitObligationStack<'o, 'tcx>,
1111 ) -> Result<EvaluationResult, OverflowError> {
1112 // In intercrate mode, whenever any of the generics are unbound,
1113 // there can always be an impl. Even if there are no impls in
1114 // this crate, perhaps the type would be unified with
1115 // something from another crate that does provide an impl.
1117 // In intra mode, we must still be conservative. The reason is
1118 // that we want to avoid cycles. Imagine an impl like:
1120 // impl<T:Eq> Eq for Vec<T>
1122 // and a trait reference like `$0 : Eq` where `$0` is an
1123 // unbound variable. When we evaluate this trait-reference, we
1124 // will unify `$0` with `Vec<$1>` (for some fresh variable
1125 // `$1`), on the condition that `$1 : Eq`. We will then wind
1126 // up with many candidates (since that are other `Eq` impls
1127 // that apply) and try to winnow things down. This results in
1128 // a recursive evaluation that `$1 : Eq` -- as you can
1129 // imagine, this is just where we started. To avoid that, we
1130 // check for unbound variables and return an ambiguous (hence possible)
1131 // match if we've seen this trait before.
1133 // This suffices to allow chains like `FnMut` implemented in
1134 // terms of `Fn` etc, but we could probably make this more
1136 let unbound_input_types =
1137 stack.fresh_trait_pred.skip_binder().trait_ref.substs.types().any(|ty| ty.is_fresh());
1139 if unbound_input_types
1140 && stack.iter().skip(1).any(|prev| {
1141 stack.obligation.param_env == prev.obligation.param_env
1142 && self.match_fresh_trait_refs(
1143 stack.fresh_trait_pred,
1144 prev.fresh_trait_pred,
1145 prev.obligation.param_env,
1149 debug!("evaluate_stack --> unbound argument, recursive --> giving up",);
1150 return Ok(EvaluatedToUnknown);
1153 match self.candidate_from_obligation(stack) {
1154 Ok(Some(c)) => self.evaluate_candidate(stack, &c),
1155 Ok(None) => Ok(EvaluatedToAmbig),
1156 Err(Overflow(OverflowError::Canonical)) => Err(OverflowError::Canonical),
1157 Err(ErrorReporting) => Err(OverflowError::ErrorReporting),
1158 Err(..) => Ok(EvaluatedToErr),
1162 /// For defaulted traits, we use a co-inductive strategy to solve, so
1163 /// that recursion is ok. This routine returns `true` if the top of the
1164 /// stack (`cycle[0]`):
1166 /// - is a defaulted trait,
1167 /// - it also appears in the backtrace at some position `X`,
1168 /// - all the predicates at positions `X..` between `X` and the top are
1169 /// also defaulted traits.
1170 pub(crate) fn coinductive_match<I>(&mut self, mut cycle: I) -> bool
1172 I: Iterator<Item = ty::Predicate<'tcx>>,
1174 cycle.all(|predicate| self.coinductive_predicate(predicate))
1177 fn coinductive_predicate(&self, predicate: ty::Predicate<'tcx>) -> bool {
1178 let result = match predicate.kind().skip_binder() {
1179 ty::PredicateKind::Clause(ty::Clause::Trait(ref data)) => {
1180 self.tcx().trait_is_coinductive(data.def_id())
1182 ty::PredicateKind::WellFormed(_) => true,
1185 debug!(?predicate, ?result, "coinductive_predicate");
1189 /// Further evaluates `candidate` to decide whether all type parameters match and whether nested
1190 /// obligations are met. Returns whether `candidate` remains viable after this further
1195 fields(depth = stack.obligation.recursion_depth),
1198 fn evaluate_candidate<'o>(
1200 stack: &TraitObligationStack<'o, 'tcx>,
1201 candidate: &SelectionCandidate<'tcx>,
1202 ) -> Result<EvaluationResult, OverflowError> {
1203 let mut result = self.evaluation_probe(|this| {
1204 let candidate = (*candidate).clone();
1205 match this.confirm_candidate(stack.obligation, candidate) {
1208 this.evaluate_predicates_recursively(
1210 selection.nested_obligations().into_iter(),
1213 Err(..) => Ok(EvaluatedToErr),
1217 // If we erased any lifetimes, then we want to use
1218 // `EvaluatedToOkModuloRegions` instead of `EvaluatedToOk`
1219 // as your final result. The result will be cached using
1220 // the freshened trait predicate as a key, so we need
1221 // our result to be correct by *any* choice of original lifetimes,
1222 // not just the lifetime choice for this particular (non-erased)
1225 if stack.fresh_trait_pred.has_erased_regions() {
1226 result = result.max(EvaluatedToOkModuloRegions);
1232 fn check_evaluation_cache(
1234 param_env: ty::ParamEnv<'tcx>,
1235 trait_pred: ty::PolyTraitPredicate<'tcx>,
1236 ) -> Option<EvaluationResult> {
1237 // Neither the global nor local cache is aware of intercrate
1238 // mode, so don't do any caching. In particular, we might
1239 // re-use the same `InferCtxt` with both an intercrate
1240 // and non-intercrate `SelectionContext`
1241 if self.is_intercrate() {
1245 let tcx = self.tcx();
1246 if self.can_use_global_caches(param_env) {
1247 if let Some(res) = tcx.evaluation_cache.get(&(param_env, trait_pred), tcx) {
1251 self.infcx.evaluation_cache.get(&(param_env, trait_pred), tcx)
1254 fn insert_evaluation_cache(
1256 param_env: ty::ParamEnv<'tcx>,
1257 trait_pred: ty::PolyTraitPredicate<'tcx>,
1258 dep_node: DepNodeIndex,
1259 result: EvaluationResult,
1261 // Avoid caching results that depend on more than just the trait-ref
1262 // - the stack can create recursion.
1263 if result.is_stack_dependent() {
1267 // Neither the global nor local cache is aware of intercrate
1268 // mode, so don't do any caching. In particular, we might
1269 // re-use the same `InferCtxt` with both an intercrate
1270 // and non-intercrate `SelectionContext`
1271 if self.is_intercrate() {
1275 if self.can_use_global_caches(param_env) {
1276 if !trait_pred.needs_infer() {
1277 debug!(?trait_pred, ?result, "insert_evaluation_cache global");
1278 // This may overwrite the cache with the same value
1279 // FIXME: Due to #50507 this overwrites the different values
1280 // This should be changed to use HashMapExt::insert_same
1281 // when that is fixed
1282 self.tcx().evaluation_cache.insert((param_env, trait_pred), dep_node, result);
1287 debug!(?trait_pred, ?result, "insert_evaluation_cache");
1288 self.infcx.evaluation_cache.insert((param_env, trait_pred), dep_node, result);
1291 /// For various reasons, it's possible for a subobligation
1292 /// to have a *lower* recursion_depth than the obligation used to create it.
1293 /// Projection sub-obligations may be returned from the projection cache,
1294 /// which results in obligations with an 'old' `recursion_depth`.
1295 /// Additionally, methods like `InferCtxt.subtype_predicate` produce
1296 /// subobligations without taking in a 'parent' depth, causing the
1297 /// generated subobligations to have a `recursion_depth` of `0`.
