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_vars() {
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| predicate.is_coinductive(self.tcx()))
1177 /// Further evaluates `candidate` to decide whether all type parameters match and whether nested
1178 /// obligations are met. Returns whether `candidate` remains viable after this further
1183 fields(depth = stack.obligation.recursion_depth),
1186 fn evaluate_candidate<'o>(
1188 stack: &TraitObligationStack<'o, 'tcx>,
1189 candidate: &SelectionCandidate<'tcx>,
1190 ) -> Result<EvaluationResult, OverflowError> {
1191 let mut result = self.evaluation_probe(|this| {
1192 let candidate = (*candidate).clone();
1193 match this.confirm_candidate(stack.obligation, candidate) {
1196 this.evaluate_predicates_recursively(
1198 selection.nested_obligations().into_iter(),
1201 Err(..) => Ok(EvaluatedToErr),
1205 // If we erased any lifetimes, then we want to use
1206 // `EvaluatedToOkModuloRegions` instead of `EvaluatedToOk`
1207 // as your final result. The result will be cached using
1208 // the freshened trait predicate as a key, so we need
1209 // our result to be correct by *any* choice of original lifetimes,
1210 // not just the lifetime choice for this particular (non-erased)
1213 if stack.fresh_trait_pred.has_erased_regions() {
1214 result = result.max(EvaluatedToOkModuloRegions);
1220 fn check_evaluation_cache(
1222 param_env: ty::ParamEnv<'tcx>,
1223 trait_pred: ty::PolyTraitPredicate<'tcx>,
1224 ) -> Option<EvaluationResult> {
1225 // Neither the global nor local cache is aware of intercrate
1226 // mode, so don't do any caching. In particular, we might
1227 // re-use the same `InferCtxt` with both an intercrate
1228 // and non-intercrate `SelectionContext`
1229 if self.is_intercrate() {
1233 let tcx = self.tcx();
1234 if self.can_use_global_caches(param_env) {
1235 if let Some(res) = tcx.evaluation_cache.get(&(param_env, trait_pred), tcx) {
1239 self.infcx.evaluation_cache.get(&(param_env, trait_pred), tcx)
1242 fn insert_evaluation_cache(
1244 param_env: ty::ParamEnv<'tcx>,
1245 trait_pred: ty::PolyTraitPredicate<'tcx>,
1246 dep_node: DepNodeIndex,
1247 result: EvaluationResult,
1249 // Avoid caching results that depend on more than just the trait-ref
1250 // - the stack can create recursion.
1251 if result.is_stack_dependent() {
1255 // Neither the global nor local cache is aware of intercrate
1256 // mode, so don't do any caching. In particular, we might
1257 // re-use the same `InferCtxt` with both an intercrate
1258 // and non-intercrate `SelectionContext`
1259 if self.is_intercrate() {
1263 if self.can_use_global_caches(param_env) {
1264 if !trait_pred.needs_infer() {
1265 debug!(?trait_pred, ?result, "insert_evaluation_cache global");
1266 // This may overwrite the cache with the same value
1267 // FIXME: Due to #50507 this overwrites the different values
1268 // This should be changed to use HashMapExt::insert_same
1269 // when that is fixed
1270 self.tcx().evaluation_cache.insert((param_env, trait_pred), dep_node, result);
1275 debug!(?trait_pred, ?result, "insert_evaluation_cache");
1276 self.infcx.evaluation_cache.insert((param_env, trait_pred), dep_node, result);
1279 /// For various reasons, it's possible for a subobligation
1280 /// to have a *lower* recursion_depth than the obligation used to create it.
1281 /// Projection sub-obligations may be returned from the projection cache,
1282 /// which results in obligations with an 'old' `recursion_depth`.
1283 /// Additionally, methods like `InferCtxt.subtype_predicate` produce
1284 /// subobligations without taking in a 'parent' depth, causing the
1285 /// generated subobligations to have a `recursion_depth` of `0`.
1287 /// To ensure that obligation_depth never decreases, we force all subobligations
1288 /// to have at least the depth of the original obligation.
1289 fn add_depth<T: 'cx, I: Iterator<Item = &'cx mut Obligation<'tcx, T>>>(
1294 it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
1297 fn check_recursion_depth<T>(
1300 error_obligation: &Obligation<'tcx, T>,
1301 ) -> Result<(), OverflowError>
1303 T: ToPredicate<'tcx> + Clone,
1305 if !self.infcx.tcx.recursion_limit().value_within_limit(depth) {
1306 match self.query_mode {
1307 TraitQueryMode::Standard => {
1308 if let Some(e) = self.infcx.tainted_by_errors() {
1309 return Err(OverflowError::Error(e));
1311 self.infcx.err_ctxt().report_overflow_obligation(error_obligation, true);
1313 TraitQueryMode::Canonical => {
1314 return Err(OverflowError::Canonical);
1321 /// Checks that the recursion limit has not been exceeded.
1323 /// The weird return type of this function allows it to be used with the `try` (`?`)
1324 /// operator within certain functions.
1326 fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V>(
1328 obligation: &Obligation<'tcx, T>,
1329 error_obligation: &Obligation<'tcx, V>,
1330 ) -> Result<(), OverflowError>
1332 V: ToPredicate<'tcx> + Clone,
1334 self.check_recursion_depth(obligation.recursion_depth, error_obligation)
1337 fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
1339 OP: FnOnce(&mut Self) -> R,
1341 let (result, dep_node) =
1342 self.tcx().dep_graph.with_anon_task(self.tcx(), DepKind::TraitSelect, || op(self));
1343 self.tcx().dep_graph.read_index(dep_node);
1347 /// filter_impls filters constant trait obligations and candidates that have a positive impl
1348 /// for a negative goal and a negative impl for a positive goal
1349 #[instrument(level = "debug", skip(self, candidates))]
1352 candidates: Vec<SelectionCandidate<'tcx>>,
1353 obligation: &TraitObligation<'tcx>,
1354 ) -> Vec<SelectionCandidate<'tcx>> {
1355 trace!("{candidates:#?}");
1356 let tcx = self.tcx();
1357 let mut result = Vec::with_capacity(candidates.len());
1359 for candidate in candidates {
1360 // Respect const trait obligations
1361 if obligation.is_const() {
1364 ImplCandidate(def_id) if tcx.constness(def_id) == hir::Constness::Const => {}
1366 ParamCandidate(trait_pred) if trait_pred.is_const_if_const() => {}
1368 ProjectionCandidate(_, ty::BoundConstness::ConstIfConst) => {}
1370 AutoImplCandidate => {}
1371 // generator / future, this will raise error in other places
1372 // or ignore error with const_async_blocks feature
1373 GeneratorCandidate => {}
1374 FutureCandidate => {}
1375 // FnDef where the function is const
1376 FnPointerCandidate { is_const: true } => {}
1377 FnPointerCandidate { is_const: false } => {
1378 if let ty::FnDef(def_id, _) = obligation.self_ty().skip_binder().kind() && tcx.trait_of_item(*def_id).is_some() {
1379 // Trait methods are not seen as const unless the trait is implemented as const.
1380 // We do not filter that out in here, but nested obligations will be needed to confirm this.