1299 /// To ensure that obligation_depth never decreases, we force all subobligations
1300 /// to have at least the depth of the original obligation.
1301 fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>(
1306 it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
1309 fn check_recursion_depth<T>(
1312 error_obligation: &Obligation<'tcx, T>,
1313 ) -> Result<(), OverflowError>
1315 T: ToPredicate<'tcx> + Clone,
1317 if !self.infcx.tcx.recursion_limit().value_within_limit(depth) {
1318 match self.query_mode {
1319 TraitQueryMode::Standard => {
1320 if let Some(e) = self.infcx.tainted_by_errors() {
1321 return Err(OverflowError::Error(e));
1323 self.infcx.err_ctxt().report_overflow_obligation(error_obligation, true);
1325 TraitQueryMode::Canonical => {
1326 return Err(OverflowError::Canonical);
1333 /// Checks that the recursion limit has not been exceeded.
1335 /// The weird return type of this function allows it to be used with the `try` (`?`)
1336 /// operator within certain functions.
1338 fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V>(
1340 obligation: &Obligation<'tcx, T>,
1341 error_obligation: &Obligation<'tcx, V>,
1342 ) -> Result<(), OverflowError>
1344 V: ToPredicate<'tcx> + Clone,
1346 self.check_recursion_depth(obligation.recursion_depth, error_obligation)
1349 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1351 OP: FnOnce(&mut Self) -> R,
1353 let (result, dep_node) =
1354 self.tcx().dep_graph.with_anon_task(self.tcx(), DepKind::TraitSelect, || op(self));
1355 self.tcx().dep_graph.read_index(dep_node);
1359 /// filter_impls filters constant trait obligations and candidates that have a positive impl
1360 /// for a negative goal and a negative impl for a positive goal
1361 #[instrument(level = "debug", skip(self, candidates))]
1364 candidates: Vec<SelectionCandidate<'tcx>>,
1365 obligation: &TraitObligation<'tcx>,
1366 ) -> Vec<SelectionCandidate<'tcx>> {
1367 trace!("{candidates:#?}");
1368 let tcx = self.tcx();
1369 let mut result = Vec::with_capacity(candidates.len());
1371 for candidate in candidates {
1372 // Respect const trait obligations
1373 if obligation.is_const() {
1376 ImplCandidate(def_id) if tcx.constness(def_id) == hir::Constness::Const => {}
1378 ParamCandidate(trait_pred) if trait_pred.is_const_if_const() => {}
1380 ProjectionCandidate(_, ty::BoundConstness::ConstIfConst) => {}
1382 AutoImplCandidate => {}
1383 // generator / future, this will raise error in other places
1384 // or ignore error with const_async_blocks feature
1385 GeneratorCandidate => {}
1386 FutureCandidate => {}
1387 // FnDef where the function is const
1388 FnPointerCandidate { is_const: true } => {}
1389 ConstDestructCandidate(_) => {}
1391 // reject all other types of candidates
1397 if let ImplCandidate(def_id) = candidate {
1398 if ty::ImplPolarity::Reservation == tcx.impl_polarity(def_id)
1399 || obligation.polarity() == tcx.impl_polarity(def_id)
1401 result.push(candidate);
1404 result.push(candidate);
1408 trace!("{result:#?}");
1412 /// filter_reservation_impls filter reservation impl for any goal as ambiguous
1413 #[instrument(level = "debug", skip(self))]
1414 fn filter_reservation_impls(
1416 candidate: SelectionCandidate<'tcx>,
1417 obligation: &TraitObligation<'tcx>,
1418 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1419 let tcx = self.tcx();
1420 // Treat reservation impls as ambiguity.
1421 if let ImplCandidate(def_id) = candidate {
1422 if let ty::ImplPolarity::Reservation = tcx.impl_polarity(def_id) {
1423 if let Some(intercrate_ambiguity_clauses) = &mut self.intercrate_ambiguity_causes {
1425 .get_attr(def_id, sym::rustc_reservation_impl)
1426 .and_then(|a| a.value_str());
1427 if let Some(value) = value {
1429 "filter_reservation_impls: \
1430 reservation impl ambiguity on {:?}",
1433 intercrate_ambiguity_clauses.insert(
1434 IntercrateAmbiguityCause::ReservationImpl {
1435 message: value.to_string(),
1446 fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result<(), Conflict> {
1447 debug!("is_knowable(intercrate={:?})", self.is_intercrate());
1449 if !self.is_intercrate() || stack.obligation.polarity() == ty::ImplPolarity::Negative {
1453 let obligation = &stack.obligation;
1454 let predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
1456 // Okay to skip binder because of the nature of the
1457 // trait-ref-is-knowable check, which does not care about
1459 let trait_ref = predicate.skip_binder().trait_ref;
1461 coherence::trait_ref_is_knowable(self.tcx(), trait_ref)
1464 /// Returns `true` if the global caches can be used.
1465 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1466 // If there are any inference variables in the `ParamEnv`, then we
1467 // always use a cache local to this particular scope. Otherwise, we
1468 // switch to a global cache.
1469 if param_env.needs_infer() {
1473 // Avoid using the master cache during coherence and just rely
1474 // on the local cache. This effectively disables caching
1475 // during coherence. It is really just a simplification to
1476 // avoid us having to fear that coherence results "pollute"
1477 // the master cache. Since coherence executes pretty quickly,
1478 // it's not worth going to more trouble to increase the
1479 // hit-rate, I don't think.
1480 if self.is_intercrate() {
1484 // Otherwise, we can use the global cache.
1488 fn check_candidate_cache(
1490 mut param_env: ty::ParamEnv<'tcx>,
1491 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1492 ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
1493 // Neither the global nor local cache is aware of intercrate
1494 // mode, so don't do any caching. In particular, we might
1495 // re-use the same `InferCtxt` with both an intercrate
1496 // and non-intercrate `SelectionContext`
1497 if self.is_intercrate() {
1500 let tcx = self.tcx();
1501 let mut pred = cache_fresh_trait_pred.skip_binder();
1502 pred.remap_constness(&mut param_env);
1504 if self.can_use_global_caches(param_env) {
1505 if let Some(res) = tcx.selection_cache.get(&(param_env, pred), tcx) {
1509 self.infcx.selection_cache.get(&(param_env, pred), tcx)
1512 /// Determines whether can we safely cache the result
1513 /// of selecting an obligation. This is almost always `true`,
1514 /// except when dealing with certain `ParamCandidate`s.
1516 /// Ordinarily, a `ParamCandidate` will contain no inference variables,
1517 /// since it was usually produced directly from a `DefId`. However,
1518 /// certain cases (currently only librustdoc's blanket impl finder),
1519 /// a `ParamEnv` may be explicitly constructed with inference types.
1520 /// When this is the case, we do *not* want to cache the resulting selection
1521 /// candidate. This is due to the fact that it might not always be possible
1522 /// to equate the obligation's trait ref and the candidate's trait ref,
1523 /// if more constraints end up getting added to an inference variable.
1525 /// Because of this, we always want to re-run the full selection
1526 /// process for our obligation the next time we see it, since
1527 /// we might end up picking a different `SelectionCandidate` (or none at all).
1528 fn can_cache_candidate(
1530 result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1532 // Neither the global nor local cache is aware of intercrate
1533 // mode, so don't do any caching. In particular, we might
1534 // re-use the same `InferCtxt` with both an intercrate
1535 // and non-intercrate `SelectionContext`
1536 if self.is_intercrate() {
1540 Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => !trait_ref.needs_infer(),
1545 #[instrument(skip(self, param_env, cache_fresh_trait_pred, dep_node), level = "debug")]
1546 fn insert_candidate_cache(
1548 mut param_env: ty::ParamEnv<'tcx>,
1549 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1550 dep_node: DepNodeIndex,
1551 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1553 let tcx = self.tcx();
1554 let mut pred = cache_fresh_trait_pred.skip_binder();
1556 pred.remap_constness(&mut param_env);
1558 if !self.can_cache_candidate(&candidate) {
1559 debug!(?pred, ?candidate, "insert_candidate_cache - candidate is not cacheable");
1563 if self.can_use_global_caches(param_env) {
1564 if let Err(Overflow(OverflowError::Canonical)) = candidate {
1565 // Don't cache overflow globally; we only produce this in certain modes.
1566 } else if !pred.needs_infer() {
1567 if !candidate.needs_infer() {
1568 debug!(?pred, ?candidate, "insert_candidate_cache global");
1569 // This may overwrite the cache with the same value.
1570 tcx.selection_cache.insert((param_env, pred), dep_node, candidate);
1576 debug!(?pred, ?candidate, "insert_candidate_cache local");
1577 self.infcx.selection_cache.insert((param_env, pred), dep_node, candidate);
1580 /// Matches a predicate against the bounds of its self type.
1582 /// Given an obligation like `<T as Foo>::Bar: Baz` where the self type is
1583 /// a projection, look at the bounds of `T::Bar`, see if we can find a
1584 /// `Baz` bound. We return indexes into the list returned by
1585 /// `tcx.item_bounds` for any applicable bounds.
1586 #[instrument(level = "debug", skip(self), ret)]
1587 fn match_projection_obligation_against_definition_bounds(
1589 obligation: &TraitObligation<'tcx>,
1590 ) -> smallvec::SmallVec<[(usize, ty::BoundConstness); 2]> {
1591 let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
1592 let placeholder_trait_predicate =
1593 self.infcx.replace_bound_vars_with_placeholders(poly_trait_predicate);
1594 debug!(?placeholder_trait_predicate);
1596 let tcx = self.infcx.tcx;
1597 let (def_id, substs) = match *placeholder_trait_predicate.trait_ref.self_ty().kind() {
1598 ty::Projection(ref data) => (data.item_def_id, data.substs),
1599 ty::Opaque(def_id, substs) => (def_id, substs),
1602 obligation.cause.span,
1603 "match_projection_obligation_against_definition_bounds() called \
1604 but self-ty is not a projection: {:?}",
1605 placeholder_trait_predicate.trait_ref.self_ty()
1609 let bounds = tcx.bound_item_bounds(def_id).subst(tcx, substs);
1611 // The bounds returned by `item_bounds` may contain duplicates after
1612 // normalization, so try to deduplicate when possible to avoid
1613 // unnecessary ambiguity.
1614 let mut distinct_normalized_bounds = FxHashSet::default();
1619 .filter_map(|(idx, bound)| {
1620 let bound_predicate = bound.kind();
1621 if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) =
1622 bound_predicate.skip_binder()
1624 let bound = bound_predicate.rebind(pred.trait_ref);
1625 if self.infcx.probe(|_| {
1626 match self.match_normalize_trait_ref(
1629 placeholder_trait_predicate.trait_ref,
1632 Ok(Some(normalized_trait))
1633 if distinct_normalized_bounds.insert(normalized_trait) =>
1640 return Some((idx, pred.constness));
1648 /// Equates the trait in `obligation` with trait bound. If the two traits
1649 /// can be equated and the normalized trait bound doesn't contain inference
1650 /// variables or placeholders, the normalized bound is returned.
1651 fn match_normalize_trait_ref(
1653 obligation: &TraitObligation<'tcx>,
1654 trait_bound: ty::PolyTraitRef<'tcx>,
1655 placeholder_trait_ref: ty::TraitRef<'tcx>,
1656 ) -> Result<Option<ty::PolyTraitRef<'tcx>>, ()> {
1657 debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
1658 if placeholder_trait_ref.def_id != trait_bound.def_id() {
1659 // Avoid unnecessary normalization
1663 let Normalized { value: trait_bound, obligations: _ } = ensure_sufficient_stack(|| {
1664 project::normalize_with_depth(
1666 obligation.param_env,
1667 obligation.cause.clone(),
1668 obligation.recursion_depth + 1,
1673 .at(&obligation.cause, obligation.param_env)
1674 .define_opaque_types(false)
1675 .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
1676 .map(|InferOk { obligations: _, value: () }| {
1677 // This method is called within a probe, so we can't have
1678 // inference variables and placeholders escape.
1679 if !trait_bound.needs_infer() && !trait_bound.has_placeholders() {
1688 fn where_clause_may_apply<'o>(
1690 stack: &TraitObligationStack<'o, 'tcx>,
1691 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
1692 ) -> Result<EvaluationResult, OverflowError> {
1693 self.evaluation_probe(|this| {
1694 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1695 Ok(obligations) => this.evaluate_predicates_recursively(stack.list(), obligations),
1696 Err(()) => Ok(EvaluatedToErr),
1701 /// Return `Yes` if the obligation's predicate type applies to the env_predicate, and
1702 /// `No` if it does not. Return `Ambiguous` in the case that the projection type is a GAT,
1703 /// and applying this env_predicate constrains any of the obligation's GAT substitutions.
1705 /// This behavior is a somewhat of a hack to prevent over-constraining inference variables
1706 /// in cases like #91762.
1707 pub(super) fn match_projection_projections(
1709 obligation: &ProjectionTyObligation<'tcx>,
1710 env_predicate: PolyProjectionPredicate<'tcx>,
1711 potentially_unnormalized_candidates: bool,
1712 ) -> ProjectionMatchesProjection {
1713 let mut nested_obligations = Vec::new();
1714 let infer_predicate = self.infcx.replace_bound_vars_with_fresh_vars(
1715 obligation.cause.span,
1716 LateBoundRegionConversionTime::HigherRankedType,
1719 let infer_projection = if potentially_unnormalized_candidates {
1720 ensure_sufficient_stack(|| {
1721 project::normalize_with_depth_to(
1723 obligation.param_env,
1724 obligation.cause.clone(),
1725 obligation.recursion_depth + 1,
1726 infer_predicate.projection_ty,
1727 &mut nested_obligations,
1731 infer_predicate.projection_ty
1736 .at(&obligation.cause, obligation.param_env)
1737 .define_opaque_types(false)
1738 .sup(obligation.predicate, infer_projection)
1739 .map_or(false, |InferOk { obligations, value: () }| {
1740 self.evaluate_predicates_recursively(
1741 TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
1742 nested_obligations.into_iter().chain(obligations),
1744 .map_or(false, |res| res.may_apply())
1748 let generics = self.tcx().generics_of(obligation.predicate.item_def_id);
1749 // FIXME(generic-associated-types): Addresses aggressive inference in #92917.