1385 ConstDestructCandidate(_) => {}
1387 // reject all other types of candidates
1393 if let ImplCandidate(def_id) = candidate {
1394 if ty::ImplPolarity::Reservation == tcx.impl_polarity(def_id)
1395 || obligation.polarity() == tcx.impl_polarity(def_id)
1397 result.push(candidate);
1400 result.push(candidate);
1404 trace!("{result:#?}");
1408 /// filter_reservation_impls filter reservation impl for any goal as ambiguous
1409 #[instrument(level = "debug", skip(self))]
1410 fn filter_reservation_impls(
1412 candidate: SelectionCandidate<'tcx>,
1413 obligation: &TraitObligation<'tcx>,
1414 ) -> SelectionResult<'tcx, SelectionCandidate<'tcx>> {
1415 let tcx = self.tcx();
1416 // Treat reservation impls as ambiguity.
1417 if let ImplCandidate(def_id) = candidate {
1418 if let ty::ImplPolarity::Reservation = tcx.impl_polarity(def_id) {
1419 if let Some(intercrate_ambiguity_clauses) = &mut self.intercrate_ambiguity_causes {
1421 .get_attr(def_id, sym::rustc_reservation_impl)
1422 .and_then(|a| a.value_str());
1423 if let Some(value) = value {
1425 "filter_reservation_impls: \
1426 reservation impl ambiguity on {:?}",
1429 intercrate_ambiguity_clauses.insert(
1430 IntercrateAmbiguityCause::ReservationImpl {
1431 message: value.to_string(),
1442 fn is_knowable<'o>(&mut self, stack: &TraitObligationStack<'o, 'tcx>) -> Result<(), Conflict> {
1443 debug!("is_knowable(intercrate={:?})", self.is_intercrate());
1445 if !self.is_intercrate() || stack.obligation.polarity() == ty::ImplPolarity::Negative {
1449 let obligation = &stack.obligation;
1450 let predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
1452 // Okay to skip binder because of the nature of the
1453 // trait-ref-is-knowable check, which does not care about
1455 let trait_ref = predicate.skip_binder().trait_ref;
1457 coherence::trait_ref_is_knowable(self.tcx(), trait_ref)
1460 /// Returns `true` if the global caches can be used.
1461 fn can_use_global_caches(&self, param_env: ty::ParamEnv<'tcx>) -> bool {
1462 // If there are any inference variables in the `ParamEnv`, then we
1463 // always use a cache local to this particular scope. Otherwise, we
1464 // switch to a global cache.
1465 if param_env.needs_infer() {
1469 // Avoid using the master cache during coherence and just rely
1470 // on the local cache. This effectively disables caching
1471 // during coherence. It is really just a simplification to
1472 // avoid us having to fear that coherence results "pollute"
1473 // the master cache. Since coherence executes pretty quickly,
1474 // it's not worth going to more trouble to increase the
1475 // hit-rate, I don't think.
1476 if self.is_intercrate() {
1480 // Otherwise, we can use the global cache.
1484 fn check_candidate_cache(
1486 mut param_env: ty::ParamEnv<'tcx>,
1487 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1488 ) -> Option<SelectionResult<'tcx, SelectionCandidate<'tcx>>> {
1489 // Neither the global nor local cache is aware of intercrate
1490 // mode, so don't do any caching. In particular, we might
1491 // re-use the same `InferCtxt` with both an intercrate
1492 // and non-intercrate `SelectionContext`
1493 if self.is_intercrate() {
1496 let tcx = self.tcx();
1497 let mut pred = cache_fresh_trait_pred.skip_binder();
1498 pred.remap_constness(&mut param_env);
1500 if self.can_use_global_caches(param_env) {
1501 if let Some(res) = tcx.selection_cache.get(&(param_env, pred), tcx) {
1505 self.infcx.selection_cache.get(&(param_env, pred), tcx)
1508 /// Determines whether can we safely cache the result
1509 /// of selecting an obligation. This is almost always `true`,
1510 /// except when dealing with certain `ParamCandidate`s.
1512 /// Ordinarily, a `ParamCandidate` will contain no inference variables,
1513 /// since it was usually produced directly from a `DefId`. However,
1514 /// certain cases (currently only librustdoc's blanket impl finder),
1515 /// a `ParamEnv` may be explicitly constructed with inference types.
1516 /// When this is the case, we do *not* want to cache the resulting selection
1517 /// candidate. This is due to the fact that it might not always be possible
1518 /// to equate the obligation's trait ref and the candidate's trait ref,
1519 /// if more constraints end up getting added to an inference variable.
1521 /// Because of this, we always want to re-run the full selection
1522 /// process for our obligation the next time we see it, since
1523 /// we might end up picking a different `SelectionCandidate` (or none at all).
1524 fn can_cache_candidate(
1526 result: &SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1528 // Neither the global nor local cache is aware of intercrate
1529 // mode, so don't do any caching. In particular, we might
1530 // re-use the same `InferCtxt` with both an intercrate
1531 // and non-intercrate `SelectionContext`
1532 if self.is_intercrate() {
1536 Ok(Some(SelectionCandidate::ParamCandidate(trait_ref))) => !trait_ref.needs_infer(),
1541 #[instrument(skip(self, param_env, cache_fresh_trait_pred, dep_node), level = "debug")]
1542 fn insert_candidate_cache(
1544 mut param_env: ty::ParamEnv<'tcx>,
1545 cache_fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
1546 dep_node: DepNodeIndex,
1547 candidate: SelectionResult<'tcx, SelectionCandidate<'tcx>>,
1549 let tcx = self.tcx();
1550 let mut pred = cache_fresh_trait_pred.skip_binder();
1552 pred.remap_constness(&mut param_env);
1554 if !self.can_cache_candidate(&candidate) {
1555 debug!(?pred, ?candidate, "insert_candidate_cache - candidate is not cacheable");
1559 if self.can_use_global_caches(param_env) {
1560 if let Err(Overflow(OverflowError::Canonical)) = candidate {
1561 // Don't cache overflow globally; we only produce this in certain modes.
1562 } else if !pred.needs_infer() {
1563 if !candidate.needs_infer() {
1564 debug!(?pred, ?candidate, "insert_candidate_cache global");
1565 // This may overwrite the cache with the same value.
1566 tcx.selection_cache.insert((param_env, pred), dep_node, candidate);
1572 debug!(?pred, ?candidate, "insert_candidate_cache local");
1573 self.infcx.selection_cache.insert((param_env, pred), dep_node, candidate);
1576 /// Matches a predicate against the bounds of its self type.
1578 /// Given an obligation like `<T as Foo>::Bar: Baz` where the self type is
1579 /// a projection, look at the bounds of `T::Bar`, see if we can find a
1580 /// `Baz` bound. We return indexes into the list returned by
1581 /// `tcx.item_bounds` for any applicable bounds.
1582 #[instrument(level = "debug", skip(self), ret)]
1583 fn match_projection_obligation_against_definition_bounds(
1585 obligation: &TraitObligation<'tcx>,
1586 ) -> smallvec::SmallVec<[(usize, ty::BoundConstness); 2]> {
1587 let poly_trait_predicate = self.infcx.resolve_vars_if_possible(obligation.predicate);
1588 let placeholder_trait_predicate =
1589 self.infcx.replace_bound_vars_with_placeholders(poly_trait_predicate);
1590 debug!(?placeholder_trait_predicate);
1592 let tcx = self.infcx.tcx;
1593 let (def_id, substs) = match *placeholder_trait_predicate.trait_ref.self_ty().kind() {
1594 ty::Alias(_, ty::AliasTy { def_id, substs, .. }) => (def_id, substs),
1597 obligation.cause.span,
1598 "match_projection_obligation_against_definition_bounds() called \
1599 but self-ty is not a projection: {:?}",
1600 placeholder_trait_predicate.trait_ref.self_ty()
1604 let bounds = tcx.bound_item_bounds(def_id).subst(tcx, substs);
1606 // The bounds returned by `item_bounds` may contain duplicates after
1607 // normalization, so try to deduplicate when possible to avoid
1608 // unnecessary ambiguity.