1750 // If this type is a GAT, and of the GAT substs resolve to something new,
1751 // that means that we must have newly inferred something about the GAT.
1752 // We should give up in that case.
1753 if !generics.params.is_empty()
1754 && obligation.predicate.substs[generics.parent_count..]
1756 .any(|&p| p.has_non_region_infer() && self.infcx.shallow_resolve(p) != p)
1758 ProjectionMatchesProjection::Ambiguous
1760 ProjectionMatchesProjection::Yes
1763 ProjectionMatchesProjection::No
1767 ///////////////////////////////////////////////////////////////////////////
1770 // Winnowing is the process of attempting to resolve ambiguity by
1771 // probing further. During the winnowing process, we unify all
1772 // type variables and then we also attempt to evaluate recursive
1773 // bounds to see if they are satisfied.
1775 /// Returns `true` if `victim` should be dropped in favor of
1776 /// `other`. Generally speaking we will drop duplicate
1777 /// candidates and prefer where-clause candidates.
1779 /// See the comment for "SelectionCandidate" for more details.
1780 fn candidate_should_be_dropped_in_favor_of(
1782 victim: &EvaluatedCandidate<'tcx>,
1783 other: &EvaluatedCandidate<'tcx>,
1786 if victim.candidate == other.candidate {
1790 // Check if a bound would previously have been removed when normalizing
1791 // the param_env so that it can be given the lowest priority. See
1792 // #50825 for the motivation for this.
1793 let is_global = |cand: &ty::PolyTraitPredicate<'tcx>| {
1794 cand.is_global() && !cand.has_late_bound_regions()
1797 // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
1798 // `DiscriminantKindCandidate`, `ConstDestructCandidate`
1799 // to anything else.
1801 // This is a fix for #53123 and prevents winnowing from accidentally extending the
1802 // lifetime of a variable.
1803 match (&other.candidate, &victim.candidate) {
1804 (_, AutoImplCandidate) | (AutoImplCandidate, _) => {
1806 "default implementations shouldn't be recorded \
1807 when there are other valid candidates"
1811 // FIXME(@jswrenn): this should probably be more sophisticated
1812 (TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => false,
1815 (BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_), _) => true,
1816 (_, BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_)) => false,
1818 (ParamCandidate(other), ParamCandidate(victim)) => {
1819 let same_except_bound_vars = other.skip_binder().trait_ref
1820 == victim.skip_binder().trait_ref
1821 && other.skip_binder().constness == victim.skip_binder().constness
1822 && other.skip_binder().polarity == victim.skip_binder().polarity
1823 && !other.skip_binder().trait_ref.has_escaping_bound_vars();
1824 if same_except_bound_vars {
1825 // See issue #84398. In short, we can generate multiple ParamCandidates which are
1826 // the same except for unused bound vars. Just pick the one with the fewest bound vars
1827 // or the current one if tied (they should both evaluate to the same answer). This is
1828 // probably best characterized as a "hack", since we might prefer to just do our
1829 // best to *not* create essentially duplicate candidates in the first place.
1830 other.bound_vars().len() <= victim.bound_vars().len()
1831 } else if other.skip_binder().trait_ref == victim.skip_binder().trait_ref
1832 && victim.skip_binder().constness == ty::BoundConstness::NotConst
1833 && other.skip_binder().polarity == victim.skip_binder().polarity
1835 // Drop otherwise equivalent non-const candidates in favor of const candidates.
1842 // Drop otherwise equivalent non-const fn pointer candidates
1843 (FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => true,
1845 // Global bounds from the where clause should be ignored
1846 // here (see issue #50825). Otherwise, we have a where
1847 // clause so don't go around looking for impls.
1848 // Arbitrarily give param candidates priority
1849 // over projection and object candidates.
1851 ParamCandidate(ref cand),
1854 | GeneratorCandidate
1856 | FnPointerCandidate { .. }
1857 | BuiltinObjectCandidate
1858 | BuiltinUnsizeCandidate
1859 | TraitUpcastingUnsizeCandidate(_)
1860 | BuiltinCandidate { .. }
1861 | TraitAliasCandidate
1862 | ObjectCandidate(_)
1863 | ProjectionCandidate(..),
1864 ) => !is_global(cand),
1865 (ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref cand)) => {
1866 // Prefer these to a global where-clause bound
1867 // (see issue #50825).
1873 | GeneratorCandidate
1875 | FnPointerCandidate { .. }
1876 | BuiltinObjectCandidate
1877 | BuiltinUnsizeCandidate
1878 | TraitUpcastingUnsizeCandidate(_)
1879 | BuiltinCandidate { has_nested: true }
1880 | TraitAliasCandidate,
1881 ParamCandidate(ref cand),
1883 // Prefer these to a global where-clause bound
1884 // (see issue #50825).
1885 is_global(cand) && other.evaluation.must_apply_modulo_regions()
1888 (ProjectionCandidate(i, _), ProjectionCandidate(j, _))
1889 | (ObjectCandidate(i), ObjectCandidate(j)) => {
1890 // Arbitrarily pick the lower numbered candidate for backwards
1891 // compatibility reasons. Don't let this affect inference.
1892 i < j && !needs_infer
1894 (ObjectCandidate(_), ProjectionCandidate(..))
1895 | (ProjectionCandidate(..), ObjectCandidate(_)) => {
1896 bug!("Have both object and projection candidate")
1899 // Arbitrarily give projection and object candidates priority.
1901 ObjectCandidate(_) | ProjectionCandidate(..),
1904 | GeneratorCandidate
1906 | FnPointerCandidate { .. }
1907 | BuiltinObjectCandidate
1908 | BuiltinUnsizeCandidate
1909 | TraitUpcastingUnsizeCandidate(_)
1910 | BuiltinCandidate { .. }
1911 | TraitAliasCandidate,
1917 | GeneratorCandidate
1919 | FnPointerCandidate { .. }
1920 | BuiltinObjectCandidate
1921 | BuiltinUnsizeCandidate
1922 | TraitUpcastingUnsizeCandidate(_)
1923 | BuiltinCandidate { .. }
1924 | TraitAliasCandidate,
1925 ObjectCandidate(_) | ProjectionCandidate(..),
1928 (&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
1929 // See if we can toss out `victim` based on specialization.
1930 // While this requires us to know *for sure* that the `other` impl applies
1931 // we still use modulo regions here.
1933 // This is fine as specialization currently assumes that specializing
1934 // impls have to be always applicable, meaning that the only allowed
1935 // region constraints may be constraints also present on the default impl.
1936 let tcx = self.tcx();
1937 if other.evaluation.must_apply_modulo_regions() {
1938 if tcx.specializes((other_def, victim_def)) {
1943 if other.evaluation.must_apply_considering_regions() {
1944 match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
1945 Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
1946 // Subtle: If the predicate we are evaluating has inference
1947 // variables, do *not* allow discarding candidates due to
1948 // marker trait impls.
1950 // Without this restriction, we could end up accidentally
1951 // constraining inference variables based on an arbitrarily
1952 // chosen trait impl.