1609 let mut distinct_normalized_bounds = FxHashSet::default();
1614 .filter_map(|(idx, bound)| {
1615 let bound_predicate = bound.kind();
1616 if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) =
1617 bound_predicate.skip_binder()
1619 let bound = bound_predicate.rebind(pred.trait_ref);
1620 if self.infcx.probe(|_| {
1621 match self.match_normalize_trait_ref(
1624 placeholder_trait_predicate.trait_ref,
1627 Ok(Some(normalized_trait))
1628 if distinct_normalized_bounds.insert(normalized_trait) =>
1635 return Some((idx, pred.constness));
1643 /// Equates the trait in `obligation` with trait bound. If the two traits
1644 /// can be equated and the normalized trait bound doesn't contain inference
1645 /// variables or placeholders, the normalized bound is returned.
1646 fn match_normalize_trait_ref(
1648 obligation: &TraitObligation<'tcx>,
1649 trait_bound: ty::PolyTraitRef<'tcx>,
1650 placeholder_trait_ref: ty::TraitRef<'tcx>,
1651 ) -> Result<Option<ty::PolyTraitRef<'tcx>>, ()> {
1652 debug_assert!(!placeholder_trait_ref.has_escaping_bound_vars());
1653 if placeholder_trait_ref.def_id != trait_bound.def_id() {
1654 // Avoid unnecessary normalization
1658 let Normalized { value: trait_bound, obligations: _ } = ensure_sufficient_stack(|| {
1659 project::normalize_with_depth(
1661 obligation.param_env,
1662 obligation.cause.clone(),
1663 obligation.recursion_depth + 1,
1668 .at(&obligation.cause, obligation.param_env)
1669 .define_opaque_types(false)
1670 .sup(ty::Binder::dummy(placeholder_trait_ref), trait_bound)
1671 .map(|InferOk { obligations: _, value: () }| {
1672 // This method is called within a probe, so we can't have
1673 // inference variables and placeholders escape.
1674 if !trait_bound.needs_infer() && !trait_bound.has_placeholders() {
1683 fn where_clause_may_apply<'o>(
1685 stack: &TraitObligationStack<'o, 'tcx>,
1686 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
1687 ) -> Result<EvaluationResult, OverflowError> {
1688 self.evaluation_probe(|this| {
1689 match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
1690 Ok(obligations) => this.evaluate_predicates_recursively(stack.list(), obligations),
1691 Err(()) => Ok(EvaluatedToErr),
1696 /// Return `Yes` if the obligation's predicate type applies to the env_predicate, and
1697 /// `No` if it does not. Return `Ambiguous` in the case that the projection type is a GAT,
1698 /// and applying this env_predicate constrains any of the obligation's GAT substitutions.
1700 /// This behavior is a somewhat of a hack to prevent over-constraining inference variables
1701 /// in cases like #91762.
1702 pub(super) fn match_projection_projections(
1704 obligation: &ProjectionTyObligation<'tcx>,
1705 env_predicate: PolyProjectionPredicate<'tcx>,
1706 potentially_unnormalized_candidates: bool,
1707 ) -> ProjectionMatchesProjection {
1708 let mut nested_obligations = Vec::new();
1709 let infer_predicate = self.infcx.replace_bound_vars_with_fresh_vars(
1710 obligation.cause.span,
1711 LateBoundRegionConversionTime::HigherRankedType,
1714 let infer_projection = if potentially_unnormalized_candidates {
1715 ensure_sufficient_stack(|| {
1716 project::normalize_with_depth_to(
1718 obligation.param_env,
1719 obligation.cause.clone(),
1720 obligation.recursion_depth + 1,
1721 infer_predicate.projection_ty,
1722 &mut nested_obligations,
1726 infer_predicate.projection_ty
1731 .at(&obligation.cause, obligation.param_env)
1732 .define_opaque_types(false)
1733 .sup(obligation.predicate, infer_projection)
1734 .map_or(false, |InferOk { obligations, value: () }| {
1735 self.evaluate_predicates_recursively(
1736 TraitObligationStackList::empty(&ProvisionalEvaluationCache::default()),
1737 nested_obligations.into_iter().chain(obligations),
1739 .map_or(false, |res| res.may_apply())
1743 let generics = self.tcx().generics_of(obligation.predicate.def_id);
1744 // FIXME(generic-associated-types): Addresses aggressive inference in #92917.
1745 // If this type is a GAT, and of the GAT substs resolve to something new,
1746 // that means that we must have newly inferred something about the GAT.
1747 // We should give up in that case.
1748 if !generics.params.is_empty()
1749 && obligation.predicate.substs[generics.parent_count..]
1751 .any(|&p| p.has_non_region_infer() && self.infcx.shallow_resolve(p) != p)
1753 ProjectionMatchesProjection::Ambiguous
1755 ProjectionMatchesProjection::Yes
1758 ProjectionMatchesProjection::No
1762 ///////////////////////////////////////////////////////////////////////////
1765 // Winnowing is the process of attempting to resolve ambiguity by
1766 // probing further. During the winnowing process, we unify all
1767 // type variables and then we also attempt to evaluate recursive
1768 // bounds to see if they are satisfied.
1770 /// Returns `true` if `victim` should be dropped in favor of
1771 /// `other`. Generally speaking we will drop duplicate
1772 /// candidates and prefer where-clause candidates.
1774 /// See the comment for "SelectionCandidate" for more details.
1775 fn candidate_should_be_dropped_in_favor_of(
1777 victim: &EvaluatedCandidate<'tcx>,
1778 other: &EvaluatedCandidate<'tcx>,
1781 if victim.candidate == other.candidate {
1785 // Check if a bound would previously have been removed when normalizing
1786 // the param_env so that it can be given the lowest priority. See
1787 // #50825 for the motivation for this.
1789 |cand: &ty::PolyTraitPredicate<'tcx>| cand.is_global() && !cand.has_late_bound_vars();
1791 // (*) Prefer `BuiltinCandidate { has_nested: false }`, `PointeeCandidate`,
1792 // `DiscriminantKindCandidate`, `ConstDestructCandidate`
1793 // to anything else.
1795 // This is a fix for #53123 and prevents winnowing from accidentally extending the
1796 // lifetime of a variable.
1797 match (&other.candidate, &victim.candidate) {
1798 (_, AutoImplCandidate) | (AutoImplCandidate, _) => {
1800 "default implementations shouldn't be recorded \
1801 when there are other valid candidates"
1805 // FIXME(@jswrenn): this should probably be more sophisticated
1806 (TransmutabilityCandidate, _) | (_, TransmutabilityCandidate) => false,
1809 (BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_), _) => true,
1810 (_, BuiltinCandidate { has_nested: false } | ConstDestructCandidate(_)) => false,
1812 (ParamCandidate(other), ParamCandidate(victim)) => {
1813 let same_except_bound_vars = other.skip_binder().trait_ref
1814 == victim.skip_binder().trait_ref
1815 && other.skip_binder().constness == victim.skip_binder().constness
1816 && other.skip_binder().polarity == victim.skip_binder().polarity
1817 && !other.skip_binder().trait_ref.has_escaping_bound_vars();
1818 if same_except_bound_vars {
1819 // See issue #84398. In short, we can generate multiple ParamCandidates which are
1820 // the same except for unused bound vars. Just pick the one with the fewest bound vars
1821 // or the current one if tied (they should both evaluate to the same answer). This is
1822 // probably best characterized as a "hack", since we might prefer to just do our
1823 // best to *not* create essentially duplicate candidates in the first place.
1824 other.bound_vars().len() <= victim.bound_vars().len()
1825 } else if other.skip_binder().trait_ref == victim.skip_binder().trait_ref
1826 && victim.skip_binder().constness == ty::BoundConstness::NotConst
1827 && other.skip_binder().polarity == victim.skip_binder().polarity
1829 // Drop otherwise equivalent non-const candidates in favor of const candidates.
1836 // Drop otherwise equivalent non-const fn pointer candidates
1837 (FnPointerCandidate { .. }, FnPointerCandidate { is_const: false }) => true,
1839 // Global bounds from the where clause should be ignored
1840 // here (see issue #50825). Otherwise, we have a where
1841 // clause so don't go around looking for impls.
1842 // Arbitrarily give param candidates priority
1843 // over projection and object candidates.
1845 ParamCandidate(ref cand),
1848 | GeneratorCandidate
1850 | FnPointerCandidate { .. }
1851 | BuiltinObjectCandidate
1852 | BuiltinUnsizeCandidate
1853 | TraitUpcastingUnsizeCandidate(_)
1854 | BuiltinCandidate { .. }
1855 | TraitAliasCandidate
1856 | ObjectCandidate(_)
1857 | ProjectionCandidate(..),
1858 ) => !is_global(cand),
1859 (ObjectCandidate(_) | ProjectionCandidate(..), ParamCandidate(ref cand)) => {
1860 // Prefer these to a global where-clause bound
1861 // (see issue #50825).
1867 | GeneratorCandidate
1869 | FnPointerCandidate { .. }
1870 | BuiltinObjectCandidate
1871 | BuiltinUnsizeCandidate
1872 | TraitUpcastingUnsizeCandidate(_)
1873 | BuiltinCandidate { has_nested: true }
1874 | TraitAliasCandidate,
1875 ParamCandidate(ref cand),
1877 // Prefer these to a global where-clause bound
1878 // (see issue #50825).
1879 is_global(cand) && other.evaluation.must_apply_modulo_regions()
1882 (ProjectionCandidate(i, _), ProjectionCandidate(j, _))
1883 | (ObjectCandidate(i), ObjectCandidate(j)) => {
1884 // Arbitrarily pick the lower numbered candidate for backwards
1885 // compatibility reasons. Don't let this affect inference.
1886 i < j && !needs_infer
1888 (ObjectCandidate(_), ProjectionCandidate(..))
1889 | (ProjectionCandidate(..), ObjectCandidate(_)) => {
1890 bug!("Have both object and projection candidate")
1893 // Arbitrarily give projection and object candidates priority.
1895 ObjectCandidate(_) | ProjectionCandidate(..),
1898 | GeneratorCandidate
1900 | FnPointerCandidate { .. }
1901 | BuiltinObjectCandidate
1902 | BuiltinUnsizeCandidate
1903 | TraitUpcastingUnsizeCandidate(_)
1904 | BuiltinCandidate { .. }
1905 | TraitAliasCandidate,
1911 | GeneratorCandidate
1913 | FnPointerCandidate { .. }
1914 | BuiltinObjectCandidate
1915 | BuiltinUnsizeCandidate
1916 | TraitUpcastingUnsizeCandidate(_)
1917 | BuiltinCandidate { .. }
1918 | TraitAliasCandidate,
1919 ObjectCandidate(_) | ProjectionCandidate(..),
1922 (&ImplCandidate(other_def), &ImplCandidate(victim_def)) => {
1923 // See if we can toss out `victim` based on specialization.
1924 // While this requires us to know *for sure* that the `other` impl applies
1925 // we still use modulo regions here.
1927 // This is fine as specialization currently assumes that specializing
1928 // impls have to be always applicable, meaning that the only allowed
1929 // region constraints may be constraints also present on the default impl.
1930 let tcx = self.tcx();
1931 if other.evaluation.must_apply_modulo_regions() {
1932 if tcx.specializes((other_def, victim_def)) {
1937 if other.evaluation.must_apply_considering_regions() {
1938 match tcx.impls_are_allowed_to_overlap(other_def, victim_def) {
1939 Some(ty::ImplOverlapKind::Permitted { marker: true }) => {
1940 // Subtle: If the predicate we are evaluating has inference
1941 // variables, do *not* allow discarding candidates due to
1942 // marker trait impls.
1944 // Without this restriction, we could end up accidentally
1945 // constraining inference variables based on an arbitrarily
1946 // chosen trait impl.
1948 // Imagine we have the following code:
1951 // #[marker] trait MyTrait {}
1952 // impl MyTrait for u8 {}
1953 // impl MyTrait for bool {}
1956 // And we are evaluating the predicate `<_#0t as MyTrait>`.
1958 // During selection, we will end up with one candidate for each
1959 // impl of `MyTrait`. If we were to discard one impl in favor
1960 // of the other, we would be left with one candidate, causing
1961 // us to "successfully" select the predicate, unifying
1962 // _#0t with (for example) `u8`.
1964 // However, we have no reason to believe that this unification
1965 // is correct - we've essentially just picked an arbitrary
1966 // *possibility* for _#0t, and required that this be the *only*
1969 // Eventually, we will either:
1970 // 1) Unify all inference variables in the predicate through
1971 // some other means (e.g. type-checking of a function). We will
1972 // then be in a position to drop marker trait candidates
1973 // without constraining inference variables (since there are
1974 // none left to constrain)
1975 // 2) Be left with some unconstrained inference variables. We
1976 // will then correctly report an inference error, since the
1977 // existence of multiple marker trait impls tells us nothing
1978 // about which one should actually apply.
1989 // Everything else is ambiguous
1993 | GeneratorCandidate
1995 | FnPointerCandidate { .. }
1996 | BuiltinObjectCandidate
1997 | BuiltinUnsizeCandidate
1998 | TraitUpcastingUnsizeCandidate(_)
1999 | BuiltinCandidate { has_nested: true }
2000 | TraitAliasCandidate,
2003 | GeneratorCandidate
2005 | FnPointerCandidate { .. }
2006 | BuiltinObjectCandidate
2007 | BuiltinUnsizeCandidate
2008 | TraitUpcastingUnsizeCandidate(_)
2009 | BuiltinCandidate { has_nested: true }
2010 | TraitAliasCandidate,
2015 fn sized_conditions(
2017 obligation: &TraitObligation<'tcx>,
2018 ) -> BuiltinImplConditions<'tcx> {
2019 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2021 // NOTE: binder moved to (*)
2022 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2024 match self_ty.kind() {
2025 ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
2036 | ty::GeneratorWitness(..)