1954 // Imagine we have the following code:
1957 // #[marker] trait MyTrait {}
1958 // impl MyTrait for u8 {}
1959 // impl MyTrait for bool {}
1962 // And we are evaluating the predicate `<_#0t as MyTrait>`.
1964 // During selection, we will end up with one candidate for each
1965 // impl of `MyTrait`. If we were to discard one impl in favor
1966 // of the other, we would be left with one candidate, causing
1967 // us to "successfully" select the predicate, unifying
1968 // _#0t with (for example) `u8`.
1970 // However, we have no reason to believe that this unification
1971 // is correct - we've essentially just picked an arbitrary
1972 // *possibility* for _#0t, and required that this be the *only*
1975 // Eventually, we will either:
1976 // 1) Unify all inference variables in the predicate through
1977 // some other means (e.g. type-checking of a function). We will
1978 // then be in a position to drop marker trait candidates
1979 // without constraining inference variables (since there are
1980 // none left to constrain)
1981 // 2) Be left with some unconstrained inference variables. We
1982 // will then correctly report an inference error, since the
1983 // existence of multiple marker trait impls tells us nothing
1984 // about which one should actually apply.
1995 // Everything else is ambiguous
1999 | GeneratorCandidate
2001 | FnPointerCandidate { .. }
2002 | BuiltinObjectCandidate
2003 | BuiltinUnsizeCandidate
2004 | TraitUpcastingUnsizeCandidate(_)
2005 | BuiltinCandidate { has_nested: true }
2006 | TraitAliasCandidate,
2009 | GeneratorCandidate
2011 | FnPointerCandidate { .. }
2012 | BuiltinObjectCandidate
2013 | BuiltinUnsizeCandidate
2014 | TraitUpcastingUnsizeCandidate(_)
2015 | BuiltinCandidate { has_nested: true }
2016 | TraitAliasCandidate,
2021 fn sized_conditions(
2023 obligation: &TraitObligation<'tcx>,
2024 ) -> BuiltinImplConditions<'tcx> {
2025 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2027 // NOTE: binder moved to (*)
2028 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2030 match self_ty.kind() {
2031 ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
2042 | ty::GeneratorWitness(..)
2046 | ty::Dynamic(_, _, ty::DynStar)
2048 // safe for everything
2049 Where(ty::Binder::dummy(Vec::new()))
2052 ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
2054 ty::Tuple(tys) => Where(
2055 obligation.predicate.rebind(tys.last().map_or_else(Vec::new, |&last| vec![last])),
2058 ty::Adt(def, substs) => {
2059 let sized_crit = def.sized_constraint(self.tcx());
2060 // (*) binder moved here
2061 Where(obligation.predicate.rebind({
2065 .map(|ty| sized_crit.rebind(*ty).subst(self.tcx(), substs))
2070 ty::Projection(_) | ty::Param(_) | ty::Opaque(..) => None,
2071 ty::Infer(ty::TyVar(_)) => Ambiguous,
2075 | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2076 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
2081 fn copy_clone_conditions(
2083 obligation: &TraitObligation<'tcx>,
2084 ) -> BuiltinImplConditions<'tcx> {
2085 // NOTE: binder moved to (*)
2086 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2088 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2090 match *self_ty.kind() {
2091 ty::Infer(ty::IntVar(_))
2092 | ty::Infer(ty::FloatVar(_))
2095 | ty::Error(_) => Where(ty::Binder::dummy(Vec::new())),
2104 | ty::Ref(_, _, hir::Mutability::Not)
2105 | ty::Array(..) => {
2106 // Implementations provided in libcore
2113 | ty::Generator(_, _, hir::Movability::Static)
2115 | ty::Ref(_, _, hir::Mutability::Mut) => None,
2118 // (*) binder moved here
2119 Where(obligation.predicate.rebind(tys.iter().collect()))
2122 ty::Generator(_, substs, hir::Movability::Movable) => {
2123 if self.tcx().features().generator_clone {
2124 let resolved_upvars =
2125 self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
2126 let resolved_witness =
2127 self.infcx.shallow_resolve(substs.as_generator().witness());
2128 if resolved_upvars.is_ty_var() || resolved_witness.is_ty_var() {
2129 // Not yet resolved.
2135 .chain(iter::once(substs.as_generator().witness()))
2136 .collect::<Vec<_>>();
2137 Where(obligation.predicate.rebind(all))
2144 ty::GeneratorWitness(binder) => {
2145 let witness_tys = binder.skip_binder();
2146 for witness_ty in witness_tys.iter() {
2147 let resolved = self.infcx.shallow_resolve(witness_ty);
2148 if resolved.is_ty_var() {
2152 // (*) binder moved here
2153 let all_vars = self.tcx().mk_bound_variable_kinds(
2154 obligation.predicate.bound_vars().iter().chain(binder.bound_vars().iter()),
2156 Where(ty::Binder::bind_with_vars(witness_tys.to_vec(), all_vars))
2159 ty::Closure(_, substs) => {
2160 // (*) binder moved here
2161 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
2162 if let ty::Infer(ty::TyVar(_)) = ty.kind() {
2163 // Not yet resolved.
2166 Where(obligation.predicate.rebind(substs.as_closure().upvar_tys().collect()))
2170 ty::Adt(..) | ty::Projection(..) | ty::Param(..) | ty::Opaque(..) => {
2171 // Fallback to whatever user-defined impls exist in this case.
2175 ty::Infer(ty::TyVar(_)) => {
2176 // Unbound type variable. Might or might not have
2177 // applicable impls and so forth, depending on what
2178 // those type variables wind up being bound to.
2184 | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2185 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
2190 /// For default impls, we need to break apart a type into its
2191 /// "constituent types" -- meaning, the types that it contains.
2193 /// Here are some (simple) examples:
2195 /// ```ignore (illustrative)
2196 /// (i32, u32) -> [i32, u32]
2197 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2198 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2199 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2201 #[instrument(level = "debug", skip(self), ret)]
2202 fn constituent_types_for_ty(
2204 t: ty::Binder<'tcx, Ty<'tcx>>,
2205 ) -> ty::Binder<'tcx, Vec<Ty<'tcx>>> {
2206 match *t.skip_binder().kind() {
2215 | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
2217 | ty::Char => ty::Binder::dummy(Vec::new()),
2223 | ty::Projection(..)
2225 | ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2226 bug!("asked to assemble constituent types of unexpected type: {:?}", t);
2229 ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => {
2230 t.rebind(vec![element_ty])
2233 ty::Array(element_ty, _) | ty::Slice(element_ty) => t.rebind(vec![element_ty]),
2235 ty::Tuple(ref tys) => {
2236 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2237 t.rebind(tys.iter().collect())
2240 ty::Closure(_, ref substs) => {
2241 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
2245 ty::Generator(_, ref substs, _) => {
2246 let ty = self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
2247 let witness = substs.as_generator().witness();
2248 t.rebind([ty].into_iter().chain(iter::once(witness)).collect())
2251 ty::GeneratorWitness(types) => {
2252 debug_assert!(!types.has_escaping_bound_vars());
2253 types.map_bound(|types| types.to_vec())
2256 // For `PhantomData<T>`, we pass `T`.