2040 | ty::Dynamic(_, _, ty::DynStar)
2042 // safe for everything
2043 Where(ty::Binder::dummy(Vec::new()))
2046 ty::Str | ty::Slice(_) | ty::Dynamic(..) | ty::Foreign(..) => None,
2048 ty::Tuple(tys) => Where(
2049 obligation.predicate.rebind(tys.last().map_or_else(Vec::new, |&last| vec![last])),
2052 ty::Adt(def, substs) => {
2053 let sized_crit = def.sized_constraint(self.tcx());
2054 // (*) binder moved here
2055 Where(obligation.predicate.rebind({
2059 .map(|ty| sized_crit.rebind(*ty).subst(self.tcx(), substs))
2064 ty::Alias(..) | ty::Param(_) => None,
2065 ty::Infer(ty::TyVar(_)) => Ambiguous,
2069 | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2070 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
2075 fn copy_clone_conditions(
2077 obligation: &TraitObligation<'tcx>,
2078 ) -> BuiltinImplConditions<'tcx> {
2079 // NOTE: binder moved to (*)
2080 let self_ty = self.infcx.shallow_resolve(obligation.predicate.skip_binder().self_ty());
2082 use self::BuiltinImplConditions::{Ambiguous, None, Where};
2084 match *self_ty.kind() {
2085 ty::Infer(ty::IntVar(_))
2086 | ty::Infer(ty::FloatVar(_))
2089 | ty::Error(_) => Where(ty::Binder::dummy(Vec::new())),
2098 | ty::Ref(_, _, hir::Mutability::Not)
2099 | ty::Array(..) => {
2100 // Implementations provided in libcore
2107 | ty::Generator(_, _, hir::Movability::Static)
2109 | ty::Ref(_, _, hir::Mutability::Mut) => None,
2112 // (*) binder moved here
2113 Where(obligation.predicate.rebind(tys.iter().collect()))
2116 ty::Generator(_, substs, hir::Movability::Movable) => {
2117 if self.tcx().features().generator_clone {
2118 let resolved_upvars =
2119 self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
2120 let resolved_witness =
2121 self.infcx.shallow_resolve(substs.as_generator().witness());
2122 if resolved_upvars.is_ty_var() || resolved_witness.is_ty_var() {
2123 // Not yet resolved.
2129 .chain(iter::once(substs.as_generator().witness()))
2130 .collect::<Vec<_>>();
2131 Where(obligation.predicate.rebind(all))
2138 ty::GeneratorWitness(binder) => {
2139 let witness_tys = binder.skip_binder();
2140 for witness_ty in witness_tys.iter() {
2141 let resolved = self.infcx.shallow_resolve(witness_ty);
2142 if resolved.is_ty_var() {
2146 // (*) binder moved here
2147 let all_vars = self.tcx().mk_bound_variable_kinds(
2148 obligation.predicate.bound_vars().iter().chain(binder.bound_vars().iter()),
2150 Where(ty::Binder::bind_with_vars(witness_tys.to_vec(), all_vars))
2153 ty::Closure(_, substs) => {
2154 // (*) binder moved here
2155 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
2156 if let ty::Infer(ty::TyVar(_)) = ty.kind() {
2157 // Not yet resolved.
2160 Where(obligation.predicate.rebind(substs.as_closure().upvar_tys().collect()))
2164 ty::Adt(..) | ty::Alias(..) | ty::Param(..) => {
2165 // Fallback to whatever user-defined impls exist in this case.
2169 ty::Infer(ty::TyVar(_)) => {
2170 // Unbound type variable. Might or might not have
2171 // applicable impls and so forth, depending on what
2172 // those type variables wind up being bound to.
2178 | ty::Infer(ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2179 bug!("asked to assemble builtin bounds of unexpected type: {:?}", self_ty);
2184 /// For default impls, we need to break apart a type into its
2185 /// "constituent types" -- meaning, the types that it contains.
2187 /// Here are some (simple) examples:
2189 /// ```ignore (illustrative)
2190 /// (i32, u32) -> [i32, u32]
2191 /// Foo where struct Foo { x: i32, y: u32 } -> [i32, u32]
2192 /// Bar<i32> where struct Bar<T> { x: T, y: u32 } -> [i32, u32]
2193 /// Zed<i32> where enum Zed { A(T), B(u32) } -> [i32, u32]
2195 #[instrument(level = "debug", skip(self), ret)]
2196 fn constituent_types_for_ty(
2198 t: ty::Binder<'tcx, Ty<'tcx>>,
2199 ) -> ty::Binder<'tcx, Vec<Ty<'tcx>>> {
2200 match *t.skip_binder().kind() {
2209 | ty::Infer(ty::IntVar(_) | ty::FloatVar(_))
2211 | ty::Char => ty::Binder::dummy(Vec::new()),
2217 | ty::Alias(ty::Projection, ..)
2219 | ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_)) => {
2220 bug!("asked to assemble constituent types of unexpected type: {:?}", t);
2223 ty::RawPtr(ty::TypeAndMut { ty: element_ty, .. }) | ty::Ref(_, element_ty, _) => {
2224 t.rebind(vec![element_ty])
2227 ty::Array(element_ty, _) | ty::Slice(element_ty) => t.rebind(vec![element_ty]),
2229 ty::Tuple(ref tys) => {
2230 // (T1, ..., Tn) -- meets any bound that all of T1...Tn meet
2231 t.rebind(tys.iter().collect())
2234 ty::Closure(_, ref substs) => {
2235 let ty = self.infcx.shallow_resolve(substs.as_closure().tupled_upvars_ty());
2239 ty::Generator(_, ref substs, _) => {
2240 let ty = self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
2241 let witness = substs.as_generator().witness();
2242 t.rebind([ty].into_iter().chain(iter::once(witness)).collect())
2245 ty::GeneratorWitness(types) => {
2246 debug_assert!(!types.has_escaping_bound_vars());
2247 types.map_bound(|types| types.to_vec())
2250 // For `PhantomData<T>`, we pass `T`.
2251 ty::Adt(def, substs) if def.is_phantom_data() => t.rebind(substs.types().collect()),
2253 ty::Adt(def, substs) => {
2254 t.rebind(def.all_fields().map(|f| f.ty(self.tcx(), substs)).collect())
2257 ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => {
2258 // We can resolve the `impl Trait` to its concrete type,
2259 // which enforces a DAG between the functions requiring
2260 // the auto trait bounds in question.
2261 t.rebind(vec![self.tcx().bound_type_of(def_id).subst(self.tcx(), substs)])
2266 fn collect_predicates_for_types(
2268 param_env: ty::ParamEnv<'tcx>,
2269 cause: ObligationCause<'tcx>,
2270 recursion_depth: usize,
2271 trait_def_id: DefId,
2272 types: ty::Binder<'tcx, Vec<Ty<'tcx>>>,
2273 ) -> Vec<PredicateObligation<'tcx>> {
2274 // Because the types were potentially derived from
2275 // higher-ranked obligations they may reference late-bound
2276 // regions. For example, `for<'a> Foo<&'a i32> : Copy` would
2277 // yield a type like `for<'a> &'a i32`. In general, we
2278 // maintain the invariant that we never manipulate bound
2279 // regions, so we have to process these bound regions somehow.
2281 // The strategy is to:
2283 // 1. Instantiate those regions to placeholder regions (e.g.,
2284 // `for<'a> &'a i32` becomes `&0 i32`.
2285 // 2. Produce something like `&'0 i32 : Copy`
2286 // 3. Re-bind the regions back to `for<'a> &'a i32 : Copy`
2290 .skip_binder() // binder moved -\
2293 let ty: ty::Binder<'tcx, Ty<'tcx>> = types.rebind(*ty); // <----/
2295 let placeholder_ty = self.infcx.replace_bound_vars_with_placeholders(ty);
2296 let Normalized { value: normalized_ty, mut obligations } =
2297 ensure_sufficient_stack(|| {
2298 project::normalize_with_depth(
2306 let placeholder_obligation = predicate_for_trait_def(
2314 obligations.push(placeholder_obligation);
2320 ///////////////////////////////////////////////////////////////////////////
2323 // Matching is a common path used for both evaluation and
2324 // confirmation. It basically unifies types that appear in impls
2325 // and traits. This does affect the surrounding environment;
2326 // therefore, when used during evaluation, match routines must be
2327 // run inside of a `probe()` so that their side-effects are
2333 obligation: &TraitObligation<'tcx>,
2334 ) -> Normalized<'tcx, SubstsRef<'tcx>> {
2335 let impl_trait_ref = self.tcx().bound_impl_trait_ref(impl_def_id).unwrap();
2336 match self.match_impl(impl_def_id, impl_trait_ref, obligation) {
2337 Ok(substs) => substs,
2339 // FIXME: A rematch may fail when a candidate cache hit occurs
2340 // on thefreshened form of the trait predicate, but the match
2341 // fails for some reason that is not captured in the freshened
2342 // cache key. For example, equating an impl trait ref against
2343 // the placeholder trait ref may fail due the Generalizer relation
2344 // raising a CyclicalTy error due to a sub_root_var relation
2345 // for a variable being generalized...
2346 self.infcx.tcx.sess.delay_span_bug(
2347 obligation.cause.span,
2349 "Impl {:?} was matchable against {:?} but now is not",
2350 impl_def_id, obligation
2353 let value = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
2354 let err = self.tcx().ty_error();
2355 let value = value.fold_with(&mut BottomUpFolder {
2361 Normalized { value, obligations: vec![] }
2366 #[instrument(level = "debug", skip(self), ret)]
2370 impl_trait_ref: EarlyBinder<ty::TraitRef<'tcx>>,
2371 obligation: &TraitObligation<'tcx>,
2372 ) -> Result<Normalized<'tcx, SubstsRef<'tcx>>, ()> {
2373 let placeholder_obligation =
2374 self.infcx.replace_bound_vars_with_placeholders(obligation.predicate);
2375 let placeholder_obligation_trait_ref = placeholder_obligation.trait_ref;
2377 let impl_substs = self.infcx.fresh_substs_for_item(obligation.cause.span, impl_def_id);
2379 let impl_trait_ref = impl_trait_ref.subst(self.tcx(), impl_substs);
2380 if impl_trait_ref.references_error() {
2384 debug!(?impl_trait_ref);
2386 let Normalized { value: impl_trait_ref, obligations: mut nested_obligations } =
2387 ensure_sufficient_stack(|| {
2388 project::normalize_with_depth(
2390 obligation.param_env,
2391 obligation.cause.clone(),
2392 obligation.recursion_depth + 1,
2397 debug!(?impl_trait_ref, ?placeholder_obligation_trait_ref);
2399 let cause = ObligationCause::new(
2400 obligation.cause.span,
2401 obligation.cause.body_id,
2402 ObligationCauseCode::MatchImpl(obligation.cause.clone(), impl_def_id),
2405 let InferOk { obligations, .. } = self
2407 .at(&cause, obligation.param_env)
2408 .define_opaque_types(false)
2409 .eq(placeholder_obligation_trait_ref, impl_trait_ref)
2410 .map_err(|e| debug!("match_impl: failed eq_trait_refs due to `{e}`"))?;
2411 nested_obligations.extend(obligations);
2413 if !self.is_intercrate()
2414 && self.tcx().impl_polarity(impl_def_id) == ty::ImplPolarity::Reservation
2416 debug!("reservation impls only apply in intercrate mode");
2420 Ok(Normalized { value: impl_substs, obligations: nested_obligations })
2423 fn fast_reject_trait_refs(
2425 obligation: &TraitObligation<'tcx>,
2426 impl_trait_ref: &ty::TraitRef<'tcx>,
2428 // We can avoid creating type variables and doing the full
2429 // substitution if we find that any of the input types, when
2430 // simplified, do not match.
2431 let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsPlaceholder };
2432 iter::zip(obligation.predicate.skip_binder().trait_ref.substs, impl_trait_ref.substs)
2433 .any(|(obl, imp)| !drcx.generic_args_may_unify(obl, imp))
2436 /// Normalize `where_clause_trait_ref` and try to match it against
2437 /// `obligation`. If successful, return any predicates that
2438 /// result from the normalization.
2439 fn match_where_clause_trait_ref(
2441 obligation: &TraitObligation<'tcx>,
2442 where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
2443 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
2444 self.match_poly_trait_ref(obligation, where_clause_trait_ref)
2447 /// Returns `Ok` if `poly_trait_ref` being true implies that the
2448 /// obligation is satisfied.
2449 #[instrument(skip(self), level = "debug")]
2450 fn match_poly_trait_ref(
2452 obligation: &TraitObligation<'tcx>,
2453 poly_trait_ref: ty::PolyTraitRef<'tcx>,
2454 ) -> Result<Vec<PredicateObligation<'tcx>>, ()> {
2456 .at(&obligation.cause, obligation.param_env)
2457 // We don't want predicates for opaque types to just match all other types,
2458 // if there is an obligation on the opaque type, then that obligation must be met
2459 // opaquely. Otherwise we'd match any obligation to the opaque type and then error
2461 .define_opaque_types(false)
2462 .sup(obligation.predicate.to_poly_trait_ref(), poly_trait_ref)
2463 .map(|InferOk { obligations, .. }| obligations)
2467 ///////////////////////////////////////////////////////////////////////////
2470 fn match_fresh_trait_refs(
2472 previous: ty::PolyTraitPredicate<'tcx>,
2473 current: ty::PolyTraitPredicate<'tcx>,
2474 param_env: ty::ParamEnv<'tcx>,
2476 let mut matcher = ty::_match::Match::new(self.tcx(), param_env);
2477 matcher.relate(previous, current).is_ok()
2482 previous_stack: TraitObligationStackList<'o, 'tcx>,
2483 obligation: &'o TraitObligation<'tcx>,
2484 ) -> TraitObligationStack<'o, 'tcx> {
2485 let fresh_trait_pred = obligation.predicate.fold_with(&mut self.freshener);
2487 let dfn = previous_stack.cache.next_dfn();
2488 let depth = previous_stack.depth() + 1;
2489 TraitObligationStack {
2492 reached_depth: Cell::new(depth),
2493 previous: previous_stack,
2499 #[instrument(skip(self), level = "debug")]
2500 fn closure_trait_ref_unnormalized(
2502 obligation: &TraitObligation<'tcx>,
2503 substs: SubstsRef<'tcx>,
2504 ) -> ty::PolyTraitRef<'tcx> {
2505 let closure_sig = substs.as_closure().sig();
2507 debug!(?closure_sig);
2509 // NOTE: The self-type is an unboxed closure type and hence is
2510 // in fact unparameterized (or at least does not reference any
2511 // regions bound in the obligation).