2257 ty::Adt(def, substs) if def.is_phantom_data() => t.rebind(substs.types().collect()),
2259 ty::Adt(def, substs) => {
2260 t.rebind(def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect())
2263 ty::Opaque(def_id, substs) => {
2264 // We can resolve the `impl Trait` to its concrete type,
2265 // which enforces a DAG between the functions requiring
2266 // the auto trait bounds in question.
2267 t.rebind(vec![self.tcx().bound_type_of(def_id).subst(self.tcx(), substs)])
2272 fn collect_predicates_for_types(
2274 param_env: ty::ParamEnv<'tcx>,
2275 cause: ObligationCause<'tcx>,
2276 recursion_depth: usize,
2277 trait_def_id: DefId,
2278 types: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
2279 ) -> Vec<PredicateObligation<'tcx>> {
2280 // Because the types were potentially derived from
2281 // higher-ranked obligations they may reference late-bound
2282 // regions. For example, `for<'a> Foo<&'a i32> : Copy` would
2283 // yield a type like `for<'a> &'a i32`. In general, we
2284 // maintain the invariant that we never manipulate bound
2285 // regions, so we have to process these bound regions somehow.
2287 // The strategy is to:
2289 // 1. Instantiate those regions to placeholder regions (e.g.,
2290 // `for<'a> &'a i32` becomes `&0 i32`.
2291 // 2. Produce something like `&'0 i32 : Copy`
2292 // 3. Re-bind the regions back to `for<'a> &'a i32 : Copy`
2296 .skip_binder() // binder moved -\
2299 let ty: ty::Binder<'tcx, Ty<'tcx>> = types.rebind(*ty); // <----/
2301 let placeholder_ty = self.infcx.replace_bound_vars_with_placeholders(ty);
2302 let Normalized { value: normalized_ty, mut obligations } =
2303 ensure_sufficient_stack(|| {
2304 project::normalize_with_depth(
2312 let placeholder_obligation = predicate_for_trait_def(
2320 obligations.push(placeholder_obligation);
2326 ///////////////////////////////////////////////////////////////////////////
2329 // Matching is a common path used for both evaluation and
2330 // confirmation. It basically unifies types that appear in impls
2331 // and traits. This does affect the surrounding environment;
2332 // therefore, when used during evaluation, match routines must be
2333 // run inside of a `probe()` so that their side-effects are
2339 obligation: &TraitObligation<'tcx>,
2340 ) -> Normalized<'tcx, SubstsRef<'tcx>> {
2341 let impl_trait_ref = self.tcx().bound_impl_trait_ref(impl_def_id).unwrap();
2342 match self.match_impl(impl_def_id, impl_trait_ref, obligation) {
2343 Ok(substs) => substs,
2345 // FIXME: A rematch may fail when a candidate cache hit occurs
2346 // on thefreshened form of the trait predicate, but the match
2347 // fails for some reason that is not captured in the freshened
2348 // cache key. For example, equating an impl trait ref against
2349 // the placeholder trait ref may fail due the Generalizer relation
2350 // raising a CyclicalTy error due to a sub_root_var relation
2351 // for a variable being generalized...
2352 self.infcx.tcx.sess.delay_span_bug(
2353 obligation.cause.span,
2355 "Impl {:?} was matchable against {:?} but now is not",
2356 impl_def_id, obligation
2359 let value = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
2360 let err = self.tcx().ty_error();
2361 let value = value.fold_with(&mut BottomUpFolder {
2367 Normalized { value, obligations: vec![] }
2372 #[instrument(level = "debug", skip(self), ret)]
2376 impl_trait_ref: EarlyBinder<ty::TraitRef<'tcx>>,
2377 obligation: &TraitObligation<'tcx>,
2378 ) -> Result<Normalized<'tcx, SubstsRef<'tcx>>, ()> {
2379 let placeholder_obligation =
2380 self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
2381 let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
2383 let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
2385 let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs);
2387 debug!(?impl_trait_ref);
2389 let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
2390 ensure_sufficient_stack(|| {
2391 project::normalize_with_depth(
2393 obligation.param_env,
2394 obligation.cause.clone(),
2395 obligation.recursion_depth + 1,
2400 debug!(?impl_trait_ref, ?placeholder_obligation_trait_ref);
2402 let cause = ObligationCause::new(
2403 obligation.cause.span,
2404 obligation.cause.body_id,
2405 ObligationCauseCode::MatchImpl(obligation.cause.clone(), impl_def_id),
2408 let InferOk { obligations, .. } = self
2410 .at(&cause, obligation.param_env)
2411 .define_opaque_types(false)
2412 .eq(placeholder_obligation_trait_ref, impl_trait_ref)
2413 .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{e}`"))?;
2414 nested_obligations.extend(obligations);
2416 if !self.is_intercrate()
2417 && self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation
2419 debug!("reservation impls only apply in intercrate mode");
2423 Ok(Normalized { value: impl_substs, obligations: nested_obligations })
2426 fn fast_reject_trait_refs(
2428 obligation: &TraitObligation<'tcx>,
2429 impl_trait_ref: &ty::TraitRef<'tcx>,
2431 // We can avoid creating type variables and doing the full
2432 // substitution if we find that any of the input types, when
2433 // simplified, do not match.
2434 let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
2435 iter::zip(obligation.predicate.skip_binder().trait_ref.substs, impl_trait_ref.substs)
2436 .any(|(obl, imp)| !drcx.generic_args_may_unify(obl, imp))
2439 /// Normalize `where_clause_trait_ref` and try to match it against
2440 /// `obligation`. If successful, return any predicates that
2441 /// result from the normalization.
2442 fn match_where_clause_trait_ref(
2444 obligation: &TraitObligation<'tcx>,
2445 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
2446 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
2447 self.match_poly_trait_ref(obligation, where_clause_trait_ref)
2450 /// Returns `Ok` if `poly_trait_ref` being true implies that the
2451 /// obligation is satisfied.
2452 #[instrument(skip(self), level = "debug")]
2453 fn match_poly_trait_ref(
2455 obligation: &TraitObligation<'tcx>,
2456 poly_trait_ref: ty::PolyTraitRef<'tcx>,
2457 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
2459 .at(&obligation.cause, obligation.param_env)
2460 // We don't want predicates for opaque types to just match all other types,
2461 // if there is an obligation on the opaque type, then that obligation must be met
2462 // opaquely. Otherwise we'd match any obligation to the opaque type and then error
2464 .define_opaque_types(false)
2465 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
2466 .map(|InferOk { obligations, .. }| obligations)
2470 ///////////////////////////////////////////////////////////////////////////
2473 fn match_fresh_trait_refs(
2475 previous: ty::PolyTraitPredicate<'tcx>,
2476 current: ty::PolyTraitPredicate<'tcx>,
2477 param_env: ty::ParamEnv<'tcx>,
2479 let mut matcher = ty::_match::Match::new(self.tcx(), param_env);
2480 matcher.relate(previous, current).is_ok()
2485 previous_stack: TraitObligationStackList<'o, 'tcx>,
2486 obligation: &'o TraitObligation<'tcx>,
2487 ) -> TraitObligationStack<'o, 'tcx> {
2488 let fresh_trait_pred = obligation.predicate.fold_with(&mut self.freshener);
2490 let dfn = previous_stack.cache.next_dfn();
2491 let depth = previous_stack.depth() + 1;
2492 TraitObligationStack {
2495 reached_depth: Cell::new(depth),
2496 previous: previous_stack,
2502 #[instrument(skip(self), level = "debug")]
2503 fn closure_trait_ref_unnormalized(
2505 obligation: &TraitObligation<'tcx>,
2506 substs: SubstsRef<'tcx>,
2507 ) -> ty::PolyTraitRef<'tcx> {
2508 let closure_sig = substs.as_closure().sig();
2510 debug!(?closure_sig);
2512 // NOTE: The self-type is an unboxed closure type and hence is
2513 // in fact unparameterized (or at least does not reference any
2514 // regions bound in the obligation).
2515 let self_ty = obligation
2519 .expect("unboxed closure type should not capture bound vars from the predicate");
2521 closure_trait_ref_and_return_type(
2523 obligation.predicate.def_id(),
2526 util::TupleArgumentsFlag::No,
2528 .map_bound(|(trait_ref, _)| trait_ref)
2531 /// Returns the obligations that are implied by instantiating an
2532 /// impl or trait. The obligations are substituted and fully
2533 /// normalized. This is used when confirming an impl or default
2535 #[instrument(level = "debug", skip(self, cause, param_env))]
2536 fn impl_or_trait_obligations(
2538 cause: &ObligationCause<'tcx>,
2539 recursion_depth: usize,
2540 param_env: ty::ParamEnv<'tcx>,
2541 def_id: DefId, // of impl or trait
2542 substs: SubstsRef<'tcx>, // for impl or trait
2543 parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
2544 ) -> Vec<PredicateObligation<'tcx>> {
2545 let tcx = self.tcx();
2547 // To allow for one-pass evaluation of the nested obligation,
2548 // each predicate must be preceded by the obligations required
2550 // for example, if we have:
2551 // impl<U: Iterator<Item: Copy>, V: Iterator<Item = U>> Foo for V
2552 // the impl will have the following predicates:
2553 // <V as Iterator>::Item = U,
2554 // U: Iterator, U: Sized,
2555 // V: Iterator, V: Sized,
2556 // <U as Iterator>::Item: Copy
2557 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2558 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2559 // `$1: Copy`, so we must ensure the obligations are emitted in
2561 let predicates = tcx.bound_predicates_of(def_id);
2562 debug!(?predicates);
2563 assert_eq!(predicates.0.parent, None);
2564 let mut obligations = Vec::with_capacity(predicates.0.predicates.len());
2565 for (predicate, span) in predicates.0.predicates {
2567 let cause = cause.clone().derived_cause(parent_trait_pred, |derived| {
2568 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
2570 impl_def_id: def_id,
2574 let predicate = normalize_with_depth_to(
2579 predicates.rebind(*predicate).subst(tcx, substs),
2582 obligations.push(Obligation { cause, recursion_depth, param_env, predicate });
2589 impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
2590 fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2591 TraitObligationStackList::with(self)
2594 fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx> {
2598 fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2602 /// Indicates that attempting to evaluate this stack entry
2603 /// required accessing something from the stack at depth `reached_depth`.
2604 fn update_reached_depth(&self, reached_depth: usize) {
2606 self.depth >= reached_depth,
2607 "invoked `update_reached_depth` with something under this stack: \
2608 self.depth={} reached_depth={}",
2612 debug!(reached_depth, "update_reached_depth");
2614 while reached_depth < p.depth {
2615 debug!(?p.fresh_trait_pred, "update_reached_depth: marking as cycle participant");
2616 p.reached_depth.set(p.reached_depth.get().min(reached_depth));
2617 p = p.previous.head.unwrap();
2622 /// The "provisional evaluation cache" is used to store intermediate cache results
2623 /// when solving auto traits. Auto traits are unusual in that they can support
2624 /// cycles. So, for example, a "proof tree" like this would be ok:
2626 /// - `Foo<T>: Send` :-
2627 /// - `Bar<T>: Send` :-
2628 /// - `Foo<T>: Send` -- cycle, but ok
2629 /// - `Baz<T>: Send`
2631 /// Here, to prove `Foo<T>: Send`, we have to prove `Bar<T>: Send` and
2632 /// `Baz<T>: Send`. Proving `Bar<T>: Send` in turn required `Foo<T>: Send`.
2633 /// For non-auto traits, this cycle would be an error, but for auto traits (because
2634 /// they are coinductive) it is considered ok.
2636 /// However, there is a complication: at the point where we have
2637 /// "proven" `Bar<T>: Send`, we have in fact only proven it
2638 /// *provisionally*. In particular, we proved that `Bar<T>: Send`
2639 /// *under the assumption* that `Foo<T>: Send`. But what if we later
2640 /// find out this assumption is wrong? Specifically, we could
2641 /// encounter some kind of error proving `Baz<T>: Send`. In that case,
2642 /// `Bar<T>: Send` didn't turn out to be true.
2644 /// In Issue #60010, we found a bug in rustc where it would cache
2645 /// these intermediate results. This was fixed in #60444 by disabling
2646 /// *all* caching for things involved in a cycle -- in our example,
2647 /// that would mean we don't cache that `Bar<T>: Send`. But this led
2648 /// to large slowdowns.
2650 /// Specifically, imagine this scenario, where proving `Baz<T>: Send`
2651 /// first requires proving `Bar<T>: Send` (which is true:
2653 /// - `Foo<T>: Send` :-
2654 /// - `Bar<T>: Send` :-
2655 /// - `Foo<T>: Send` -- cycle, but ok
2656 /// - `Baz<T>: Send`
2657 /// - `Bar<T>: Send` -- would be nice for this to be a cache hit!
2658 /// - `*const T: Send` -- but what if we later encounter an error?
2660 /// The *provisional evaluation cache* resolves this issue. It stores
2661 /// cache results that we've proven but which were involved in a cycle
2662 /// in some way. We track the minimal stack depth (i.e., the
2663 /// farthest from the top of the stack) that we are dependent on.
2664 /// The idea is that the cache results within are all valid -- so long as
2665 /// none of the nodes in between the current node and the node at that minimum
2666 /// depth result in an error (in which case the cached results are just thrown away).
2668 /// During evaluation, we consult this provisional cache and rely on
2669 /// it. Accessing a cached value is considered equivalent to accessing
2670 /// a result at `reached_depth`, so it marks the *current* solution as
2671 /// provisional as well. If an error is encountered, we toss out any
2672 /// provisional results added from the subtree that encountered the
2673 /// error. When we pop the node at `reached_depth` from the stack, we
2674 /// can commit all the things that remain in the provisional cache.
2675 struct ProvisionalEvaluationCache<'tcx> {
2676 /// next "depth first number" to issue -- just a counter
2679 /// Map from cache key to the provisionally evaluated thing.
2680 /// The cache entries contain the result but also the DFN in which they
2681 /// were added. The DFN is used to clear out values on failure.