2512 let self_ty = obligation
2516 .expect("unboxed closure type should not capture bound vars from the predicate");
2518 closure_trait_ref_and_return_type(
2520 obligation.predicate.def_id(),
2523 util::TupleArgumentsFlag::No,
2525 .map_bound(|(trait_ref, _)| trait_ref)
2528 /// Returns the obligations that are implied by instantiating an
2529 /// impl or trait. The obligations are substituted and fully
2530 /// normalized. This is used when confirming an impl or default
2532 #[instrument(level = "debug", skip(self, cause, param_env))]
2533 fn impl_or_trait_obligations(
2535 cause: &ObligationCause<'tcx>,
2536 recursion_depth: usize,
2537 param_env: ty::ParamEnv<'tcx>,
2538 def_id: DefId, // of impl or trait
2539 substs: SubstsRef<'tcx>, // for impl or trait
2540 parent_trait_pred: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
2541 ) -> Vec<PredicateObligation<'tcx>> {
2542 let tcx = self.tcx();
2544 // To allow for one-pass evaluation of the nested obligation,
2545 // each predicate must be preceded by the obligations required
2547 // for example, if we have:
2548 // impl<U: Iterator<Item: Copy>, V: Iterator<Item = U>> Foo for V
2549 // the impl will have the following predicates:
2550 // <V as Iterator>::Item = U,
2551 // U: Iterator, U: Sized,
2552 // V: Iterator, V: Sized,
2553 // <U as Iterator>::Item: Copy
2554 // When we substitute, say, `V => IntoIter<u32>, U => $0`, the last
2555 // obligation will normalize to `<$0 as Iterator>::Item = $1` and
2556 // `$1: Copy`, so we must ensure the obligations are emitted in
2558 let predicates = tcx.bound_predicates_of(def_id);
2559 debug!(?predicates);
2560 assert_eq!(predicates.0.parent, None);
2561 let mut obligations = Vec::with_capacity(predicates.0.predicates.len());
2562 for (predicate, span) in predicates.0.predicates {
2564 let cause = cause.clone().derived_cause(parent_trait_pred, |derived| {
2565 ImplDerivedObligation(Box::new(ImplDerivedObligationCause {
2567 impl_def_id: def_id,
2571 let predicate = normalize_with_depth_to(
2576 predicates.rebind(*predicate).subst(tcx, substs),
2579 obligations.push(Obligation { cause, recursion_depth, param_env, predicate });
2586 impl<'o, 'tcx> TraitObligationStack<'o, 'tcx> {
2587 fn list(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2588 TraitObligationStackList::with(self)
2591 fn cache(&self) -> &'o ProvisionalEvaluationCache<'tcx> {
2595 fn iter(&'o self) -> TraitObligationStackList<'o, 'tcx> {
2599 /// Indicates that attempting to evaluate this stack entry
2600 /// required accessing something from the stack at depth `reached_depth`.
2601 fn update_reached_depth(&self, reached_depth: usize) {
2603 self.depth >= reached_depth,
2604 "invoked `update_reached_depth` with something under this stack: \
2605 self.depth={} reached_depth={}",
2609 debug!(reached_depth, "update_reached_depth");
2611 while reached_depth < p.depth {
2612 debug!(?p.fresh_trait_pred, "update_reached_depth: marking as cycle participant");
2613 p.reached_depth.set(p.reached_depth.get().min(reached_depth));
2614 p = p.previous.head.unwrap();
2619 /// The "provisional evaluation cache" is used to store intermediate cache results
2620 /// when solving auto traits. Auto traits are unusual in that they can support
2621 /// cycles. So, for example, a "proof tree" like this would be ok:
2623 /// - `Foo<T>: Send` :-
2624 /// - `Bar<T>: Send` :-
2625 /// - `Foo<T>: Send` -- cycle, but ok
2626 /// - `Baz<T>: Send`
2628 /// Here, to prove `Foo<T>: Send`, we have to prove `Bar<T>: Send` and
2629 /// `Baz<T>: Send`. Proving `Bar<T>: Send` in turn required `Foo<T>: Send`.
2630 /// For non-auto traits, this cycle would be an error, but for auto traits (because
2631 /// they are coinductive) it is considered ok.
2633 /// However, there is a complication: at the point where we have
2634 /// "proven" `Bar<T>: Send`, we have in fact only proven it
2635 /// *provisionally*. In particular, we proved that `Bar<T>: Send`
2636 /// *under the assumption* that `Foo<T>: Send`. But what if we later
2637 /// find out this assumption is wrong? Specifically, we could
2638 /// encounter some kind of error proving `Baz<T>: Send`. In that case,
2639 /// `Bar<T>: Send` didn't turn out to be true.
2641 /// In Issue #60010, we found a bug in rustc where it would cache
2642 /// these intermediate results. This was fixed in #60444 by disabling
2643 /// *all* caching for things involved in a cycle -- in our example,
2644 /// that would mean we don't cache that `Bar<T>: Send`. But this led
2645 /// to large slowdowns.
2647 /// Specifically, imagine this scenario, where proving `Baz<T>: Send`
2648 /// first requires proving `Bar<T>: Send` (which is true:
2650 /// - `Foo<T>: Send` :-
2651 /// - `Bar<T>: Send` :-
2652 /// - `Foo<T>: Send` -- cycle, but ok
2653 /// - `Baz<T>: Send`
2654 /// - `Bar<T>: Send` -- would be nice for this to be a cache hit!
2655 /// - `*const T: Send` -- but what if we later encounter an error?
2657 /// The *provisional evaluation cache* resolves this issue. It stores
2658 /// cache results that we've proven but which were involved in a cycle
2659 /// in some way. We track the minimal stack depth (i.e., the
2660 /// farthest from the top of the stack) that we are dependent on.
2661 /// The idea is that the cache results within are all valid -- so long as
2662 /// none of the nodes in between the current node and the node at that minimum
2663 /// depth result in an error (in which case the cached results are just thrown away).
2665 /// During evaluation, we consult this provisional cache and rely on
2666 /// it. Accessing a cached value is considered equivalent to accessing
2667 /// a result at `reached_depth`, so it marks the *current* solution as
2668 /// provisional as well. If an error is encountered, we toss out any
2669 /// provisional results added from the subtree that encountered the
2670 /// error. When we pop the node at `reached_depth` from the stack, we
2671 /// can commit all the things that remain in the provisional cache.
2672 struct ProvisionalEvaluationCache<'tcx> {
2673 /// next "depth first number" to issue -- just a counter
2676 /// Map from cache key to the provisionally evaluated thing.
2677 /// The cache entries contain the result but also the DFN in which they
2678 /// were added. The DFN is used to clear out values on failure.
2680 /// Imagine we have a stack like:
2682 /// - `A B C` and we add a cache for the result of C (DFN 2)
2683 /// - Then we have a stack `A B D` where `D` has DFN 3
2684 /// - We try to solve D by evaluating E: `A B D E` (DFN 4)
2685 /// - `E` generates various cache entries which have cyclic dependencies on `B`
2686 /// - `A B D E F` and so forth
2687 /// - the DFN of `F` for example would be 5
2688 /// - then we determine that `E` is in error -- we will then clear
2689 /// all cache values whose DFN is >= 4 -- in this case, that
2690 /// means the cached value for `F`.