2683 /// Imagine we have a stack like:
2685 /// - `A B C` and we add a cache for the result of C (DFN 2)
2686 /// - Then we have a stack `A B D` where `D` has DFN 3
2687 /// - We try to solve D by evaluating E: `A B D E` (DFN 4)
2688 /// - `E` generates various cache entries which have cyclic dependencies on `B`
2689 /// - `A B D E F` and so forth
2690 /// - the DFN of `F` for example would be 5
2691 /// - then we determine that `E` is in error -- we will then clear
2692 /// all cache values whose DFN is >= 4 -- in this case, that
2693 /// means the cached value for `F`.
2694 map: RefCell<FxHashMap<ty::PolyTraitPredicate<'tcx>, ProvisionalEvaluation>>,
2696 /// The stack of args that we assume to be true because a `WF(arg)` predicate
2697 /// is on the stack above (and because of wellformedness is coinductive).
2698 /// In an "ideal" world, this would share a stack with trait predicates in
2699 /// `TraitObligationStack`. However, trait predicates are *much* hotter than
2700 /// `WellFormed` predicates, and it's very likely that the additional matches
2701 /// will have a perf effect. The value here is the well-formed `GenericArg`
2702 /// and the depth of the trait predicate *above* that well-formed predicate.
2703 wf_args: RefCell<Vec<(ty::GenericArg<'tcx>, usize)>>,
2706 /// A cache value for the provisional cache: contains the depth-first
2707 /// number (DFN) and result.
2708 #[derive(Copy, Clone, Debug)]
2709 struct ProvisionalEvaluation {
2711 reached_depth: usize,
2712 result: EvaluationResult,
2715 impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> {
2716 fn default() -> Self {
2717 Self { dfn: Cell::new(0), map: Default::default(), wf_args: Default::default() }
2721 impl<'tcx> ProvisionalEvaluationCache<'tcx> {
2722 /// Get the next DFN in sequence (basically a counter).
2723 fn next_dfn(&self) -> usize {
2724 let result = self.dfn.get();
2725 self.dfn.set(result + 1);
2729 /// Check the provisional cache for any result for
2730 /// `fresh_trait_ref`. If there is a hit, then you must consider
2731 /// it an access to the stack slots at depth
2732 /// `reached_depth` (from the returned value).
2735 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
2736 ) -> Option<ProvisionalEvaluation> {
2739 "get_provisional = {:#?}",
2740 self.map.borrow().get(&fresh_trait_pred),
2742 Some(*self.map.borrow().get(&fresh_trait_pred)?)
2745 /// Insert a provisional result into the cache. The result came
2746 /// from the node with the given DFN. It accessed a minimum depth
2747 /// of `reached_depth` to compute. It evaluated `fresh_trait_pred`
2748 /// and resulted in `result`.
2749 fn insert_provisional(
2752 reached_depth: usize,
2753 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
2754 result: EvaluationResult,
2756 debug!(?from_dfn, ?fresh_trait_pred, ?result, "insert_provisional");
2758 let mut map = self.map.borrow_mut();
2760 // Subtle: when we complete working on the DFN `from_dfn`, anything
2761 // that remains in the provisional cache must be dependent on some older
2762 // stack entry than `from_dfn`. We have to update their depth with our transitive
2763 // depth in that case or else it would be referring to some popped note.
2766 // A (reached depth 0)
2768 // B // depth 1 -- reached depth = 0
2769 // C // depth 2 -- reached depth = 1 (should be 0)
2772 // D (reached depth 1)
2773 // C (cache -- reached depth = 2)
2774 for (_k, v) in &mut *map {
2775 if v.from_dfn >= from_dfn {
2776 v.reached_depth = reached_depth.min(v.reached_depth);
2780 map.insert(fresh_trait_pred, ProvisionalEvaluation { from_dfn, reached_depth, result });
2783 /// Invoked when the node with dfn `dfn` does not get a successful
2784 /// result. This will clear out any provisional cache entries
2785 /// that were added since `dfn` was created. This is because the
2786 /// provisional entries are things which must assume that the
2787 /// things on the stack at the time of their creation succeeded --
2788 /// since the failing node is presently at the top of the stack,
2789 /// these provisional entries must either depend on it or some
2791 fn on_failure(&self, dfn: usize) {
2792 debug!(?dfn, "on_failure");
2793 self.map.borrow_mut().retain(|key, eval| {
2794 if !eval.from_dfn >= dfn {
2795 debug!("on_failure: removing {:?}", key);
2803 /// Invoked when the node at depth `depth` completed without
2804 /// depending on anything higher in the stack (if that completion
2805 /// was a failure, then `on_failure` should have been invoked
2808 /// Note that we may still have provisional cache items remaining
2809 /// in the cache when this is done. For example, if there is a
2812 /// * A depends on...
2813 /// * B depends on A
2814 /// * C depends on...
2815 /// * D depends on C
2818 /// Then as we complete the C node we will have a provisional cache
2819 /// with results for A, B, C, and D. This method would clear out
2820 /// the C and D results, but leave A and B provisional.
2822 /// This is determined based on the DFN: we remove any provisional
2823 /// results created since `dfn` started (e.g., in our example, dfn
2824 /// would be 2, representing the C node, and hence we would
2825 /// remove the result for D, which has DFN 3, but not the results for
2826 /// A and B, which have DFNs 0 and 1 respectively).
2828 /// Note that we *do not* attempt to cache these cycle participants
2829 /// in the evaluation cache. Doing so would require carefully computing
2830 /// the correct `DepNode` to store in the cache entry:
2831 /// cycle participants may implicitly depend on query results
2832 /// related to other participants in the cycle, due to our logic
2833 /// which examines the evaluation stack.
2835 /// We used to try to perform this caching,
2836 /// but it lead to multiple incremental compilation ICEs
2837 /// (see #92987 and #96319), and was very hard to understand.
2838 /// Fortunately, removing the caching didn't seem to
2839 /// have a performance impact in practice.
2840 fn on_completion(&self, dfn: usize) {
2841 debug!(?dfn, "on_completion");
2843 for (fresh_trait_pred, eval) in
2844 self.map.borrow_mut().drain_filter(|_k, eval| eval.from_dfn >= dfn)
2846 debug!(?fresh_trait_pred, ?eval, "on_completion");
2851 #[derive(Copy, Clone)]
2852 struct TraitObligationStackList<'o, 'tcx> {
2853 cache: &'o ProvisionalEvaluationCache<'tcx>,
2854 head: Option<&'o TraitObligationStack<'o, 'tcx>>,
2857 impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> {
2858 fn empty(cache: &'o ProvisionalEvaluationCache<'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2859 TraitObligationStackList { cache, head: None }
2862 fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2863 TraitObligationStackList { cache: r.cache(), head: Some(r) }
2866 fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2870 fn depth(&self) -> usize {
2871 if let Some(head) = self.head { head.depth } else { 0 }
2875 impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> {
2876 type Item = &'o TraitObligationStack<'o, 'tcx>;
2878 fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2885 impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
2886 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2887 write!(f, "TraitObligationStack({:?})", self.obligation)
2891 pub enum ProjectionMatchesProjection {