2691 map: RefCell<FxHashMap<ty::PolyTraitPredicate<'tcx>, ProvisionalEvaluation>>,
2693 /// The stack of args that we assume to be true because a `WF(arg)` predicate
2694 /// is on the stack above (and because of wellformedness is coinductive).
2695 /// In an "ideal" world, this would share a stack with trait predicates in
2696 /// `TraitObligationStack`. However, trait predicates are *much* hotter than
2697 /// `WellFormed` predicates, and it's very likely that the additional matches
2698 /// will have a perf effect. The value here is the well-formed `GenericArg`
2699 /// and the depth of the trait predicate *above* that well-formed predicate.
2700 wf_args: RefCell<Vec<(ty::GenericArg<'tcx>, usize)>>,
2703 /// A cache value for the provisional cache: contains the depth-first
2704 /// number (DFN) and result.
2705 #[derive(Copy, Clone, Debug)]
2706 struct ProvisionalEvaluation {
2708 reached_depth: usize,
2709 result: EvaluationResult,
2712 impl<'tcx> Default for ProvisionalEvaluationCache<'tcx> {
2713 fn default() -> Self {
2714 Self { dfn: Cell::new(0), map: Default::default(), wf_args: Default::default() }
2718 impl<'tcx> ProvisionalEvaluationCache<'tcx> {
2719 /// Get the next DFN in sequence (basically a counter).
2720 fn next_dfn(&self) -> usize {
2721 let result = self.dfn.get();
2722 self.dfn.set(result + 1);
2726 /// Check the provisional cache for any result for
2727 /// `fresh_trait_ref`. If there is a hit, then you must consider
2728 /// it an access to the stack slots at depth
2729 /// `reached_depth` (from the returned value).
2732 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
2733 ) -> Option<ProvisionalEvaluation> {
2736 "get_provisional = {:#?}",
2737 self.map.borrow().get(&fresh_trait_pred),
2739 Some(*self.map.borrow().get(&fresh_trait_pred)?)
2742 /// Insert a provisional result into the cache. The result came
2743 /// from the node with the given DFN. It accessed a minimum depth
2744 /// of `reached_depth` to compute. It evaluated `fresh_trait_pred`
2745 /// and resulted in `result`.
2746 fn insert_provisional(
2749 reached_depth: usize,
2750 fresh_trait_pred: ty::PolyTraitPredicate<'tcx>,
2751 result: EvaluationResult,
2753 debug!(?from_dfn, ?fresh_trait_pred, ?result, "insert_provisional");
2755 let mut map = self.map.borrow_mut();
2757 // Subtle: when we complete working on the DFN `from_dfn`, anything
2758 // that remains in the provisional cache must be dependent on some older
2759 // stack entry than `from_dfn`. We have to update their depth with our transitive
2760 // depth in that case or else it would be referring to some popped note.
2763 // A (reached depth 0)
2765 // B // depth 1 -- reached depth = 0
2766 // C // depth 2 -- reached depth = 1 (should be 0)
2769 // D (reached depth 1)
2770 // C (cache -- reached depth = 2)
2771 for (_k, v) in &mut *map {
2772 if v.from_dfn >= from_dfn {
2773 v.reached_depth = reached_depth.min(v.reached_depth);
2777 map.insert(fresh_trait_pred, ProvisionalEvaluation { from_dfn, reached_depth, result });
2780 /// Invoked when the node with dfn `dfn` does not get a successful
2781 /// result. This will clear out any provisional cache entries
2782 /// that were added since `dfn` was created. This is because the
2783 /// provisional entries are things which must assume that the
2784 /// things on the stack at the time of their creation succeeded --
2785 /// since the failing node is presently at the top of the stack,
2786 /// these provisional entries must either depend on it or some
2788 fn on_failure(&self, dfn: usize) {
2789 debug!(?dfn, "on_failure");
2790 self.map.borrow_mut().retain(|key, eval| {
2791 if !eval.from_dfn >= dfn {
2792 debug!("on_failure: removing {:?}", key);
2800 /// Invoked when the node at depth `depth` completed without
2801 /// depending on anything higher in the stack (if that completion
2802 /// was a failure, then `on_failure` should have been invoked
2805 /// Note that we may still have provisional cache items remaining
2806 /// in the cache when this is done. For example, if there is a
2809 /// * A depends on...
2810 /// * B depends on A
2811 /// * C depends on...
2812 /// * D depends on C
2815 /// Then as we complete the C node we will have a provisional cache
2816 /// with results for A, B, C, and D. This method would clear out
2817 /// the C and D results, but leave A and B provisional.
2819 /// This is determined based on the DFN: we remove any provisional
2820 /// results created since `dfn` started (e.g., in our example, dfn
2821 /// would be 2, representing the C node, and hence we would
2822 /// remove the result for D, which has DFN 3, but not the results for
2823 /// A and B, which have DFNs 0 and 1 respectively).
2825 /// Note that we *do not* attempt to cache these cycle participants
2826 /// in the evaluation cache. Doing so would require carefully computing
2827 /// the correct `DepNode` to store in the cache entry:
2828 /// cycle participants may implicitly depend on query results
2829 /// related to other participants in the cycle, due to our logic
2830 /// which examines the evaluation stack.
2832 /// We used to try to perform this caching,
2833 /// but it lead to multiple incremental compilation ICEs
2834 /// (see #92987 and #96319), and was very hard to understand.
2835 /// Fortunately, removing the caching didn't seem to
2836 /// have a performance impact in practice.
2837 fn on_completion(&self, dfn: usize) {
2838 debug!(?dfn, "on_completion");
2840 for (fresh_trait_pred, eval) in
2841 self.map.borrow_mut().drain_filter(|_k, eval| eval.from_dfn >= dfn)
2843 debug!(?fresh_trait_pred, ?eval, "on_completion");
2848 #[derive(Copy, Clone)]
2849 struct TraitObligationStackList<'o, 'tcx> {
2850 cache: &'o ProvisionalEvaluationCache<'tcx>,
2851 head: Option<&'o TraitObligationStack<'o, 'tcx>>,
2854 impl<'o, 'tcx> TraitObligationStackList<'o, 'tcx> {
2855 fn empty(cache: &'o ProvisionalEvaluationCache<'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2856 TraitObligationStackList { cache, head: None }
2859 fn with(r: &'o TraitObligationStack<'o, 'tcx>) -> TraitObligationStackList<'o, 'tcx> {
2860 TraitObligationStackList { cache: r.cache(), head: Some(r) }
2863 fn head(&self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2867 fn depth(&self) -> usize {
2868 if let Some(head) = self.head { head.depth } else { 0 }
2872 impl<'o, 'tcx> Iterator for TraitObligationStackList<'o, 'tcx> {
2873 type Item = &'o TraitObligationStack<'o, 'tcx>;
2875 fn next(&mut self) -> Option<&'o TraitObligationStack<'o, 'tcx>> {
2882 impl<'o, 'tcx> fmt::Debug for TraitObligationStack<'o, 'tcx> {
2883 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2884 write!(f, "TraitObligationStack({:?})", self.obligation)
2888 pub enum ProjectionMatchesProjection {